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Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

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time, attenuation correction in SPECT using transmission sources is promising, butit is still under development and its ultimate clinical utility is not yet proven.<strong>The</strong> pixels from the transverse image slices may be reordered to produce coronaland sagittal images. For cardiac imaging, it is desirable to produce oblique imagesoriented either parallel (vertical and horizontal long axis images) or perpendicular(short axis images) to the long axis of the left ventricle. Because there is considerableanatomic variation among patients regarding the orientation of the longaxis of the left ventricle, the long axis of the heart must be marked on the displayedimage.<strong>The</strong> most commonly used collimator <strong>for</strong> SPECT is the high-resolution parallel-holecollimator. However, specialized collimators have been developed <strong>for</strong> SPECT. <strong>The</strong>fan-beam collimator, shown in Fig. 22-7, is a hybrid of the converging and the parallel-holecollimators. Because it is a parallel-hole collimator in the y-direction, eachrow of pixels in a projection image corresponds to a single transaxial slice of the subject.In the x-direction, it is a converging collimator, with spatial resolution-efficiencycharacteristics that are superior to those of a parallel-hole collimator (seeChapter 21, Fig. 21-13). Because a fan-beam collimator is a converging collimatorin the cross-axial direction, its FOV decreases with distance from the collimator. Forthis reason, the fan-beam collimator is mainly used <strong>for</strong> brain SPECT; if the collimatoris used <strong>for</strong> body SPECT, portions of the body are excluded from the FOY,creating artifacts in the reconstructed images.To reduce the limitations imposed on SPECT by collimation and limited time perview, camera manufacturers provide SPECT systems with two or three scintillation\\\\\\\\\\llllIJI1777~

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