TABLE 3.10—Desirable characteristics of x-ray beam energy and exposure rate.
3.1.7 Exposure Control
Accurate control of the exposure is essential for providing consistent images within the optimal range of optical densities.
220.127.116.11 Automatic Exposure Control. Reliable automatic exposure control (AEC) systems are essential for high-quality mammogra-phy and should be designed to operate in all imaging modes (grid, nongrid and magnification) and with all imaging attachments, such as coned-down compression devices (ACR, 1993). A properly designed AEC provides better control over image optical densities than does manual exposure control. Radiographic density cannot be predicted with the required accuracy from compressed breast thickness or firmness on compression and shows only a poor correlation with patient age (Swann et al., 1987). The AEC device should either automatically compensate for changes in imaging formats or disallow exposure until appropriate technique factors are set. The need for an effective and reliable AEC system is especially acute for large volume screening practices, particularly for those that depend on delayed batch processing of the mammographic films.
Mammographic AEC has been described by various authors (Barnes, 1994; LaFrance et al., 1988). In general, the sensors in such systems are located behind the image receptor (screen-film cassette). The sensor may be a phosphor coupled to a photomulti-plier tube, but will more typically be an ionization chamber or a solid-state detector. The sensor produces a current proportional to the exposure rate of the radiation incident on it and the current charges a capacitor (Figure 3.8). In the cases of the ionization chamber and the solid-state detector, an intermediate amplification step is required. The voltage across the capacitor is then proportional to the exposure to the sensor (and therefore to the
patient). This voltage is compared to a reference voltage, and when the two voltages are equal the exposure is terminated.
Early AEC provided with dedicated mammographic units exhibited various performance problems (Kimme-Smith et al., 1987; LaFrance et al., 1988; NCRP, 1986). The sources of these problems have been identified (LaFrance et al., 1988) as beam hardening, film reciprocity law failure, and sensor dark current. Beam hardening is the dominant effect causing the AEC to terminate the exposure too soon. As breast thickness or density or both increases or as operating potential increases, the x-ray beam exiting the breast becomes more penetrating. Consequently, an increased proportion of the x-ray beam is transmitted through the image receptor and exposes the AEC sensor. This higher exposure rate causes the AEC to terminate the exposure too soon with the result that film densities decrease (Figure 3.9).
Modern AEC appropriately compensates for variations in the selected operating potential and breast density and thickness. This can be accomplished through circuit modifications which incorporate a nonlinear amplification step (LaFrance et al., 1988). The result is that short exposure times become shorter and long exposure times become longer. Also, microprocessor controlled AECs have been introduced (Frederick et al., 1991) which correct for changes in operating potential and breast density and thickness in a variety of ways. Compensation for operating potential can be accomplished by including the preset kilovolt peak as a factor in
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