Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

simulations and experimental measurements, to meet the desired exposure timerange for breast thickness and breast composition.8.3 COMPRESSION, SCATTEREDRADIATION,AND MAGNIFICATIONBreast compression is a necessary part of the mammography examination. Firmcompression reduces overlapping anatomy and decreases tissue thickness of thebreast (Fig. 8-16A). This results in fewer scattered x-rays, less geometric blurring ofanatomic structures, and lower radiation dose to the breast tissues. Achieving a uniformbreast thickness lessens exposure dynamic range and allows the use of highercontrast film.Compression is achieved with a compression paddle, a flat Lexan plate attachedto a pneumatic or mechanical assembly (Fig. 8-16B). The compression paddleshould match the size of the image receptor (I8 X 24 cm or 24 X 30 cm), be flatand parallel to the breast support table, and not deflect from a plane parallel to thereceptor stand by more than 1.0 cm at any location when compression is applied.A right-angle edge at the chest wall produces a flat, uniform breast thickness whencompressed with a force of 10 to 20 newtons (22 to 44 pounds). One exception isthe spot compression exam. A smaller compression paddle area (-5 cm diameter)reduces even further the breast thickness in a specific breast area and redistributesthe breast tissue for improved contrast and anatomic rendition, as illustrated in Fig.8-16C. Spot compression is extremely valuable in delineating anatomy and achievingminimum thickness in an area of the breast presenting with suspicious findingson previous images.Typically, a hands-free (e.g., foot-pedal operated), power-driven device, operablefrom both sides of the patient, adjusts the compression paddle. In addition, amechanical adjustment control near the paddle holder allows fine adjustments ofcompression. While firm compression is not comfortable for the patient, it is oftenthe difference between a clinically acceptable and unacceptable image.X-rays transmitted through the breast contain primary and scattered radiation. Primaryradiation retains the information regarding the attenuation characteristics ofthe breast and delivers the maximum possible subject contrast. Scattered radiationis an additive, slowly varying radiation distribution that degrades subject contrast.If the maximum subject contrast without scatter is Co = ~P/p, the maximum contrastwith scatter is+~rCs=CO(lwhere S is the amount of scatter, P is the amount of primary radiation, and SIP isthe scatter to primary ratio. The quantity modifying Co is the scatter degradation factoror contrast reduction factor as explained in Chapter 6. The amount of scatter inmammography increases with increasing breast thickness and breast area, and is relativelyconstant with kVp. Typical SIP ratios are plotted in Fig. 8-17. For a typical