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The introduction of electrocardiographic (ECG)-gated myocardial single-photon emission computed tomography (SPECT) in the 1990s expanded the application of myocardial perfusion imaging to routinely include the assessment of LV systolic function.1 The development and use of this technique occurred when technetium-based imaging agents were placed into clinical use as these tracers provided much higher counts, and therefore image quality for measuring function. This was a critical development in the evolution of myocardial perfusion imaging. Currently, the majority of myocardial perfusion studies performed in the United States use gated SPECT technology and Tc-99m tracers. A more recent development has been the evaluation of ventricular dyssynchrony by SPECT, providing further data beyond myocardial perfusion. This chapter will discuss both aspects of ventricular assessment.
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PRINCIPLES OF ECG-GATED SPECT IMAGING
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Technical Considerations
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Gated SPECT images can be acquired using single- or multiple-detector cameras. More recently, dual-headed cameras in the 90-degree configuration have been preferred, as images can be acquired in half the time required using a single-headed system without sacrificing image quality. The majority of gated SPECT imaging is performed with high-resolution parallel hole collimators for Tc-99m studies, while all-purpose collimators are used for thallium-201 (Tl-201) studies. A 180-degree imaging arc (45-degree right anterior oblique to 45-degree left posterior oblique projections) with a circular orbit is most commonly used, although noncircular (body contour) orbits can also be used. The most common detector rotation mode is the "continuous step and shoot" acquisition method, in which the detector records events when stationary at each projection, and then rotates (moves) to the next projection. A "continuous" acquisition mode is also available. The standard image matrix size for gated and nongated SPECT imaging is 64 × 64 pixels, with pixel sizes of 5 to 7 mm. This size offers adequate image resolution for interpretation and quantitation of both Tl-201 and Tc-99m tomograms. Contemporary computers possess adequate processing speed and internal hard disk space to process and store large amounts of scintigraphic data. Acquisition computers are usually separate from processing computers to allow for efficient laboratory operations. In addition, unsophisticated, relatively inexpensive, three-lead gating devices are provided by manufacturers to supply the trigger to the acquisition computer.2
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Gated SPECT Acquisition and Processing
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In an ECG-gated acquisition, a three-lead ECG provides the R-wave trigger to the acquisition computer, with two successive R-wave peaks on the ECG defining a cardiac cycle. Counts from each phase of the cardiac cycle are binned to a corresponding temporal "frame" within the computer. Perfusion projection images are obtained from summation of the individual frames (Fig. 11-1).1 There is a trade-off between the temporal resolution of gated Tc-99m-sestamibi images and the count density of the individual frames. Gating of myocardial perfusion is usually performed at 8 or 16 frames per R–R interval per projection to maintain the count density using ...