Signal-to-Noise Ratio and MR Tissue Parameters in Human Brain Imaging at 3, 7, and 9.4 Tesla Using Current Receive Coil Arrays

Purpose: Relaxation times, transmit homogeneity, signal-to-noise ratio (SNR) and parallel imaging g-factor were determined in the human brain at 3T, 7T, and 9.4T, using standard, tight-fitting coil arrays. Methods: The same human subjects were scanned at all three field strengths, using identical sequence parameters and similar 31- or 32-channel receive coil arrays. The SNR of three-dimensional (3D) gradient echo images was determined using a multiple replica approach and corrected with measured flip angle and T-2* distributions and the T-1 of white matter to obtain the intrinsic SNR. The g-factor maps were derived from 3D gradient echo images with several GRAPPA accelerations. Results: As expected, T-1 values increased, T-2* decreased and the B-1-homogeneity deteriorated with increasing field. The SNR showed a distinctly supralinear increase with field strength by a factor of 3.10+/-0.20 from 3T to 7T, and 1.76+/-0.13 from 7T to 9.4T over the entire cerebrum. The g-factors did not show the expected decrease, indicating a dominating role of coil design. Conclusion: In standard experimental conditions, SNR increased supralinearly with field strength (SNR similar to B-0(1.65)). To take full advantage of this gain, the deteriorating B1-homogeneity and the decreasing T-2* have to be overcome. (C) 2015 Wiley Periodicals, Inc.