Ernest V. Garcia, Ph.D, Emory University School of Medicine Atlanta, Georgia, USA
Myocardial perfusion SPECT imaging has enjoyed widespread clinical use because of its well documented diagnostic accuracy for detecting coronary artery disease. Recently, innovative designs of dedicated cardiac scanners promise to further improve image quality and study efficiency as well as diagnostic quality. These scanners’ designs have in common that all available detectors are constrained to imaging just the cardiac field of view. Figure 1 shows how 8 detectors surrounding the patient are all simultaneously imaging the heart. These new designs vary in the number and type of scanning or stationary detectors, and whether scintillation crystals or solid state detectors are used. They all have in common the potential for a 5 to 10 fold increase in count sensitivity at no loss or even a gain in resolution, resulting in the potential for acquiring a stress myocardial perfusion scan injected with a standard dose in 2 minutes or less. Recent software improvements in image reconstruction take into account the loss of resolution with distance inherent in parallel-hole collimators. Using this knowledge in conjunction with the imaging properties of the system allows for a mathematical correction of this resolution degradation known as resolution recovery. At the same time, noise is suppressed because additional counts are now correctly considered rather than treated as noise. Because resolution recovery actually reduces noise while improving spatial resolution as compared to FBP, resolution recovery can yield reconstructed images from studies acquired in less time with the same signal/noise as FBP images reconstructed from studies acquired for longer times. This advance results in another 2 to 4 fold reduction in acquisition time at a further improvement in accuracy.
Another software advance has been the use of phase analysis to measure LV dyssynchrony from ECG-gated myocardial perfusion studies. Dr. Ji Chen has reported how we quantify LV dyssynchrony as the regional time delays in the onset of mechanical contraction (OMC) over the LV myocardium. Using Fourier analysis the 3D count distributions are extracted from each of the 8 LV short-axis datasets to generate a phase array (3D regional phases). The calculated phase array describes the regional OMC of the myocardium in 3D. with 0° corresponding to the peak of the R wave and one R-R interval corresponding to 360° (Figure 2). Our approach has been validated using Echo tissue Doppler imaging as a gold standard.