T2 weighted images as a useful tool in determining myocardial viability in patients performing cardiac magnetic resonance imaging

We read the article published by Sozzi et al. [1] with great interest and we must thank the authors for such a descriptive study on this case which emphasises the importance of cardiac magnetic resonance imaging (MRI) as an evolving tool that is capable of delineating nonviable or infracted myocardium from potentially salvageable myocardium, with the additional advantages over the usual nuclear and PET methods of having high spatial resolution and shorter examination time. We would like to add another point specific to the case mentioned by the authors.

The patient presented with an inferior STEMI but had a previous history of an anterior myocardial infarction. Those groups of patients can be particularly challenging in interpretation of their cardiac MRI. Areas of acute or chronic infarction may be difficult to distinguish using cine or delayed enhancement MR images. Both acute and chronic infarctions show bright areas on delayed enhancement MRI. If the infarction is transmural and chronic, the myocardium will show thinning as mentioned in the case presented by the authors.

However, enhancement of the myocardium on MRI can be non-specific and requires knowledge with respect to the clinical setting. We advise combining knowledge from coronary catheterisation, which may aid interpretation of the MRI results. For example, patients with large acute myocardial infarctions usually have micro-vascular obstruction with delayed first pass enhancement on MRI. However, a patient with chronic infarction and total occlusion of a coronary territory may also show delayed first pass enhancement in that coronary distribution. Both types of patients will have delayed enhancement on MR images obtained 10–20 min after injection of the gadolinium agent.

If the clinical setting is ambiguous, we advise the use of T2 weighted images (rather than T1 in this case) as these may prove very useful in increasing specificity for acute versus chronic infarction. T2 weighted imaging relies upon local dephasing of spins following the application of the transverse energy pulse. The contrast of a T2 weighted image is predominantly dependent on T2 and using a long echo time will increase the T2 dependence. Therefore T2 weighted image contrast state is approached by imaging with a TR long compared to tissue T1 (to reduce T1 contribution to image contrast) and a TE between the longest and shortest tissue T2s of interest. Water has a very high T2 constant, therefore has very high T2 signal and thus appears bright on a T2 contrast image. Consequently, T2 weighted MR images depict more clearly the distribution of oedema in acute myocardial infarction, which is not present in scar/fibrosis.

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