MRI assessment of left ventricle myocardial viability in patients with chronic coronary artery disease in comparison with single photon emission computed tomography

Background: Coronary artery disease (CAD) is highly prevalent in developed countries and is a leading cause of death. Diagnostic imaging plays an important role in the proper assessment and management of CAD. Myocardial perfusion is evaluated by SPECT in most patients today; however, this technique exposes patients to radiation and its diagnostic accuracy is sometimes limited by relatively low spatial resolution and artifacts from photon scatter and tissue attenuation. Scintigraphy defects may not be apparent until 10 g of tissue is infarcted. Thus, because a sizable threshold of damage is required, SPECT may miss small or subendocardial MI. Advances in rapid magnetic resonance imaging technology and its application to cardiac imaging have shown that MR imaging has tremendous potential for evaluation of cardiac disease. Practical advantages of cardiac MRI include the lack of ionizing radiation, a shortened examination time (25 to 40 min), good safety and tolerability profile, and detection of small subendocardial infarcts.
Aim: assessment of myocardial viability in patients with chronic coronary artery disease by magnetic resonance imaging in comparison with single photon emission computed tomography (SPECT).
Patients and Methods: This prospective study include examination of 10 adult patients (each patient has 3 coronary arteries, so we examined 30 coronary arteries and their myocardial territories) known to have chronic ischemic heart disease in Cairo university by MRI and SPECT with conventional angiography as a standard reference. MRI done by using 1.5T machine, using SENSE (sensitivity encoding) cardiac coil (6 element phased-array coil, receive only), functional cine images, first pass perfusion images and delayed enhancement images were acquired. SPECT study was done by 2-day exercise/rest gated SPECT imaging with Tc–99m sestamibi. Conventional coronary angiography was done using a trans-femoral approach to selectively inject the left and right coronary systems sequentially. The results of cardiac MRI subdivided into three groups: (a) Myocardial ischemia was defined as: either 1- Cardiac segment with motion abnormality or perfusion deficit at first-pass perfusion MR imaging consistent with no hyper-enhancement at delayed-enhancement MR imaging. Or 2- Cardiac segment of subendocardial enhancement (25 % thickness scar) with motion abnormality. (b) Myocardial scarring is defined as: either 1- Cardiac segment with myocardial delayed enhancement having ≥ 75% thickness scar. Or 2- Myocardial thickness less than 6 mm in diastole. (c) Mixed myocardial ischemia and scarring is defined as: Cardiac segment with myocardial delayed enhancement of near 50 % (>25% and <75%) thickness scarring. The results of SPECT were obtained by comparison between resting and exercise images to detect areas with fixed defects (scar) and those with reversible defects (ischemia). A defect was considered to be fixed (scarred) when there was no change between the stress and rest images, partially reversible (mixed ischemia and scar) when there was an improvement in tracer uptake of at least 1 grade between stress and rest images, totally reversible (ischemic) when there was normalization of uptake at rest images. In coronary angiography a reduction of the luminal diameter 70% or more in a major epicardial coronary artery or the major branches was considered to be a relevant stenosis. The angiographic results were classified as one-, two-, or three-vessel disease.
Results:Both CMR and SPECT are accurate in detection of diseased coronary arteries with statistically significant P values. The overall patient based sensitivity, specificity, PPV, NPV and accuracy of cardiac MRI for the detection of diseased coronary artery  were 94% (15 of 16 territories), 100% (14 of 14), 100% (15 of 15), 93% (14 of 15) and 97% (29 out of 30) respectively while the overall sensitivity, specificity, PPV, NPV and accuracy of SPECT for the detection of coronary artery stenosis were 88% (14 of 16), 93% (13 of 14), 93% (14 of 15), 87% (13 of 15) and 90% (27 out of 30) respectively. Both modalities CMR and SPECT show no statistically significant difference in detection of ischemic and scarred left ventricle myocardial segments although the sensitivity of MRI was higher than SPECT in detection of transmural scarring. As regard detection of complications CMR detected 1 intraventricular thrombus that was missed by SPECT.
Conclusion:  CMR is an accurate tool for assessment of coronary artery disease and myocardial viability as regard ischemia, scarring with great capability for detection of transmural extension of scarring and prediction of recovery. MRI can detect complications like ventricular thrombus that was missed by SPECT. Other advantages of MRI are time saving, lack of ionizing radiation and detection of left ventricular volumetrics and wall motion abnormalities.
*= radiodiagnosis department, ^=internal medicine department- faculty of medicine-sohag unevirsity.