In vivo chronic myocardial infarction characterization by spin locked cardiovascular magnetic resonance (Journal of Cardiovascular Magnetic Resonance)

Background: Late gadolinium enhanced (LGE) cardiovascular magnetic resonance (CMR) is frequentlyused to evaluate myocardial viability, estimate total infarct size and transmurality, but is notalways straightforward is and contraindicated in patients with renal failure because of the riskof nephrogenic systemic fibrosis. T2- and T1-weighted CMR alone is however relativelyinsensitive to chronic myocardial infarction (MI) in the absence of a contrast agent. Theobjective of this manuscript is to explore T1rho-weighted rotating frame CMR techniques forinfarct characterization without contrast agents. We hypothesize that T1rho CMR accuratelymeasures infarct size in chronic MI on account of a large change in T1rho relaxation timebetween scar and myocardium. Methods: 7Yorkshire swine underwent CMR at 8 weeks post-surgical induction of apical orposterolateral myocardial infarction. Late gadolinium enhanced and T1rho CMR wereperformed at high resolution to visualize MI. T1rho-weighted imaging was performed with aB1 = 500 Hz spin lock pulse on a 3 T clinical MR scanner. Following sacrifice, the heart wasexcised and infarct size was calculated by optical planimetry. Infarct size was calculated forall three methods (LGE, T1rho and planimetry) and statistical analysis was performed. T1rhorelaxation time maps were computed from multiple T1rho-weighted images at varying spinlock duration. Results: Mean infarct contrast-to-noise ratio (CNR) in LGE and T1rho CMR was 2.8 +/- 0.1 and 2.7 +/- 0.1.The variation in signal intensity of tissues was found to be, in order of decreasing signalintensity, LV blood, fat and edema, infarct and healthy myocardium. Infarct size measured byT1rho CMR (21.1% +/- 1.4%) was not significantly different from LGE CMR (22.2% +/- 1.5%) orplanimetry (21.1% +/- 2.7%; p < 0.05).T1rho relaxation times were T1rhoinfarct = 91.7 ms in theinfarct and T1rhoremote = 47.2 ms in the remote myocardium. Conclusions: T1rho-weighted imaging using long spin locking pulses enables high discrimination betweeninfarct and myocardium. T1rho CMR may be useful to visualizing MI without the need forexogenous contrast agents for a wide range of clinical cardiac applications such as todistinguish edema and scar tissue and tissue characterization of myocarditis and ventricularfibrosis.