Volumetric imaging with homogenised excitation and static field at 9.4 T

OBJECTIVES: To overcome the challenges of B(0) and RF excitation inhomogeneity at ultra-high field MRI, a workflow for volumetric B(0) and flip-angle homogenisation was implemented on a human 9.4 T scanner. MATERIALS AND METHODS: Imaging was performed with a 9.4 T human MR scanner (Siemens Medical Solutions, Erlangen, Germany) using a 16-channel parallel transmission system. B(0)- and B(1)-mapping were done using a dual-echo GRE and transmit phase-encoded DREAM, respectively. B(0) shims and a small-tip-angle-approximation kT-points pulse were calculated with an off-line routine and applied to acquire T(1)- and T(2)(*)-weighted images with MPRAGE and 3D EPI, respectively. RESULTS: Over six in vivo acquisitions, the B(0)-distribution in a region-of-interest defined by a brain mask was reduced down to a full-width-half-maximum of 0.10 ± 0.01 ppm (39 ± 2 Hz). Utilising the kT-points pulses, the normalised RMSE of the excitation was decreased from CP-mode’s 30.5 ± 0.9 to 9.2 ± 0.7 % with all B(1)(+) voids eliminated. The SNR inhomogeneities and contrast variations in the T(1)- and T(2)(*)-weighted volumetric images were greatly reduced which led to successful tissue segmentation of the T(1)-weighted image. CONCLUSION: A 15-minute B(0)- and flip-angle homogenisation workflow, including the B(0)- and B(1)-map acquisitions, was successfully implemented and enabled us to reduce intensity and contrast variations as well as echo-planar image distortions in 9.4 T images.