بسم الله الرحمن الرحيم و الشمس تجري لمستقر لها ذلك تقدير العزيز العليم صدق الله العظيم

Chapter 1 Concepts of Medical Instrumentation Essential of Medical Imaging

Basic X-Ray Circuit

Simplified diagram (Khan Figure 3.3) Consists of two parts: High-voltage circuit – provides x-ray tube accelerating potential for the electrons and the low-voltage circuit to supply heating current to the filament.Since the voltage applied between the cathode and the anode is high enough to accelerate all the electrons across to the target, the filament temperature or filament current controls the tube current.

The filament supply for electron emission usually consists of 10 V at about 6 A. As shown in Fig. 3.3, this can be accomplished by using a step-down transformer in the AC line voltage. The filament current can be adjusted by varying the voltage applied to the filament. Since a small change in this voltage or filament current produces a large change in electron emission or the current (Fi-g. 3. lo), a special kind of transformer is used which eliminates normal variations in line voltage.

The high voltage to the x-ray tube is supplied by the step-up transformer (Fig. 3.3). The primary of this transformer is connected to an auto transfoormer and a rheostat. The function of the auto transformer is to provide a stepwise adjustment in voltage

focal spot An important requirement of the anode design is the optimum size of the target area from which the x-rays are emitted. This area, which is called the focal spot, should be as small as possible for producing sharp radiographic images. However, smaller focal spsots generate more heat per unit area of target and, therefore, limit currents and exposure. In therapy tubes, relatively larger focal spots are acceptable since the radiographic image quality is not the overriding concern.

Focal spot size The apparent size of the focal spot can be reduced by the principle of line focus, illustrated in Fig The target is mounted on a steeply inclined surface of the anode. The apparent side n is equal to A sin Ɵ, where A is the side of the actual focal spot at an angle Ɵ with respect to the electron beam. Since the other side of the actual focal spot is perpendicular to the electron, its apparent length remains the same as the original. The dimensions of the actual focal spot are chosen so that the apparent focal spot results in an approximate square. Therefore, by making the target angle Ɵ small, side a can be reduced to a desired size. In diagnostic radiology, the target angles are quite small (6-17 degrees) to produce apparent focal spot sizes ranging from 0.1 x 0.1 to 2 x 2 mm. In most therapy

Heel Effect Heel effect –Reduction in x-ray intensity on anode side of x-ray tube caused by increased x-ray absorption due to obliquity –Figure 2-11 of Christensen

Production of x-rays beam Cameron, 1979 The main components of a modern x-rays unit are (1) a source of electrons – a filament or cathode (2) an evacuated space where to speed up the electrons (3) a high positive potential to accelerate the negative electrons and (4) a target or anode which the electrons strike to produce x-rays as shown in fig (2-2).

The intensity of the x-ray beam produced when the electrons strike the anode is highly dependent on the anode material in general the higher the atomic number (z) of the target, the more efficiently x-rays are produced. The target material used should be also having a high melting point since the heat produced when the electrons are stopped in the surface of target is substantial. Nearly all x-ray tubes use tungsten targets. The (z) of tungsten is 74 and the melting point of it is about 3400 o C.

Concepts of Gamma Camera Physics for Medical Imaging,2002

The multihole collimator The patient is positioned as shown in Fig. 5.2 close to the collimator. This consists of a lead disk, typicaaly 25 mm thick and up to 400 mm in diameter. It is drilled with some closely packed circular or hexagonal holes, each 2.5 mm in diameter. They are seperated by septa 0.3 mm thick which absorb all but a few percent of the rays attempting to pass through them obliquely. (The half value layer (HVL) of lead for 99m Tc gamma rays is 0.3 mm. )

–Each hole only accepts gamma rays from a narrow channel, thus locating any radioactive source along its line of sight. For example, in Fig. 5.2, ray a is accepted by the collimator, and ray b rejected. However, other

However, other rays such as c can be scattered in the body and then pass through the collimator. These rays will have less energy and so can be rejected later by energy discrimination(see pulse height analyzer below)

The Crystal Instead of an equally large number of tiny detectors, behind the collimator there lies a single large phosphor crystal some 500 mm in diameter and 9-12 mm thick, made of sodium iodide (thalium)activated with a trace of. Having a high atomic number Z= 53) and density., it absorbs about 90 % of Tc-99m gamma rays, principally by the photoelectic process; but only some 30 % of those from I-131.

It is fragile and easily damaged by temperature changes. To protect it from light and the atmosphere ( it is hygroscopic) it is encapsulated in an aluminium cylinder with one flat Pyrex face.each gamma photon (such as photon a in Fig. 5.2) when absorbed by the crystal produces a flash of light, shown by the dashed lines diagram. The flash contains some 5000 light photons which travel in all directions and last less than a microsecond.

About 4000 of them emerge from the farther flat surfaces, having been reflected off the other faces, which are coated with a reflecting titanium compound. The distribution of light leaving ray passed through, and is measured by uup to 91 mathed photomultipliers, closely packed in a hexagonal array. Five of these are shown, diagrammatically in fig Transfer of light from the crystal to the photomultipliers, in Fig Transfer of light from the crystal to the photomultipliers may be maximized by a light guide a flate transparent plate.

Photomultipliers Each photomultiplier (Fig.5.3) consists of an evacuated glass envelope containg ( on a photocathode coated with a material Which absorbs light and emits photoelectrons. One electron per five or 10 light photons.

Pulse height analyzer and Resolution