Hepatic radiotracer: Still difficult to suppress?

Abstract

Coronary artery disease, CAD, is the leading cause of death and disability in most developed nations and of increasing prevalence in developing nations worldwide. Single-photon emission computed tomography SPECT myocardial perfusion imaging (SPECT MPI), with radiotracer 99mTc-methoxyisobutylisonitrile (MIBI), provides an accurate, safe, and clinically useful tool for the diagnosis, prognostication, and management of patients with known or suspected CAD. SPECT MPI serves as the most common imaging modality for assessment for possible coronary ischemia with approximately 15-20 million cases annually. Nevertheless, imaging artifacts may limit SPECT accuracy including motion artifacts, attenuation artifacts from overlying soft tissue structures such as breast, or altered myocardial tracer appearance from excessive bowel or liver tracer. Whereas ancient physicians sought to repress excessive biliary activity and balance the four humors, modern nuclear cardiologists too seek to suppress hepatic photon emission. In particular, retained tracer uptake in the liver may overlie portions of the heart and could result in Compton scatter that may decrease sensitivity and specificity of the exam both obscuring small regions of diseased myocardium as well as false-positive appearance of perfusion defects. In this issue of the Journal, Dr. Sood and colleagues share valuable insight from a small single-center randomized controlled trial of 118 patients clinically referred to SPECT MPI and administered either the choleretic agent ursodeoxycholic acid (UDCA) versus placebo. Their results demonstrate a simple, safe, cheap, and novel method to reduce hepatic MIBI artifacts impacting SPECT MPI, which importantly could improve nuclear cardiology accuracy, reduce the need for confirmatory testing such as invasive or noninvasive angiography, and potentially improve patient outcomes. SPECT accuracy has improved from initial reports of planar SPECT with Thallium-201 tracer to the present day. In part, this improvement results from not only improved tracer, MIBI compared with Tl-201, but also acquisition protocols, hardware, software post-processing, repositioning, and CT attenuation correction (AC). Nevertheless, SPECT accuracy remains imperfect with additional need for continued improvement. A recent meta-analysis comparing SPECT MPI to invasive FFR reported a pooled patient-level sensitivity of 0.74 (0.670.79) and specificity of 0.79 (0.74-0.83). Advancements such as AC can reduce many soft tissue artifacts to improve specificity, but not all sites employ the technology. As noted in the 2018 ASNC SPECT MPI guideline by Dr. Dorbala et al, artifacts that appear to either increase or decrease myocardial counts result from the common problem of extracardiac tracer activity in bowel or liver, and ‘‘There is currently no reliable correction for such artifacts, and attenuation algorithms can even exacerbate them.’’ Similarly, repositioning such as the addition of prone imaging reduces some subdiaphragmatic artifacts especially bowel uptake, but this technique also may prove insufficient or is under-utilized. For example, in the IAEA Nuclear Cardiology Protocols Cross-Sectional Study (INCAPS), Dr. Einstein and colleagues reported significant variability in the use of best practices such as AC or repositioning, which occurred worldwide at 67 percent of nuclear cardiology laboratories surveyed. Experienced technicians may Reprint requests: Edward A. Hulten, Cardiology Service, Department of Medicine, Fort Belvoir Community Hospital, DeWitt Loop, 9300, Fort Belvoir, VA 22060; [email protected] J Nucl Cardiol 2020;27:2349–50. 1071-3581/$34.00 Copyright 2019 American Society of Nuclear Cardiology.