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EQUIPMENT

Real-time radiography systems are available in a number of configurations and sizes ranging from tabletop models to units that fill a large room. A typical system is shown in figure 3. The main components of a real-time radiograph system are shown in Figure 4.

Figure 3. Photograph of an example of a real-time radiograph system.

Figure 4. Schematic showing the various components of a real-time system.

 

The usual source of radiation for RTR systems is the X-ray generator. The main reason that an X-ray generator is used is because image intensifiers are relatively inefficient at converting radiation to light, and thus more flux is required than an isotope can offer. Additionally, X-ray energy and current must be adjustable to permit the correct exposure. (Time cannot be adjusted in this instance because the image is being viewed in real-time.) With radioactive isotope sources there is also the disadvantage that few wavelengths of different energies exist, and therefore the beam tends to be monochromatic compared with the beam from an X-ray machine. As discussed previously, the resolution in RTR systems is generally less than with film radiography. A way to improve resolution is to use a smaller focal-spot to reduce the penumbra. This will be discussed in more detail later.

Smaller spot sizes are especially advantageous in instances where magnification of object or region of the object is necessary. Magnification involves the use of minifocus and microfocus X-ray tubes. One common way to differentiate between conventional, minifocus, and microfocus is to say that conventional units have focal-spots greater than 200 microns; minifocus units have focal-spots ranging from 50 microns to 200 microns (.050 mm to .2 mm); and microfocus have focal-spots smaller than 50 microns. As indicated, the cost of some microfocus tubes can exceed $100,000. Although this component must be selected carefully, it will enable the best resolution possible. Some manufacturers combine two filaments of different sizes to make a dual-focus tube. This usually involves a conventional and a minifocus spot-size and adds flexibility to the system.

An important point to keep in mind is that the milli amperage used in mini- and microfocus units is smaller than that used in conventional tubes. Amperage determines the intensity of X-rays. Such small focal-spots cannot withstand a great deal of heat. An X-ray generation feature offsetting the lower current levels is that mini- and micro-units usually are constant potential. This means that voltage is delivered at a very constant level, with very little ripple. In a conventional X-ray tube, voltages differ and the maximum X-ray output occurs when voltage peaks. Output tends, therefore, to vary with time. Because constant potential systems are essentially always at maximum output, the X-ray output over a given period of time is greater than that of a conventional tube at the same settings. The final result being that the output of constant potential units can be greater than the conventional X-ray tube.)

 

Figure 5. Photograph of a conventional x-ray used in a RTR system.