Unit 1 (Medical Physics) PDF

1 PHYSICS FOR BIO MEDICAL ENGINEERING UNIT I LOW ENERGY ELECTROMAGNETIC SPECTRUM AND ITS MEDICAL APPLICATION PHYSICS OF LIGHT Light propagated through space and exhibited the properties of electromag...

1 PHYSICS FOR BIO MEDICAL ENGINEERING UNIT I LOW ENERGY ELECTROMAGNETIC SPECTRUM AND ITS MEDICAL APPLICATION PHYSICS OF LIGHT Light propagated through space and exhibited the properties of electromagnetic radiation. The wavelength of EM radiation ranges from 10-14m to about 108m. Visible light occupies only a narrow band from about 400-700 nm. Speed of light  The speed at which light travels in a vacuum is approximately 3 × 108 ms−1, or 186000 miles per second. Its speed in a transparent medium is always less than this, and the ratio of the speed in vacuum to that in the medium is known as its index of refraction, n,  2 Refraction and Reflection at boundary When light travels from one medium into another its frequency cannot change but its velocity must. The wavelength changes to accommodate the change in velocity. At a fixed frequency, wavelength is proportional to speed and therefore the ratio of the wavelength in vacuum to the wavelength in a medium is also equal to the index of refraction. If the light meets a boundary at an oblique angle then its direction must change if the wavelength is to change by the appropriate factor. The angle through which the light is refracted can be calculated from simple geometry, and the relationship between the angle of incidence and the angle of refraction is determined by Snell’s law The relative intensities of the incident, reflected and refracted light depend on the properties of the media and on the angle of incidence. If the angle at which the light strikes its surfaces is greater than the critical angle, it is called total internal reflection. A consequence of Snell’s law is that a spherical boundary will focus parallel light rays onto a single point.  3 This property is the basis of the geometrical optics of the eye, and of our design of corrective optical devices for defective vision. Two portions of spherical boundaries, as represented by a simple biconvex lens, will also focus parallel incident light rays to a point. If the index of refraction of the material of a thin lens is n2 and that of the surrounding medium is n1, the focal length, f of the lens is given by Where Ra is the radius of curvature of the first surface of the lens and Rb is that of the second surface, measured from the same side of the lens. This equation is known as the lens-makers’ equation.  4 Interference of light waves Light waves interact and interfere with each other in just the same way as do sound waves. The relative phase of the waves determines whether the interference is constructive, increasing the intensity, or destructive, reducing the intensity. Fig. 3.19. Condition for constructive and destructive interference The point sources are at A and B, and they are separated by a distance s. A screen is placed parallel to and at a distance S from the line connecting the two sources. Light falling at a point C on the screen has to travel a different distance from each source. From simple geometry, assuming that the distance of the screen from the sources is large compared to all other distances, the difference in lengths of the paths BC and AC is There will be constructive interference when this distance represents a whole number of wavelengths of the light. Hence the conditions for constructive interference are This implies that the distance between bright peaks will be Sλ/s. Dark troughs corresponding to destructive interference lie half-way between the bright peaks.  5 Diffraction The spreading of the light from the slit is called diffraction, and the pattern of fringes on the screen is the diffraction pattern. The width of the bright central fringe can be taken as a measure of the diffraction. The half- angle, θ, at which the beam appears to diverge can be approximated by the relationship Where λ is the incident wavelength and w is the width of the slit. Complete diffraction occurs when the width of the slit approaches the wavelength of the incident wave. The physical process of diffraction does have consequences for our acuity of vision. The pupil is an aperture of finite size, and diffraction occurs as light passes through it. Consequently, there will always be some diffusion of the intensity of ‘a light ray’ about the nominal point at which it strikes the retina. Diffraction at a circular aperture produces a similar effect, but with rings rather than parallel fringes. Measurement of Light and Its Units Wavelengths of light are used to be measured in microns (1μ=10-6m) or in angstroms (1A=10- 10m) but at present the recommended unit is the nanometer (1nm=10-9m). Ultraviolet light has wavelengths from about 100 to 400 nm; visible light extends from about 400 to 700 nm; and IR light extends from about 700 to over 104 nm. Each of these categories is further subdivided according to wavelength. For example, UV-C has wavelengths from about 100 to 290 nm, UV-B has wavelengths from 290 to 320 nm, and UV-A has wavelengths from 320 to 400 nm. Visible light is measured in photometric units that relate how light is seen by the average human eye. In photometry the quantity of light striking a surface is called illuminance and the intensity of a light source is called its luminance.  6 All light radiation, including UV and IR radiation can be measured in radiometric units. In radiometry the quantity of light striking a surface is called irradiance and the intensity of a light source is its radiance. In the electromagnetic radiation spectrum, light has wavelengths much shorter than TV and radio waves but much longer than x-rays and gamma rays. Visible light has energies ranging from about 2 eV up to about 4 eV. For comparison, the energy of a typical x-ray photon used in medicine is about 50 keV. A steradian (sr) is a unit for a solid angle. LIMITS OF VISION AND COLOR VISION AN OVERVIEW The visible light spectrum ranges from about 380 to 740 nanometers. Spectral colors (colors that are produced by a narrow band of wavelengths) such as red, orange, yellow, green, cyan, blue, and violet can be found in this range. Structure and Functions of the Human Eye The human eyes are the most complicated sense organs in the human body. From the muscles and tissues to nerves and blood vessels, every part of the human eye is responsible for a certain action. Furthermore, contrary to popular belief, the eye is not perfectly spherical; instead, it is two separate segments fused together. It is made up of several muscles and tissues that come together to form a roughly spherical structure. From an anatomical perspective, the human eye can be broadly classified into external structure and internal structure  7 The External Structure of an Eye The parts of the eye that are visible externally include the following:- Sclera: It is a white visible portion. It is made up of dense connective tissue and protects the inner parts. Conjunctiva: It lines the sclera and is made up of stratified squamous epithelium. It keeps our eyes moist and clear and provides lubrication by secreting mucus and tears. Cornea: It is the transparent, anterior or front part of our eye, which covers the pupil and the iris. The main function is to refract the light along with the lens. Iris: It is the pigmented, coloured portion of the eye, visible externally. The main function of the iris is to control the diameter of the pupil according to the light source. Pupil: It is the small aperture located in the centre of the Iris. It allows light to enter and focus on the retina. The Internal Structure of an Eye  8 The internal components of an eye are: Lens: It is a transparent, biconvex, lens of an eye. The lens is attached to the ciliary body by ligaments. The lens along with the cornea refracts light so that it focuses on the retina. Retina: It is the innermost layer of the eye. It is light sensitive and acts as a film of a camera. Three layers of neural cells are present in them, they are ganglion, bipolar and photoreceptor cells. It converts the image into electrical nerve impulses for the visual perception by the brain Optic nerve: It is located at the posterior portion of the eyes. The optic nerves carry all the nerve impulses from the retina to the human brain for perception. Aqueous Humour: It is a watery fluid present between the cornea and the lens. It nourishes the eye and keeps it inflated. Vitreous Humour: it is a transparent, jelly-like substance present between the lens and the retina. It contains water (99%), collage, proteins, etc. The main function of vitreous humour is to protect the eyes and maintain its spherical shape. Limits of Vision Visual Acuity If the angle between two light rays passing through the optical centre is too small, we will not be able to distinguish between them with respect to location. The minimum angle at which resolution is just possible is called the visual angle, and the inverse of the visual angle, measured in minutes of arc, is our visual acuity. The most commonly applied test of visual acuity has been the Snellen chart. One version of this chart consists of a series of 11 rows of letters, progressively smaller from the top. When it is viewed from a distance of 20 ft the letters on the eighth line are just distinguishable by a person of good vision: the distinguishing characteristics of the letters on this line form a visual angle of 1 min of arc at 20 ft. A person with 20/20 vision can read the letters of the eighth line at 20 ft. The lines above the eighth are marked with greater distances, again at which they are just discernible by the person with good vision. A person with 20/40 vision can read at 20 ft the line (in fact the fifth) that with good vision is readable at 40 ft. Note that the visual acuity expressed as a ratio is dimensionless, and the distances equally well be expressed in metres or any other unit.  9 Many Snellen charts are now marked in metres and 6/6 vision (recorded at 6 m) is the equivalent of 20/20 when measured in feet. There are alternative tests, based for example on patterns of lines or grids of squares. The Snellen chart, in particular, is quite crude because some letters are distinguishable just by their general shape, and are therefore easier to read than others. Under ideal conditions a person with excellent vision might achieve a visual acuity of two, implying that their visual angle of resolution is 0.5 min. For the normal eye the optical centre is about 17 mm in front of the retina, and the cones are 2 μm apart. This implies that the maximum visual angle is about 0.4 min, and the upper bound on visual acuity is 2.5. There is an order of magnitude reduction in visual acuity at 10◦ of arc from the fovea. The brain appears to be able to compensate for this by scanning the scene in front of us and building up a high resolution image within the brain. Visual Sensitivity The rods are much more sensitive than the cones. In terms of luminance, the cones do not function below about 0.001 cd m−2, and our vision is entirely dependent on the rods. The optimal sensitivity of the rods is to light at a wavelength of about 510 nm. Hecht directed light at this wavelength at an area of high rod concentration (away from the fovea). He demonstrated that, for a number of observers, the average threshold was about 100 photons arriving at the cornea. He further calculated that only about 48% of these would arrive at the retina: 4% would be reflected at the cornea, 50% of those remaining would be absorbed in the media within the eye. Of the 48% getting through, 20% would be absorbed by the rhodopsin to create a visual stimulus (the remainder would either have been absorbed by the neural components before reaching the photoreceptors or would miss the rods entirely and be absorbed by the black pigment behind). In total then, only about 10% of the light arriving at the retina, or about ten photons, actually generates the visual stimulus.  10 COLOR VISION Colour is a psychophysical property of light, in that it is associated with visual perception. The two attributes of a light wave that governs the perception is its wavelength and the intensity. When light is mixed together, the spectral composition of the resulting combination is considered to be important. This effectively gives three parameters that is used to describe a colour. The duration of exposure to the light and time of exposure to the stimulus might also be important. Putting aside the time element, the remaining three parameters have been represented by Munsell as a double cone. The vertical axis represents the intensity of the colour. The circumferential coordinate represents the hue: it is dependent on the wavelength, and is what we would normally think of as the determinant of colour. The horizontal axes define the saturation of the colour, and reflect the spectral composition. At the outer extreme the light is of only one pure colour, and at the inner extreme all wavelengths are present. The vertical axis represents brilliance, which is a property of intensity. At the bottom there is no light, and the colour is black. At the top all wavelengths are present at maximum intensity, and the resulting colour is white. All other points on the vertical axis represent shades of grey. It is less useful in predicting the outcome of combining different colours. The rules of combination are not simple. Pre-dating the Munsell description is the Young–Helmholtz trichromatic theory. Young originally observed that all colours could be made up from three primary ones: his chosen primaries were red, green and blue. He postulated that the eye contained three different types of nerve receptor, and that the brain made up composite colours by combination of signals.  11 Helmholtz did not initially accept Young’s theory, because he was aware that some colours could not apparently be produced from pure monochromatic (single wavelength) primaries. He later realized that the receptors might not be ‘pure’ in that they might have overlapping spectral response curves, and that this could explain the discrepancies in experimental results. Essentially the trichromatic theory uses the ‘concentration’ of three colours as a mathematical basis rather than three parameters such as brilliance, saturation and hue. Simple rules for the combination of colours using addition and subtraction can readily be developed on this basis. The choice of primaries for a trichromatic combination is arbitrary, and any three ‘independent’ colours will serve the purpose. The Young–Helmholtz theory suggests that we can write any colour, C, of intensity c as a linear sum of the three primaries, Cc = Rr + Gg + bB The intensities of the colours (c, r, g and b) can be measured in any standard photometric units, such as lumens.  12 The total light flux must be the sum of the components, and so c = r + g + b it turns out that there are some colours that cannot be matched in this way. A saturated blue–green is one example of a colour that cannot be produced by a combination of red, green and blue light. What is possible, however, is to refocus the red beam so that it falls onto the original blue– green spot, and then to match the resulting colour with a combination of blue and green. Cc + Rr = Gg + Bb In principle the two equations are identical, except that we have to accommodate the notion of a negative coefficient of a colour in the trichromatic equation. Chromaticity diagrams Chromaticity diagrams provide a two-dimensional representation of a colour. The parameter that is sacrificed is brightness. It is assumed that the basic determinants of colour are the relative intensities of the three chosen primaries. It is assumed that one unit of colour is produced by a particular relative mixture of the primaries, irrespective of their absolute magnitudes. In this case a relative form of the trichromatic equation can be written; Changing each of the intensities by the same factor will produce the same colour. The three coefficients are not independent: any one of them can be obtained by subtracting the sum of the other two from unity. This means that we can choose any pair of the coefficients as the independent variables and represent the colour as a point in a single plane. The resulting graph is called a chromaticity diagram. All colours can be represented on this diagram, each occupying a point in the plane.  13 We should recognize that some colours will not lie within the triangle shown because negative coefficients would be required to produce them. Straight lines on the chromaticity diagram have special properties: all colours on a straight line can be represented as a linear combination of the two monochromatic colours at its extremes. The monochromatic colours either side of a line through the white spot are complementary. If we stare at a red object for some time, and then look at a white piece of paper, we see a blue– green image of the object. This fits in with the trichromatic theory, because we would anticipate that the photochemicals associated with red vision have been ‘used up’, and therefore we will get a higher response to the white light from the photosensors associated with the blue and green reception. The triangular envelope formed by the connection with straight lines of points on a chromaticity diagram defines all of the colours that can be produced using an additive combination of the colours represented by the points. This has immediate application in the fields of television, cinema and colour printing.  14 NON-IONIZING ELECTOMAGNETIC RADIATION Non-ionizing (or non-ionising) radiation refers to any type of electromagnetic radiation that does not carry enough energy per quantum (photon energy) to ionize atoms or molecules—that is, to completely remove an electron from an atom or molecule. Electromagnetic Radiation (EMR): EMR is defined as energy that is transmitted at the speed of light through oscillating electric and magnetic fields. EMR refers to the waves (or their quanta, photons) of the electromagnetic field, propagating (radiating) through space, carrying electromagnetic radiant energy. Examples: Radio waves, microwaves, infrared, (visible) light, ultraviolet, X-rays, and gamma rays. Ionizing Radiation: Ionizing radiation is any type of particle or electromagnetic wave that carries enough energy to ionize or remove electrons from an atom to produce ion pair. Examples: X-rays, and gamma rays. Non-Ionizing Radiation: Non-ionizing (or non-ionising) radiation refers to any type of electromagnetic radiation that does not carry enough energy per quantum (photon energy) to ionize atoms or molecules—that is, to completely remove an electron from an atom or molecule. Examples: Radio waves, microwaves, infrared, (visible) light, ultraviolet. 15  16 TISSUE AS A LEAKY DIELECTRIC If two electrodes are placed over the abdomen and the electrical impedance is measured between them over a wide range of frequencies then the results obtained might be as shown in figure 2. Figure 2. Electrical impedance as a function of frequency for two electrodes placed on the Abdomen The results will depend somewhat upon the type and size of electrodes, particularly at the lowest frequencies, and exactly where the electrodes are placed. However, the result is mainly a function of the tissue properties. The impedance always drops with increasing frequency. Let us consider tissue as a lossy dielectric. We know that conductors, which have free charge carriers, and of insulators, which have dielectric properties as a result of the movement of bound charges under the influence of an applied electric field. An insulator cannot also be a conductor. Tissues though contain both free and bound charges, and thus exhibit simultaneously the properties of a conductor and a dielectric. If we consider tissue as a conductor, we have to include a term in the conductivity to account for the redistribution of bound charges in the dielectric. Conversely, if we consider the tissue as a dielectric, we have to include a term in the permittivity to account for the movement of free charges. The two approaches must, of course, lead to identical results. Considering the interaction between electromagnetic waves and tissue by exploring the properties of dielectrics. The use of dielectrics which are insulators in cables and electronic components. A primary requirement of these dielectrics is that their conductivity is very low (104 Sm−1). Intermediate between metals and insulators are semiconductors (conduction by excitation of holes and electrons) with conductivities in the range 100–10−4 S m−1, and electrolytes (conduction by ions in solution) with conductivities of the order of 100– 102 S m−1.  17 Tissue can be considered as a collection of electrolytes contained within membranes of assorted dimensions. None of the constituents of tissue can be considered to have ‘pure’ resistance or capacitance the two properties are inseparable. Let us considering slabs of an ideal conductor and an ideal insulator, each with surface area A and thickness x (see figure 3). Figure 3. Slabs of an insulator, on the left, and a conductor, on the right. The capacitance of the insulator and the resistance of the conductor are given If the dielectric has relative permittivity εr then the slab has a capacitance C = ε0εrA/x. The conductance of the slab is G = σA/x, where the conductivity is σ. The conductivity σ is the current density due to unit applied electric field (from J = σE), and the permittivity of free space ε0 is the charge density due to unit electric field, from Gauss’ law. The relative permittivity εr = Cm/C0, where C0 is the capacitance of a capacitor in vacuo, and Cm is the capacitance with a dielectric completely occupying the region containing the electric field. In tissue, both of these properties are present, so we take as a model a capacitor with a parallel conductance, as shown in figure 4.  18 Figure 4. Tissue with both capacitive and resistive properties in parallel. The capacitance and resistance of the two arms are marked The equations C = ε0εrA/x and G = σA/x define the static capacitance and conductance of the dielectric, i.e. the capacitance and conductance at zero frequency. If we apply an alternating voltage to our real dielectric, the current will lead the voltage. Clearly, if G = 0, the phase angle θ = π/2, i.e. the current leads the voltage by π/2, as we would expect for a pure capacitance. If C = 0, current and voltage are in phase, as expected for a pure resistance. For our real dielectric, the admittance is given by Y * = G + jωC, where the * convention has been used to denote a complex variable (this usage is conventional in dielectric theory). We can, as a matter of convenience, define a generalized permittivity ε* = ε’ – jε” which includes the effect of both the resistive and capacitive elements in our real dielectric. Where ε’ is the real part and ε” is the imaginary part. We can relate the generalized permittivity to the model of the real dielectric by considering the admittance,  19 By analogy with an ideal capacitance C which has admittance jωC, we can define the complex capacitance C* of the real dielectric, From this it can be seen that we can consider the properties of our non-ideal capacitor as being the result of inserting a dielectric with a relative permittivity ε* in an ideal capacitor C. The real part ε’ is the relative permittivity εr of the ideal capacitor, and the imaginary part jε” is associated with the resistive properties. We now have a means of handling real dielectrics which is analogous to that for ideal dielectrics. We can also consider the admittance in terms of a complex conductivity, The complex permittivity and complex conductivity are related by We are thus able to relate the behaviour of the conductivity and permittivity. Note that as the frequency tends to zero, the complex conductivity becomes purely real, and in the high-frequency limit, the complex permittivity becomes purely real. We would thus expect the conductivity to be dominant at low frequencies, and the permittivity to be dominant at high frequencies.  20 OVERVIEW OF NON-IONIZING RADIATION EFFECTS Low frequency effects Biological tissue contains free charge carriers so that it is meaningful to consider it as an electrical conductor and to describe it in terms of conductivity. Bound charges are also present in tissue so that dielectric properties also exist and can be expected to give rise to displacement currents when an electric field is applied. Biological tissue also contains mechanisms for the active transport of ions This is an important mechanism in neural function and also in membrane absorption processes, such as those which occur in the gastro-intestinal tract. Conductivity is the dominant factor when relatively low-frequency (less than 100 kHz) electric fields are applied to tissue. Frequency–dependent effects The electrical properties of a material can be characterized by an electrical conductivity σ and permittivity ε. If a potential V is applied between the opposite faces of a unit cube of the material then conduction current Ic and displacement current Id will flow, where and where ε0 is the dielectric permittivity of free space with the value 8.854×10−12 Fm−1. If V is sinusoidally varying then Id is given by where f is the frequency of the sinusoidal potential Ic increases only slowly with increasing frequency and indeed at frequencies up to 100 kHz conductivity is almost constant. Id increases much more rapidly with increasing frequency and above about 107 Hz the displacement current exceeds the conduction current. The region around 10 Hz is generally considered to arise from dielectric dispersion associated with tissue interfaces such as membranes; the region around 1 MHz is associated with the capacitance of cell membranes; the region around 1010 Hz represents the dielectric dispersion associated with polarizability of water molecules in tissue.  21 Neural Effects If low-frequency currents are passed between a pair of electrodes placed on the skin then a current can be found at which sensation occurs. In general, this threshold of sensation rises with increasing frequency of applied current. Three fairly distinct types of sensation occur as frequency increases.  At very low frequencies (below 0.1 Hz) individual cycles can be discerned and a ‘stinging sensation’ occurs underneath the electrodes. The major effect is thought to be electrolysis at the electrode/tissue interface where small ulcers can form with currents as low as 100 μA. The application of low-frequency currents can certainly cause ion migration and this is the mechanism of iontophoresis. Current densities within the range 0–10 A m−2 have been used to administer local anaesthetics through the skin, and also therapeutic drugs for some skin disorders. The applied potential acts as a forcing function that can cause lipid soluble drugs to penetrate the stratum corneum. Sweat ducts are the principal paths for ion movement.  At frequencies above 10 Hz, electrolysis effects appear to be reversible and the dominant biological effect is that of neural stimulation. If the electrodes are placed over a large nerve trunk such as the ulnar or median, then the first sensation arises from the most rapidly conducting sensory fibres. If the amplitude of the current is increased, then more slowly conducting fibres are stimulated and motor contractions occur. Stimulation over a nerve trunk arises as a result of depolarization at a node of Ranvier. The capacitance of a single node is of the order 10 pF such that a charge of 10−12 C is required to remove the normally occurring polarization potential of about 0.1 V. 10−12 C can be delivered as a current of 10−9 A for 1 ms. However, when the current is delivered through relatively distant surface electrodes only a very small fraction of the current will pass into a particular node of Ranvier.  At frequencies above about 10 kHz the current necessary to cause neural stimulation is such that heating of the tissue is the more important biological effect. Displacement currents are usually negligible within the range 10–100 kHz and therefore the I2R losses are dominant. The major biological effects within our frequency range of interest are therefore electrolysis, neural stimulation and heating.  22 The threshold will depend upon the electrode area as there is ample evidence to show that current density rather than current is the important parameter. However, the relative magnitude of the three effects we have considered is not changed when current density rather than current is used. A typical value of current density at threshold and 50 Hz is 2 A m−2. Cardiac Stimulation: Fibrillation In many cases the patient is directly connected to the equipment so that in cases of a fault electrical current may flow through the patient. The response of the body to low-frequency alternating current depends on the frequency and the current density. Low-frequency current (up to 1 kHz) which includes the main commercial supply frequencies (50 Hz and 60 Hz) can cause:  prolonged tetanic contraction of skeletal and respiratory muscles;  arrest of respiration by interference with the muscles that control breathing;  Heart failure due to ventricular fibrillation (VF). The skin can have a resistance as high as 1 MΩ (dry skin) falling to 1 kΩ (damp skin). Internally, the body resistance is about 50Ω. Internal conduction occurs mainly through muscular pathways. Ohm’s law can be used to calculate the current. For example, for a person with damp skin touching both terminals of a constant voltage 240 V source (or one terminal and ground in the case of mains supply), the current would be given by I = V/R = 240/2050 = 117 mA, which is enough to cause ventricular fibrillation (VF). Indirect Cardiac Stimulation The threshold of current perception is about 1 mA, when a tingling sensation is felt. At 5 mA, sensory nerves are stimulated. Above 10 mA, it becomes increasingly difficult to let go of the conductor due to muscle contraction. At high levels the sustained muscle contraction prevents he victim from releasing their grip. When the surface current reaches about 70– 100 mA the co ordinate electrical control of the heart may be affected, causing ventricular fibrillation (VF). The fibrillation may continue after the current is removed and will result in death after a few minutes if it persists. Larger currents of several amperes may cause respiratory paralysis and burns due to heating effects.  23 The whole of the myocardium contracts at once producing cardiac arrest. However, when the current stops the heart will not fibrillate, but will return to normal co-ordinated pumping. This is due to the cells in the heart all being in an identical state of contraction. This is the principle behind the defibrillator where the application of a large current for a very short time will stop ventricular fibrillation. Direct Cardiac Stimulation Currents of less than 1 mA, although below the level of perception for surface currents, are very dangerous if they pass internally in the body in the region of the heart. They can result in ventricular fibrillation and loss of pumping action of the heart. Currents can enter the heart via pacemaker leads or via fluid-filled catheters used for pressure monitoring. The smallest current that can produce VF, when applied directly to the ventricles, is about 50 μA. 0.5 mA limit for leakage currents from normal equipment is below the threshold of perception, but above the VF threshold for currents applied to the heart. Ventricular Fibrillation VF occurs when heart muscle cells coming out of their refractory period are electrically stimulated by the fibrillating current and depolarize, while at the same instant other cells, still being in the refractory period, are unaffected The cells depolarizing at the wrong time propagate an impulse causing other cells to depolarize at the wrong time. Thus, the timing is upset and the heart muscles contract in an uncoordinated fashion. The heart is unable to pump blood and the blood pressure drops. Death will occur in a few minutes due to lack of oxygen supply to the brain. To stop fibrillation, the heart cells must be electrically co-ordinated by use of a defibrillator. The threshold at which VF occurs is dependent on the current density through the heart, regardless of the actual current. As the cross-sectional area of a catheter decreases, a given current will produce increasing current densities, and so the VF threshold will decrease.  24 Higher Frequency Effects Surgical Diathermy / electro surgery The technique uses an electric arc struck between a needle and tissue in order to cut the tissue. The arc, which has a temperature in excess of 1000 °C, disrupts the cells in front of the needle so that the tissue parts as if cut by a knife; with suitable conditions of electric power the cut surfaces do not bleed at all. If blood vessels are cut these may continue to bleed and current has to be applied specifically to the cut ends of the vessel by applying a blunt electrode and passing the diathermy current for a second, or two or by gripping the end of the bleeding vessel with artery forceps and passing diathermy current from the forceps into the tissue until the blood has coagulated sufficiently to stop any further bleeding. Diathermy can therefore be used both for cutting and coagulation. The current from the ‘live’ or ‘active’ electrode spreads out in the patient’s body to travel to the ‘indifferent’, ‘plate’ or ‘patient’ electrode which is a large electrode in intimate contact with the patient’s body. Only at points of high current density, i.e. in the immediate vicinity of the active electrode, will coagulation take place; further away the current density is too small to have any effect. Although electricity from the mains supply would be capable of stopping bleeding, the amount of current needed (a few hundred milliamperes) would cause such intense muscle activation that it would be impossible for the surgeon to work and would be likely to cause the patient’s heart to stop. The current used must therefore be at a sufficiently high frequency that it can pass through tissue without activating the muscles. Diathermy Equipment Diathermy machines operate in the radio-frequency (RF) range of the spectrum, typically 0.4–3 MHz. Diathermy works by heating body tissues to very high temperatures. The current densities at the active electrode can be 10 A cm−2. The total power input can be about 200 W. The power density in the vicinity of the cutting edge can be thousands of W cm−3, falling to a small fraction of a W cm−3 a few centimetres from the cutting edge.  25 The massive temperature rises at the edge (theoretically thousands of °C) cause the tissue fluids to boil in a fraction of a second. The cutting is a result of rupture of the cells. An RF current follows the path of least resistance to ground. This would normally be via the plate (also called dispersive) electrode. However, if the patient is connected to the ground via the table or any attached leads from monitoring equipment, the current will flow out through these. The current density will be high at these points of contact, and will result in surface burns (50 mA cm−2 will cause reddening of the skin; 150 mA cm−2 will cause burns). Even if the operating table is insulated from earth, it can form a capacitor with the surrounding metal of the operating theatre due to its size, allowing current to flow. Inductive or capacitive coupling can also be formed between electrical leads, providing other routes to ground. Heating effects If the whole body or even a major part of the body is exposed to an intense electromagnetic field then the heating produced might be significant. The body normally maintains a stable deep-body temperature within relatively narrow limits (37.4 }1 °C) even though the environmental temperature may fluctuate widely. The normal minimal metabolic rate for a resting human is about 45Wm−2 (4.5 Mw cm−2), which for an average surface area of 1.8 m2 gives a rate of 81 W for a human body. Blood perfusion has an important role in maintaining deep-body temperature. The rate of blood flow in the skin is an important factor influencing the internal thermal conductance of the body: the higher the blood flow and hence, the thermal conductance, the greater is the rate of transfer of metabolic heat from the tissues to the skin for a given temperature difference. Blood flowing through veins just below the skin plays an important part in controlling heat transfer surface temperatures will be affected by vessels carrying blood at a temperature higher or lower than the surrounding tissue provided the vessels are within a few millimetres of the skin surface.  26 APPLICATIONS Thermography Infrared thermography is the process of using a thermal imager to detect radiation (heat) coming from an object, converting it to temperature and displaying an image of the temperature distribution. Images of the detected temperature distribution are called thermograms, and they make it possible to see heatproducing objects invisible to the naked eye. It's widley-used in predictive maintenance and condition monitoring. IRT has been successfully used in diagnosis of breast cancer, diabetes neuropathy and peripheral vascular disorders. It has also been used to detect problems associated with gynecology, kidney transplantation, dermatology, heart, neonatal physiology, fever screening and brain imaging.