Free-breathing myocardial T2* mapping using GRE-EPI and automatic Non-rigid motion correction

Abstract

BackgroundMeasurement of myocardial T2* is becoming widely used in the assessment of patients at risk for cardiac iron overload. The conventional breath-hold, ECG-triggered, segmented, multi-echo gradient echo (MGRE) sequence used for myocardial T2* quantification is very sensitive to respiratory motion and may not be feasible in patients who are unable to breath-hold. We propose a free-breathing myocardial T2* mapping approach that combines a single-shot gradient-echo echo-planar imaging (GRE-EPI) sequence for T2*-weighted image acquisition with automatic non-rigid motion correction (MOCO) of respiratory motion between single-shot images.MethodsECG-triggered T2*-weighted images at different echo times were acquired by a black-blood, single-shot GRE-EPI sequence during free-breathing. A single image at a single TE is acquired in each heartbeat. Automatic non-rigid MOCO was applied to correct for in-plane respiratory motion before pixel-wise T2* mapping. In a total of 117 patients referred for clinical cardiac magnetic resonance exams, the free-breathing MOCO GRE-EPI sequence was compared to the breath-hold segmented MGRE approach. Image quality was scored independently by 2 experienced observers blinded to the particular image acquisition strategy. T2* measurements in the interventricular septum and in the liver were compared for the two methods in all cases with adequate image quality.ResultsT2* maps were acquired in all 117 patients using the breath-hold MGRE and the free-breathing MOCO GRE-EPI approaches, including 8 patients with myocardial iron overload and 25 patients with hepatic iron overload. The mean image quality of the free-breathing MOCO GRE-EPI images was scored significantly higher than that of the breath-hold MGRE images by both reviewers. Out of the 117 studies, 21 breath-hold MGRE studies (17.9 % of all the patients) were scored to be less than adequate or very poor by both reviewers, while only 2 free-breathing MOCO GRE-EPI studies were scored to be less than adequate image quality. In a comparative evaluation of the images with at least adequate quality, the intra-class correlation coefficients for myocardial and liver T2* were 0.868 and 0.986 respectively (p < 0.001), indicating that the T2* measured by breath-hold MGRE and free-breathing MOCO GRE-EPI were in close agreement. The coefficient of variation between the breath-hold and free-breathing approaches for myocardial and liver T2* were 9.88 % and 9.38 % respectively. Bland-Altman plots demonstrated good absolute agreement of T2* in the interventricular septum and the liver from the free-breathing and breath-hold approaches (mean differences -0.03 and 0.16 ms, respectively).ConclusionThe free-breathing approach described for T2* mapping using MOCO GRE-EPI enables accurate myocardial and liver T2* measurements and is insensitive to respiratory motion.

Accelerated free‐breathing whole‐heart 3D T2 mapping with high isotropic resolution

“…Furthermore, 2D sequences, typically acquired during breath‐holding, regularly suffer from respiratory and cardiac motion between the T 2 ‐weighted images, due primarily to imperfect breath‐holding or variable heart rate . Although robust nonrigid motion‐correction techniques have been proposed to correct for residual motion and improve map quality, such techniques are relatively complex, computationally expensive, and only correct for in‐plane motion …”

Assessment of cardiac function, blood flow and myocardial tissue relaxation parameters at 0.35 T

“…Reduced field inhomogeneities and susceptibility differences associated with lower B0 resulted in reduced spatial variability in T2* maps at 0.35 T compared with 1.5 and 3 T. However, spatial variability in T1 and T2 maps was greater at lower B0, probably due to reduced SNR. Increasing the number of averages and performing retrospective motion correction may further reduce the spatial variability observed in these parametric maps at 0.35 T 61,62 . The spatial variability for quantitative mapping was determined under the assumption that the underlying pixel‐wise parametric estimates are homogeneous within the septal ROI.…”

Free-breathing T2* mapping for MR myocardial iron assessment at 3 T

“…To this aim, breath-holding GRE sequences are commonly used [27]. However, these sequences are quite sensitive to respiratory artefacts and always require adequate breath-holding during image acquisition [28]. Thus, this standard technique is not feasible in patients who are unable to perform breathholding commands, especially in young children.…”