Myocardial viability assessment by cardiac MRI
Cardiac magnetic resonance (CMR) imaging can be used to assess myocardial viability. Preserved myocardial wall thickness of more than 5.5 mm has a good sensitivity of 95%, but low specificity for detecting myocardial viability on CMR.
Delayed or late gadolinium enhancement (LGE) on CMR indicates myocardial scar. If the extent of scar is less as indicated by less than 50% transmural extent of hyperenhancement indicates viability. If 4 or more dysfunctional segments show viability, it has a good sensitivity of 95%, again with low specificity of 45%.
Dobutamine cine MRI is useful in assessing the contractile reserve of the myocardium. Improvement in myocardial thickening of more than 2 mm with low dose dobutamine CMR is indicative of viability.
CMR has the advantage of an accurate assessment of the extent of myocardial scar with superior spatial resolution. Wall thickness can be accurately measured by CMR. The image quality is not limited by characteristics of patient chest wall as in echocardiography. There is good interobserver and intraobserver agreement in the assessment of LGE. CMR has good sensitivity and a fair specificity.
Some of the limitations of CMR are its high cost, limited availability, longer imaging time and restrictions in patients with cardiac implantable electronic devices (CIED). Claustrophia of patients may sometimes prevent CMR imaging. Gadolinium enhancement is not suitable in those with low glomerular filtration rate of below 30 ml per minute. Irregular heart rhythms make gating difficult. Sick patients may find breathholding difficult. While performing dobutamine stress, there are technical difficulties in monitoring the patient in the MRI suite.