One of the most important developments in single photon emission computed tomography (SPECT) myocardial perfusion imaging (MPI) is the ability to acquire these studies in conjunction with electrocardiographic (EGG) gating. This has provided the ability to simultaneously assess ventricular perfusion and function. Initially developed in the late 1980s, it has now evolved into a standard for MPI in the United States. The American Society of Nuclear Cardiology in its position paper from March 1999 recommends the routine incorporation of EGG gating during SPECT cardiac perfusion scintigraphy.1 By providing simultaneous assessment of perfusion and function in a single-injection, single-acquisition sequence, it adds to the quality control of myocardial perfusion SPECT, and improves its diagnostic accuracy and prognostic value. This chapter will describe the technical aspects, interpretive sequence, and clinical data supporting the use of ECG-gated SPECT imaging.
Gated SPECT images can be acquired using single- or multiple-detector cameras. More recently, dual-headed cameras in the 90° 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° imaging arc (45° right anterior oblique to 45° left posterior oblique projections) with a circular orbit is most commonly used, although non-circular (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 while moving as well as 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 non-gated SPECT imaging is 64 × 64 pixels, in conjunction with pixel sizes of 5–7 mm. This size offers adequate image resolution for interpretation and quantitation of both Tl-201 and Tc-99m tomograms. Computers with adequate processing speed and internal hard disk space are needed 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
Gated SPECT Acquisition and Processing
In an ECG-gated acquisition, a three-lead EGG provides the R-wave trigger to the acquisition computer, with two successive R-wave peaks on the EGG 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 frames3 (Fig. 7-1). 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 ...