Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

megabytes, and four times that for a four-image screening exam. Research studiesindicate the possibility of relaxing digital acquisition to 50 f.lm and even further to100 f.lm pixel size. (The FDA has approved a full-field digital mammography devicewith a 100 f.lm pixel aperture in early 2000.) Digital systems outperform screen-filmreceptors in contrast detection experiments.Disadvantages of digital mammography include image display and system cost.Soft copy (video) displays currently lack the luminance and spatial resolution to displayan entire mammogram at full fidelity. Films produced by laser cameras are usedfor interpretation. The extremely high cost of full-field digital mammography systems(currently about five times more expensive than a comparable screen-film system)hinders their widespread implementation.X-ray mammography is the technique of choice for detecting nonpalpable breastcancers. However, the risk of carcinogenesis from the radiation dose to the breast isof much concern, particularly in screening examinations, because of the large numberof women receiving the exams. Thus, the monitoring of dose is important andis required yearly by the MQSA.The glandular tissue is most always the site of carcinogenesis, and thus the preferreddose index is the average glandular dose. Because the glandular tissues receivevarying doses depending on their depths from the skin entrance site of the x-raybeam, estimating the dose is not trivial. The midbreast dose, the dose delivered to theplane of tissue in the middle of the breast, was the radiation dosimetry benchmarkuntil the late 1970s. The midbreast dose is typically lower than the average glandulardose and does not account for variation in breast tissue composition.Average Glandular DoseThe average glandulardose, Dg, is calculated from the following equation:Dg = DgN X XESEwhere XESEis the entrance skin exposure (ESE) in roentgens, and DgN is an ESE toaverage glandular dose conversion factor with units of mGy/R or mrad/R. An airfilledionization chamber measures the ESE for a given kVp, mAs, and beam quality.The conversion factor DgN is determined by experimental and computer simulationmethods and depends on radiation quality (kVp and HVL), x-ray tube targetmaterial, filter material, breast thickness, and tissue composition. Table 8-6 listsDgN values for a 50% adipose, 50% glandular breast tissue composition of 4.5-cmthickness as a function of HVL and kVp for a Mo/Mo target/filter combination.For a 26-kVp technique with a HVL of 0.35 mm AI, the average glandular dose isapproximately 17% of the measured ESE (Table 8-6). For higher average x-ray energies(e.g., Mo/Rh, Rh/Rh, W/Mo, W/Rh), conversion tables specific to the generatedx-ray spectrum must be used, as the DgNvalues increase due to the higher effectiveenergy of the beam. DgN decreases with an increase in breast thickness forconstant beam quality and breast composition. This is because the beam is rapidlyattenuated and the glandular tissues furthest from the entrance receive much lessdose in the thicker breast (e.g., DgN = 220 mrad/R for 3-cm thickness versus 110