Contributions of nuclear cardiology to diagnosis and prognosis of patients with coronary artery disease.

In the past decade, significant advances have been made in the ability to image the heart with radionuclide tracers under stress and resting conditions in patients with suspected or known coronary artery disease (CAD) for the detection of ischemia, determination of prognosis, assessment of myocardial viability, preoperative risk assessment for patients undergoing noncardiac surgery, and evaluation of the efficacy of revascularization in patients undergoing coronary artery bypass surgery or an interventional procedure.1 For many years, planar imaging and SPECT with Tl constituted the only scintigraphic techniques available for detecting CAD and assessing prognosis in patients undergoing stress perfusion imaging. The major limitation of Tl scintigraphy is the high false-positive rate observed in many laboratories, which is attributed predominantly to image attenuation artifacts and variants of normal that are interpreted as defects consequent to a significant coronary artery stenosis. Although quantification of Tl images improves specificity, the false-positive rate remains problematic, particularly in women and in obese patients. Breast attenuation artifacts in women are sometimes difficult to distinguish from perfusion abnormalities secondary to inducible ischemia or myocardial scar. In recent years, new Tc-labeled perfusion agents have been introduced into clinical practice to enhance the specificity of SPECT and to provide additional information regarding regional and global left ventricular systolic function via ECG gating of images. It was immediately apparent that the quality of images obtained with these new Tc-labeled radionuclides was superior to that of images obtained with Tl because of the more favorable physical characteristics of Tc imaging with a gamma camera. With Tc, doses of '10 to 20 times higher than those that are feasible with Tl can be administered, yielding images with higher count density. Tc demonstrates less scatter and attenuation than Tl, which is associated with fewer image artifacts in patients with no underlying CAD. Perhaps most importantly, Tc-sestamibi or Tc-tetrofosmin imaging allows easy gated acquisition, permitting the simultaneous evaluation of regional systolic thickening, global left ventricular function, and myocardial perfusion.2 Experimental studies have shown that the uptake of these new Tc tracers is proportional to regional blood flow but plateaus earlier than Tl at hyperemic flows because extraction is reduced. It should be pointed out that the first-pass myocardial extraction fraction of Tc-tetrofosmin is lower than Tc-sestamibi by '10% to 15%, which may contribute to its lower sensitivity for detecting mild to moderate stenoses with vasodilator stress. A new Tc-labeled perfusion agent, Tc-N-NOET, presently under investigation shows a higher first-pass myocardial extraction than Tc-sestamibi or Tc-tetrofosmin and redistributes over time similar to Tl.3