Physics of Biological Membrane PDF

‫ﻣﺮﻛﺰ اﻟﺸﺎﻣﻞ‬ PHYSICS OF BIOLOGICAL MEMBRANE 1 Electrolytes are inorganic compounds, where Water is often necessary for the ionization or molecule split. The dissolved compound...

‫ﻣﺮﻛﺰ اﻟﺸﺎﻣﻞ‬ PHYSICS OF BIOLOGICAL MEMBRANE 1 Electrolytes are inorganic compounds, where Water is often necessary for the ionization or molecule split. The dissolved compounds in liquids (water) are broken down into ions. Ions are atoms, which have accepted more electrons (negative charges Ion) or lost electrons (positive charges Ion). When solid NaCl is dissolved in water, water splits the NaCl molecules into free Na + _ and Cl- _ ions, the solution so formed is called an electrolyte. The ions in electrolyte solution, generally are more free to move (have a higher mobility) than in solids. Living tissue is electrically an electrolytic conductor, where both intracellular and extracellular liquids contain ions free to migrate. Electrolyte solution has positive and negative ions, if two electrodes with potential difference are inserted into this electrolyte solution, ion will be attracted to the electrode with opposite charge. The positively charged ions move toward the cathode while the negatively charged ions move toward the anode. Therefore an electric field forms between the two electrodes and such movement of ions conducts an electric current through the electrolyte solution Figure (1). Figure (1) 1 / 75  2 Cell Membrane The basic building block of the human body is the living cell, and a prerequisite for its life is that it is surrounded by an electrolyte solution. Cells appear as transparent medium when viewed through a light microscope. In living tissue important communication control is implemented by hormones and nerves. Some cells are not excitable, for instance the cells of adipose and connective tissue or blood. They are passive, not under nerve control, and only weakly polarized. However, nerve, muscle and gland cells are polarized and excitable; within a 1/1000 second (1 m. second) such cells may react on trigger signals such as electrical, mechanical or chemical energy. The excitation of a cell is accompanied by an action potential. The action potential is the basic bioelectric event and signal source in the body. The cell is normally surrounded by a membrane, which act as partition to separate the interior of the cell from its surrounded medium. The membrane is a bi- layer lipid membrane (BLM) shown in Figure (2). The membrane of a living cell is a most complex and dynamic system. The structure of the membrane composed of lipids and proteins with minute pores distributed through the membrane, it is a major barrier to ion flux, but embedded in the membrane are channels, ion pumps and...etc. Figure (2) 2 / 75  3 ELECTRICAL PROPERTIES OF CELL MEMBRANE: MEMBERANE VISCOSITY The viscosity of a fluid will be constant regardless of the flow field if the temperature and the density are invariant. Fluids that consist of small molecules, such as air and water, have low viscosity comparing with blood viscosity. Temperature(C°) Water Viscosity (Pa·s) 30 0.797 × 10−3 40 0.653 × 10−3 Temperature(C°) blood Viscosity (Pa·s) 37 3×10−3 to 4×10−3 Physical studies have shown that the membrane exists in a liquid- crystalline state, where the viscosity of biological membrane ranges from 0.2 poise and upwards at physiological temperatures of 37C0. RESISTANCE The resistor is a circuit component that opposes current flow. Resistance (R) is measured in units of ohm(Ω).The relation between current (I ) and voltage (V ) is given by Ohm’s law, which is V = IR Where l = length of the conductor. a = cross-sectional area. σ = specific resistance. 3 / 75  4 Materials that present a very small resistance to current flow are called conductors. Materials with a very large resistance are called insulators. In the aqueous environment of the body, salt and various other molecules dissociate into positive and negative ions. As a result, body fluids are relatively good conductors of electricity. Still, these fluids are not nearly as conductive as metals. The electrical resistivity of the internal fluid is relatively higher than the external fluid, which is due to its relatively high specific resistance, a smaller volume and so a narrow cross-sectional area, but the plasma membrane offers the highest resistance that range between 102 - 105 ohms.cm-2 and consequently limits the flow of ions through itself. Therefore, it is a far weaker conductor because of its lipid matrix. DIELECTRIC A dielectric is an insulator material, which prevents the movements of charges from the negative plate to the positive plate through it. Basically, a perfect dielectric is a substance without free charges. It reduces the intensity of the electric field between the conductors. The dielectric constant is a main factor, which play an important role in the design of the capacitor, where the high dielectric constant enhances the capacitances. In living cell, membrane capacitance has significant function. Even if the conductivity of the membrane itself is very low, the membrane is so thin that the capacitance is very high, that is due to its thickness 7 nm and its dielectric constant of 3 – 10. This represents a large dielectric strength and it acts as a dielectric, but it is not a perfect dielectric. CAPACITANCE The capacitor is a circuit element that stores electric charges. In its simplest form it consists of two conducting plates separated by a dielectric (insulator) shown in Figure (3). In a charged capacitor, positive charges are on one side of the plate, and negative charges are on the other. Capacitance (C) is measured in farads (uF.cm-2). The relation between the stored charge (Q), and the voltage across the capacitor is given by 4 / 75  5 C= Q/V Figure (3) The intracellular and extracellular fluids are electrolyte solutions contain electrolyte ions act as conducting plates. The intracellular and extracellular electrolytes are well conductive; the cell interior is nearly completely insulated from the outside by a membrane. The parallel plate membrane capacitor has a constant and relatively high capacitance per unit area of the membrane (µF.cm-2) because the membrane is extremely thin, has relatively high dielectric constant (3 - 10), and the conductive fluids (outer and inner) offer relatively large surface area towards the membrane. The parallel-plate membrane capacitor is not perfect capacitors because the membrane is not perfect dielectric and the ions can diffuse through its pores leading to dielectric loss and it needs the active membrane transport to maintain its capacitance. CELL POLARIZATION (RESTING STATE): Cell polarization is generated by the ion pumps, which pump (drive) the ions against the electrochemical gradient. This energy-consuming mechanism polarizes the cell so that the interior of excitable cells has a potential about _ 90 mV 5 / 75  6 with respect to the extracellular electrolytes Figure (4). Such a pump is a molecular device, embedded in the cell membrane, capable of generating a net electric current across the membrane. The cell membrane is somewhat permeable to potassium ions and much less to sodium ions whereas chloride ions can readily pass through the membrane ,Figure (4) The distribution of ions across the cell membrane and the nature of this membrane provide the explanation for the membrane potential. The concentration gradient for K+ facilitates its movement out of the cell via K+ channels, but its electrical gradient is in the opposite direction (inward). The Na+–K+ pump, pumps three Na+ out of the cell for every two K+ it pumps in; thus, it also contributes a small amount to the membrane potential by itself. Millions of such pumps in one cell membrane polarize the cell to steady state so that the cell is fully polarized and ready to be triggered. This is the stage before the action potential and it is known as Resting Membrane Potential. Where the measuring values of Na+ and K+ concentration are as follow: Concentration inside Concentration outside Na+ 15 150 m Mol/ l K+ 150 m Mol/ l 5.5 6 / 75  7 ACTION POTENTIAL The cell membrane is polarized at rest, with positive charges lined up along the outside of the membrane and negative charges along the inside. When a surface of a single axon or section of a cell membrane (nerve or muscle) is excited effectively and if excitation exceeded firing level either by the flow of ionic current or by some form of external energy (chemical, thermal, mechanical, electrical…etc.), changing in membrane characteristics (properties) starts; resistance decreases and sodium ions enter the cell, thus the charge in the inner surface increases in the positive direction. Then inward current is formed and this led to decrease the barrier of the membrane to sodium ions, the sodium channel gate opens and Na+ flows into the cell, the result is rush of sodium ions into the cell and the cell is depolarized Figure (5). Figure (5) The potential crosses the zero line and soon reaches its maximum positive potential. This potential is known as the action potential and is approximately say +40 mV. The cell is said to be depolarized and the process of changing from resting state to the action potential is called depolarization, which is the beginning of an action potential. Once the action potential reaches +40 mV, the membrane closes the + Na channels, which rapidly enter a closed state in order to block the movement of sodium ions. In addition, the direction of the electrical gradient for Na + is reversed during the overshoot because the membrane potential is reversed, and this limits Na+ influx. The membrane permits some of the potassium ions to leave the cell, thus the outflow of potassium constitutes an outward current, where the polarity is 7 / 75  8 abolished for a brief period and actually reverses (from positive to negative) and falls rapidly toward the resting level as shown in Figure (6). Figure (6) The cell is said to be repolarized and assumes its resting potential when full repolarization has occurred. The excess sodium ions are actively pumped out of the cell whereas the excess potassium is actively pumped into the cell. STANDARD WAVEFORM OF A. P. In the normal resting state, the potential across the nerve fiber is about - 90mV. Standard waveform is graphical recording of an action potential of a single nerve fiber, which initiate at the resting potential depolarization, and return to the resting potential after full repolarized state as shown in Figure (7). Action potential is a beginning depolarization of the membrane. After the rate of depolarization increases. The point at which this change in rate occurs is called the firing level or sometimes the threshold. 8 / 75  9 Figure (7) Action potential wave form Thereafter, the waveform rapidly reaches (zero potential) zero line to approximately +40 mV. The sharp rise and rapid fall are the spike potential of the action potential, which is nearly +40 mV. The ascending phase on the action potential, called the depolarization phase, is produced by the inward current of sodium ions whereas the descending phase called the repolarization phase is produced by the outward current of potassium ions, which represent the reduction of potential from spike potential to resting state (-90mV). The total amplitude of the action potential is 130 mV (+40 - (- 90 mV)).That is defined as the difference between the potential of the depolarized membrane at the peak of the action potential and the resting membrane potential. LOCAL RESPONSE Normally channels have closed gates. A gate can be opened by a voltage change. The voltage gated is the voltage across the cell membrane that determines whether channel is opened or remains closed. A polarized cell may suddenly become depolarized according to the stimulus and threshold intensity, where threshold intensity varies with the duration; with weak stimuli it is long, and with strong stimuli it is short. 9 / 75  10 The action potential fails to occur if the stimulus is lower than threshold intensity (around -50mV) and such event is known as local response as shown in Figure (8). Figure (8) Action potential occurs with constant amplitude and form regardless of the strength of the stimulus if the stimulus is at or above threshold intensity. The local response can be defined as the event formed in the interval between resting state and the threshold (firing level). PERIODS LATENT PERIOD The stimulus artifact is followed by an interval (latent period) that ends with the start of the action potential and corresponds to the time it takes the impulse to travel along the axon from the site of stimulation to the recording electrodes as shown in Figure (9). Figure (9) 10 / 75  11 Its duration is proportionate to the distance between the stimulating and recording electrodes and inversely proportionate to the speed of conduction. REFRACTORY PERIOD This refractory period is divided into absolute refractory period, corresponding to the period from the firing level until the first-third of the repolarized phase, thus during the depolarization phase and the first 1/3 of the repolarized phase, the nerve remains absolutely refractory. Relative refractory period starts from the end of the first 1/3 of the repolarized phase, until its last third, during the 2/3 of the repolarized phase nerve remains relative refractory as shown in Figure (10). During the absolute refractory period, no stimulus, no matter how strong, will excite the nerve, but during the relative refractory period, stronger than normal stimuli can cause excitation. Figure (10) 11 / 75  1 Transmission of Nerve Action Potential Electrical potential exists across the enveloping membrane of living tissue. Many cells such as nerve, muscles and gland have the ability to transmit a change in these potentials along their membrane. The transmission can be in either direction. Muscles fibers can transmit action potentials which result in a contraction of the muscles. The transmission of nerve action potentials is brought about by the flow of ionic current around the nerve fiber. These currents are adequate to depolarize neighboring cells or adjacent areas of the same cells. As a result a wave of a depolarization will travel along the membrane without attenuation (damping), followed by a wave of repolarization. The energy used in propagation does not from the stimulus, but is released by the nerve fiber along its length. Nerves normally conduct action potential (impulses) in one direction-toward the central nervous system in sensory fibers, and away from central nervous system in motor fibers, but all nerves can conduct action potential in both directions. The nerve action potential in one fiber is of constant amplitude and shape and that characteristics can’t be altered by changing the strength or the quality of the stimulus. Nerve action potentials have an amplitude of approximately 0.1 V (100 mV), and a duration of 1 msec. their amplitude is measured between the inside and the outside of the nerve fibers are of two types: myelinated and non- myelinted nerve fiber, based on that the transmission can be explained as follow. (A) Transmission of Nerve A. P. along a non-myelinted nerve fiber In non-myelinted nerve fibers there is no fatty sheath (myelin) and the fiber consist of a cylindrical semipermeable membrane which surrounds the axon of a nerve cell. The membrane, which separates the axoplasm from the external solution, acts as a barrier and prevents the ions in the external solution from mixing rapidly with the internal solution. The membrane has a high electrical resistance in a resting axon and electrical capacity. During the nerve action potential the conductivity of the membrane increases about 100 times, and sodium and potassium ions move down their concentration gradients. 12 / 75  2 In the normal resting state, a potential of about -90 mV exists across the nerve fiber. When a nerve cell with a long nerve fiber is stimulated at some point, the stimulus produces local depolarization which will be propagated along the nerve fiber. Due to the difference in potential between the adjacent depolarized and polarized portions of the nerve membrane, some cations flow from the depolarized to the polarized portion through the axoplasm while some anions flow in opposite direction. i.e. from the polarized to the depolarized portion through the surrounding interstitial fluid. These currents are sufficient to stimulate the next inactive portion of the membrane and consequently, the membrane of the inactive portion gets depolarized to fire an action potential there as illustrated in Figure (5). Figure (5) continuous conduction in an unmyelinated nerve These events are repeated in subsequently parts of the nerve fiber and a wave of depolarization is propagated along the fiber without attenuation followed by a wave of repolarization. After a brief period (refractory period), the nerve fiber becomes capable of transmitting a new action potential. (B) Transmission of Nerve A. P. along a myelinted nerve fiber In myelinted nerve fibers the axoplasm of the fiber is shown surrounded by bands of fatty materials called bands of myelin as shown in Figure (6). The short gaps between the bands are called the nodes of Ranvier. Myelin is a good insulator, 13 / 75  3 which enables the action potential to skip from one node to the next and if myelin is continuous along the nerve, no action potential are possible. Since myelin is relatively thick, it has a low capacitance and high resistance against the radial flow of ions across it. Figure (6-A&B) saltatory conduction in myelinated nerve At the nodes, the irritable axon membrane is in direct contact with the surrounding interstitial fluids.The arrival of an action potential depolarizes the membrane of a node by enhancing the membrane permeability to sodium ions and then to potassium ions. Due to the difference in potential between the depolarized node and the polarized node, some cations flow through the axoplasm from the depolarized node to the next polarized node and through the interstitial fluid in the reverse direction. As a result, the membrane of the inactive node gets depolarized 14 / 75  4 to fire action potential (spike) there, but such formation of action potential can’t occur in the myelinated bands. The action potential thus, groups from one node to the next. This process is called salutatory conduction which permits an action potential to be transmitted with a higher velocity a long a myelinated nerve than along a no-myelinated nerve of similar axonal diameter. Conduction Velocity In nerve fibers the rate at which an action potential moves down a fiber or is propagated from cell to cell is called conduction velocity. It varies widely, depending on nerve diameter, temperature and myelinated. It ranges from 20 to 150 msec. the conduction velocity is higher for myelinated than for non myelinated and for their nerves. The thinner the nerve fiber, the smaller is the cross-section area and therefore the higher is the resistance of the axoplasm against the ionic current. Thus the time needed for the formation of action potential (spike) in active region is prolonged therefore, the speed of the spike falls and the duration increases in proportion to the fall in the fiber diameter. Larger nerves conduct action potential faster than then nerves. In muscles, the conduction is much slower and contraction follows the development of action potential. The rate of travel of action potential through the muscle is slower with 0.2 to o.4 msec. on the average. 15 / 75 ‫ﻣﺮﻛﺰ ﺍﻟﺸﺎﻣﻞ‬ ‫‪16 / 75‬‬  1 ELECTRICAL CONDUCTION SYSTEM OF THE HEART The electrical conduction system of the heart consists of specialized tissue that spontaneously generates and distributes action potentials through the heart. The components of the conduction system are the sinoatrial node (SA node), atrioventricular node (AV node), bundle of His, and Purkinje fibers as shown in Figure1. The bundle of His divides into the right and lift bundle branches, which are continuous with the Purkinje network of small fibers within the ventricular walls. This finally balanced conduction system is subject to many faults, due to congenital abnormalities and also to acquired heart diseases. Figure (1). The electrical conduction system 17 / 75  2 Cardiac Rhythm The contraction of the cardiac muscles is initiated by the depolarization of cell membrane of cardiac muscle. Each contractile chamber of the heart produces electrical action and recovery potential associated with mechanical contraction and relaxation (recovery). The general rule is that the bioelectric event precedes the mechanical event. Thus, each contractile chamber exhibits spontaneous rhythmicity. There are certain areas (regions) within the heart muscles that display spontaneous rhythmicity. These areas are the SA node, the AV node and the His-Purkinje cells. These areas are called auto-rythmic. In an adult man, the basic depolarization rate of the SA node 70/80 times/minute, the basic AV node rate is 40/50 times/minute, and that of His-Purkinje cells is 30-40 times/minute. The auto-rhythmic area with the highest degree of rhythmcity is the SA node that determines the cardiac frequency of contraction. To understand the electrocardiogram it is necessary to know the electrical activity of the SA node cell and that of the ordinary myocardial cell. Action potential of a single SA node cell The heart’s natural pacemaker is the sinus node (SA node), a small area or region of specialized tissues that spontaneously generate action potentials at a regular rate it is located near the top of the right atrium. Figure (2) S A node cell 18 / 75  3 In the resting state, the outside of the membrane of a single SA node cell is positively charged while the inside is negatively charged, the membrane potential being -90 mV. This resting membrane potential slowly changes due to steady reduction in the permeability of the cell membrane to potassium ions. Potassium ions are actively pumped into the cell and this inflow of ions is balanced by a passive leakage of potassium ions from the cell (outflow). If this leakage is reduced, the inside of the cell will become more positive. When the potential difference across the cell membrane reaches -75 mV, the membrane becomes permeable to sodium ions (and then to potassium ions), which rush into the cell, giving the rapid depolarization. This is followed immediately by a rapid repolarization. This self- depolarization process is continuous and is conducted to the cardiac muscle leading to the regular beating of the normal heart. Action potential of the myocardial cell In the resting state, the interior of the myocardial cell is negatively charged with respect to the extracellular fluid. The resting membrane potential is -90 mV. This ionic imbalance will remain undisturbed unless an adjacent cell undergoes depolarization. The depolarization wave from the SA node causes a rapid depolarization of the myocardial cell by permitting sodium ions to enter the cell cancelling the negative potential inside. This rapid electrical discharge (depolarization) causes the cell to contract for some 200 – 350 msec. the rapid depolarization is followed by a period in which the potential changes slowly, after which the cell repolarizes, returning to its former state. The membrane potential then remains constant at - 90 mV until the next triggering depolarization wave arrives from the SA node. The cardiac muscle cell has an absolute refractory period of a bout 250 msec., starting after depolarization of the membrane, during which the membrane is completely insensitive to a stimulus. In figure no: 3 the absolute refractory period is followed by a relative refractory period that occur during repolarization, during which a larger than normal stimulus is needed to initiate depolarization. 19 / 75  4 Figure (3) normal cardiac cell The frequency of contraction is mainly determined by the speed with which the potential changes from -90 mV to - 75 mV. This rate of change is slower in the AV node cell and slower still in the His-Purkinje cells, thus giving their characteristic rhythm. Transmission of electrical signal The contraction of the heart chambers are triggered by electrical signals originated within the heart itself. Each action potential in the heart originates at the SA node. The SA node spontaneously generates and emits action potentials (signals) at a regular rate of about 70-80/min. Each of these signals triggers one cycle of the cardiac activity. To initiate a heart beat, the excitation wave from the SA node spreads through the ordinary myocardial cells of the right atrium and is conducted rapidly to the left atrium along a specialized bundle of fibers so that both atria are stimulated and contracted together within 100 msec. of the initial wave of depolarization leaving the sinus node. The contraction of the atria forces blood into the ventricles. The wavefront of excitation (depolarization wave) travels parallel to the surface of the atria toward the junction of the atria and ventricles. The depolarization wave finally terminates at the AV node, a point near the center of the heart. The atria are insulated from the 20 / 75  5 ventricles by a ring of non-conductive fibrous tissues. The only conducting tissue joining the atria to the ventricles is the AV node. The AV node has a low propagation velocity and therefore delays the spread of the excitation to ventricles by about 100 msec. to give time for the ventricles to be filled with blood. After this brief delay the excitation wave from the AV node is very rapidly conducted down through the myocardial cells of the ventricles by further specialized pathways of conductive tissue the bundle of His, the left and right bundle branch and Purkinje fibers to initiate action potentials simultaneously in the ventricles causing them to contract together. The ventricular contraction pumps out blood into the aorta from the left ventricle and into the pulmonary artery from the right ventricle. The wavefront of excitation doesn’t follow along the surface, but is perpendicular to the surface and moves from the inside to the outside of ventricular wall, terminating at the tip or apex of the heart. A wave of repolarization follows the depolarization by about 0.2 to 0.4 sec. This repolarization occurs as each cell returns to its resting potential independently. After each contraction the heart relaxes, the atria are refilled with blood, and another signal is generated in the SA node to start the next cycle of the cardiac activity. Each cycle of the cardiac activity consists of three main components: atrial depolarization, ventricular depolarization and repolarization of the cardiac muscle. These three activities are reflected in the electrocardiogram. Electrocardiogram A convenient method of studying the cardiac cycle employs a technique called electrocardiogram, which is a record of the electrical activity of the heart obtained with body surface electrodes. When electrodes are placed on or in the heart to detect the electrical activity of the heart, the technique is called cardiac electrogram. The electrocardiogram, as conventionally recorded, reflects the varying potential difference between a pair of electrodes. This potential difference is a small and time-dependent. The two masses of the heart-atria and ventricles are made up of muscles fibers. Each of which produces its own action potential. The temporal and the spatial summation of the activities (action potential) of all the myocardial fibers results in the electrocardiogram, abbreviated ECG, which controls the 21 / 75  6 contraction of the heart. Figure no: 4 shows the basic waveform of the normal electrocardiogram as it appears when recorded from the surface of the body. Figure (4) normal ECG The horizontal segment of this waveform is designated as the baseline. The normal electrocardiogram consists of a series of deflection from the baseline produces by the depolarization and repolarization of the cardiac muscles cells. The first low amplitude, broad upward deflection (P- wave) represents atrial depolarization. This signal causes the atria to contract, forcing the blood into the ventricles. The actual contraction of the atria follows the P-wave by a fraction of second. The start of the P-wave is the beginning of depolarization of the SA node. After an interval of 0.1 – 0.2 msecond from the onset of the P wave there is a bigger or (main) deflection known as the QRS complex or simply QRS wave. It indicates the depolarization of the large ventricular tissue mass and the repolarization of the atria, which occur almost simultaneously. The QRS waveform consists of three separate waves, the Q wave, R wave and S wave. It begins as a short downward deflection (Q -wave) followed by a sharp upward spike (R-wave) and ends as a down deflection at the base (S-wave). The QRS complex is followed by low amplitude, broader upward deflection (T-wave), which is produced by ventricular repolarization. It occurs at the end of the ventricular 22 / 75  7 contraction and causes the relaxation of the ventricles. Thus, the electrocardiogram is composed of both depolarization and repolarization. The waveform of the ECG signal represents the variations in the ECG potentials during the cardiac cycle. The configuration of the ECG and the duration of the various waves carry important physiological and diagnostic information. The shape of the ECG depends strongly on the location (placements) of the measuring electrodes with respect to the heart and the condition of the heart. In healthy individuals the ECG remain reasonably constant, even though the heart rate changes with the demand of the body. Any pathological condition that disturbs the electrical activity of the heart will produce characteristic damages in one ore more of the waves such as changes in the magnitude, intervals or duration of the wave. Thus, understanding the normal ECG pattern is clinically important. ‫ﻣﺮﻛﺰ ﺍﻟﺸﺎﻣﻞ‬ 23 / 75  1 THE ELECTRICAL FUNDMENTAL OF THE ECG The ECG is graphical recording of the electrical potentials generated in association with heart activity. In 1901 Einthoven first described the use of a string galvanometer to detect the presence of electrical activity in the adult heart that resulted in the birth of electrocardiography. The ECG shown in Figure1 is an ideal tool for the cardiologists. It is non-invasive, inexpensive, easy to use and it yields a wealth of information. Figure 1 normal ECG In the normal human heart, the valves maintain a unidirectional flow of blood with minimal frictional resistance, whilst almost completely preventing reverse flow. The left ventricular free wall and the septum are much thicker than the right ventricular wall as shown in Figure 2. This is logical since the pumping action that forces blood out to the body and lungs are provided by the ventricles. This ventricular contractile phase is called the systole, while the atrium is responsible for storing the blood during this time. After ventricular contraction, the atria contract to force blood into relaxing ventricles. This ventricular relaxation phase is called diastole. There are two types of electrocardiography invasive, which use to measure the cardiac signal direct from the heart surface, and the common method that is the 24 / 75  2 non-invasive electrocardiography; it has been used to obtain valuable clinical information about the patient well being by applying the electrodes on the chest surface. The current generated by the heart spreads out on the body surface regularly. That is due to the properties of the cardiac cells, automaticity and the ability to trigger electrical signals. Figure 2 the anatomy of the heart In order to measure the ECG signal, different electrodes can be used for this purpose (usually and the most used is piece of silver or it may be covered with silver chloride Ag/AgCl) where the contact area of the skin should be covered with a gel regarding good conduction between the electrode and the skin surface. It used also in stress ECG to reduce the motion artifact. In addition to that there are electrocardiographic leads system, consists of 12 leads are used for recording the electrical activity of the heart. Each lead is responsible for recording a cardiac signal from special selected location on the chest. These signals are known and written as follow: I, II, III - aVR, aVL, aVF - V1,V2, V3, V4, V5 and V6. 25 / 75  3 1-Einthoven’s method (bipolar): I, II, III is called also Einthoven’s triangle or can be known as limb leads. This type of measurements also is identified as bipolar configuration. These ECG electrodes are placed on the right and left arms and the left leg, where the voltages across the three pairs of these electrodes are monitored. It represents the potential difference between each two electrodes, and are called I (VI = Vleft arm − Vright arm), II (VII = Vleft leg − Vright arm), and III (VIII = Vleft leg − Vleft arm), These three electrodes act as if they probe at the vertices of a triangle, which is usually called the Einhoven’s triangle, as shown in Figure 3. Because the arms and legs do not have new sources of electric fields and the tissue in each is a conductor, the probes on the arms actually sense the same voltages as if they were instead placed on the respective shoulders and the probe on the leg has the same voltage as if it were placed on the bottom of the torso (upper body) near the pubic area, an electrode is positioned on the right leg (not shown in this figure) to serve as an electrical ground. Figure 3 Einthoven’s method I represents the electrical potential between R(RA) and L(LA) II represents the electrical potential between R(RA) and F(LL) III represents the electrical potential between L(LA) and F(LL) Then the sum of these potentials as close circuit equal zero I - II +III =0 II = I + III 26 / 75  4 Lead I or 1 = LA – RA Lead II or 2 = LL – RA Lead III or 3 = LL – LA This is known as Einthoven’s Law. Thus if any two of the three potential differences are measured the third can be calculated. 2 - Goldberger’s method (augmented): In a unipolar configuration, one electrode is placed over the region of interest and the other electrode is located at some distance away from the tissue as shown in Figure 4. aVR represents the electrical potential between the right arm and indifferent electrode, which connect between LA and LL. aVL represents the electrical potential between the left arm and indifferent electrode, which connect between RA and LL. aVF represents the electrical potential between the left leg and indifferent electrode, which connect between LA and RA. Figure 4. Goldberger’s method These electrodes are usually referred to as the exploratory and indifferent electrodes (point between each two resistance), respectively. Unipolar signals represent electrical activity from an entire region and only the electrical events at the exploratory electrode. 27 / 75  5 3 - wilson’s method (unipolar): It is unipolar method used for recording the electrical potential from 6 point on the chest, V1, V2, V3, V4, V5, and V6. In standard ECG registering, the indifferent electrode is replaced by Wilson’s terminal, and Figure 5 illustrates all the locations of the three methods Einthoven, Goldberger and Wilson. Figure 5. Th e 12 ECG leads and the Einthoven’s triangle. The electrical potential should be measured between each one of V1, V2, V3, V4, V5, and V6 electrode with the Wilson Terminal (WT) for all three electrodes together, the right arm, left arm and left leg as shown in Figure 6 b. The electric current generated by the heart is conducted through the wires or transmitted wireless by radio to the recording device, which consists fundamentally of an amplifier that magnifies the electric signals and a galvanometer that moves the inscription needle. The needle moves in accordance with the magnitude of the electrical potential generated by the patient’s heart. The needle inscribes a positive or negative deflection, depending on whether the explorer electrode of a given lead faces the head or the tail of the depolarisation or repolarisation vector. 28 / 75  6 Figure 6. Location of the precordial leads: (a) frontal plane with the scheme of unipolar register.WT stands for Wilson’s terminal, (b) galvanometric recorder. There are different types of the ECG needles, which can be used for ECG recording (thermal traces, pigment traces or color paste traces), where thermal traces needle is the most use. In addition to that the ECG recording device are divided according the number of the needles required (designed), one channel, three channels or six channels ECG recorder. These recorders usually used 25 mm/s recorder speed, where 50 mm/s speed is used for special cases only. At the end of the electrocardiography process the data can be recorded on thermal paper, display on monitor (CRT) or stored for analyzing in digital form (memory). How to calculate the heart rate (HR): Calculating the HR is one of the ways to assist the physician to diagnose the situation of the Patient, different methods are used to calculate the HR, the main step for all these methods is to detect the R peaks of the QRS complexes, using special algorithm for real time, or manually for ECG paper according Figure 7. After detecting the R peaks, the time can be calculated as follow (a) ECG in digital form : T = t(R2) - t(R1) or T = the time between two adjacent R peaks in (sec). 29 / 75  7 HR = (1/T)*Fs*60 (beat/minute) this method usually used in research areas. Figure 7. intervals and segements of ECG signal (b) ECG on thermal paper: The area of this paper is divided to square units (5mm x 5mm), each unit consists of 25 small squares, and the area of each small square is 1mm x 1mm. All these squares are separated by red Lines. In order to calculate the HR, the doctor should determine the locations of the R peak and the location of the next R peak, then calculates the number of the squares (1mm x 1mm) between the two R peaks, where the time required for 1mm is 0.04 (second), then the HR can be calculated as follow: 1 *60 (beat/minute) HR = {[number of the squares * 0.04(sec)]} In addition to that there are another possibilities for calculating the HR using different techniques, such as Phonocardiography (PCG) Magnetocardiogram (MCG) and Doppler Ultrasound. The nature of the ECG signal usually is weak (in milivolt), due to the noise from the electrical sources and other bioelectric interference, some kind of signal 30 / 75  8 processing is included in the electrocardiograph device to enhance the ECG signal by attenuating the noise. An example of such signal with and without noise is illustrated in Figure 8. In order to avoid and reduce the interference or the noise, some rolls should be followed: the patient and the recorder should be grounded very well, patient should be relaxed and in supine position and breathing should be calmly. Figure 8. ECG signal before and after processing There are two Patient cable standards for distinguishing the ECG leads by mark or color according to their location on the body surface as in Table1. Table1: standards electrodes Patient cable, IEC standard patient cable, AHA standard Marked plug color electrode location Marked plug color R red right arm (RA) white L yellow left arm (LA) black F green left leg (LL) red N black right leg (RL) green C1…C6 chest V1…V6 chest 31 / 75 ‫ﻣﺮﻛﺰ اﻟﺸﺎﻣﻞ‬ ‫‪32 / 75‬‬ 1 HIGH FREQUENCY CURRENTS IN MEDICINE “DIATHERMY” Electro-surgery system replaces some previous (old) tools to stop hemorrhage (bleeding) such as cauterizing and using hot oil. The art of surgery has been practiced for thousands of years but bleeding was always a problem. Cutting into flesh in order to remove a body part or foreign object or to repair some kind of damage, resulted in many small and large blood vessels being severed. Larger vessels could sometimes be tied off, but the smaller ones were impossible, and the surgeon and patient just had to wait until the body's natural clotting mechanism stemmed the flow, hopefully before the patient bled to death. Besides this rather inconvenient side effect, bleeding obscured (covered) the surgical site, making it hard for the surgeon to see what he was doing. The use of high frequency current offers a number of important advantages. The separation of tissues by electric current always takes place immediately in front of the cutting edge. Electric cutting therefore does not require any application of force. Instead it facilitates elegant and effortless surgery. The electrode virtually melts through the tissue instantaneously and seals capillary and other vessels, thus preventing contamination (infection) by bacteria. A simplified method of coagulation saves valuable time since bleeding can be arrested immediately by touching the spot briefly with the coagulating electrode. A high frequency apparatus is regarded as standard equipment in the operating theatre. Now a day Diathermy can be used to facilitate the effort of the surgeon instead of using scalpel in the field of surgery. There are two main modes used of the diathermy system: 33 / 75 2 a) Cutting mode Cutting soft tissue. b) Coagulation mode Coagulation of blood vessels. Electro-surgery equipment: This medical device is one of the most important equipment in hospital (Figure 1). The idea with RF is to create a lesion by generating localized heat at the location of interest. The main electrode in such systems is the smaller electrode (hand-piece), which is also known as the “active” electrode, is connected to the RF generator. (Figure 1) Modern unit consists of the following parts; the main unit includes the heart of the electronic system, in addition to the front panel at the 34 / 75 3 front side of the unit and the wires connected to the main unit (two electrodes, foot switch and ground). a) - Main unit: Box made of hard plastic or metal, includes power supply, the heart of the electronic system that is the logic and control part which produces the basic signal and provides various timing signals for the cutting, coagulation, haemostasis modes of operation and a stable multi- vibrator generates 500 kHz square pulses. b) - Front panel: it mainly used for input data and consists of different touch switches for selecting type of function (cutting or coagulation) or for selecting proper values of functions according to the patient body (decreasing or increasing). c) - Two electrodes are connected to the main unit with wires, active electrode and dispersive electrode in addition to the dispersive electrode and foot switch. Mono-polar Technique: Active electrode consists of long wire ends with the holder or handle (pen shape), where the surgical electrode should be fixed in a hole at the tope of this holder (Figure 2). (Figure 2) Consequently, the electrodes are used either with electrode handles with finger-tip switch, or without finger-tip switch, then foot switch 35 / 75 4 replace it in triggering electric current. Depending upon the intensity and duration of the current, a high local increase in heat will be obtained. The tissue changes due to drying and limited coagulation. Different types of surgical electrodes are available and one only can be fixed there as shown in Figure (3). It is connected to the RF generator, which provides high frequency current according to the requirement of the patient operation for cutting or coagulation. (Figure 3) The wire that carries the RF to the active electrode is shielded by a low dielectric loss insulator to prevent heating of undesired areas. In the monopolar technique the current flows from the active electrode through the patient to the neutral electrode (patient plate) from which it returns to the generator. Bi-polar Technique: 36 / 75 5 In this technique, two electrodes are used. The current in this case flows through the tissue between the tips of the two electrodes and returns to the generator without passage through the patient. The bipolar surgery is not only safer than monopolar but is also more precise since the current only flows locally at the specific site where it is actually required for heat generation. In addition, the risk of inadvertent burning of the patient at the patient plate is very low. Therefore, the bi-polar technique is becoming a method of choice wherever possible. In some procedures, especially those performed laparoscopically a return electrode arrangement is not practical. In such circumstances, a bipolar arrangement is used, in which the source and sink of current are close to each other. This is usually in the form of forceps or pincers in which one jaw is source and the other sink. This makes a separate return electrode and associated wiring unnecessary, but is only useful for small structures such as fallopian tubes or intestinal polyps. Solid-state machines mostly incorporate an independent bi-polar RF generator for microsurgery procedures offering a fine output power control. The output waveform is a damped sinusoid at a repetition lower frequency, which has power output about 20 W. Dispersive electrode: It is a plate, which has square shape of different sizes; this plate which is connected directly to the surface of the skin has large area, where the currents passing through the body can be collected in this plate has low density due to the large area of the plate comparing with the active electrode. There is no universal agreement over the safe effective area of the indifferent electrode necessary for diathermy current to exit without 37 / 75 6 causing a rise in skin temperature. Indifferent electrodes are currently available in the range from 50 to 200 cm2. The most common reason for faulty performance of an electro-surgical unit is improper placement of the indifferent electrode. This electrode must be placed in firm contact with a fleshy portion of the patient and as near as possible to the operating site. Foot switch: The surgeon usually holds the active electrode using the pen holder (insulator), which protect the surgeon fingers during operations, so the surgeon uses the foot switch to trigger the high frequency current from the generator to the active electrode. The spark-gap generator produces damped high frequency current which is specifically suitable for the coagulation of all kinds of tissues. The mixing of both these currents signifies one of the most important possibilities for use in electro-surgery. By blending the currents of the tube and spark-gap generator, the degree of coagulation of wound edges may be chosen according to the requirements. The concurrent (simultaneous) use of continuous radio-frequency current for cutting and a burst wave radio- frequency for coagulation is called Haemostasis mode. With the invention of the transistor (high voltage power transistor), solid state transistor makes the pure continuous cutting available in the field of surgery. This lead to new generation of electrosurgery equipments, which are developed; these new equipments can be used for both cutting and coagulation. Developed Solid-state generators have replaced a substantial number of vacuum tube and spark-gap units 38 / 75 7 (generators). Cut waveform, coagulate waveform and blend waveform are shown in Figure 4. (Figure 4) Power output: In electro-surgery the selection of power depends strongly on the type of the tissue. Electro-surgical devices are currently capable of delivering up to 500 W of power at fundamental output frequencies ranging from 0.3 to 3 MHz. and in some units, significant harmonic components exist beyond 5 MHz. Low power is used for dermatology, dentistry and ophthalmic surgery. Some procedures such as urological operations need high power for cutting tissue that is Due to the solutions (saline and urine) around this tissue. Units of maximum power output of around 170 watts are becoming common. 39 / 75 8 The temperatures produced at the points at which the electric arcs contact the tissue like microscopic flashes of lightning are so high that the tissue is immediately evaporated or burned away. A voltage of approximately 200 V is required in order to produce the electric arc between a metal electrode and biological tissue. If the voltage is less than 200 V, the electric arcs cannot be triggered and the tissue cannot be cut. The voltage suitable for cutting biological tissue ranges between 200V - 500V. If the voltage rises above 500V, the electric arcs become so intense that the tissue is increasingly carbonized and the cutting electrode may be damaged. The maximum output voltage can become so high (above 600°C) that severe carbonization occurs. Conversely, the minimum value of the output voltage can become so low (below 200°C) that cutting action is not achieved. Biological tissue is coagulated by thermal means if the requisite temperature is maintained at around 70 C0. In order to facilitate identification of each mode of operation, the machines incorporate an audio tone generator. The tone signals are derived from the counter at 1 kHz (coagulatioti), 500 Hz (cutting) and 250 Hz (haemostasis). With experimentation, it was found that certain frequencies of electricity caused more nerve and muscle stimulation than others, and ESUs were designed to avoid these frequencies. Most units operate at frequencies in the range above 200 KHz. Since it is impossible for a cell to depolarize at this rate, the electrical resistance of the tissue produces localized intracellular heating without the accompanying muscle contraction. Different types of diathermy electrodes are shown in Figure 5. 40 / 75 9 (Figure 5) Cutting of the soft tissue: By applying a continuous radio frequency (RF) current to the active electrode, radio frequency current will be concentrated at the tip of the electrode, which means that a current with high density is formed at that tip of the electrode (see Figure 6). The heating occurs at a depth of penetration of a few millimeters around the electrode tip, as result the temperature of the site under the tip increases rapidly. Once it reaches to the vaporization level the process of cutting starts. This process depends on the rupturing of the cellular fluids, thus as the electrode is advanced the tissue will be separated with the similar rate, so the surgeon should advance the active electrode in comparative rate (5 to 10 cm/sec) in order to avoid spoiling the adjacent tissue. 41 / 75 10 Figure (6) Coagulation of blood vessel: Divergent tool such as a pair of forceps is the proper electrode for coagulating the blood vessel by clamping its edges, so that the bleeding can be stopped easier with less time than stitching. In some cases the electrosurgical unit produces the coagulation current, which is applied to the active electrode connected to a forceps, which clamps the vessel, and actually can be acts as coagulating electrode (Figure 7). Thus the effect of the heating by the RF high frequency current causes the coagulation. The site and type of tissue to be operated plays an important role on the surgeon option in selecting the type of current, the shape of electrode and the power needed. 42 / 75 11 Figure (7) Automated Electro-surgical Systems In a conventional electro-surgical unit, there is a considerable fluctuation of the output voltage throughout the period of the cut. The cause of this undesirable fluctuation is linked to the following factors: 1- Size and Shape of the cutting Electrode: 2- Type and Speed of cutting: The cutting quality is determined by the speed with which the electrode is moved (quick or slow) and by the type of cut (superficial or deep) 3- Different Tissue Properties: The tissue with a high resistance (fat) or tissue with a low electric resistance (nerves and blood vessels) has a strong influence on the quality of the cut. The variations in the output voltage due to the above factors considerably affect the quality of the cut. The automatic control (microprocessor controlled automated systems) operates on two different criteria: - Voltage control: whereby the selected voltage is controlled and held constant. - Spark control: by which the selected spark intensity is held constant. 43 / 75 12 The microprocessor-controlled machine also provides the following coagulation modes: - Soft coagulation: In this, no electric arcs are produced between the coagulation electrode and the tissue, which is in direct contact with the tissue to be coagulated. during the entire coagulation process to prevent the tissue from becoming carbonized. - Forced Coagulation: This is characterized by the fact that electric arcs are intentionally generated between the coagulation electrode and the tissue in order to obtain deeper coagulation than could be achieved with soft coagulation, particularly when using thinner or smaller electrodes. Spray Coagulation: In this, electric arcs are deliberately produced between the spray electrode and tissue so that direct contact between electrode and tissue becomes unnecessary. Spray coagulation is used both for surface coagulation and haemostasis of vessels not directly accessible to coagulation electrodes, such as those hidden in bone fissures. These types of diathermy electrodes are shown in Figure 8. Figure 8 44 / 75 13 Lateral Heat : During an electrosurgical procedure, inadvertent (unintended) heating of tissue adjacent to the surgical site is possible. This lateral heating results from the resistance of the adjacent cells to RF wave current flow. By controlling the electrode size, the time it contacts the tissue, and the type and intensity of the current, thus heating can be minimized. The electrode should be in contact with the tissue for a maximum of (1 to 2) seconds. At the proper current setting the RF wave passes through the tissue and produces a slight rise in temperature, which causes the volatilization of one cell layer and leaves adjacent cell layers intact (undamaged). If the current is set too low, it will result in drag (the inability of the electrode to cut the tissue efficiently); if it is set too high, it will create sparking and result in excessive heat at the tissue site. Absolute familiarization (accommodation) with the unit is crucial to achieve optimal results and also to avoid scar tissue, which may be formed by the heat delay in the same area ‫ﻣﺮﻛﺰ ﺍﻟﺸﺎﻣﻞ‬ 45 / 75  RADIATION X-RAYS: X-rays are a form of electromagnetic radiations like light, radio waves, and so on. X-rays were discovered accidentally by Wilhelm Roentgen, a German physicist. He found out that most of the materials allowed the new ray to pass through and also left a shadow on a photographic plate. Since Roentgen’s days, x-rays have found very widespread uses and are used across different fields such as radiology, geology, crystallography. In the field of radiology, x-rays are used in digital X-ray, fluoroscopy, angiography, computed tomography (CT). Today, many of the noninvasive surgeries are performed under x-ray guidance. An x-ray machine consists of an x-ray tube, control unit and high voltage generator. X-RAY TUBE CONSTRUCTION: An x-ray tube consists of the following major parts. They are an anode, cathode, and an evacuated glass tube (made of pyrex glass) to hold these parts together (Figure 1). Figure 1 46 / 75  The cathode is the negative terminal that produces electrons that will be accelerated toward the anode. The filament is heated by passing current, which generates electrons by a process of thermionic emission. The number of electrons produced is proportional to the current impressed upon it. The filament is generally made from tungsten to withstand high temperatures. The electron produced is focused by the focusing cup, which is maintained at the same negative potential as the cathode. The glass enclosure in which the x-ray is generated is evacuated, so that the electrons do not interact with other molecules and can also be controlled independently and precisely. The focusing cup is maintained at a very high potential in order to accelerate the electrons produced by the filament. The anode is the positive electrode and is bombarded by the fast-moving electron. A tungsten target is fixed on the anode, which is generally made from copper, so that the heat produced by the bombardment of the electron can be properly dissipated. The fast- moving electrons knock out the electrons from the inner shells of the tungsten target. This process results in generation of x-rays. CATHODE: The cathode in an x-ray tube consists of a filament and a focusing cup. a-) The Filament: The filament is the source of electrons within the x-ray tube. It is a coil of tungsten wire about 2mm in diameter. It is mounted on two stiff wires that support it and carry the electric current. The filament is heated by the flow of current and emits electrons at a rate proportional to the temperature of the filament. The filament lays in a focusing cup with negatively charged concave reflector. b-) The Focusing Cup: It has cylindrical metallic shape, which electro-statically focuses the electrons emitted by the filament into a narrow beam directed at a small rectangular area on the anode called the focal spot. The electrons move in this direction because they are repelled by the negatively charged cathode and attracted to the positively charged anode. The x-ray tube is evacuated to prevent 47 / 75 collision of the moving electrons with gas molecules, which would significantly reduce their speed. This also prevents oxidation. ANODE: The anode made of a tungsten target embedded in a copper stem. The purpose of the target in an x-ray tube is to convert the kinetic energy of the electrons generated from the filament into x-ray photons. The target is made of tungsten, a material that has several characteristics of an ideal target material. It has a high atomic number (Z = 74), high melting point (3700 C0) and high thermal conductivity. A target made of a high atomic number material is best because it is most efficient in producing x rays. Due to the heat generated at the anode, the requirement for a target (anode) with a high melting point is clear. Tungsten also has high thermal conductivity, thus dissipating heat into the copper stem. Both cathode and anode lie within an evacuated glass envelope or tube. This glass envelope is immersed in an electrical insulating material usually oil, which lie within an electrically grounded metal housing (metal case) called the head of the x-ray machine. Thus the oil is used to reduce the heat and to isolate the metal case from the high voltage. The design of metal case allows the x ray to pass from a small window on the tube only. There are two types of the anode, a stationary and a rotating anode. The rotating anode has a longer life than the stationary anode, as the area exposed to the electrons varies continuously. X-RAY GENERATION: There are two different types of spectra (Figure 2) generated when x-rays are produced by the electron beam generated from the filament and bombard the anode. Most of the kinetic energy of the electron beam goes into heating the target, a small number of incident electron produces x-ray by giving up their kinetic energy in the following processes: a) - The general radiation or Bremsstrahlung “Braking” spectrum, which is a continuous radiation. b)- The characteristic radiation, which is a discrete spectrum. 48 / 75  Figure 2 Bremsstrahlung spectrum: When the fast-moving electron produced by the cathode moves very close to the nucleus of the tungsten atom, and strongly deflected in its path by the attraction of the nucleus as illustrated in Figure 3, the electron decelerates and the loss of energy is emitted as radiation. This spectrum is referred as the Bremsstrahlung spectrum. Figure 3 X-rays produced by the energy of the Bremsstrahlung interaction may have energy in the range of 0 to max energy of the incident electron. 49 / 75  Characteristic spectrum (Discrete): The second type of radiation spectrum (Figure 4) results from the interaction of incident electron with an electron orbit within the tungsten structure, which called the characteristic radiation (characteristic X-ray). The fast-moving electrons could eject the electron from the k- shell or L shell (inner shell) of the tungsten atom. Since this shell is unstable due to the ejection of the electron, the vacancy is filled by an electron from the outer shell. This is accompanied by the release of x-ray energy. Figure 4 production of characteristic radiation The energy and wavelength of the electron are dependent on the binding energy of the electron whose position is filled. Thus the bombarding electron must have energy of more than the energy of the ejected electron (electron shell). In the x-ray machine the x-rays of low energy are blocked by special filters and would not become a part from the useful x-ray beam. 50 / 75  FACTORS CONTROLLING THE X-RAY BEAM The x-ray beam emitted from an x-ray tube may be modified by altering the exposure time, tube current “mA” and tube voltage “kVp”. Where selecting values of all these factors depends mainly on the body of the patient (fat or thin). X-ray is an invisible light, which is able to penetrate, ionize and may be scattered, passed or absorbed in the penetrated medium. This invisible light has the following properties: a-) FLUORESCENCE: Some metallic salts of crystalline form (for example zinc-cadmium sulfide or cadmium iodide) emit visible light if irradiated. Fluorescence describes the fact that light is emitted only during irradiation. This can be used in intensifying screen (usually the film is fixed between two papers of intensifying screen), where the end result in this case is a hardcopy of the image or can be used in image intensifier (I.I) for continuous imaging. b-) PHOTOGRAPHIC EFFECT: X-ray is found to be blacken photographic film; this property is utilized in film badge dosimeter, which monitoring the x ray dose received by radiographer and radiologist in x ray department. c-) PENETRATION: X-rays are able to penetrate materials which depend on their atomic number and density; these materials include tissue which is called soft tissue and bone which is known as hard tissue. Based on that x ray can be used with high penetrating power or little penetrating power, which are known as hard x ray and soft x ray respectively. Penetration and absorption play an important role to produce image with good contrast for the inner structure of the human body, while scattered radiation fogs the image. d-) CHEMICAL EFFECT: If a beam of x ray irradiates a matter, interaction will occur and produce chemical changes due to the absorbed energy. For example (ionization chamber) X-ray energy will ionize 51 / 75 the gas (air), this energy converted in kinetic energy of electrons, which cause them to move resulting in current (Ic). This chemical change is the base on which the radiation dose meter is designed. An examples of such dose meters is: * Pen dose meter e-) BIOLOGICAL EFFECT: X ray as ionizing radiation can affect living cells. Most of the damages are resulted of biochemical reaction, which are triggered by the ionization ability of the penetrating radiation. In the cell liquid “about 70% water” short living toxic substances are produced. It may destroy the cell (indirect damage). If the safe level is exceeded then radiation will cause:  Somatic damage.  Late Somatic damage. In addition to the medical diagnosis, the knowledge of these properties (effects) has led to two new applications of x ray in medical area are:  Radiation therapy.  Radiation protection. FLUOROSCOPY: It is a continuous radiation used for imaging internal organs in order to examine, diagnose, treat these organs, and many of the noninvasive surgeries are performed under x-ray guidance providing a new “eye” to the surgeons. The image intensifier allowed intensification of the light emitted by the input phosphor; it could be safely and effectively used to produce a system that could generate and detect x-rays and also produce images fit enough for human consumption using a series of lens and camera to display the image on a TV or it can be recorded on a film. CONTRAST EXAMINATION Barium and iodinated compounds are used as an aid for imaging internal organs this is due to their high atomic numbers and high densities compared with soft tissue. When contrast compound fills the kidney or when barium fills the colon, these internal organs are readily visualized on the radiograph. 52 / 75  INTERACTIONS OF X RAYS WITH MATTER The intensity of an x-ray beam is reduced by interaction with the matter it encounters. This attenuation results from interactions of individual photons in the beam with atoms in the absorber. The x-ray photons are either absorbed or scattered out of the beam. In absorption, photons ionize absorber atoms and convert their energy into kinetic energy of the absorber electrons. In scattering, photons are ejected (change the direction) out of the primary beam. Photons in an x-ray beam interact with the object primarily by Compton scattering, in which case the scattered photon may strike the film and degrade the radiographic image by causing film fog, or photoelectric absorption. Relatively few photons undergo coherent scattering within the object or pass through the object without interacting and expose the film. In x-ray beam there are three means of beam attenuation: (1) coherent scattering, (2) Compton scattering and photoelectric absorption (3). COHERENT SCATTERING: Coherent scattering (also known as classical, elastic, or Thompson scattering) may occur when a low-energy incident photon passes near an outer electron of an atom (which has a low binding energy). The incident photon interacts with the electron by causing it to become momentarily excited at the same frequency as the incoming photon (Figure 1). Figure (1) 53 / 75  The net result of classical scattering is a change in direction of x-ray without a change in its energy. There is no energy transfer and therefore no ionization. Most classically scattered x-ray are scattered in the forward direction. COMPTON SCATTERING: In Figure (2) Compton scattering occurs when a photon interacts with an outer orbital electron. In this interaction the incident photon collides with an outer electron, which receives kinetic energy and recoils from the point of impact. The path of the incident photon is deflected by its interaction and is scattered from the site of the collision. The energy of the scattered photon equals the energy of the incident photon minus the sum of the kinetic energy gained by the recoil electron and its binding energy. Scattered photons continue on their new paths, causing further ionizations. Similarly, the recoil electrons also give up their energy by ionizing other atoms. Figure (2) The probability of a Compton interaction is directly proportional to the electron density of the absorber. The number of electrons in bone is greater than in soft tissue; therefore the probability of Compton scattering is correspondingly greater in bone than in tissue. Compton interaction is disadvantageous because it causes nonspecific film darkening. Scattered photons darken the film while carrying no useful information because their paths are altered. Both Coherent and Compton scattering are of no useful diagnostic information, Backscatter radiation is an example of such scattered radiation. 54 / 75  PHOTOELECTRIC ABSORPTION: Photoelectric absorption is critical in diagnostic imaging. This process occurs when an incident photon collides (interact) with an inner electron in an atom of the absorbing medium as shown in Figure (3). At this point the incident photon ceases to exist. The electron is ejected from its shell and becomes a recoil electron (photoelectron). The kinetic energy imparted to the recoil electron is equal to the energy of the incident photon minus that used to overcome the binding energy of the electron. The absorbing atom is now ionized because it has lost an electron. In the case of atoms with low atomic numbers (e.g., those in most biologic molecules), the binding energy is small. As a result the recoil electron acquires most of the energy of the incident photon. An atom that has participated in a photoelectric interaction is ionized as a result of the loss of an electron. Recoil electrons ejected during photoelectric absorption travel only short distances in the absorber before they give up their energy through secondary ionizations. Figure (3) As a consequence, all the energy of incident photons that undergo photoelectric interaction is deposited in the patient. Although this is beneficial in producing high-quality radiographs, because no scattered radiation fogs the film, it is potentially deleterious (harmful) for patients because of increased radiation absorption. The probability of photoelectric interaction is directly proportional with the third power of the atomic number of the absorber. For example, because the effective atomic number of compact bone (Z = 13.8) is greater than that of soft tissue (Z = 7.4), the probability that a photon will be absorbed by photoelectric interaction in bone is approximately 6.5 times greater than in an equal thickness of soft tissue. This difference in the absorption makes the production of a radiographic image possible. 55 / 75  DIFFERENTIAL ABSORPTION: The importance of photoelectric absorption and Compton scattering in diagnostic radiography relates to differences in the way photons are absorbed by various anatomic structures. The number of photoelectric and Compton interactions is greater in hard tissues than in soft tissues. As a consequence, the photons in the beam exit the patient are more after passing through soft tissue than through hard tissue. Thus while the incident beam, the beam striking the patient, is spatially homogenous, the remnant beam, the beam that exits the patient, is spatially heterogeneous. This remnant beam strikes the image receptor (film), resulting in greater exposure of the film behind soft tissue than behind hard tissues. This is differential exposure of the film that allows a radiograph to reveal the morphology of bone, and soft tissues. The photoelectric absorption of x-ray results in bright areas of a radiograph such as those corresponding to bone; while other x-ray penetrate the body and transmitted with no inter action whatever, these result in dark areas of a radiograph, which means that an x-ray image results from the difference between those x-ray absorbed photoelectrically and those not absorbed at all. Based on these interactions of x rays with matter and according to the physical properties of the x-ray, some special techniques are developed and applied in other different field of medical area. Examples of such techniques are mentioned in the following paragraph. ANGIOGRAPHY: A digital angiographic system consists of an x-ray tube, an image intensifier-based detector, a video camera to record the image and a computer to process the acquired image. The system is similar to fluoroscopy except that it is primarily used to visualize blood vessels using a contrast as shown in Figure (4). The x-ray tube must have big focal spot to prevent tube loading due to constant generation of x-ray. It must also provide a constant output over time. The computer controls the whole imaging chain and also performs digital subtraction angiography (DSA) on the images obtained. The computer controls the x-ray technique so that uniform exposure is obtained across all images. 56 / 75  Figure (4) Digital subtraction angiography (a) image without contrast material (b) image with contrast material (c) the difference image. The computer obtains the first set of images without the injection of contrast and stores them as a mask image. Subsequent images obtained under the injection of contrast are stored and subtracted from the mask image to obtain the image with the vessel alone. MAMMOGRAPHY: An example of soft tissue radiography is mammography that is radiographic examination of the breast, where the breast cancer is the leading cause of cancer death in women. Normal breast cancer consists of different principal types of tissue. Conventional radiographic technique is useless for imaging the breast that is due to: *)- The similarity of the mass density *)- The effective atomic number for soft tissue components of the breast. Therefore x-ray mammography requires a low kVp technique. As kVp technique is reduced, however the penetrability of the x-ray beam is also reduced, which in turn requires an increase in the mAs. If kVp is too low, inordinately high mAs may be required, and this could be unacceptable because of the increased patient dose. Technique factors of kVp are taken in 57 / 75 consideration, which are employed as an effective compromise between the increasing the dose at low kVp rang and reduced the image quality at high kVp range. The incident radiation must be very low energy. Thus the x-ray tube is operated at voltage below 28 kV. They have typically anode (target) of molybdenum (Z = 42), in contrast to tungsten (Z = 74) target of conventional x- ray machine. In addition to that compression is of particular importance in mammography and should be employed there. Some advantages resulting from the use of vigorous compression as follow: - A compressed breast is of more uniform thickness. - Tissues near the chest wall are less apt to be underexposed and tissues near the nipple are less apt to be overexposed. By using vigorous compression, all object structures are brought closer to the image plane and focal spot blur of image is reduced caused by motion. - Absorption unsharpness, radiation dose and scattered radiation are all reduced. All dedicated mammographic x-ray machine have a built in stiff compression device that is parallel with the surface of the image receptor (film). Breast compression produces a greatly improved image by increasing contrast, spreading tissue structure and minimizing the distance between the breast tissue and the image receptor. COMPUTED TOMOGRAPHY: The fluoroscopy and angiography discussed so far produce a planar projection image, which is a shadow of the part of the body under x-rays. The image may also contain other organs/structures that impede the ability to make a clear diagnosis. In such cases, a slice through the patient would provide an unimpeded view of the organ of interest. The system that produces this virtual slice is called the CT “there are 4 generation of the CT scan”. It uses the x-ray source to produce selectional or slice images. The radiographic film is replaced by detectors “very sensitive crystals”. When the tube rotate around the patient the detectors receive the x-ray beam through the patient and measured its intensity then convert this into digital form to produce penetration and attenuation profile of region being examined, which are stored and can be manipulated by computer. The computer calculate the absorption at point on matrix, which is called pixel “points”, where the area being imaged by each pixel has definite volume depending 58 / 75 on the thickness of the tomographic slice this is voxel “the thickness of the slice is usually between 1.5mm – 6mm or 10mm” as shown in Figure (5). This numeric information is converted into gray scale, which represents different tissue densities, this allowing a visual image to be generated. Figure (5) The normal x-ray systems studied so far produce a shadow or projection of the object on a 2D plane and, hence, the 3D depth information is permanently lost. CT solves that problem by acquiring x-ray images all around the object. A computer then processes these images to produce a map of the original object using a process called reconstruction. SURGICAL X-RAY: Surgical x-ray is another technique used in operation rooms and C- arm is an example of this technique, where many of the noninvasive surgeries are performed under x-ray guidance such as kidney stone destruction, foreign body localization and orthopedics surgery. 59 / 75  RADIATION THERAPY: X-ray can affect living cells, which may lead to sickness such as cancer, this type of damage can be treated by exposing these cancer cells with x-radiation using different x-ray doses, where the application range of the energy depends on the effect depth. Radiation can be divided as follow: (5-60) kv for surface (15-60) kv for medium depth (80-250) kv for deep tissue. Energies up to 400 kV were used before, which is higher than the power of the conventional x-ray machine (40 kV to 125~150 kV). RADIATION PROTECTION: In order to limit the effect of ionizing radiation on the human body, exposure limits have been established for contact with radiation, based on international findings. The limits are intended to prevent the risk of radiation damage leading to sickness be coming. They form the basis for measures in radiation protection. The radiation protection subdivides rooms, in which work is carried out with radiation, into radiation protective zones. In control area doses can be between 15 mSv up to a maximum of 50 mSv per year. Persons working in these areas are subject to continuous medical supervision. To prevent risk of radiation, two steps should be taken in consideration:  X-ray room must be covered with material of high density, which is able to absorb the radiation (usually the lead is used due to its properties high density and high atomic number).  The radiographer or radiologist should wear protective clothing manufactured from material containing lead (apron).These persons must carry out measurements of their personal dose using techniques as shown in Figure (6): 1- Pen dosimeter 2- Film badge. Fig.6 60 / 75  ULTRASOUND Sound waves are produced by vibrating sources. The vibrating sources cause the adjacent molecules in the air to be compressed and expanded depending on the movement of the source and oscillated with the frequency of the source. Sound wave can be transmitted through many materials, such as air, water, wood and biological tissue but the sound cannot travel in a vacuum. The resultant air compressions are accompanied by increases in the pressure. Here zero pressures refer to equilibrium, usually the atmospheric pressure, if we are considering a sound wave in air. Places where particles are squeezed together are referred to as regions of compression and the pressure here is greater than zero. Places where particles are expanded are referred to as regions of rarefaction (expansion) and the pressure here is less than zero. Sound waves are generally classified based on the frequency of the waves. Infrasound waves are less than 20 Hz and cannot be heard by humans. Audible sound falls in the range between 20 Hz and 20,000 Hz. Any sound wave frequency above the limit of human hearing is technically considered ultrasound. However, in diagnostic ultrasound generally the most used frequencies are ranging from 1 MHz up to 20 MHz. Why high frequency? It gives the ability to determine small objects. It gives the penetrability of the beam. The beam become more collimated and directional ULTRASOUND IMAGING ADVANTAGES: The transducer has two functions, it is used to send and receive the reflected signal from the tissue of interest. One of the advantages of ultrasound imaging is that it produces high-resolution images that rival (competitor) another relatively common imaging modality: x-ray imaging, plus it can produce real-time images. Ultrasound is a very valuable diagnostic tool in medical disciplines such as cardiology, obstetrics, gynecology, surgery, pediatrics, radiology, and neurology. 61 / 75 ACOUSTIC IMPEDANCE: The acoustic impedance is resulted from the production of the material density and the speed of the sound in the material. The significance of this quantity is its role in determining the amplitude of reflected and transmitted waves at an interface. Z = P*V. kg/ m2s (10-6) ) ATTENUATION: As waves travel through a medium that contains molecules packed in various densities, the molecules will be brought into oscillation and, hence, energy is depleted from the wave propagation, Attenuation of waves in biological tissues can occur by absorption, refraction, diffraction, scattering, or reflection. Since the ultrasound transducer can only detect sound waves that are returned to the crystal, the absorption of sound in the body tissue decreases the intensity of sound waves that can be detected. Since most soft tissues transmit ultrasound at nearly the same velocities, refraction of ultrasound is usually a minor problem, although sometimes 62 / 75 organs can appear to be displaced or have an incorrect shape due to the refraction of the ultrasound waves coming from or going to the transducer. Attenuation in soft tissue is highly dependent on the ultrasonic frequency. In most cases attenuation is nearly proportional to the frequency. REFLECTION: Whenever an ultrasound beam is incident on an interface formed by two materials having different acoustic impedance, in general, some of the energy in the beam will be reflected and the reminder transmitted (Figure 1). The amplitude of the reflected wave depends on the difference between the acoustic impedance of the two materials forming the interface. Pt1 Pt2 Pt3 P0 P1R P2R P3R Figure 1 In fact at most soft tissue-soft tissue interface in the body the reflection coefficient is fairly small and most of the sound is transmitted through the interface. In case, that the beam is almost completely reflected. This illustrates the difficulty in transmitting 63 / 75 ultrasound beyond any tissue to air interface. Nearly total reflection results in virtually no sound beyond the interface. The complete reflections at air interfaces also explain the need for a coupling medium, such as gel, between the ultrasound transducer and the tissue of the patient during ultrasound examinations. Soft tissue-to-bone is also strong reflector. Reflection of sound beam occurs whenever the beam is incident on an interface formed by two tissues having different acoustic impedance. The acoustic impedance could be cause by a change in speed of sound, a change in densities or both. SCATTERING: If an ultrasound beam is incident on a highly irregular or rough surface such as kidney (Figure 2A) or on particles with small size comparable or small compared with wave length, the incident beam is scattered in all direction example of small scatters include red blood cells and small structures distributed throughout the Parenchyma of most organs (Heterogeneous tissue such as liver) (Figure 2B). B A (Figure 2) ULTRASOUND WAVES PROPERTIES: As mentioned before, ultrasound imaging has reasonable similarities to x-ray imaging. On the other hand, there are considerable differences between ultrasound imaging and with x-ray imaging. Ultrasound has no reported side effects for biological 64 / 75 imaging applications, whereas x-ray ionizes molecules and atoms since its energy approaches the atomic binding energies. The speed of an ultrasound wave is different in different tissues, whereas electromagnetic waves only have a relatively slight difference in speed of propagation between most biological tissues. Informally speaking, the wave spends more time passing through one type of tissue than another one. For instance, the speed of ultrasonic waves in soft tissue is much less than the speed of electromagnetic waves (including x-ray and light): for example, Vsound = 1540 m/s versus Velectromagnetic = 2.9979 × 108 m/s; however, a small but detectable variation in the speed of sound is capable of providing detailed structural information. The reason why the changes in speed of sound propagation are detected is the reflection from boundaries with different densities on either side. TRANSDUCER: Several component compromise the transducer (Figure 3). The case provides structural support for the internal filling and mechanical support so the device can be manipulated by hand. The face of the transducer assembly is a protective acoustic window designed to match the active crystal and transmit the ultrasound beam through acoustic coupling to the patient. A matching layer with acoustic impedance is used to improve ultrasound transmission into tissue by reducing surface reflectivity. 65 / 75  Figure 3 Ultrasound waves can be produced in different modes of operation. The most often used mechanism is piezoelectrically. In piezoelectric ultrasound generation, a class of molecules with an unequal distribution of electric charges can be driven to oscillation by applying an external alternating electric field. As a result, the medium made up of these molecules changes shape in harmony at the rhythm of the alternating current through the medium. Common transducer materials are, for instance, barium titanate, and lead zirconate–titanate,. The most commonly used ceramic material is a lead zirconate– titanate crystal. This crystal is sandwiched between two electrodes that provide a voltage across the thickness of the crystal. The crystal will hence change (Figure 4) shape when a voltage is applied to it. Conversely, any ultrasonic transducer material will also produce a voltage when it changes shape. The piezoelectric crystals produce sound waves with only milliwatts of power, which is still appropriate for diagnostic use. Since the emitted and received levels of acoustic power are very small, the intensity of the sound waves often provides a better mechanism to describe the magnitude of the sound. 66 / 75  Figure 4 Consequently, the piezoelectric crystals are backed by material designed to damp the movement of the crystal so that, when the electric stimulus is removed, the crystal will cease motion immediately. The focusing plays an important roll in determining and reaching to the tissue of interest. Focusing the beam is accomplished by shaping the crystal, the face and using acoustic lens. OPERATION MODE: The pressure waves collected by the piezoelectric detector followed by conversion into an electronic signal can subsequently be recorded on an oscilloscope. The oscilloscope can represent the received signal in the following modes of operation: A- mode stands for amplitude mode, and B-mode represents brightness mode. Additional operational modes are M-mode or TM-mode (Time-mode). These modes can be divided as follow: (A) - STATIC IMAGING MODES: 1) - A-MODE IMAGING: 67 / 75  A-mode (Figure 5) is used when distances need to be calculated in relative accuracy. Examples of A-mode imaging are found in neurology and in ophthalmology. In neurology the mid-line echo of the separation between the left and the right side of the brain often needs to be determined. The mid-line echo needs to be measured exactly in between both the sides of the scull; any deviation is an indication of abnormal case. This method is fast and relatively inexpensive and accurate enough for a first diagnosis. TRANSDUCER P0 P d h P1R 1R 2R 3R 4R Figure 5 In ophthalmology, the measurement of eye dimensions: length of the eye, position of the lens, and the location of foreign objects, can be measured in A-mode imaging. However, most of these diagnoses can also be done with greater precision and not much more expensively by laser interferometry measurement. 2) - B-MODE IMAGING 68 / 75  In B-mode (Figure 6) imaging the amplitude of the collected electronic signal is represented by a relative brightness of the tracking dot on the screen. d h A-MODE d B-MODE Figure 6 In A mode display, the height of the blip is proportional to the intensity of the reflected wave. If that blip is now squeezed down to a dot on CRT (screen) display, its brightness will be proportional to the intensity of the reflected wave, this type of display is called B-mode “brightness mode”. This has two dimensional sights. In the so called B-mode ultrasound scan (brightness mode), the line along which the ultrasound scan is made is varied in angle and a two dimensional image is created. By scanning this two dimensional plane continuously; a moving image is created of the fetus and for instance the heart. The B-mode is used to study the abdominal imaging. 69 / 75  1 ULTRASOUND (A) - DYNAMIC IMAGING MODES : 1) - M-Mode or TM-Mode Imaging If a