Temporal SNR optimization through RF coil combination in fMRI: The more, the better?

For functional MRI with a multi-channel receiver RF coil, images are often reconstructed channel by channel, resulting into multiple images per time frame. The final image to analyze usually is the result of the covariance Sum-of-Squares (covSoS) combination across these channels. Although this reconstruction is quasi-optimal in SNR, it is not necessarily the case in terms of temporal SNR (tSNR) of the time series, which is yet a more relevant metric for fMRI data quality. In this work, we investigated tSNR optimality through voxel-wise RF coil combination and its effects on BOLD sensitivity. An analytical solution for an optimal RF coil combination is described, which is somewhat tied to the extended Krueger-Glover model involving both thermal and physiological noise covariance matrices. Compared experimentally to covSOS on four volunteers at 7T, the method yielded great improvement of tSNR but, surprisingly, did not result into higher BOLD sensitivity. Solutions to improve the method such as for example the t-score for the mean recently proposed are also explored, but result into similar observations once the statistics are corrected properly. Overall, the work shows that data-driven RF coil combinations based on tSNR considerations alone should be avoided unless additional and unbiased assumptions can be made.

General gradient delay correction method in bipolar multispoke RF pulses using trim blips.

PURPOSE: To correct with gradient trim blips for gradient delays in bipolar-spoke RF pulses in slice-selective and slab-selective excitations, compatible with tilted acquisitions and anisotropic delays. THEORY: The effect of small gradient delays with respect to RF pulses results in a dephasing of the second RF spoke, proportional to the slab-selection gradient amplitude and the distance of the slice center from the magnet isocenter. Accordingly, adding a trim blip along the corresponding logical gradient axis between the two spokes compensates for the same dephasing, and therefore cancels the gradient delay effects, regardless of position and orientation. METHODS: Gradient delays on different axes were first measured on a 7T system based on an imaging method. Parallel transmission universal bipolar spokes were designed offline to mitigate the RF field inhomogeneity problem in the human brain. Trim blips were used to compensate for the known delays, which was validated with flip angle and temporal SNR measurements on two different volunteers at 7 T. RESULTS: Pulses corrected with trim blips greatly reduced gradient delay effects. Acquisitions made with corrected and noncorrected pulses showed good fidelity with simulations. CONCLUSIONS: Unlike time or phase-shifting approaches, trim blip-based methods apply to all possible bipolar spoke scenarios such as slice excitations, slab excitations, and anisotropy in the gradient delays.