Fundamentals of Radiology and Imaging PDF

# Fundamentals of Radiology and Imaging ## UNIT - I ### 1.1 Measurement The laws of physics are expressed in terms of physical quantities. Measurements of any physical quantity is a number and any idea about its magnitude is given by its unit. Thus, we see that the measurement of any physical qua...

# Fundamentals of Radiology and Imaging ## UNIT - I ### 1.1 Measurement The laws of physics are expressed in terms of physical quantities. Measurements of any physical quantity is a number and any idea about its magnitude is given by its unit. Thus, we see that the measurement of any physical quantity consists of two parts\: 1. Numeric\: The numeric part is the measure of the physical quantity. It is purely a number. 2. The unit employed depends upon the nature of the physical quantity. Example\: Length of a table is 1.50 metres. It simply means\: 1. The length is 1.50 times the unit chosen (i.e., metre) 2. The unit chosen for measurement of length is metre. The direct comparison with standard unit is called direct measurement using a metre scale. On the other hand, the distance of the moon from the surface of earth cannot be measured by direct method, therefore, the indirect method of measurement become necessary. The distance of moon can be measured by sending a radar signal (or laser beam) to the moon and then receiving its echo. If 't' is the time taken by the radar signal to go to the moon and come back, then Distance of moon = Speed of signal x Total time i.e., s = Cxt/2. Here C = 3 x 10^8 m/second. Some units which are frequently used for short and large length are\: 1 Fermi = 1.0 x 10^-15 metres 1 Angstrom = 10^-10 metres 1 Light year = 9.46 x 10^15 metres (i.e. distance travelled by light in one year) 1 Parsec = 3.08 x 10^16 metres (distance corresponding to an annual parallax of one second) If θ is small, then sin θ = θ = 1/r = CBV $$ \theta = \frac{CB}{r} = \frac{1.5 \times 10^{11} \times 180 \times 3600}{3600} (CB = 1.5 \times 10^{11} m) $$ Now, by definition, r = 1 parsec $$ \frac{1.5 \times 10^{11} \times 180 \times 3600}{1} = 3.08 \times 10^{16} m $$ A universally accepted unit is associated with each physical quantity and a physical standard must possess the following desirable characteristics\: 1. It should be well defined and should be of a suitable size. 2. It should be easily and accurately reproducible. 3. It should not be affected by change of temperature, pressure, time and other physical standard. 4. It should be easily comparable with other similar units. ### 1.2 Basic absolute system of unit There are five absolute system of units as given below\: 1. CGS (centimetre - gram - second) 2. MKS (metre - kilogram - second) 3. SI (International system of units) 4. FPS (foot-pound - second) 5. Indian old system (man or mound, seer, chhatak, tola, masa, rani). CGS, MKS and SI are frequently used. #### CGS system:- In this system length is measured in centimetre which is 1/100th part of metre, weight is measured in gram which is 1/1000 part of kilogram, and time is measured in seconds. #### MKS system In this system length is measured in metre, weight is measured in kilogram and time is rated in second. The two systems are from France. #### Sl system: - It is the French version for international system of units. This system was approved in 1960 by all the countries of the world. This system covers all the physical quantities in all the branches of physics like mechanics, heat, optics, electricity, electronics and magnetism, etc. The world conference has recommended the following seven basic units. 1. **Metre**: It is a bar of platinum-iridium alloy kept at the International Bureau of Weights and Measures at Sevres near Paris. The distance between the two lines engraved on gold plugs near ends of the bar at the temperature of 0°C is called one metre. A similar bar is also kept at National Physical Laboratory, Pusa, Delhi at temperature 273.16 K at 1 bar pressure. All other metres are caliberated against this metre. The standard metre is now defined as length equal to 1650763.73 wavelength of red orange light emitted by krypton isotope of atomic mass number 86. This is more satisfactory standard than the platinum iridium metre as krypton atoms are easily available in any laboratory. Moreover, wavelength of light is not affected by temperature, time and environment. We use optical interferometer to measure wavelength of light. 2. **Kilogram**: The standard for mass is kilogram, which is the mass of cylinder of platinum-iridium alloy in a controlled environment at Sevres near Paris and NPL, Delhi. 3. **Second**: One day is defined as the time taken by the earth to complete one rotation about its own axis. The day has been divided into 24 hours, each hour is further divided into 60 minutes and each minute into 60 seconds i.e. one second is 1/86400 part of a day. Since a day is not constant but varies throughout the year, from season to season hence a second has been defined as 1/31556925.9747 part of the solar year 1900 AD. Now the standard of time is based upon the atomic clock. This redefined second is equal to the time taken by cesium atom of atomic mass number 133 to vibrate 9192631770 times. 4. **Ampere**: In the field of electricity the basic quantity is current intensity, and its unit is ampere. It is defined as the current flowing through two thin infinite length parallel wires and placed in vacuum at one metre from each other which produces a force of 2 x 10^-7 Newton per metre length, between them. The electromagnetic unit of current is based on the magnetic effect of an electric current. It is defined as that current which flowing in a wire of unit length bent into the arc of a circle of unit radius produces at the centre a magnetic field of unit intensity. Thus, intensity of magnetic field = gauss = 1°C=r L = length of wire in coil in cm C = Current R = Radius of coil For practical purposes we use ampere which is 1/10th of EM unit = 10^-1 EMU = 3 x 10^9 ESU. Thus, 1 EM Unit = 10 Amperes 5. **Kelvin**: Kelvin is the unit for the measurement of temperature in SI system. The zero of Kelvin scale is chosen as -273° C and is known as absolute zero. Each degree on this scale is equal to one unit on Celsius scale. The boiling point of water is 373 K. Any temperature say 'T' degree C corresponds to T = (1+273) K on the Kelvin scale. 6. **Candela**: It is the unit for the measurement of luminous intensity or illuminating power of a source of light. 7. **Mole (mol)**: It is the unit for amount of substance. ### 1.3 Fundamental and derived units It is difficult to remember a separate unit for each physical quantity. Thus, we regard some physical quantities as fundamental quantities. Their units are fundamental units. All physical units can be expressed in terms of only seven fundamental units of length, mass, time, temperature, luminous intensity, electric current and amount of substance. All other physical quantities are defined in terms of these fundamental quantities and their units as derived units, for example watt (W). It is the unit of power and coulomb (C) is the unit of charge. Watt is the unit of electric power and may be defined as the power required to maintain a current of 1 ampere flowing under a potential drop of 1 volt. So watt = ampere x volt. $$ 1 Coulomb = 1 Ampere \times 1 Second $$ Matter\: It is defined as anything that occupies space and has inertia. It was in about 400 BC that Democritus first suspected for matter having invisible and indivisible particles called atoms. ### 1.4 Nature of electric charge All charges occurring in nature are positive or negative integral multiples of basic unit of electric charge. This basic unit is taken as the magnitude of the charge on an electron. The symbol used for the amount of charge of an electrons is 'e' which is a unit for measuring charges. Since electron is negatively charged, hence charge on an electron is 'e' that on a proton is +ve while charge on a neutron is zero. The exchange of electrons constitutes the flow of the electric current. An electric current (in a metallic conductor) then may be defined as a stream or exchange of electrons and the flow is always from the negative to positive. #### Electric Charge is Additive The total electric charge on an extended body is the algebraic sum of the charges located at different points or in different regions of it. This is similar to additive property of mathematics. #### Conservation of Electric Charge :- If some amount of matter is isolated in a certain region of space, and no matter either enters or leaves this region by moving across its boundary, then whatever other changes may occur in the matter inside, its total electric charge will not change. This is known as law of conservation of electric charge. This law is similar to conservation of law of energy. ### Potential and Potential Difference :- The electric field around a charged particle can be described not only by a vector i.e. electric field strength E but also by a scaler quantity the electric potential V. We know that the electric field due to a point charge falls inversely as the square of distance. At very large distance the field becomes extremely weak and a charge (40) placed there experiences practically no force. We call such a distance at which the electric field becomes inappreciable or zero as infinite distance and the region beyond this as infinity. Now let us bring a charge qo from infinity towards a point P in a direction opposite to the field. For this we have to do certain amount of work against the repulsive force due to the field. As a consequence of this the potential energy of charge qo increases from zero to some finite value. Thus a charge qo acquires certain amount of potential energy when it arrives at the point P. The potential energy of the charge qo placed at the point P of the field will depend on the charge as well on the field intensity. Now, if we consider the ...potential energy of a unit charge (qo = 1), we find that it depends on some characteristics of the field alone which we call as electric potential (Electric potential at a point is the amount of work done in bringing a coulomb of positive charge from infinity to the point under consideration. If the potential at a point is V, then it means that the work done in bringing +1 C charge from infinity to that point is V joules. From this it follows that the work done W in bringing a charge +40 from infinity to the point under consideration is given by $$ W = qoV \text{ or } V = W/q_o $$ Hence the unit of electric potential is joule per coulomb which we call as volt (V). If the work done in bringing a unit +ve charge from infinity to a certain point is 1 joule we say that the potential of the point is 1 volt. ### 1.5 Current And Resistance #### Static and Current Electricity: - Static electric charges present on the surface of a conductor constitute static electricity while the motion or dynamics of charges gives rise to current electricity. Electric current is defined as rate of flow of electric charge or electric charge flowing through a conductor per second or I = Q/t, the unit of electric current is ampere (A). Ampere is defined if 1 coulomb of charge flows through a point in one second, then the current is one ampere. $$ 1 Ampere = 1 Coulomb \times 1 Second $$ Conventional direction of electric current is taken as the direction of flow of positive charge (i.e. opposite to the direction of flow of electrons. It is established that current in metals is carried by negatively charged electrons. ### 1.6 Resistance It is the property of a substance by virtue of which it offers hinderance to the flow of charge. When electrons flow through a substance, they collide with the ions and their kinetic energy is converted into heat or light. The symbol of resistance is R and its unit is ohm. It measures the opposition offered by conductor to flow of charges through it. If resistance is more, current would be less. #### Ohm's Law of Electrical Circuit :- It states that the electrical current 'I' flowing through a conductor is directly proportional to the potential difference V across its end, provided other physical conditions like temperature, remain the same. Mathematically, V∞ I or V = IR where R is constant of proportionality called resistance of the conductor, conductance is the reciprocal of resistance. R = 1/C and the electrical conductivity is the reciprocal of electrical resistivity. Ohm's law is not a fundamental law of nature. In many cases the relation between voltage and current is not linear. The substances which do not obey Ohm's law are called non- ohmic substances. #### Failure of Ohm's Law in case of Conductors: - When current flowing through a conductor is increased the conductor becomes hotter and its resistance increases, which is given by the relation\: #### Failure of Ohm's law in case of Superconductor: - Superconductors are those substances which offer no resistance to the flow of current. We know, as the temperature decreases the conductivity of conductor increases (as resistance decreases). Near absolute zero the conductivity of conductors becomes very high (below 10° K and 90° K for YBa2Cu3 7°K). These substances lose their electrical resistivity entirely and become superconductor. This large change in the conductivity is not due to the ionic vibrations that cause deflection of moving electron. The reason is that the electrons are not independent in a superconductor, but are mutually coherent i.e. they form cooperative clouds of electrons. Ionic vibrations are unable to detect this cooperative cloud of electrons. Now-a-days more and more substances have been discovered which exhibit superconductivity even at higher temperatures. We are trying to get superconductor at room temperature and their use will save a lot of power losses during transmission of electric power from one place to another and in transformers. ### 1.6 Current Current is of two kinds, AC and DC. Alternating current (Fig. 1.1) is that current in which the magnitude & direction change periodically but in the case of direct current (Fig. 1.2) the magnitude & direction remain the same. The ...direct current is also called steady current. It is obtained from DC generators, cells, batteries, storage cell, accumulators & induction coils. The graph is straight line parallel to potential axis. In direct current alternating EMF is rectified by use of commutator plates and direct current flows in the external circuit. When a direct current at a given voltage passes through a conductor, a certain amount of heat is generated. For an alternating current to develop the same amount of heat when its peak voltage is the same as the direct current voltage, the current must be increased in the ratio of 2\:1. ### 1.7 Electric Power, Ammeter And Voltmeter #### Electric Power It is defined as the rate of consumption of electric energy i.e., the rate at which electrical energy is being converted into some other form. If a potential difference of V volts is applied in an electric circuit and a current I amperes flows for time t seconds, the electrical energy consumed $$ W = VIt \text{ joules} $$ Electric power, $$ P =\frac{W}{t} = VI \text{ watts} $$ The bigger unit of power is kilowatt (1000 watts) and megawatt (10^5 watts). #### Electrical Energy It is defined as the total electrical energy consumed when a current I flows through a conductor for given time t. Electrical energy = VIt = Pto know The unit of electrical energy is joule (1 watt x 1 second). It is a very small unit. The practical unit of electrical energy is 1 kilowatt hour (1 kWh). $$ 1 kWh = 1 kW \times 1 hr = 1000 \text{ unit watt} \times 3600 \text{ sec} = 3.6 \times 10^6 \text{ watt sec} = 3.6 \times 10^6 \text{ joules} $$ 1 kWh is also known as one unit of electric energy consumed in our household appliances. Resistance is introduced in a circuit with the help of resistance box. We use resistance with a wide range of values in electrical and electronic circuits. Rheostat is used in the electrical circuit for providing varying resistance as a potential divider joined in the circuit in such a way that the current enters at its positive terminal and comes out from its negative terminal. #### Voltmeter It is an instrument which is used to measure potential difference (pd) between any two points of a conductor or in any part of a circuit. It is similar to an ammeter in its construction. Actually speaking an ideal voltmeter should have an infinite resistance. It is always connected in parallel to the conductor or the source of emf, where potential difference is to be measured. Its positive terminal is connected to positive terminal of cell and negative terminal is connected to negative terminal of cell. Galvanometer (moving coil type) is a sensitive galvanometer of medium resistance. A scale uniformly divided into 30 dimensions on either side of the central mark is attached to it. The galvanometer is generally used for detecting current in null method. It consists of a coil of wire mounted on a sharp point and is placed between the poles of a strong magnet. Two hair springs serve as load for the current. A light pointer is fixed to the coil and it moves over a scale. #### Ammeter It is galvanometer having low resistance in parallel called a shunt. It is used for measuring current in a circuit and is graduated so as to read the current directly in amperes or fraction of an ampere. A coil of low resistance is joined in parallel with the galvanometer. An ammeter is always placed in series with the circuit. Since the resistance of ammeter is low, its insertion in a circuit does not alter the actual value of current in the circuit (Fig. 1.4.) ...The electromagnetic induction, self and mutual, production of AC generator, transformers. magnetic lines of force, magnetic flux and its unit: magnetic density. ### Magnetic Density The magnetic flux density at any point may be defined as the number of lines of magnetic induction passing through a unit area held at right angles to the lines at that point and is represented by the vector B. It is expressed in webers per square metre. Wb/m2. Here 1 wb/m² = 10^4 gauss = 1 Tesla ### lonization It is the process of breaking the neutral gas atom with \positively and negatively charged particles. ### 1.8 Atomic structure 1. According to Rutherford scattering experiment an atom consists of a small massive central core called nucleus. 2. The size of the nucleus is negligible as compared with the size of the atom (10^-10 m). 3. The nucleus is surrounded by suitable number of electrons such that total negative charge is equal to the positive charge of the nucleus. These electrons revolve in various orbits around the nucleus. Bohr gave three postulates for electron motion in orbit. 1. Electron revolve in certain definite orbits satisfying the quantum conditions. The attraction between electrons and nucleus balances the centrifugal force due to circular motion or electrons. 2. The total of angular momentum of the moving electron is an integral multiple of h/2л. 3. When an electron jumps from lower to higher orbit then an atom emits discrete energy photons. Nucleus contains proton (+ve) and charge of electron (ve) so that atom may remain neutral. The magnitude of the charge on a proton is equal to that of the charge on an electron 1.602 × 10^-19 coulombs. Mass of proton is 1.6726 x 10^-27 kg. It is 1836 times more than that of electron. So mass of an atom is mainly due to its nucleus. Chadwick in 1932 discovered another type of particle called neutron present in the nucleus. Neutron is a neutral particle. It has no electrical charge, its mass is 1.6749 x 10^-27 kg slightly more than that of a proton. The number of protons in a nucleus determines its charge. This number is atomic number and is represented by letter Z. The number of nucleus (proton) and neutrons taken together in a nucleus is called its mass number and it is represented by A. Density of nucleus is the same of all the atoms of all substances = 2.4 x 10^17 kg M. #### Isotopes Atoms which have same atomic number Z but have different mass number are called isotopes. for example, deuterium nucleus has one proton and one neutron. #### Isobar Have same value of atomic mass number A but different values of Z (atomic number). Example of isobar Ca18, C20 and Argon18 Аг40. ## UNIT - II ### 2.1 Electromagnetic Induction Let us take a case of copper wire whose ends are connected to a sensitive galvanometer. If the north pole of a powerful bar magnet is now quickly inserted in the coil, galvanometer shows a momentary deflection showing induced current is set up in the circuit. But if the magnet is taken away from the coil, the needle moves in opposite direction. Similarly, if two coils are placed near each other, the induced current is generated in the second coil at Make and Break. If the strength of the current is changed with the help of rheostat, the induced current is still observed in the circuit. The experiment shows that the induced emf is produced only when the magnetic line of force or magnetic field near the coil is changing. It is known as the electromagnetic induction. Faraday gave the following laws governing the phenomenon of electromagnetic induction. 1. **First law**: It states that whenever the number of magnetic lines of force (magnetic flux) linked with a closed-circuit change, an induced emf is produced in the coil and induced current flows through the circuit and lasts so long as the change lasts. According to this law an increase in the number of lines of force produce an inverse current and the decrease in the number of lines of force produce a direct current. 2. **Second law**: It states that magnitude of induced electromagnetic force produced in a coil is directly proportional to the rate of change of magnetic flux linked with it. If Φ is the magnetic flux linked with the coil at any instant 't', then the induced emf E produced in the coil is $$ E \propto \frac{d\phi}{dt} \text{ or } E = - \frac{d\phi}{dt} $$ If coil contains N number of turns then $$ E = -N \frac{d\phi}{dt} $$ If the magnetic flux changes from 0 to 2 in time 'r', the induced emf in a coil of N turn is $$ E = -\frac{N(\phi_2-\phi_1)}{t} $$ The negative sign is due to Lenz's law, E is in volts. ### 3. Lenz's Law of Electromagnetic Induction Lenz's law gives the direction of induced emf. It states that the direction of the induced emf is such that it opposes the changes or the cause which produces it. If the current in the primary is increasing, the direction of induced current in the secondary will be in the opposite direction and tries to decrease the magnetic flux in the circuit. When a magnet moves towards the solenoid, the direction of induced current is such that it tries to oppose the movement of the magnet towards the solenoid. For example, if the north pole of a bar magnet approaches the coil, the magnetic lines of force linked with it increase and emf is induced in the coil. The induced current flowing in coil is anticlockwise and the face of the coil facing the north pole of the bar magnet also become north pole. The two north poles, one of the magnet and other of the coil, repel each other. This shows that the direction of induced current is such that it opposes the cause which produces it. From the above fact it is understood that Lenz's law is consequence of conservation of energy because there is force of repulsion between the two north poles and in order to move the magnet towards the coil, some additional mechanical energy has to be applied. This additional mechanical energy is converted into electrical energy and when north pole of the magnet moves away from the coil, then there is a force of attraction between the south pole of the coil and north pole of the magnet. To overcome this force of attraction additional mechanical energy has to be applied which is converted into the electrical energy which appears in the form of induced current that produces deflection in galvanometer. An experiment shows the demonstration of Lenz's rule. Take a coil of large number of turns with an extra-long iron rod as its core. A solid aluminium ring is slipped over the iron rod and above the coil. When an alternating emf is applied to the coil, the aluminium ring jumps up into the air. The induced current in the ring opposes field in the coil and due repulsion takes place in both directions of current. ### Fleming's Right-Hand Rule Stretch and hold the thumb, the forefinger and the central finger of your right hand mutually perpendicular to each other. If the forefinger points in the direction of the magnetic field and the thumb in the direction of conduction, then the central finger will point in the direction of induced current. #### Eddy currents The induced currents produced inside a block of metal when either the block moves in the magnetic field or the magnetic flux through the stationary metal block changes are called eddy currents. The direction of eddy current is given by Lenz's law. Since M = E, when dI/dt = 1 $$ E = M \frac{dI}{dt} $$ Let E= 1 volt, and d//dt = 1 A/s Thus, 1 henry is defined as the mutual inductance of two coils when a current varies at a rate of 1 A/sec in one coil, producing an induced emf of one volt across the other coil. ### Self-inductance When there is only one coil and varying current flows through it, then the magnetic field is associated with the coil. The magnetic flux linked with the coil is directly proportional to the current through the coil i.e φ = LI where L is called self-inductance of the coil. When current changes with time the magnetic flux also changes with time i.e. $$ \frac{d\phi}{dt} = L \frac{dI}{dt} $$ We know, dΦ/dt = -E Hence, the induced emf or the back $$ emf E = -L \frac{dI}{dt} $$ If I = 1, then E = -L $$ \frac{dI}{dt} $$ Hence the self-inductance of the circuit is equal to the back emf (opposing emf) produced in the circuit due to a unit rate of change of current in the circuit. The SI unit of self-inductance is Henry (H). Since Φ = LI, when Φ = 1 wb, I= 1 A then L = 1 Henry. Thus, the self-inductance of a circuit is one henry if a magnetic flux of 1 weber is produced when 1 ampere current flows through it. #### Expression for self-inductance of long solenoid :- The self-inductance L of a long solenoid of length l, arns N and area of cross-section A and permeability μ. $$ L = \mu. B A = N I \mu . A i $$ Since B = NI/l therefore, flux through each turn Since L = I, therefore, L = ΦI Total flux through N turns $$ \phi = INI\mu AI = NI^2 \mu A I $$ The phenomenon of radioactivity is shown by nucleus of high atomic mass. In case of nucleus of high atomic mass, the force of repulsion between charged proton increase and hence the binding energy decreases. The nucleus then becomes unstable. Radioactive substance emits three kinds of rays\: (1) a particle, (2) ẞ particle, and (3) y rays. When radiations emitted by radioactive substances are kept inside a small hole drilled in a lead box and subjected to perpendicular magnetic field and allowed to fall on a photo- graphic plate then three spots are found on a photographic plate which shows under the influence of magnetic field the radiation split into three parts a rays. B rays, and y rays. The rays are positively charged particles which have nucleus of helium (He). The B rays are stream of electrons and Y rays (photons) are electromagnetic waves. Generally, a, B are deflected by electron field. They have effect on photographic plates. They cause ionization. y rays produce fluorescence in barium platinocynide. y rays are diffracted by X-rays. B rays also produce fluorescence. Brays penetrating power is higher than X-rays. X-rays produce intense ionization on photographic plate. Penetrating power of a rays is maximum as well as their speed also. X-rays were discovered on November 8, 1895 by Sir Wilhelm Conard Roentgen, a German physicist, while studying in high voltage discharge phenomenon in a partially vacuum tube (Crooke's tube). He noticed fluorescence of a barium platino- cyanide screen lying several feet away in the same room. He named the rays causing it as X-rays as he did not know their exact nature at that time. During following years, he and many other workers studied the nature and properties of X-rays, which are as follows\: 1. X-rays are electromagnetic rays which behave as waves as well as particles. 2. They possess great penetrating power. 3. Being electromagnetic waves, they are electrically neutral. 4. However, they can cause ionisation of gases indirectly as they can remove orbital electrons from atoms. 5. Their wavelength ranges from 0.1-0.5 Å having energy levels of 25 keV to 125 keV. 6. They travel at the speed of light i.e. 3 x 10^8 m/s. 7. They travel in a straight line, like light rays, however, unlike light rays, they cannot be focussed by a lens 8. They liberate minute amounts of heat on passing through matter 9. They cause fluorescence of certain crystals. 10. They cause photographic effect on silver halide crystals, which is a chemical change. 11. They can produce other chemicals as well as biological changes mainly by ionisation and excitation. ## UNIT - III ### 3.1 Production of X-Rays: - X-rays are produced by energy conversion when fast moving electrons from filament of X-ray tube interact with the tungsten anode (target). The kinetic energy of the electron in passing across the voltage V is increased by\: $$ E = eV $$ As the electron kinetic energy is increased by raising kVp both intensity and energy of X-ray beam increases. At 100 mA, for example, 6 x 10^17 electrons travel from cathode to anode of X-ray tube every second. In an X-ray unit operating at 70 kVp, for example, each electron arrives at the target with a maximum kinetic energy of 70 keV. Since there are 1.6 x 10^-16 J/keV, this energy is equivalent to\: $$ (70 keV) (1.6 \times 10^{-16} J/keV) = 1.12 \times 10^{-14} J $$ Since 1/2 me Vé ² = 1.12 x 10^4, where me → mass of electron, V. → velocity gained by electron By putting the value of mass of electrons we find that the velocity of electron is 1.6 x 10^7 m/sec arriving at the target at 70 keV. Hence at 70 kV,, maximum speed of electron would be approximately 50% of velocity of light since as electron approaches the speed of light its mass increases. So speed of electron would be slightly lesser than that calculated as above. The distance between the filament and target is only 1 to 3 cm. Therefore, it takes the electron something like 10^-10 sec to go from cathode to anode When these high speed electrons hit the heavy metal atom of target, they transfer their kinetic energy to the target atom. These interactions occur within a very small depth of penetration into the target. As they occur, these high-speed electrons slow down and finally come nearly to rest. Then they are conducted through X-ray anode assembly and out into associated electronics circuitry. The high-speed electrons interact with either orbital electron or nuclei of target atoms. The interactions result in the conversion of kinetic energy into thermal energy (heat) and electromagnetic energy in the form of infrared radiation and X-rays. #### Heat production: - The high-speed electrons interact with the outer shell electrons of the target atom but do not transfer sufficient energy of these outer shell electrons to ionize them. Instead the outer shell electronsare simply raised to an excited state. when outer shell electrons immediately drop back to their normal energy level there is emission of infrared radiation or heat the constant excitation and return of outer shell electrons are responsible for heat generated in the anode of X-ray tubes generally,more than 99%of kinetic energy of high speed electrons is converted to heat which leaves less than 1% available for the production of radiation the production the production of heat production of heat in the anode increases directly with increasing tube current Doubling the circuit current doubles the quantity of heat produced heat production also increasing directly with increasing kVp at least in diagnostic range. The efficiency of X-ray production is independent of the current the efficiency of X-ray production increases with increasing kVp. At 60 kVp only 0.5% of electron kinetic energy is converted to X-rays At 100 kVp, approximately 1% is converted to X-rays. #### Thermionic emission A substance which readily releases electrons when heated is known as thermionic emitter and the process responsible for the release of electrons is called thermionic emission In the atoms, which make up the material the outer shell electrons are loosely bound than the inner electrons because they are further away from the nucleus. The applications of heat to a body increases the kinetic energy (KE) of its atoms and so increases the violence of their collisions. As a result of these collisions the outer electrons may be dislodged from the atom electrons so released near the outer of the body travel only a relatively short distance but they are released near the surface of the material In thermionic emission, this is the mechanism by which the electrons are emitted higher the temperature of a body higher will be the number of electrons that will have sufficient energy to break free from the influence of the surface of the atom of the body. #### Bremsstrahlung radiation When an energic electron that completely avoids the orbital electron may come sufficiently close to nucleus of atom under its influence. Because electron is negatively charged and nucleus is positively charged, there is an electrostatic force of attraction between them. This coulombian force is very strong because nucleus contains many protons and distance between nucleus and high-speed electron is very small. So, when an energic electron passes by the nucleus, it is decelerated, resulting in loss ...Historically, the early X-ray tubes were cold cathode tubes. Such a tube consisted of a glass bulb in which a partial vacuum had been produced, leaving a small amount of gas. In it were sealed two electrodes, one negative (cathode) and other positive (anode). The cathode terminal was not heated. Application of high voltage across the terminals causes ionization of the gas in the tube with release of a stream of electrons which produced X-ray upon striking the positive terminal. The gas tube is now obsolete not only because of its inefficiency but also because tube mA could not be changed independently of KV making it difficult to control X-ray quantity and quality independently. In 1913, W.D. Coolidge invented a new type of tube based on radiographically different principle. This was the hot cathode tube and so kV and mA were controlled indepen- dently. Modern tubes are based on this principle. ...of kinetic energy of electron. This loss of kinetic energy is converted into X-rays known as bremsstrahlung X-rays. The amount of energy lost by each electron during the bremsstrahlung process and thus energy of X-rays that produced is