Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

For cardiac imaging, by comparison, the 23-cm (9-inch) image intensifier is adequatefor imaging the heart, and its smaller size allows tighter positioning. In additionto the largest FOV; which is determined by the physical size of the II, most lIshave several magnification modes. Magnification is produced (Fig. 9-7) by pushinga button that changes the voltages applied to the electrodes in the II, and this resultsin different electron focusing. As the magnification factor increases, a smaller areaon the input of the II is visualized. When the magnification mode is engaged, thecollimator also adjusts to narrow the x-ray beam to the smaller field of view.As discussed above, the brightness gain of the image intensifier decreases as themagnification increases. The automatic brightness control circuitry (discussed later)compensates for the dimmer image by boosting the x-ray exposure rate. Theincrease in the exposure rate is equal to the ratio of FOV areas. Take for example a30-cm (12-inch) II, which has 23-cm (9-inch) and 18-cm (7-inch) magnificationmodes. Switching from the 12-inch mode to the 9-inch mode will increase the x-ray exposure rate by a factor of (12/9)2 = 1.8, and going from the 12-inch to the 7-inch mode will increase the exposure rate by a factor of (12/7)2 = 2.9. Therefore, asa matter of radiation safety, the fluoroscopist should use the largest field of view (theleast magnification) that will facilitate the task at hand.The contrast ratio is an indirect measurement of the veiling glare of an image intensifier.With a constant input x-ray exposure rate to the II, the light intensity at thecenter of the output phosphor is measured using a light meter. A thick, 2.5-cmdiameterlead disk is then placed in the center of the input phosphor, blocking theradiation to the input and theoretically cutting the light at the center of the outputphosphor to zero. However, light produced in the periphery of the II will scatter,and consequently some light intensity will be measured at the center of the outputphosphor. The contrast ratio is simply the ratio of light intensity, with and withoutthe lead disk being present. Contrast ratios of 15 to 30 are commonly found.FIGURE 9-7. In normal operation of the image intensifier (left), electrons emittedby the photocathode over the entire surface of the input window are focused ontothe output phosphor, resulting in the maximum field of view (FOV) of the II. Magnificationmode (right) is achieved by pressing a button that modulates the voltagesapplied to the five electrodes, which in turn changes the electronic focusing suchthat only electrons released from the smaller diameter FOV are properly focusedonto the output phosphor.