Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

equiring a compensatory increase in radiation dose. If the developer temperature istoo high or immersion time too long, an increase in film speed occurs, permittinga lower dose; however, the film contrast is likely to be reduced while film fog andquantum mottle are increased. Stability of the developer may also be compromisedat higher-than-recommended temperatures. The manufacturer's guidelines for optimalprocessor settings and chemistry specifications should be followed.Film SensitometryBecause of the high sensitivity of mammography film quality to slight changes inprocessor performance, routine monitoring of proper film contrast, speed, and baseplus fog values is important. A film-processor quality control program is requiredby MQSA regulations, and daily sensitometric tests prior to the first clinical imagesmust verifY acceptable performance. A sensitometer, densitometer, thermometer,and monitoring chart are tools necessary to complete the processor evaluation (Fig.8-23). (Sensitometers and densitometers are discussed in Chapter 7.) A filmobtained from a box of film reserved for quality control (QC) testing (the QC filmeliminates variations between different lots of film from the manufacturer) isexposed using a calibrated sensitometer with a spectral output similar to the phosphor(e.g., Gd 2 0 2 S:Tb screens largely emit green light). The processor then developsthe film. On the film, the optical densities produced by the calibrated lightintensity strip of the sensitometer are measured with a film densitometer. Datapoints corresponding to the base plus fog optical density, a speed index step aroundan OD of 1.0, and a density difference (an index of contrast) between two steps of~0.5 and ~2.0 OD are plotted on the processor QC chart. Action limits (horizontaldashed lines on the graphs shown in Fig. 8-24) define the range of acceptableprocessor performance. If the test results fall outside of these limits, correctiveaction must be taken. Developer temperature measurements are also a part of thedaily processor quality control testing. Typical processor developer temperatures are~35°C (95°F). With good daily quality control procedures and record keeping,processor problems can be detected before they harm clinical image quality.Variation in film sensitivity often occurs with different film lots (a "lot" is a specificmanufacturing run). A "crossover" measurement technique must be usedbefore using a new box of film for QC. This requires the sensitometric evaluationof five films from the nearly empty box of film and five films from the new box offilm. Individual OD steps on the old and new group of films are measured dens itometrically,averages are determined, and a difference is calculated. A new baselineis adjusted from the old baseline by this positive or negative value. Action limits arethen established for the new baseline level.A film characteristic curve and gradient curve (the gradient is the rate of changeof the characteristic curve) are shown in Fig. 8-25. The gradient curve shows thefilm contrast as a function of incident exposure. Sometimes more useful is the plotof the gradient versus film optical density (Fig. 8-26), depicting the contrastresponse of the film at different levels of optical density. Small changes in the shapeof the characteristic curve can lead to relatively large changes in the gradient versusOD curve, making this a sensitive method for monitoring processor and filmresponse over time.