Myocardial perfusion imaging’s major utility centers around diagnosis of coronary artery disease. In this setting, the overall sensitivity of the test ranges from 70-95% and specificity from 70-85% for static (planar) imaging versus three-dimensional single-photon computed tomography imaging techniques (SPECT), respectively.
A second diagnostic question concerns coronary artery stenoses. Stress perfusion imaging is used to define the severity or physiologic significance of a stenosis already identified by coronary angiography.
Perioperative risk assessment is another use of myocardial perfusion studies. Dipyridamole myocardial perfusion imaging with thallium-201 is highly predictive of either perioperative or postoperative cardiac-related death and/or myocardial infarction in patients with reversible perfusion defects prior to surgery. These reversible defects are the equivalent of ischemic tissue or myocardium at-risk, of which perfusion imaging assesses the tissue quantity and severity.
Myocardial perfusion imaging has been used to evaluate and define prognosis following acute myocardial infarction, identifying fixed defects (suggesting myocardial infarction) as well as reversible defects in other vascular distributions are risk factors for subsequent myocardial events. Dipyridamole-thallium-201 stress myocardial perfusion imaging early following a myocardial infarction (within 48-96 hours) may be useful for identifying patients at greatest risk for subsequent myocardial events, especially when reversible defects are identified within the supposed ECG documented territory.
Myocardial perfusion imaging with thallium-201 has been used to identify living tissue that will recover function following resupply of normal coronary blood flow termed viability.‚ Subsequent coronary revascularization with angioplasty or coronary artery bypass surgery then achieves reversal of wall motion abnormalities and complete recovery of cardiac systolic function.
Compare the radionuclide tracers used to assess myocardial perfusion.
Thallium-201, a potassium analog, is the most widely used and standard myocardial perfusion agent. It is primarily distributed within the myocardium via regional myocardial blood flow but requires cell integrity and intact sodium-potassium ATPase pump activity for intracellular uptake. Thallium-201 is less than optimal for imaging large patients or those with prominent soft tissue outside their chest, due to the low inherent y-energy (80 KeV) of the radionuclide, a major factor determining image resolution.
New technetium-based agents have been developed that employ the improved energy and imaging characteristics of technetium-99m. One such agent is technetium-99m-methoxy-isobutyl-isonitrile (Tc-MIBI), a cationic compound which is distributed via myocardial blood flow and traverses the cell membrane to concentrate within the mitochondrial wall. Tc-MIBI is similar to thallium-201 in its myocardial physiologic perfusion properties, although it possesses improved imaging characteristics resulting from the greater y-photon energy emission (140 KeV).
Both perfusion agents have similar sensitivity for detecting significant coronary artery disease, although Tc-MIBI has a slightly higher specificity due to its improved image quality. Technetium agents lend themselves to techniques which simultaneously assess myocardial perfusion and myocardial systolic performance from the same study.