Non-contrast T1 and T2 relaxometry characterizes reperfusion injury of acute MI in swine
Journal of Cardiovascular Magnetic Resonance volume 17, Article number: W14 (2015)
Reperfusion injury in acute myocardial infarction (MI) results in edema, necrosis, microvascular obstruction (MVO), and intramyocardial hemorrhage (IMH), the latter presents an interesting clinical target.  Cardiovascular MRI has been shown capable of characterizing all of these tissue components. Other than MVO, which is currently detected by flow-deficient regions in contrast enhanced imaging, all other tissue components can be identified by T1 and T2 (T2*). Theoretically, the byproducts of blood breakdown observed with IMH lead to decreased T1 and T2 (T2*).  Conversely, free water accumulation (edema) and necrosis lead to increased T1 and T2.  Hence, direct and quantitative measurement of relaxation rates is promising in myocardial tissue characterization, avoiding ambiguity typical of weighted images (i.e. T2-weighted spin-echo), undesired signal loss from T2* (weighted) images or the uncertainty introduced by contrast agent kinetics. Hypothesis: Combined T1 and T2 mapping can characterize reperfused MI without contrast agents.
MI was induced in swine by 1 (N=3) or 2 (N=3) hr balloon occlusion of the LAD after the first diagonal, with MRI 7-9 days post MI (Achieva TX, Philips). Relaxometry: 3D respiratory navigator-gated T2-mapping ; 2D Breath-hold T1-mapping (MOLLI) . Clinical standard: breath-hold black-blood T2W TSE (BB-T2-STIR) ; early (3 min post) gadolinium-enhanced images (EGE) using PSIR and 0.2 mmol/kg Magnevist. . IMH was identified in T2W images/T1/T2 maps as areas of hypointensity surrounded by hyperintense signal/T1/T2 representing edema. MVO was defined in EGE images as hypointense areas surrounded by enhanced MI. The co-localization of tissue types among techniques was examined.
IMH was detected in all animals with 2 hr occlusions, identified by decreased T1 and T2, and was spatially consistent with the hypoenhanced core in BB-T2-STIR and with MVO in EGE. Edema was observed in all animals (elevated T1and T2). (Fig. 1)
Planimetry showed that relative to remote myocardium, T1 and T2 of edema were significantly higher (p < 0.001 and p <1e-5, respectively), while within IMH T1 was lower (p = 0.001) and T2 the same (p = 0.28). (Fig. 2)
Though either T1 or T2 can be used to separate tissues, combined T1 and T2 mapping may allow for more accurate detection of IMH in reperfusion injury, without variability from contrast kinetics, or BB-T2-STIR artifacts.  Based on a small number of animals, T2 was superior in edema detection, while T1 performed better in IMH detection. Combined relaxometry may identify tissues with better specificity than individual and may help clarify the link between MVO and IMH. High-resolution relaxometry may be necessary to avoid partial volume.
Funded in part by the American Heart Association - 11SDG5280025.
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Ding, H., Schär, M., Schuleri, K.H. et al. Non-contrast T1 and T2 relaxometry characterizes reperfusion injury of acute MI in swine. J Cardiovasc Magn Reson 17 (Suppl 1), W14 (2015). https://doi.org/10.1186/1532-429X-17-S1-W14