An Initial Clinical Evaluation of Quantitative Susceptibility Mapping for Quantitative Characterisation of Intramyocardial Hemorrhage

Objective
To characterise the performance of quantitative susceptibility mapping (QSM) for the detection of hemorrhagic iron in the context of ST elevation myocardial infarction (STEMI).

Background
Iron deposition following intramyocardial hemorrhage in STEMI has a mechanistic role in adverse left ventricular remodelling. Current MRI-based methods for measuring myocardial iron in STEMI, including T2*, T1, and T2 mapping, are limited. Iron-mediated T1, T2, and T2* relaxation time shortening is opposed by edema-mediated prolongation of relaxation times, hindering the accuracy of these techniques. In contrast, QSM measures intrinsic tissue magnetic susceptibility, potentially resulting in higher specificity for iron detection in the setting of infarct-related myocardial edema.

Methods
In this work, 22 patients with STEMI were scanned using QSM and T2* mapping sequences. Presence/absence of iron, image quality, and diagnostic confidence were assessed by two expert readers for QSM, T2* mapping, and T2*-weighted imaging (longest echo-time T2* map source image). The volume of intramyocardial hemorrhage was quantified for each technique and compared to the volume of microvascular obstruction determined by late-gadolinium enhancement imaging. The mass of hemorrhagic iron in each case was determined using QSM and T2* maps.

Results
In the qualitative analysis, QSM had significantly improved diagnostic confidence and image quality compared to both T2* maps and T2*-weighted images. For quantitative analysis, the volume of intramyocardial hemorrhage determined by QSM had a significantly stronger correlation with the reference standard than both T2* map and T2*-weighted imaging. There was a strong correlation between the mass of hemorrhagic iron calculated by T2* map and QSM.

Conclusion
This work demonstrates, in a patient population, the opportunity QSM presents for improving the assessment of hemorrhagic iron in the context of STEMI. Full evaluation in a large clinical trial is now warranted.