Technetium-99 m Hexakis 2-Methoxyisobutyl Isonitrile : Human Biodistribution , Dosimetry , Safety , and Preliminary Comparison to Thallium-201 for Myocardial Perfusion Imaging

In the 1980s, Tl scintigraphy helped to introduce noninvasive assessment of myocardial perfusion as an important clinical tool for the detection and management of coronary artery disease (CAD). Monovalent cations such as Tl exhibit pharmacokinetics that are very suitable for the assessment of regional myocardial perfusion. Early uptake based on a high fi rst-pass extraction fraction of the radioisotope correlates linearly with myocardial blood fl ow over a wide fl ow range, allowing the sensitive detection of impaired coronary fl ow reserve. At later time points, the tracer washes out differentially, producing a redistribution pattern that has experimentally and clinically been linked to myocardial tissue viability. Therefore, the use of one tracer such as Tl has successfully addressed several clinical questions in patients with acute and chronic CAD and has established nuclear cardiology as a subspecialty of both nuclear medicine and cardiology (1). However, Tl has drawbacks due to its low photon energy. Image quality is less than optimal, which was especially appreciated as nuclear cardiology moved to SPECT. In addition, the long physical half-life of Tl is associated with a relatively high radiation exposure, which was further aggravated by the technique of reinjection as a means to enhance the redistribution process for the detection of regional tissue viability (2). For this reason, the advent of Tc-labeled blood fl ow tracers was very much welcomed by the nuclear cardiology community. Initial animal studies, however, showed that the physiologic characteristics in terms of fi rst-pass extraction fraction were less optimal than those of Tl (3). But the higher photon energy and the ease of preparation by commercially available Tc generators and kits overcame these limitations. First clinical experience showed that the diagnostic accuracy to detect regional CAD was quite comparable to that of Tl imaging (4). Since Tc-sestamibi does not redistribute, imaging can be performed for a long period after tracer injection, still refl ecting blood fl ow distribution at the time of injection. This slow redistribution has been exploited for assessment of patients with acute myocardial infarction, leading to measurement of infarct Received Jun. 11, 2020; revision accepted Jun. 15, 2020. For correspondence or reprints contact: Markus Schwaiger, Technical University of Munich, Ismaningerstrasse 22, Munich, 806275 Germany. E-mail: markus.schwaiger@tum.de COPYRIGHT © 2020 by the Society of Nuclear Medicine and Molecular Imaging. DOI: 10.2967/jnumed.120.251439