Particles/radioactivity: Technetium decay and its cardiac application

Technetium-99m (Tc-99m), a nuclear isomer of technetium in the excited state, has numerous applications in medicine. Technetium-99m is the decay product of molybdenum-99 and undergoes gamma decay to form the ground state of technetium-99. Ground-state technetium-99 can further undergo decay to form ruthenium (element 44). The general synthesis and decay schematic is illustrated in Figure 1.

Figure 1. Tc-99m production and nuclear decay schematic with respective half lives for the decay reactions

The primary reaction of interest is the process by which Tc-99m decays to Tc-99. This reaction is utilized in cardiac perfusion imaging, where Tc-99m is injected directly into the bloodstream of the patient. Once it reaches the myocardium, it distributes proportionally to the myocardial blood flow. An instrument used to capture gamma rays emitted by the Tc-99m decay is employed to graphically depict the distribution of radioactive Tc-99m in blood.

Figure 2 shows the results of a Tc-99m myocardial perfusion study conducted on the same patient at rest and during exercise ("stress"). Three views of the heart are taken.

Figure 2. Tc-99m myocardial perfusion study conducted on the same patient during rest and activity

A study conducted at 25 cardiology centers compared the effectiveness of thallium-201 and technetium-99m perfusion tests in detecting myocardial under-perfusion among patients with different degrees of confirmed coronary artery blockage ("stenosis"). In this study, patients with confirmed 30%, 50%, and 75% coronary artery stenosis underwent sequential thallium-201 and technetium-99m perfusion studies. Their images were read by a panel of blinded cardiology experts to determine whether signs of perfusion defect in the myocardial tissue were evident in the imaging study. The study results comparing the mean detection rate of perfusion defects utilizing the two different imaging methods are shown in Figure 3.

Figure 3. Comparison of mean detection rates of perfusion defects utilizing thallium-201 and technetium-99m in patients with confirmed coronary artery stenosis