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Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

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pool image sequence, using T c-99m-Iabeled red blood cells. <strong>The</strong> image sequence isacquired from a left anterior oblique (LAO) projection, with the camera positioned atthe angle demonstrating the best separation of the two ventricles, after sufficient timehas elapsed <strong>for</strong> the administered activity to reach a uni<strong>for</strong>m concentration in the blood.<strong>The</strong> calculation of the LVEF is based on the assumption that the counts fromleft ventricular activity are approximately proportional to the ventricular volumethroughout the cardiac cycle. A ROI is first drawn around the left ventricular cavity,and a TAC is obtained by superimposing this ROI over all images in thesequence. <strong>The</strong> first image in the sequence depicts end diastole, and the image containingthe least counts in the ROI depicts end systole. <strong>The</strong> total left ventricularcounts in the end-diastolic and end-systolic images are determined. Some programsuse the same ROI around the left ventricle <strong>for</strong> both images, whereas, in other programs,the ROI is drawn separately in each image to better fit the varying shape ofthe ventricle. Un<strong>for</strong>tunately, each of these counts is due not only to activity in theleft ventricular cavity but also to activity in surrounding tissues, chambers, and greatvessels. To compensate <strong>for</strong> this "crosstalk" (commonly called "background activity"),another ROI is drawn just beyond the wall of the left ventricle, avoiding active

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