Magnetic field strength dependent SNR gain at the center of a spherical phantom and up to 11.7T

Abstract

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 3 T and 11.7 T.

Methods: The SNR was measured at the center of a spherical water saline phantom with a gradient-recalled echo sequence. Measurements were performed at NeuroSpin at 3, 7, and 11.7 T. The phantom was then shipped to Maastricht University and then to the University of Minnesota for additional data points at 7, 9.4, and 10.5 T. Experiments were carried out with the exact same type of birdcage volume coil (except at 3 T, 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 with B₀ ^2.13 according to ultimate intrinsic SNR theory.

Conclusions: By using quasi-identical experimental setups (RF volume coil, phantom, electrical properties, and protocol), this work reports experimental data between 3 T and 11.7 T, enabling the comparison with SNR theories in which conductivity and permittivity can be assumed to be 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 about 32 elements for central SNR in the same spherical phantom.

Keywords: field strength; signal-to-noise ratio; volume coil.

FIGURE 1

Photographs of the RF volume coils and phantom used for the measurements. The 7T, 9.4T, 10.5T, and 11.7T coils had strictly the same design and dimensions. The same exact phantom was shipped from one site to the next to keep experimental conditions as identical as possible

FIGURE 2

Measured B₁ ⁺ field profile versus field strength (central axial view). The values are normalized with respect to the maximum at the center

FIGURE 3

Measured SNR at the center of the phantom versus field strength in a multicentric study performed at Neurospin (NS), Maastricht University (MBIC), and University of Minnesota (CMRR) compared with ultimate intrinsic SNR (uiSNR) calculations. A, Untransformed data points with fit (raw SNR measurements, aside from flip angle, T₁, T₂*, and preamplifier NF corrections). B, Log–log representation of the same data with its linear fit. The 95% confidence intervals are indicated in the legends for the exponent of the fits. The UiSNR equivalent results are provided in (C) and (D)

FIGURE 4

ultimate intrinsic SNR (uiSNR) versus electrical properties. The values are reported for the center of the sphere and are normalized with respect to the values found with the phantom properties (ε_r = 76 and σ = 1 S/m) at 3 T (A), 7 T (B) and 11.7 T (C)