Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

CompressionpaddleGridScreen-filmPhototimerdetectorFIGURE 8-3. A dedicated mammography system has many unique attributes.Components of a typical system (excluding the generator and userconsole) are shown.8.1 X-RAY TUBE DESIGNCathode and Filament CircuitThe mammographic x-ray tube is typically configured with dual filaments in afocusing cup that produce 0.3 and 0.1 mm nominal focal spot sizes. A small focalspot minimizes geometric blurring and maintains spatial resolution necessary formicro calcification detection. An important difference in mammographic tube operationcompared to conventional radiographic operation is the low operating voltage,below 35 kilovolt peak (kVp). The space charge effict (Chapter 5) causes a nonlinearrelationship between the filament current and the tube current. Feedbackcircuits adjust the filament current as a function of kV to deliver the desired tubecurrent, which is 100 mA (±25 mA) for the large (0.3 mm) focal spot and 25 mA(± 10 mA) for the small (0.1 mm) focal spot.Mammographic x-ray tubes use a rotating anode design. Molybdenum is the mostcommon anode material, although rhodium and tungsten targets are also used.Characteristic x-ray production is the major reason for choosing molybdenum andrhodium. For molybdenum, characteristic radiation occurs at 17.5 and 19.6 keY,and for rhodium, 20.2 and 22.7 keY.As with conventional tubes, the anode disk is mounted on a molybdenum stemattached to a bearing mounted rotor and external stator, and rotation is accom-