Radiology Physics CH 1 : X-RAY TUBE DESIGN
The production of X-ray needs the following: (i) electron source (cathode), (ii) target to stop the electrons (anode), (iii) high voltage supply to accelerate electrons, (iv) vacuum and (v) tube insert (glass envelope). Electron can be produced either by ionization in gas or by thermionic emission. The electron source acts as a cathode and the target acts as an anode. The high voltage is applied in between the cathode and anode. This voltage accelerates the electrons to a higher velocity; as a result the electron will possess high kinetic energy. When the electrons are stopped by a target, the electron kinetic energy is converted into X-ray energy and thus X-rays are produced. The equipment having all the above requirements is called an X-ray tube.
The tube should be designed in such way that, it should withstand voltages from 20–150 kV and currents up to 1000 mA. In radiography, tube current may vary from 100 to 1000 mA, whereas in fluoroscopy, it is 1–5 mA. In addition, exposure time has to be varied over a wider range.
CATHODE
The cathode is made of tungsten wire in the form of helical filament, surrounded by a focusing cup (Fig. 3.5). Tungsten is used as filament material because of its high melting point, low vapor pressure, good ductility (easily drawn into fine wire) and low work function (4.5 eV). Tungsten exhibits thermionic emission well below its melting point. The filament is made of tungsten wire, about 0.2 mm in diameter,that is coiled to form a vertical spiral about 0.2 cm in diameter and 1 cm in length. The coil format provides large surface area for electron emission.
The filament circuit supplies a voltage of 8–12 V and selectable filament current of 3–7 amperes. Electrical resistance to electron flow heats the filament to very high temperature, releasing surface electron through thermionic emission process. The rate of emission depends on the temperature and it can be adjusted by the filament current. A trace of thorium in the filament not only increases the efficiency, but also prolongs the filament life. If the applied voltage between anode and cathode is zero, the electrons form a cloud near the cathode, which is called space charge. As the applied voltage increases, the electrons are accelerated towards the anode, helps the production of X-rays. The focusing cup controls the width of the electron distribution, and directs the electron toward the target. Usually, the focusing cup is at the same potential as the filament, which is called unbiased X-ray tube. Usually, X-ray tubes are provided with two filaments of different length. Selection of a particular filament determines the focal spot length or area.
SPACE CHARGE EFFECT
To under stand the operation of an X-ray tube, it is essential to know how the tube current depends upon the tube voltage, for a given filament excitation (Fig. 3.6). When the applied kV is zero or small, the electrons surrounding the filament forms a cloud, resulting in space charge effect. These electrons tend to repel electron back into the filament and hence the tube current is very small. As the kVp is increased, (0–40 kV) the effect of space charge reduces gradually and the tube current also increases. This is called space charge limited region. In this region, the tube current strongly dependent on applied kV, for a constant filament current.
Above 40 kVp, the space charge effect is overcome, and the tube current is controlled by the filament current. This is called the saturation or emission limited region. In this region, the tube current undergoes little change with an increase in tube voltage. The tube current is 5–10 times lesser than the filament current in this region.
Most of the X-ray tubes are operating in between the space charge region and saturation region. Thus, the tube current is determined by both kV and filament current. In the space charge region (< 40 kV), the tube current is influenced by the applied voltage only. To deliver the selected tube current, a space charge compensating circuit is used.
This circuit also corrects the small increase in tube current, at higher applied voltage (> 40 kV).
The above characteristics of the tube (curves) are depend upon many factors, including the distance between anode-cathode, the configuration of focusing cup, the focal spot size, and the filament temperature. Particularly the change of potential of the focusing cup, will drastically alter the curves.
ANODE
The anode is the target electrode, which is maintained at a positive potential. The target material should possess the following properties: (i) high melting point to withstand high temperature, (ii) high atomic number to increase the X-ray production efficiency, (iii) high thermal conductivity to dissipate heat quickly, (iv) low vapor pressure at high temperature to prevent the evaporation of target material, and (v) easily machined to make smooth surface. Tungsten (W) is the metal widely used as target because of its high
melting point, 3387°C and high atomic number, 74. However, its thermal conductivity is low (174 Wm–1K–1) and hence tungsten is embedded over a thick block of copper. The thermal conductivity of copper is 400 Wm–1K–1, so that the heat will be removed very quickly to the surrounding. The vapor pressure of tungsten is 5000 kPa, which is low, by which it releases less vapor into the vacuum. In the stationary anode, the tungsten is a square or rectangular plate of 2 or 3 mm thick and dimension greater than 1 cm. However, rotating anode design employs disk of 75–200 mm diameter, with beveled edges. Large diameter anodes are used in CT and fluoroscopy, which increases the heat capacity and heat dissipation. However, they are prone for mechanical damage, which is prevented by making radial slots in the anode. The above
type of anode is called stress relieved anodes.
The anode has a tendency to crack under severe stress caused by heating. Therefore, tungsten-rhenium alloy (90% tungsten + 10%
rhenium) is always used, which makes the target tougher and reduces surface pitting. Molybdenum (Mo, Z = 42) and rhodium (Rh, Z = 45) are commonly used as anode materials for mammographic X-ray tubes. These targets are capable of giving characteristic X-rays, suitable for soft tissue contrast studies.
FOCAL SPOT SIZE
The area of target with in which the electrons are absorbed and X-rays are generated is called focal spot or focal area. If the focal
area is very small, penumbra will be lesser, and the picture sharpness will be good, but heat removal is difficult. On the other hand, if the
focal area is large, heat will be removed quickly, penumbra is larger and the picture sharpness is bad. This can be compromised by careful design of the tube.
Usually, focal spot is defined in two ways namely, actual and effective focal spots. The actual focal spot size is the area on the anode that is struck by electrons. The effective focal spot size is the length and width of the emitted X-ray beam as projected down the central axis of the X-ray tube. The effective focal spot length is always smaller than the actual focal spot. The relation between them is given as follows:
Effective focal length = Actual focal length × sin
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