![]() A small effective focal spot can thus be obtained with the resultant heat being spread over of a broad area of anode material. Its diameter, R, can be up to 120 mm and it is generally spun at speeds of about 3,000 rpm during the exposure. The situation is illustrated in Figure 2.3 on the right where an end-on and side-on view of an anode disk is shown. This spins a disc of tungsten during the exposure so that the electron beam strikes an annular region of its surface rather than being concentrated into a small area, as in the stationary anode design we’ve considered up to now. The second design feature to be incorporated is an anode rotated by an electric motor. 2.3: End-on and side-views of a rotating anode. This is called the effective size of the focal spot.Īnode angles are typically 6-15 o in modern XRTs, with the small angled tubes being used for angiography, for instance, where fine detail imaging is required.įig. The size of the electron beam focal spot is therefore reduced by angling the anode so that an apparently smaller X-ray source is obtained. The same consideration can be applied to the width of the electron beam, so that the heat it generates can as a result be dispersed over a broader region of the anode and a fine X-ray focal spot can still be obtained. From below, the size of ab appears to be shortened to that of cd depending on the sine of the angle, θ, of the anode. Here we see an exploded view of the anode surface being struck by an electron beam of height, ab. The first one is based on the Line Focus Principle and is illustrated in Figure 2.2. 2.2: Illustration of the Line Focus Principle. Two design features have been incorporated into XRT design to address these conflicting requirements.įig. Modern diagnostic XRTs are more complex that the simple arrangement illustrated in Figure 2.1, mainly because fine focal spots are required to produce good spatial resolution in images (as discussed in the next chapter) and effective heat dissipation from the anode target is a major issue. Thirdly, remember that ‘electron flow’ is the reverse of the direction of electric current ( Michael Faraday got it wrong!), as indicated in the figure. This current is one of the factors used to control X-ray exposure and its value is generally displayed on an ammeter (labelled mA in Figure 2.1). 1 mA in fluoroscopy to over 1,000 mA in angiography), depending on the efficiency of the thermionic emission. 5 A), but the current generated in the electron beam (the so-called X-ray tube current) is much lower (e.g. Secondly, the filament supply current is quite large (e.g. It has a high melting point (over 3,000 oC) and therefore doesn’t melt under normal operating conditions.įurther points to note are firstly that the glass tube contains a vacuum so that electrons will not be absorbed and deflected by air molecules as they pass between cathode and anode. The focal spot therefore gets quite hot and it is here that the second important characteristic of tungsten comes into play. Most of the electron energy deposited in the anode is converted to heat with less than 1% actually producing X-rays. collimators not shown in the figure) are used to allow only a primary beam to escape the source and irradiate the patient. X-rays are produced in all directions from this focal spot, but beam restriction devices (e.g. The focusing cup can be used to form the electrons into a narrow beam and hence only strike a small spot on the anode target. Bremsstrahlung and Characteristic Radiation processes are involved here, as previously described. A high voltage (HV) of up to 100 kV or more is then applied and the electrons are attracted across the gap to collide with the anode at high energies, to produce X-rays. ![]() ![]() Electrons are boiled off the filament by applying an electric current so that it becomes white hot - the process is called Thermionic Emission. The anode target is generally made from tungsten separated from the filament by a small gap, although molybdenum and rhodium targets are used in Mammography. The cathode generally consists of a small coil of wire, called a filament, mounted in a focusing cup. We have described the basics of its operation in the previous chapter and will get into much more detail here.Īn XRT in its simplest form consists of an anode and cathode mounted inside an evacuated glass tube - see Figure 2.1. X-rays can be generated by instruments such as the electron synchrotron and linear accelerator but in Diagnostic Radiography are nearly always produced by a small electron accelerator called an X-ray tube (XRT). ![]()
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