The tools are ready, are we?

Abstract

In this issue of the Journal of Nuclear Cardiology, Lee and colleagues elegantly review the recent advances in the instrumentations for nuclear cardiac imaging. 1 Solid-state radiation detectors incorporating cadmium zinc telluride (CZT) become the detector of choice for dedicated cardiac imaging systems. CZT single-photon emission computed tomography (SPECT) has improved image quality and shortened acquisition time of myocardial perfusion imaging (MPI) thanks to its direct energy conversion mechanism, narrow energy resolution, and pixelated nature of electrical circuit. The measurement of absolute myocardial blood flow (MBF) as well as coronary flow reserve (CFR) using CZT SPECT is now under active investigation and is showing clinical feasibility. Multimodality imaging systems which hybridized SPECT and positron emission tomography (PET) with CT widened clinical application of cardiac imaging studies. Quantitative SPECT/CT provides additive information of coronary calcium burden, attenuation correction for both visual and quantitative assessment of myocardial perfusion, and enables quantification of myocardial blood flow and flow reserve. Cardiac PET is also an active progress. Adoption of silicon photomultipliers (SiPMs) leads to enhancements in image quality and count rates. The application of accurate time-of-flight (TOF) information reduced emission-transmission mismatch artifacts. Information of the extracted coronary arteries from CT angiography allows non-linear motion correction of PET plaque images. Replacement of photomultiplier tubes with semiconductor photosensors such as the avalanche photodiode (APD) and SiPM has made it possible to integrate PET with magnetic resonance (MR). PET/MR system is now clinically available in hybrid as well as parallel camera structure.The recently introduced nuclear cardiac imaging tools are ready to provide more information in better quality than ever before. It is fair to question whether we are ready to apply these tools in clinical fields. There are challenges to overcome before we can use the state-ofthe-art imaging tools. MEASUREMENT OF MBF AND CFRSeveral studies showed clinical feasibility of measuring MBF and CFR by dynamic image acquisition using Tc-99m sestamibi and CZT SPECT/CT cameras. 2,3 It does not directly mean that it can be accepted as a clinical tool in daily practice. Measuring MBF and CFR by cardiac PET is a mature technology but is not widely used in clinics. Several obstacles are noted. 4 First of all, resting MBF is variable according to hemodynamic and metabolic changes. MBF and CFR cannot differentiate epicardial obstructive and microvascular diseases or provide anatomical localization of coronary obstruction. The complex interplay among different MBF parameters has not been fully unveiled. Different MBF parameters stand for different coronary pathophysiologies, and are not interchangeable. 5 For example, a relative MBF ratio namely relative flow reserve (RFR) could be more suitable for the diagnosis of focal significant coronary stenosis in ...