The growing flood of technetium-99m myocardial perfusion agents. More water ... or more mud?

Assessment of myocardial viability

The prevalence of left ventricular (LV) dysfunction and resultant congestive heart failure is increasing. Patients with this condition are at high risk for cardiac death and usually have significant limitations in their lifestyles. Although there have been advances in medical therapy resulting in improved survival and well being, the best and most definitive therapy, when appropriate, is revascularization. In the setting of coronary artery disease, accounting for approximately two thirds of cases of congestive heart failure, LV dysfunction often is not the result of irreversible scar but rather caused by impairment in function and energy use of still viable-myocytes, with the opportunity for improved function if coronary blood flow is restored. Patients with LV dysfunction who have viable myocardium are the patients at highest risk because of the potential for ischemia but at the same time benefit most from revascularization. It is important to identify viable myocardium in these patients, and radionuclide myocardial scintigraphy is an excellent tool for this. Single-photon emission computed tomography perfusion scintigraphy, whether using thallium-201, Tc-99m sestamibi, or Tc-99m tetrofosmin, in stress and/or rest protocols, has consistently been shown to be an effective modality for identifying myocardial viability and guiding appropriate management. Metabolic imaging with positron emission tomography radiotracers frequently adds additional information and is a powerful tool for predicting which patients will have an improved outcome from revascularization, including some patients referred instead for cardiac transplantation. Other noninvasive modalities, such as stress echocardiography, also facilitate the assessment of myocardial viability, but there are advantages and disadvantages compared with the nuclear techniques. Nuclear imaging appears to require fewer viable cells for detection, resulting in a higher sensitivity but a lower specificity than stress echocardiography for predicting post-revascularization improvement of ventricular function. Nevertheless, it appears that LV functional improvement may not always be necessary for clinical improvement. Future directions include use of magnetic resonance imaging, as well as larger, multicenter trials of radionuclide techniques. The increasing population of patients with LV dysfunction, and the increased benefit afforded by newer therapies, will make assessment of myocardial viability even more essential for proper patient management.

Noninvasive evaluation of women with coronary artery disease

Coronary artery disease (CAD) is the leading cause of death in women. More women than men die of CAD each year, and unlike men, the death rate has not declined for women but has remained stable over the last 20 years. Despite these statistics, much less is known about the prevention, diagnosis, or treatment of CAD in women. The noninvasive diagnosis of CAD in women is difficult secondary to differences in physiology, etiology, presenting symptoms, risk factor prevalence, comorbid conditions, hormonal status, and body habitus between women and men. Echocardiography and Tc-99m sestamibi single photon emission computed tomography imaging are two noninvasive imaging techniques commonly combined with exercise or pharmacologic agents (dobutamine, adenosine, dipyridamole) that have recently evolved to address these differences. These evolutions and the role of both techniques in the diagnosis and prognosis of women with CAD will be reviewed in this article.

Comparing the image quality of myocardial perfusion agents in the clinical laboratory: small test groups and large reference populations

Myocardial perfusion studies have been performed for 6 years using technetium-99m (99Tcm)-sestamibi (MIBI). In this study we evaluated a newer agent, 99Tcm-tetrofosmin (TF), on a trial basis for 2 weeks, and compared the results from each week to those in an adjacent week of MIBI use. The routine protocol included weight-based dosing, frequent dipyridamole use, and separate-day rest and stress wherever possible. During the first week, TF was used with 'usual' image timing, i.e. stress tomography performed 30-60 min after stress, and ancillary immediate images performed 4 min after injection. For the second week, 'early' tomography was performed 15-30 min after stress. TF scans (n = 53) were compared with MIBI scans for the adjacent weeks (n = 54) and with a historical reference series (n = 1800). Blinded analysis was made of tomographic image quality, peak myocardial counts and background activity (lung and abdomen) on immediate and delayed acquisitions and on tomographic reconstructions. The TF and MIBI test groups were similar with respect to gender, weight, stress protocol, tracer doses, imaging times and scintigraphic findings. Using analysis of variance, the tomographic quality scores were similar for the two observers, with stress>rest (P<0.0001), 'usual'>'early' (P<0.001) and MIBI>TF (P<0.05). Myocardial counts were approximately 20% higher with the MIBI test group than with TF at all times after stress (P=0.001), and were similar to the reference population. MIBI with usual timing gave more favourable stress abdominal background ratios than the other three agent/timing combinations. Satisfactory images could be obtained with TF, but no apparent advantage over MIBI could be attained with earlier post-stress imaging. Subtle advantages for MIBI over TF were suggested by comparison of the small test groups. In our local imaging context, these conclusions were reinforced by a large control series.

Radionuclide tracers in the evaluation of resting myocardial ischaemia and viability

Of all the tracer techniques currently available for the detection of myocardial viability, it is the classic pattern of fluorine-18 deoxyglucose-perfusion mismatch that is clearest from the conceptual point of view and consistently gives good predictive values. Measurements of absolute rates of glucose uptake depend on the much criticized lumped constant, never validated for myocardial ischaemia, and may provide little additional information or may even be confusing because of the bi-directional changes in glucose uptake in response to increasing ischaemia. Labelled nitroimidazole compounds are currently of interest because they are "ischaemia-avid" and because they can be imaged by a gamma camera. Nevertheless, much more work is required to show whether retention of nitroimidazole in ischaemic tissue may reflect viability.