Compressed Sensing-Accelerated Free-Breathing Liver MRI at 7 T

Ultra-high field MRI facilitates imaging at high spatial resolutions, which may become important for detailed anatomical and pathological assessment of the human liver. Therefore, we aimed to advance structural liver imaging at 7 T by implementing a high-resolution, phase-shimmed, free-breathing liver scan. Six healthy participants underwent liver MRI scans at 7 T, utilizing an eight-channel parallel transmission system for phase shimming. B₀ mapping and Fourier phase-encoded dual refocusing echo acquisition mode (PE-DREAM) multichannel B₁ ⁺ mapping were performed during breath-holds at expiration. Prospectively undersampled golden-angle pseudo-spiral k-space data were acquired under free breathing, enabling retrospective respiratory binning using self-gating. Post-binning, the simultaneous autocalibrating and k-space estimation (SAKE) algorithm was employed for interpolation of a center of k-space area, prior to estimation of receive coil sensitivity maps. Image reconstruction was performed on expiration-phase data using compressed sensing, optimizing image quality by evaluating various regularization factors and numbers of respiratory bins. Finally, N4BiasFieldCorrection was applied to the resulting images. Expiration-phase image reconstruction using four bins and regularization factor values of 10^-2.5 (1.50 mm) and 10^-2.33 (1.35 mm) were found to optimize the tradeoff between sharpness, SNR, and artifacts. The optimized protocol facilitated clear visualization of the liver, blood vessels, and surrounding structures at isotropic resolutions of 1.50 and 1.35 mm in 3.5 min, without B₁ ⁺ inhomogeneity effects in the shimmed liver region. A comparison between low-resolution fully sampled free-breathing (3.5 min) and breath-hold (19 s) acquisitions demonstrated comparable sharpness and SNR. To compare the 7 T data with 3 T MRI, 3 T scans were performed for two participants. 3 T reconstructions were done similarly to 7 T, excluding N4BiasFieldCorrection. Scan-specific regularization optimization was performed for fair comparison. Compared to 3 T, 7 T showed superior vascular contrast with inflow effects not observed at 3 T. Fold-over artifacts were present in 3 T scans but were minor at 7 T. 3 T and 7 T provided comparable results, with a much higher RF channel count at 3 T. In conclusion, high-resolution expiration-phase liver imaging at 7 T with homogeneous signal can be successfully achieved using a phase-shimmed, free-breathing protocol with a golden-angle pseudo-spiral sampling pattern technique and respiratory self-gating. This approach allows detailed anatomical depiction without the limitations of breath-holding, representing a significant advancement in ultra-high field abdominal MRI.