Reliability and Dosimetry fo Computerized Tomography
Reliability and accuracy in CT diagnosis depends intimately on DOSE. Figure 1 illustrates the difference between the DOSE given by an ordinay x-ray machine and the DOSE attributable to a CT scanner. With ordinary x-rays, as successive x-rays are made the DOSE is accumulated because the same area of the body generally receives the radiation. On the other hand, the CT Scanner radiates only the section being studied and hence, when the next section is made, different tissue receives the DOSE. The DOSE is measured in units of rads, which is the energy absorbed by a unit mass of the tissue being irradiated. However, as shown in Figure 2, CT scans of successive sections can have a slight overlap. The tissue in the overlapping area does receive a slightly larger DOSE than would have been obtained if only a single section had been made. Table 1, taken from McCullough (2), gives a summary of some values of absorbed DOSE for CT scanning. The DOSE depends upon the direction of the x-ray beam and Figure 3 shows some values obtained on phantoms. Dixon et al. (2) describe a system for using film to compute the doses, otherwise the dose evaluation procedure is somewhat complex and is usually carried out by radiation physicists. Figure 4 clearly illustrates the variation in dose between single and multiple sections and from front to back of the patient.
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