What did we learn from PET/MR?

Abstract

In this JNC issue, Dr. Autio and his colleagues introduce Ga-DOTA chelate as new tracer for the delineation of myocardial perfusion as well as extracellular space in a rodent infarct model. This application has most likely been triggered by the fact that gadolinium chelates are widely being used as contrast agent in magnetic resonance imaging (MRI). In cardiac MRI (CMRI), the use of Gd-chelates represents an established technology for the work-up of patients with coronary artery disease. Quite interestingly, this class of MR contrast agents was modeled in the 1980s after a tracer principle in nuclear medicine—Tc-DTPA. The advantages of MRI in terms of high spatial resolution and lack of ionizing radiation have supported the clinical application of Gd-chelates for the imaging of myocardial perfusion, the delineation of myocardial scar as well as—in more general terms—for the detection of alterations in extracellular volume. However, there have been questions about the possible toxicity of these contrast agents as gadolinium deposits were found in brain tissue. Nevertheless, the technology remains a standard procedure in many cardiovascular imaging centers. Based on this positive experience, it is not surprising that using Ga-labelled chelates in combination with dynamic PET acquisition allows a replication of data established in the MR community. The sophisticated first-pass analysis established at the laboratory of the Turku investigators, mainly using Owater as the PET tracer, allows an almost automatic quantitative analysis yielding absolute measurements of myocardial perfusion as shown in many publications. The group at the University of Turku has already demonstrated that PET O-water studies in the normal flow range correlate with the perfusion results obtained after the intravenous bolus injection of Ga-DOTA. The paper by Dr. Autio et al raises an important question: What are the relative advantages of PET vs MR imaging for extracting biological information such as myocardial perfusion, extracellular space and myocardial infarct extension? For the clinical work-up of patients with suspected coronary artery disease the need for an accurate and robust assessment of myocardial perfusion and coronary flow reserve has been recognized for decades. Currently, the most commonly used PET tracer is rubidium-82 (Rb) because it is generator-used and allows rapid evaluation of rest and stress perfusion due to its short physical half-life of 76 seconds. In the scientific community, N-ammonia has gained acceptance since it has a high myocardial extraction coupled with a suitable physical half-life of 10 minutes to provide excellent image quality due to the high-tracer retention. The drawback of N-ammonia is the need of an onsite cyclotron, which limits the use to primarily academic institutions. The same applies to O-water, which represents a freely diffusible used for cerebral and myocardial perfusion measurements. Many publications have indicated that with both O-water and N-ammonia global as well as regional Reprint requests: Markus Schwaiger, MD, Technical University of Munich, Munich, Germany; [email protected] J Nucl Cardiol 2020;27:899–902. 1071-3581/$34.00 Copyright 2019 American Society of Nuclear Cardiology.