Dual-isotope myocardial perfusion SPECT imaging: Past, present, and future

Abstract

Myocardial perfusion SPECT imaging (MPI) is a widely available technique used to identify and quantify ischemia, infarct, and myocardial viability. The first radioisotope used was thallium (Tl)-201, introduced to nuclear medicine by Kawana et al in 1970, 1 and evaluated for MPI shortly after. 2 As a biologic analog of potassium, Tl-201 has the advantages of efficient myocardial extraction and rapid blood clearance, favoring its use as a myocardial perfusion agent. Accumulation of Tl-201 within ischemic zones is slower compared to non-ischemic regions, and reaches a peak after several hours. Thus, early post-Tl-201 injection images demonstrate flow, whereas delayed images demonstrate myocardial viability within the ischemic or infarcted zones.Tc-99m-labeled tracers were introduced during the 1980s, and technetium (Tc)-99m sestamibi became commercially available in the United States in 1990. 3 The shorter physical half-life of Tc-99m compared to Tl-201 (6 vs 73 hours) allowed the injection of a tenfold higher dose of 25 to 30 mCi compared to 3 mCi, respectively. The higher injected activity resulted in higher myocardial count density and better image quality compared to Tl-201 images, and allowed the acquisition of ECG-gated images of good quality. Moreover, lacking significant redistribution, Tc-labeled tracers offered flexibility of imaging protocols, using a 2-day stress and rest protocol, or 1-day either rest-stress or stress-rest protocols. Therefore, Tc-99m-labeled radiotracers, either sestamibi or tetrofosmin became dominant in nuclear cardiology.During the early 1990s, dual-isotope MPI was first introduced by Berman et al 4,5 using Tl-201 for rest, followed by Tc-99m sestamibi for stress imaging. This original protocol gained attention and popularity because it offered significant shortening of the entire test time from 4-5 hours to only 1.5-2 hours. In addition, it utilized advantages of both tracers for the assessment of myocardial ischemia and viability. The high count density of Tc-99m sestamibi provided good-quality gated SPECT stress images, allowing evaluation of stress perfusion deficit and post-stress left ventricular function, whereas resting Tl-201 provided assessment of resting perfusion, and the option to obtain delayed images for the assessment of myocardial viability within infarcted or severely ischemic regions. Ideally, acquisition of rest Tl-201 and stress Tc-99m should be obtained simultaneously in one SPECT acquisition. This would provide optimal image registration, save imaging time, and increase patient throughput. However, when using traditional sodium iodide (NaI) detectors and photomultipliers, the crosstalk of counts originating from the Tc-99m into the Tl-201 window is significant, and accounts for 27% of the counts acquired in the Tl-201 window. 3 This is due to the limited energy resolution of the traditional Anger cameras. These ''extra'' counts may cause overestimation of ischemia and viability. Therefore, dual-isotope protocol has been used typically in the separate-acquisiti...

Fusion-Based Neutron Generator Production of Tc-99m and Tc-101: A Prospective Avenue to Technetium Theranostics

“…Whereas 99m Tc is typically considered a purely diagnostic isotope [ 15 ], it is envisioned that 101 Tc (t 1/2 = 14.22 min) could be applied as an ephemeral theranostic agent; a beta emission (~487 keV, 90.3% [ 16 ]) for radiotherapeutics is complemented by a 307 keV (89.4%) gamma emission ideal for synchronised diagnostic scanning [ 17 ]. In addition, the 99m Tc/ 101 Tc pair may be useful in dual imaging modalities [ 18 ], where the similar chemistry, but differing nuclear properties of the two isotopes could be exploited.…”

Diagnostyka choroby niedokrwiennej serca za pomocą scyntygrafii perfuzyjnej z użyciem izotopu 99mTc-MIBI u pacjentów chorujących lub diagnozowanych w kierunku stabilnej choroby wieńcowej

“…Myocardial perfusion single photon emission computed tomography (SPECT) is a non-invasive imaging method evaluating cardiac muscle perfusion with the use of radionuclides. Since SPECT's introduction at the start of the 1990s [1,2], its role in diagnosing ischaemic heart disease has considerably increased. An improvement in the field of coronary heart disease (CAD) invasive treatment has been observed over recent decades.…”