Work

Non-contrast T1 and T2 relaxometry characterizes reperfusion injury of acute MI in swine

Year: 2015

Type: article

Abstract: 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. [ 1 ] 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 T 1 and T 2 (T 2 *). Theoretically, the byproducts of blood breakdown observed with IMH lead to decreased T 1 and T 2 (T 2 *). [ 2 ] Conversely, free water accumulation (edema) and necrosis lead to increased T 1 and T 2 . [ 2 ] Hence, direct and quantitative measurement of relaxation rates is promising in myocardial tissue characterization, avoiding ambiguity typical of weighted images (i.e. T 2 -weighted spin-echo), undesired signal loss from T 2 * (weighted) images or the uncertainty introduced by contrast agent kinetics. Hypothesis : Combined T 1 and T 2 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 T 2 -mapping [ 3 ]; 2D Breath-hold T 1 -mapping (MOLLI) [ 4 ]. Clinical standard: breath-hold black-blood T 2 W TSE (BB-T 2 -STIR) [ 5 ]; early (3 min post) gadolinium-enhanced images (EGE) using PSIR and 0.2 mmol/kg Magnevist. [ 6 ]. IMH was identified in T 2 W images/T 1 /T 2 maps as areas of hypointensity surrounded by hyperintense signal/T 1 /T 2 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 T 1 and T 2 , and was spatially consistent with the hypoenhanced core in BB-T 2 -STIR and with MVO in EGE. Edema was observed in all animals (elevated T 1 and T 2 ). (Fig. 1 ) Matched representative SAX images from swine without hemorrhage after 1 hr LAD occlusion (Case #1) (a-d) and with hemorrhage after a 2 hr occlusion (Case #2) (e-h). In Case #1, significant T 1 and T 2 elevation are present in both maps (c,d), though no MVO was observed with EGE (b) nor was a hypointense core present in T 2 W images (a). In comparison, Case #2 shows clear IMH, demarked by decrease in T 1 and T 2 , surrounded by edema, shown by increased T 1 and T 2 (g and h), all of which are excellently co-localized with that in T 2 W (e) and EGE (f). Green arrowhead indicates edema; red arrowhead indicates the core of IMH. Note that T 1 mapping is influenced by off-resonance and lower image resolution. As the result of competing effects on T 1 and T 2 from IMH and edema, partial volume averaging makes the T 2 in IMH close to that of normal myocardium. Planimetry showed that relative to remote myocardium, T 1 and T 2 of edema were significantly higher (p < 0.001 and p <1e-5, respectively), while within IMH T 1 was lower (p = 0.001) and T 2 the same (p = 0.28). (Fig. 2 ) a. ROI-based T 1 & T 2 for edema, IMH and remote myocardium from matched T 1 and T 2 maps. b. Changes in T 1 & T 2 after subtraction of the reference values of remote myocardium. Edema had higher T 2 than both remote myocardium or IMH. T 1 can be used to discriminate between edema, IMH and normal myocardium, though the distributions may overlap. Edema and IMH can be classified with higher specificity using both T 1 and T 2 . Though either T 1 or T 2 can be used to separate tissues, combined T 1 and T 2 mapping may allow for more accurate detection of IMH in reperfusion injury, without variability from contrast kinetics, or BB-T 2 -STIR artifacts. [ 7 ] Based on a small number of animals, T 2 was superior in edema detection, while T 1 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.

Source: Journal of Cardiovascular Magnetic Resonance

DOI: 10.1186/1532-429x-17-s1-w14