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

In recent years, available field strengths for MRI instruments have increased rapidly, both for human and animal applications. While 7 Tesla (T) has developed into a standard field strength for ultra-high field MR in humans, the interest in even stronger magnets remains high, causing a slow but steady increase in scanners operating at 9.4T or above. The main driving force for this trend is the gain in intrinsic signal-to-noise ratio (SNR), which is expected to grow at least linearly with field strength. However, with increasing static field, the homogeneity of the RF-fields declines, the longitudinal relaxation time T₁ increases, while T₂ and T₂^* decrease. In addition, the construction of RF coils for high frequencies becomes more and more involved, mainly because of the need to combine the multichannel receive arrays with tight-fitting multielement transmit coils needed to reach the required transmit homogeneity. This poses the question of how much of the expected gain in sensitivity can be realized in practical experiments. While in general this question has to be investigated for every application separately, because the changing magnetic field may have different effects on the contrasts to be observed in a specific experiment, the intrinsic SNR and the relaxation times as the underlying MR parameters are certainly of interest for many studies and allow predictions for various applications. In addition, the B₁ homogeneity and the g-factor of parallel imaging applications are expected to change with field strength and can play an important role in practical experiments. The aim of this study is to investigate variations in those MR parameters and in the SNR by a direct comparison between 3T, 7T, and 9.4T.
While relaxation times and transmit field inhomogeneity can be determined and their influences on the final SNR be taken into account, the effect of different receive coils cannot easily be assessed because this would require determining the absolute receive sensitivity, which, for a multi channel array coil, is hardly feasible. While at lower fields, the principle of reciprocity can be used to connect the easily measurable transmit field with the receive sensitivity of a single channel coil, this is no longer possible at 7T and above. In addition, using a birdcage or other single channel transceiver coil would ignore common experimental practice. Instead, by using highly efficient, multi-channel receive-only coil arrays with similar shape and similar number of elements that are used in many current MR studies, we try to address the actual SNR behavior in typical studies investigating the human brain.