Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

ference of 10% to 15% positive (greater exposure) or negative (lesser exposure) perstep from the neutral setting permits flexibility for unusual imaging circumstances,or allows variation for radiologist preference in film optical density. If the transmissionof photons is insufficient to trigger the comparator switch after an extendedexposure time, a backup timer terminates the exposure. For a retake, the operatormust select a higher kVp for greater beam penetrability and shorter exposure time.Inaccurate phototimer response, resulting in an under- or overexposed film, canbe caused by breast tissue composition heterogeneity (adipose versus glandular), compressedthickness beyond the calibration range (too thin or too thick), a defective cassette,a faulty phototimer detector, or an inappropriate kVp setting. Film response tovery long exposure times, chiefly in magnification studies (low mA capacity of thesmall focal spot), results in reciprocity law ftilure and inadequate film optical density.For extremely thin or fatty breasts, the phototimer circuit and x-ray generatorresponse can be too slow in terminating the exposure, causing film overexposure.The position of the phototimer detector (e.g., under dense or fatty breast tissue)can have a significant effect on film density. Previous mammograms can aid inthe proper position of the phototimer detector to achieve optimal film density forglandular areas of the breast. Most systems allow positioning in the chest wall tonipple direction, while some newer systems also allow side-to-side positioning toprovide flexibility for unusual circumstances. The size of the AEC detector is animportant consideration. A small detector (e.g., solid-state device) can be affectedby non uniformities that are not representative of the majority of breast tissue. Onthe other hand, a large area detector can be affected by unimportant tissues outsidethe area of interest. Either situation can result in over- or underexposure of thebreast image.Technique ChartTechnique charts are useful guides to determine the appropriate kVp for specificimaging tasks, based on breast thickness and breast composition (fatty versus glandulartissue fractions). Most mammographic techniques use phototiming, and theproper choice of the kVp is essential for a reasonable exposure time, defined as arange from approximately 0.5 to 2.0 seconds, to achieve an optical density of 1.5 to2.0. Too short an exposure can cause visible grid lines to appear on the image, whiletoo long an exposure can result in breast motion, either of which degrades the qualityof the image. Table 8-4 lists kVp recommendations, determined from computerTABLE 8-4. RECOMMENDED kVp TECHNIQUES, AS A FUNCTION OF BREASTCOMPOSITION AND THICKNESS IS DETERMINED WITH COMPUTERSIMULATIONS AND CLINICAL MEASUREMENTaBreast Thickness (cm)Breast Composition 2 3 4 5 6 7 8Fatty 24 24 24 24 25 27 3050/50 24 24 24 25 28 30 32Glandular 24 24 26 28 31 33 35aThe desired end point is an exposure time between 0.5 and 2.0 see. This is for a Mo target and 0.030-mm Mo filter using a 100-mA tube current.