Characterization of real B0 shim fields generated by higher order B0 shim systems of whole body human 3T and 7T MRI systems

Abstract

PurposeEnsuring optimal homogeneity of the static magnetic field (B0) is critically important for significantly enhancing the image quality, spectral resolution, and overall diagnostic accuracy of magnetic resonance imaging (MRI) and magnetic resonance spectroscopy/spectroscopic imaging (MRS/MRSI), especially at high and ultra-high field strengths. Achieving this goal relies on the deployment of advanced, higher-order shim hardware, which is indispensable for effective and precise B0 shimming. MethodsIn this study, we acquired B0 field maps to characterize the performance of each shim channel of vendor-provided B0 shim hardware integrated into whole body human 7T MRI system. To rigorously evaluate and compare the B0 field homogeneity achievable using these devices, we utilized spherical harmonic expansions to fit the measured B0 distributions for each shim channel in each of the investigated built-in B0 shim systems. This methodology enabled a systematic assessment of shim field "purity" and the effectiveness of different B0 shim configurations. ResultsWhile our analysis confirmed that the linear-order shim terms provided by the gradient coils maintained high purity, certain second-order shim terms and all third-order B0 shim coils of all vendors 7T whole body human MRI systems exhibit a high level of impurity and more specifically introduce substantial linear field components. These unintended linear contributions interfere with the linear shim fields provided by the gradient coils negating their corrective influence. As a result, the 3rd order B0 shim coils and some 2nd order B0 shim coil elements compromised the overall effectiveness of the B0 shimming process rather than improving it. Although pure higher order B0 shim fields were previously shown to substantially improve B0 field inhomogeneity, the vendor implemented versions of higher order B0 shim systems in current 7T whole-body human MRI scanners cannot achieve the desired level of uniformity and partly perform worse than first order B0 shimming. Substantial deviations between 2nd order and especially 3rd order real shim fields versus ideal shim fields demonstrate the necessity of real shim field calibration and call for improvement of future 2nd and 3rd order B0 shim coil design for human whole-body MRI scanners. Our findings offer valuable guidance for optimizing B0 shimming strategies at high and ultra-high field strengths, ultimately enhancing image quality. ConclusionRecent evaluations have demonstrated that certain second- and third-order shim coils, as implemented in current commercial 7T human whole-body MRI systems, introduce unintended lower-order (linear) field components. These field "impurities" substantially deviate from the desired higher-order spatial harmonics, thereby diminishing the orthogonality of the higher order B0 shim system. This in turn largely compromises the effectiveness of higher-order B0 shimming at whole-body 7T MRI scanners. These findings emphasize the necessity of conducting accurate shim field calibration to best utilize the current 7T B0 shim system configurations on one hand. On the other hand, improving the purity of these commercial B0 shim systems in the design phase to ensure that the higher-order terms accurately match their intended spherical harmonic profiles, particularly for the third-order shim terms--would largely enhance the achievable B0 field homogeneity at 7T. This improvement has directly clinically significant outcomes.