Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

Field of View:cm (inches)14 (5.5)20 (7.9)27 (10.6)40(15.7)1,023-Line Video:Line Pairs/mm525-Line Video:Line Pairs/mmtrast-detail phantoms under fluoroscopic imaging conditions. Contrast resolution isincreased when higher exposure rates are used, but the disadvantage is more radiationdose to the patient. The use of exposure rates consistent with the image qualityneeds of the fluoroscopic examination is an appropriate guiding principle. Fluoroscopicsystems with different dose settings (selectable at the console) allow theuser flexibility from patient to patient to adjust the compromise between contrastresolution and patient exposure.The excellent temporal resolution of fluoroscopy is its strength in comparison toradiography, and is its reason for existence. Blurring phenomena that occur in thespatial domain reduce the spatial resolution, and similarly blurring in the timedomain can reduce temporal resolution. Blurring in the time domain is typicallycalled image lag. Lag implies that a fraction of the image data from one frame carriesover into the next frame. In fact, image information from several frames cancombine together. Lag is not necessarily bad, as mentioned previously in the sectionon frame averaging. Video cameras such as the vidicon (SbS 3 target) demonstrate afair amount of lag. By blurring together several frames of image data, a higher SNRis realized because the contribution of the x-ray photons from the several images areessentially combined into one image. In addition to the lag properties of the videocamera, the human eye produces a lag of about 0.2 sec. At 30 frames/sec, this meansthat approximately six frames are blurred together by the lag of the human eye-brainsystem. Combining an image with six times the x-ray photons increases the SNR by\16 ""2.5 times, and thus the contrast resolution improves at the expense of temporalresolution. As mentioned above, some systems provide digital frame averagingfeatures that intentionally add lag to the fluoroscopic image.Whereas some lag is usually beneficial for routine fluoroscopic viewing, forreal-time image acquisition of dynamic events such as in digital subtraction angiography,lag is undesirable. With OSA and digital cine, cameras with low-lag performance(e.g., plumbicons or CCO cameras) are used to maintain temporal resolution.The clinical application strongly influences the type of fluoroscopic equipment thatis needed. Although smaller facilities may use one fluoroscopic system for a widevariety of procedures, larger hospitals have several fluoroscopic suites dedicated to