Magnetic field strength dependent signal-to-noise ratio gain at the center of a spherical phantom and up to 11.7T

Purpose: The SNR at the center of a spherical phantom of known electrical properties was measured in quasi-identical experimental conditions as a function of magnetic field strength between 3T and 11.7T.

Methods: SNR was measured at the center of a spherical water saline phantom with a GRE sequence. Measurements were performed at NeuroSpin at 3T, 7T and 11.7T. The phantom was then shipped to Maastricht University and then to the University of Minnesota for additional data points at 7T, 9.4T and 10.5T. Experiments were carried out with the exact same type of birdcage volume coil (except at 3T where a similar coil was used) to attempt at isolating the evolution of SNR with field strength alone. Phantom electrical properties were characterized over the corresponding frequency range.

Results: Electrical properties were found to barely vary over the frequency range. Removing the influence of the flip angle excitation inhomogeneity was crucial, as expected. After such correction, measurements revealed a gain of SNR growing as B₀^{1.94 ± 0.16} compared to B₀^2.13 according to ultimate intrinsic SNR theory.

Conclusion: By using quasi-identical experimental setups (RF volume coil, phantom, electrical properties and protocol), this work reports experimental data between 3T and 11.7T enabling the comparison with SNR theories where conductivity and permittivity can be assumed constant with respect to field strength. According to ultimate SNR theory, these results can be reasonably extrapolated to the performance of receive arrays with greater than ~32 elements for central SNR in the same spherical phantom.