2 - Image Contrast Loss in Double Inversion Recovery Dark-blood T2* CMR of Intramyocardial Hemorrhage

Background: Double inversion recovery (DIR) preparation is often used for dark-blood (DB) CMR. It is the preferred approach for evaluation of iron overload in Thalassemia patients because dark-blood images provide a clearer border definition between the blood pool and the myocardium. Based on these observations, several studies have used DIR T2* CMR for imaging intramyocardial hemorrhage. However, recent studies have reported that DIR T2* CMR leads to marked loss in image contrast in the assessment of hemorrhagic MI (hMI). We investigated the biophysical reasons contributing to this loss in image contrast in DIR T2* CMR. We tested the hypothesis that the loss in image contrast in DIR T2* CMR stems from the signal loss after double inversion recovery and the insufficient recovery time between DIR and readout using a validated animal model of hMI.

Methods: Multi-gradient-echo (mGRE) DIR-prepared DB T2*-weighted, mGRE bright-blood (BB) T2* weighted and LGE images were acquired in dogs (n=8) with hMI in at 3T. DB images were acquired with different trigger delays (TD) between DIR and diastolic readout. The average heartrate of dogs was 84±10 bpm. The delay times were set to 0ms (minimum), 295±5ms (modest), and 555±87ms (maximum) within one heartbeat (HB). Images were also acquired with readout in every other heartbeat to achieve longer recovery times of 1182±122 ms. BB images with readout in every and every other heartbeat were acquired as well. LGE was used to identify infarctions. Hemorrhage was identified from the one-heartbeat-readout BB T2* images (TE=13 ms) using the mean-2SD method. SNR and CNR were calculated as SNR = SI_myo / σ_air , CNR = (SI_myo - SI_hMI) / σ_air. T2* values were measured on T2* maps generated by pixel-wise exponential fitting. Paired t-test was used for comparison.

Results: Fig. 1 shows a case example from an animal with hMI on dark-blood T2*, bright blood T2* and LGE images. Compared to bright-blood images, SNR and CNR were significantly reduced in DB images in the first three groups but increased with TD (See Fig. 2). In images collected every other heartbeat (TD=1182±122 ms), SNR and CNR were enhanced to the same level of bright-blood images.

Conclusion: DIR preparation with insufficient recovery time leads to reduced SNR and CNR in T2*-weighted images of hMI, which can compromise the sensitivity for detecting hMI. Although the loss in image contrast may be mitigated by extending the recovery time, this may require longer scan time and potentially miss the blood nulling point. For these reasons, we conclude that bright-blood T2*-weighted imaging is the preferred method for imaging hMI.