V SALAI SELVAM, AP & HOD, ECE, Sriram Engg. College, Perumalpattu 1 MEDICAL ELECTRONICS UNIT IV Ionizing and non-ionizing radiations: The radiation that ionizes the gases through which it travels is known as the ionizing radiation. Examples: α-rays, β-rays, X-rays, gamma rays, cosmic rays. Among these ionizing radiations, the X-rays are widely used in the medial imaging for diagnosis. There are three different types of radiations, each with its own distinct properties. They are (i) alpha rays: they are positively charged particles (helium nuclei) with low penetrating capacity (ii) beta rays: they are negatively charged particles (electrons) with moderate penetrating capacity and (iii) gamma rays & X-rays: they are electrically neutral particles (photons) with very high penetrating capacity. The other radiations that do not ionize the gases are known as the non-ionizing radiations. Examples: visible light, infrared, radio waves Generation of ionizing radiations: Generation of X-rays: X-ray tube, principle of operation X rays are generated when fast-moving electrons are suddenly decelerated by a hard target. An X-ray tube is basically a high-vacuum diode with a heated cathode located opposite a target anode. This diode is operated in the saturated mode with a low cathode (heater) temperature. So the current through the tube does not depend on the applied anode voltage. The intensity of X rays depends on the current through the tube. This current can be varied by varying the heater (cathode) current. This in turn controls the cathode (heater) temperature. The wavelength of the X rays depends on the target material and the velocity of the electrons hitting the target. It can be varied by varying the target (anode) voltage of the tube. X-ray equipment for diagnostic purposes uses target voltages in the range of 30 to 100 kv and the current is in the range of several hundred milliamperes. These voltages are obtained from high-voltage transformers that are often mounted in oil-filled tanks to provide electrical insulation. When ac voltage is used, the X-ray tube conducts only during one halfwave and acts as its own rectifier. (Otherwise high-voltage diodes, often in voltage-doubler or multiplier configurations, are used as rectifiers.)
2 V SALAI SELVAM, AP & HOD, ECE, Sriram Engg. College, Perumalpattu For therapeutic X-ray equipment, higher radiation energies are required and hence linear or circular particle accelerators such as cyclotron and magnetron have been used to obtain electrons with sufficiently high energy. When the electrons strike the target, only a small part of their energy is converted into X rays; most of it is dissipated as heat and the target, usually made of tungsten, has a high melting point. Therefore, it is either water-cooled or air-cooled or it is in the form of a motordriven rotating cone. The electron beam is concentrated to form a small spot on the target. The X rays emerge in all directions from this spot. Collimators, made up of lead, are used to confine these X-rays into a fine beam. Diagnostic X-ray equipments: The use of X rays as a diagnostic tool is based on the fact that various components of the body have different densities for the rays. When X rays from a point source penetrate a body section, the internal structure of the body absorbs varying amounts of the radiation. The radiation that leaves the body, therefore, has a spatial intensity variation that is an image of the internal structure of the body. When, as shown in the following figure, this intensity distribution is visualized by a suitable device, a shadow image is generated that corresponds to the X-ray density of the organs in the body section. X rays normally cannot be detected directly by the human senses. Hence, indirect methods of visualization must be used to visualize X-ray images. Three different techniques are in common use: Fluoroscopy, X-ray films and Image intensifiers. Fluoroscopy: Certain metal salts glow in the dark when struck by the X-rays. The brightness of this fluorescence is a function of the radiation intensity, and cardboard pieces or glass surfaces coated with such metal salts are used to visualize X-ray images. The X-ray intensity necessary to obtain a fluoroscopic image is harmful to both the patient and the observer. If the X-ray intensity is reduced to a safer level, the fluoroscopic image becomes rather faint. Because of these inconveniences, direct fluoroscopy now has only limited use. X-ray films: X rays react with photographic emulsions. After processing in a developing solution, a film that has been exposed to X rays shows an image of the X-ray intensity. The sensitivity of this effect can be increased by the use of intensifying screens which are similar to the fluoroscopic screens. The screen is brought into close contact with the film surface so that the film is exposed to the X rays as well as to the light from the fluorescence of the screen. X-ray films, with or without intensifying screens, are packaged in light-tight cassettes in which one side is made of thin plastic that can easily be penetrated by the X rays.
V SALAI SELVAM, AP & HOD, ECE, Sriram Engg. College, Perumalpattu 3 Image intensifiers: The faint image of a fluoroscopic screen can be made brighter with the help of an electronic image intensifier, as shown in the following figure. The intensifier tube contains a fluorescent screen. Its surface is coated with a suitable material to act as a photocathode. The electron image thus obtained is projected onto a phosphor screen at the other end of the tube by means of an electrostatic lens system. The resulting brightness gain is due to the acceleration of the electrons in the lens system and the fact that the output image is smaller than the primary fluorescent image. The gain can reach an overall value of several hundred. The intensifying tube, however, is rather heavy and requires a special suspension. For this reason, a TV camera is now used frequently to pick up the intensified image. This TV picture can also^e recorded on a TV tape recorder. Often special techniques are used to obtain usable images from certain body structures. Grids: Some of the X rays entering the body of a patient are scattered and no longer travel in a straight line. The scattered X rays can cause a blurring of the X-ray image. This effect can be reduced by the use of a grid or a Bucky diaphragm. This device consists of a grid-like structure made of thin lead strips that is placed directly in front of the X-ray film. The grid absorbs the scattered X rays while those traveling in straight lines can pass. In order to prevent the grid from throwing its own shadow on the film, it is moved by a motorized drive during the exposure of the film. Contrast Media: While foreign bodies and bone absorb the X rays much more readily than soft tissue, the organs and soft tissue structures of the body do not show up well in the X-ray images. In order to make them visible on the X-ray images, they are filled with a contrast medium prior to taking the X-ray photo. Example: (i) In Pneumoencephalography, the ventricles of the brain are made visible by filling them with air. (ii) The structures of the gastrointestinal tract are made visible with the help of barium sulfate, given orally or as an enema. (iii) In angiography, the outlines of blood vessels are made visible on the X-ray image by injecting a bolus of contrast medium directly into the bloodstream in the region to be investigated. Cardiac Catheterization: Fluoroscopic techniques are used to assist the cardiac catheterization used primarily to diagnose valve deficiencies, septal defects, and other conditions of the heart characterized by hemodynamic changes.
4 V SALAI SELVAM, AP & HOD, ECE, Sriram Engg. College, Perumalpattu Three-Dimensional Visualization: A basic limitation of X-ray images is the fact that they are two-dimensional presentations of three-dimensional structures. One organ located in front of or behind another organ therefore frequently obscures details in the image of the other organ. (i) In stereoradiography, two X-ray photos are taken from different angles, and viewed in a stereo viewer to give a three-dimensional X-ray image. (ii) In tomography, the X-ray photo shows the structure of only a thin slice or section of the body. Several photos representing slices taken at different levels give three-dimensional visualization. Tomographic X-ray photos are obtained with a thin X-ray beam by moving the X-ray tube and the film cassette in opposite directions during the exposure of the film. Use of radioisotopes for diagnosis: Radioisotopes: Radioactive decay is the other source of nuclear radiation, but only a very small number of chemical elements exhibit natural radioactivity. Artificial radioactivity can be induced in other elements by exposing them to high-velocity neutrons. By introducing a high-velocity neutron into the nucleus of the atom of an element, an unstable form of the element is generated that is chemically equivalent to the original form (isotope). The unstable atom disintegrates after some time, often through several intermediate forms, until it has assumed the form of another stable element. At the moment of the disintegration, radiation is emitted. The time after which half of the original number of radioisotope atoms have decayed is called the half-life. Each radioisotope has a characteristic half-life from a few seconds to thousands of years. RADIOISOTOPES Isotope Radiation Half-Life 3 H Beta 12.3 days 14 C Beta 5570 years 51 Cr Gamma 27.8 days 99m Tc Gamma 6 hours 131 I Gamma 8.07 days 198 Au Gamma 2.7 days With the help of the emitted radiation, the path of the substance (isotope) can be traced and its concentration in various parts of the organism can be determined. The radioisotopes most frequently used for medical purposes are listed in the above table. For in-vivo (inside living body) diagnosis, the gamma-emitting isotopes must be used and the beta-emitting isotopes are used only for in-vitro (outside living body) diagnosis. This is because the gamma rays penetrate the surrounding tissues but the beta rays do not. Detection methods: Radioisotope techniques are all based on actually counting the number of nuclear disintegrations that occur in a radioactive sample during a certain time interval or on counting the radiation quanta that emerge in a certain direction during this time. Scintillation detectors or counters that utilize the light flashes caused by radiation in a suitable medium are used for the detection of radiation from isotopes. The scintillation detector consists of a suitable medium that emits light flashes on the incidence of radiation and a photomultiplier. Each radiation quantum passing the crystal causes an output pulse at the photomultiplier. The amplitude of this pulse is proportional to the energy of the radiation. This property of the scintillation detector is used to reduce the background noise (counts due to natural radioactivity) by means of a pulse-height analyzer. For in-vivo determinations, a scintillation detector with a collimator, known as collimated detector, is used. A collimator is a thick lead shield with holes for the passage of X-rays travelling in straight lines. The following figure shows the other building blocks that constitute a typical instrumentation system for medical radioisotope measurements.
V SALAI SELVAM, AP & HOD, ECE, Sriram Engg. College, Perumalpattu 5 The pulses from the photomultiplier tube are amplified and shortened before they pass through the pulse-height analyzer. A timer and gate allow the pulses that occur in a set time interval to be counted by means of a scaler (decimal counter with readout). A rate meter (frequency meter) shows the rate of the pulses. Radiation therapy: The ionizing effect of X rays is utilized in the treatment of certain diseases, especially of certain tumors. In dermatology very soft X rays (called the Grenz rays) that do not have enough penetration power are used for treatment of the skin. In the therapy of deep-seated tumors, on the other hand, very hard X rays generated using voltages much higher than those for diagnostic X rays are used. Sometimes linear accelerators or betatrons are used to obtain electrons with a very high voltage for this purpose. Changing the direction of entry of the beam in successive therapy sessions or rotating the patient during a session reduces the radiation damage to unaffected body parts while concentrating the radiation at the site of the tumor. Adverse effects of radioactive diagnosis and therapy: The effects of cumulative X-ray dosage of ionizing radiation may result in (i) mutations genetic changes resulting from damage to chromosomes (ii) physical illness vomiting, headache, dizziness, loss of hair, and burns and (iii) death destruction of vital physiological systems such as nervous, cardiovascular, respiratory, renal, and digestive systems and tissues. The radioactive diagnosis or therapy is not advised for pregnant females due to the above-said adverse effects of radiation.