Original ArticleA comparison of the image quality of full-time myocardial perfusion SPECT vs wide beam reconstruction half-time and half-dose SPECT
Introduction
Myocardial perfusion SPECT has been rightfully criticized both for the length of the imaging procedure and for the radiation dose a patient receives. The advantages of shortening image acquisition times include improved patient comfort and tolerance, a decreased opportunity for patient motion, and enhanced laboratory throughput. New camera/detector systems have been developed, incorporating semiconductor detectors and focused collimation, considerably increasing cardiac SPECT counting statistics and thereby allow for greatly reduced image acquisition times.1 Another solution has been the introduction of software methods incorporating depth-dependent resolution recovery and noise-modeling that provide good image quality despite reduced counting statistics, thereby allowing for shorter acquisition times.2,3 Our group has previously reported gated image quality equivalent to that achieved with a full-time SPECT acquisition using both “half time” Wide Beam Reconstruction® (UltraSPECT Ltd, Haifa, Israel) (WBR) or “half-time” Evolution for Cardiac® (General Electric Medical Systems, Waukesha, WI).4 Subsequently, we reported gated and summed tomogram image quality equivalent to that achieved with full-time SPECT for “quarter-time” WBR, incorporating a modified algorithm with enhanced noise modeling.5
However, in recent years the volume of myocardial perfusion SPECT scans performed in the United States has plateaued or decreased slightly, most likely due to a downturn in the economy and insurance pre-certification requirements. Therefore, the need for increased throughput has not been realized in many laboratories. In contrast, the medical community, the public, and the media have demonstrated an increased awareness and have expressed caution regarding patient radiation exposure from diagnostic medical procedures. Diagnosticians have been urged to decrease and limit patient radiation exposure whenever possible. Moreover, because of the recent downtime of several nuclear reactors that produce Mo-99, there was a shortage of Tc-99m, also prompting laboratories to decrease radiopharmaceutical activity.
Since the new software methods referenced above generate good quality images with lower counting statistics, alternatively it should be possible to decrease the myocardial perfusion radiopharmaceutical activity while maintaining the standard SPECT acquisition time to generate equally acceptable gated and summed tomographic images. We therefore postulate that with either a reduced-time or reduced-activity acquisition yielding the same counting statistics, reconstructed cardiac SPECT images should be of equivalent quality.
Section snippets
Wide Beam Reconstruction
Wide Ream Reconstruction accurately models the physics and geometry of the emission and detection processes. This method has been described in detail in previous publications.2,3 Specifically, WBR calculates the probability relations between reconstruction voxels and projection pixels based on information regarding the collimator used. This three-dimensional collimator distance response is calculated analytically for each voxel by taking into account the solid angles subtended by the collimator
Results
Summed perfusion tomograms and gated images were interpretable (image quality score ≥ 2) in all Group A and Group B patients. Perfusion defects were present in 82/156 (53%) Group A patients, and 72/160 (45%) Group B patients (P = .43). Mean left ventricular ejection fraction was 56.1 ± 14.3% (range 20-84%) in Group A patients, as per Myometrix processing of post-stress WBR images, and 48.8 ± 13.0% (range 20-78%) in Group B patients (P < .0001).
Analysis of image quality of “full-time” OSEM vs
Discussion
Considering that the “half-time” reconstruction algorithm is designed to process images with half the standard counting statistics, which can be achieved with either a “half-time” or “half-dose” acquisition, it would be anticipated that similar results could be achieved with “half-dose”, “full-time” acquisition and processing. Indeed in this study, we have demonstrated that “half-time” myocardial perfusion SPECT processed with Wide Beam Reconstruction yield similarly good-to-excellent image
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Research support: UltraSPECT Ltd., Haifa, Israel.