Purpose: Aortic stiffness is associated with various cardiovascular diseases, making it a potential imaging marker for assessing risk of cardiovascular events. Aortic MR elastography (MRE) is a non-invasive phase-contrast technique to measure aortic stiffness. Currently, gradient-recalled echo (GRE) MRE sequences are widely employed for aortic stiffness measurements. However, GRE is sensitive to signal T2* decay, leading to low signal-to-noise ratio (SNR). Moreover, multiple slices cannot be efficiently acquired using GRE, resulting in prolonged scan. Therefore, the goal of this work is to demonstrate a multi-slice spin-echo echo-planar imaging (SE-EPI) MRE sequence for measuring in vivo aortic stiffness. Specifically, we aim to compare the aortic stiffness, first harmonic amplitude (FHA) and octahedral shear strain-based SNR (OSS-SNR) obtained using SE-EPI and GRE MRE sequences.
Methods: A free-breathing cardiac-gated SE-EPI aortic MRE sequence was developed in this work. To reduce the effect of aortic blood flow on MRE phase images, the motion-encoding gradient was designed to be zero- and first-moment-nulled, making it primarily sensitive to harmonic motions. EPI ghosting and geometry distortion was effectively corrected via protocol optimization and a non-phase-encoded reference scan. All EPI and GRE MRE imaging was performed on a 3T MR scanner (Skyra, Siemens Healthcare, Erlangen, Germany) with 10 healthy volunteers (age: 26±4years).
Results: The scan time to acquire 3 sagittal slices was 50 and 150 seconds for SE-EPI and GRE, respectively. Bland-Altman analyses comparing SE-EPI to GRE measurements displayed narrow confidence interval and low mean bias (<0.1kPa), suggesting the feasibility and excellent reproducibility of SE-EPI. Moreover, SE-EPI MRE yielded significantly higher FHA and higher OSS-SNR than GRE MRE (p<0.05), demonstrating improved motion encoding sensitivity and advantages in imaging patients with high body mass index.
Conclusion: The proposed multi-slice SE-EPI aortic MRE sequence demonstrated advantages over established GRE in producing stronger FHA and OSS-SNR with shortened acquisition time.