cardiovascular disorders
Radionuclide Imaging of the Heart
Myocardial Perfusion Imaging
In myocardial perfusion imaging, IV radionuclides are taken up by cardiac tissues in rough proportion to perfusion; thus, areas of decreased uptake represent areas of relative or absolute ischemia. Attenuationof myocardial activity by overlying soft tissue may cause false-positive results. Attenuation by breast tissue in women is especially common. Attenuation by the diaphragm and abdominal contents may produce spurious inferior wall defects in both sexes but is more common among men. Attenuation is more likely with technetium-99m (99mTc) than with radioactive thallium 201 (Tl-201). Myocardial perfusion imaging is used with Stress Testing to Evaluate patients with chest pain of uncertain origin Determine the functional significance of coronary artery stenosis seen on angiography Determine the functional significance of collateral vessels seen on angiography Evaluate the success of reperfusion interventions (eg, coronary artery bypass grafting [CABG], percutaneous intervention, thrombolysis) Estimate prognosis after myocardial infarction After acute myocardial infarction, myocardial perfusion imaging can help estimate prognosis because it can show extent of the perfusion abnormality due to acute myocardial infarction, extent of scarring due to previous infarcts, and residual peri-infarct or other areas of reversible ischemia. Various protocols are used depending on the imaging agent, which include Radioactive thallium-201 (Tl-201) Technetium-99m (Tc-99m) markers (sestamibi, tetrofosmin, and teboroxime) Iodine-123 (I-123)–labeled fatty acids I-123 metaiodobenzylguanidine (MIBG) (Tl-201), which acts as a potassium analog, was the original tracer used in stress testing. It is injected at peak stress and imaged with SPECT, followed 4 hours later by injection of half the original dose during rest and by repeat SPECT. The goal of this protocol is to evaluate reversible perfusion defects that may warrant intervention. After stress testing, the perfusion imbalance between normal coronary arteries and those distal to a stenosis appears as a relative decrease in Tl-201 uptake in the areas perfused by the stenosed arteries. Sensitivity of stress testing with Tl-201 for coronary artery disease is similar whether imaging is done after Exercise stress testing or Severaltechnetium-99m(Tc-99m) myocardial perfusion markers have been developed because the imaging characteristics of Tl-201 are not ideal for the gamma camera. Markers include sestamibi (commonly used), tetrofosmin, and teboroxime (see table). Protocols include 2-day stress-rest, 1-day rest-stress, and 1-day stress-rest. Some protocols use dual isotopes (Tl-201 and Tc-99m), although this approach is expensive. With either of these markers, sensitivity is about 90%, and specificity is about 71% to detect coronary artery disease. 30 millicurie) are used, first-transit function studies ( Radionuclide Ventriculography ) may be used with perfusion imaging. Other radionuclides include iodine-123 (I-123)–labeled fatty acids, which produces cold spots where myocardium is ischemic; gallium citrate-67 (Ga-67), which accumulates in sites of active inflammation (eg, in acute inflammatory cardiomyopathy); and I-123 metaiodobenzylguanidine, a neurotransmitter analog taken up and stored in neurons of the sympathetic nervous system and used in research to evaluate heart failure, diabetes, pheochromocytoma, certain arrhythmias, and arrhythmogenic right ventricular dysplasia. 