Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

11111111 11I1 111I 1111 1111--FIGURE 10-22. While the MTF is the best description of the resolution properties ofan imaging system, for routine quality assurance purposes easier-to-measure estimatesof spatial resolution suffice. The limiting resolution of an imaging system canreliably be estimated using either line pair or star pattern phantoms.between the LSF and the MTF should be understood. Figure 10-21 illustrates threeLSFs, getting progressively worse toward the bottom of the figure. On the right columnof Fig. 10-21, the MTFs corresponding to the LSFs are shown. As the LSFgets broader (poorer resolution), the MTF curves dive down toward zero modulationmore rapidly.Spatial resolution is something that should be monitored on a routine basis formany imaging modalities. However, measuring the LSF or the MTF is usually toodetailed for rourine quality assurance purposes. For a "quick and dirty" evaluationof spatial resolution, resolution test phantoms are used. The test phantoms are usuallyline pair phantoms or star patterns (Fig. 10-22) for projection radiography, andline pair phantoms can be purchased for CT as well.The test phantoms are imaged, and the images are viewed to garner a qualitativeunderstanding of the limiting spatial resolution of the imaging system. Theseroutine measurements are often made on fluoroscopic equipment and for screenfilmand digital radiographic systems.Figure 10-23 shows three isometric "images"; each one has similar contrast, but theamount of noise increases toward the right of the figure. Noise interjects a randomFIGURE 10-23. The concept of noise is illustrated using isometric display. Alow noise image is shown on the left, with increasing amounts of noiseadded to the "images" toward the right.