Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

TABLE 8-2. REQUIREMENTS FOR MINIMUM HALF VALUE LAYER (HVL) MQSA:21 CFR (FDA REGULATIONS) PART 1020.30; ACR: WITH COMPRESSION PADDLETube Voltage(kV)MQSA-FDAkVp/100ACR kVp/100 + 0.0324262830320.240.260.280.300.320.270.290.310.330.35ACR, American College of Radiology; CFR, Code of Federal Regulations; FDA, Food and DrugAdministration; MQSA, Mammography Quality Standards Act.tube for 25 to 30 kVp, the beam is "harder" than optimal, which is likely caused bya pitted anode or aged tube, and can potentially result in reduced output and poorimage quality. ACR accreditation guidelines recommend a maximum HVL basedon the target/filter combination listed in Table 8-3. Estimation of radiation dose tothe breast requires accurate assessment of the HVL (see Radiation Dosimetry,below).Tube OutputTube output [milliroentgen/milliampere second (mR/mAs)] is a function of kVp,target, filtration, location in the field, and distance from the source. At mammographyenergies, the increase in exposure output is roughly proportional to the thirdpower of the kY, and directly proportional to the atomic number of the target (Fig.8-14). The MQSA requires that systems used after October 2002 be capable of producingan output of at least 7.0 mGy air kerma per second (800 mR/sec) whenoperating at 28 kVp in the standard (Mo/Mo) mammography mode at any SIDwhere the system is designed to operate.Fixed-size metal apertures or variable field size shutters collimate the x-ray beam.For most mammography examinations, the field size matches the film cassette sizes(e.g., 18 X 24 cm or 24 X 30 cm). The exposure switch is enabled only when thecollimator is present. Many new mammography systems have automatic collimationsystems that sense the cassette size. Variable x-y shutters on some systems allowTABLE 8-3. ACR RECOMMENDATIONS FOR MAXIMUM HVL: MAXIMUM HVL(mm AI) = kVp/100 + C FOR TARGET/FILTER COMBINATIONSTube Voltage (kV) Mo/Mo C = 0.12 Mo/Rh C = 0.19 Rh/Rh C = 0.22 W/Rh C = 0.3024 0.36 0.43 0.46 0.5426 0.38 0.45 0.48 0.5628 0.40 0.47 0.50 0.5830 0.42 0.49 0.52 0.6032 0.44 0.51 0.54 0.62