There has been a significant growth in hybrid single-photon emission computed tomography and computed tomography (SPECT–CT) and positron emission tomography computed tomography (PET–CT) systems over the past decade, driven in large part by oncologic imaging. A fortuitous byproduct of this has been the development, application, and validation of myocardial perfusion imaging (MPI) using these hybrid systems. Cardiac hybrid SPECT–CT and PET–CT systems offer distinct advantages compared with traditional SPECT or PET MPI. The CT provides for excellent attenuation correction and improves the sensitivity and specificity of MPI, CT-derived coronary artery calcium (CAC) score adds substantial incremental diagnostic and prognostic information to MPI, and hybrid scanners offer the ability to combine a physiologic assessment of perfusion, function, or metabolism with an anatomic assessment of atherosclerosis and structural heart disease. By doing so, hybrid SPECT–CT and PET–CT imaging offers unprecedented opportunities for molecular cardiology research. The primary focus of this chapter is to discuss the clinical applications of hybrid radionuclide MPI with calcium scoring and coronary CTA.
The hardware of SPECT–CT and PET–CT scanners comprises a conventional SPECT scanner or a PET scanner coupled with a CT scanner of various configurations. While all SPECT–CT and PET–CT scanners offer CT-based attenuation correction, calcium scoring (≥4 slice MDCT) and coronary CTA (≥64 slice MDCT) may be performed only on certain hybrid SPECT–CT and PET–CT scanners. Sample hybrid PET–CT and SPECT–CT protocols are shown in Figure 25-1A and B, respectively. The CAC score and/or coronary CTA study can be performed sequentially right before or after the SPECT or PET scan or at a separate setting.
Sample protocols for PET/CT (Panel A) and SPECT/CT (Panel B) myocardial perfusion imaging. CTAC, CT for AC (10 mA, 120 keV, nongated free breathing); CAC, calcium score CT scan (300 mA, 140 keV, ECG-gated CT scan with breath hold). *, optional.
ATTENUATION CORRECTION WITH CT FOR BOTH SPECT AND PET
Attenuation correction using transmission scanning employing external radioactive sources or cardiac CT improves the count uniformity of the image and helps distinguish attenuation artifacts from real defects. It also offers the possibility of stress-only imaging, with potential savings of time, cost, and radiation dose as discussed in more detail in Chapter 9. Also, accurate attenuation correction enables precise measurements of absolute radiotracer concentration in the myocardium making feasible noninvasive quantitative estimation of myocardial blood flow in mL/g/min.
External radionuclide source transmission scans pose some challenges including: (1) degradation of the radionuclide source over time which adversely impacts image quality, (2) longer acquisition time when compared with the CT transmission scans, and (3) a small additional radiation dose (in addition to the emission scan). CT attenuation correction, on the other hand, is rapid (takes a few seconds) and of excellent ...