SAR and power implications of different RF shimming strategies in the pelvis for 7T MRI

Abstract

Purpose: To determine the best radiofrequency (RF) shimming method for 7 T body imaging that provides sufficient B 1 ϩ excitation inside the target region while energy deposition (SAR) and power demands are as low as possible and that does not incorporate anatomy specific electric field information inside the patient models, as this information is not available in practice. Materials and Methods:Finite difference time domain (FDTD) simulations were used to evaluate five RF shimming strategies for the pelvis inside a body coil. The results were compared to the theoretical best solution that could be achieved if the electric field inside the patient was known.Results: Most of the RF shimming strategies were successful. However, between the different strategies a factor of two difference in average SAR reduction, a factor of three difference in local maximum SAR reduction, and a factor of 20 difference in power efficiency was observed. Phase matching was found to be the most promising RF shimming method for the body coil used and patient models. Conclusion:RF shimming can reduce the SAR and improve power efficiency in an accurate patient model without knowing the electric field. However, choosing the right method is critical to prevent unexpected behavior in local SAR deposition. THE INCREASE in main magnetic field strength for human magnetic resonance imaging (MRI) toward 7 T has shown large benefits for head imaging, where the higher signal-to-noise ratio can be used to increase scan speed and/or to obtain a higher resolution (1-4) and where the higher spectral resolution (5,6) and the increased sensitivity for various contrast mechanisms such as blood oxygenation level-dependent (BOLD) (4,7,8) have proven useful.Recent studies have shown that 7 T MRI is not limited to the head, but that imaging of the body is also possible (9 -11). However, this ultrahigh magnetic field imaging is hampered by difficulties related to the control of the radiofrequency (RF) excitation field. In the head these difficulties are not too severe and imaging is possible with traditional imaging concepts. In the body this becomes increasingly challenging due to its larger anatomical dimensions. As the Larmor frequency increases with the main magnetic field strength, the wavelength of the RF excitation field becomes smaller, which leads to more pronounced interference effects both in the RF excitation (B 1 ϩ ) field and in the accompanying electric field. The former can result in inhomogeneous B 1 ϩ fields with excitation voids in the region of interest (ROI) (9,12,13) and the latter can cause local maxima in the electric field, which result in large local energy depositions (specific absorption rate, SAR). This poses a safety concern because of possible local overheating of the patient. A second effect of the increase in Larmor frequency is that the RF penetration depth becomes smaller. A larger amount of power is therefore required to reach the center of the body with an increasing SAR as a result. In addition, the chemical shift also in...

Uniform prostate imaging and spectroscopy at 7 T: comparison between a microstrip array and an endorectal coil

“…However, small changes in the allowed repetition times are likely, and it seems sensible to include more patient models when the scanning parameters are pushed towards the limit. Ideally, this ratio should be determined for each individual patient, but this is not yet possible in a clinical environment (3).…”

“…However, spectroscopic images could be made with the endorectal transceiver using a semi-LASER sequence with MEGA [Mescher Garwood (named after the authors)] water and lipid suppression with FOV ¼ 50 Â 40 Â 50 mm 3 , acquisition matrix of 7 Â 7 Â 8 and TR/TE ¼ 1.6/56 ms (5). Clear resonances of choline, creatine and citrate can be seen (Fig.…”

“…Within these guidelines, it was possible to obtain T 1 -and T 2 -weighted images with the eight-element microstrip array. The T 1 -weighted image was made using a three-dimensional FFE sequence with a flip angle of 108, TR/TE ¼ 17/2.2 ms, FOV ¼ 400 Â 400 Â 40 mm 3 and a voxel size of 1 Â 1 Â 2 mm 3 , resulting in a total scan duration of 2.20 min at 50% of the allowed SAR (Fig. 9).…”

“…A further increase in the MR field strength from 3 to 7 T is therefore expected to improve the detection and staging of prostate cancer even further. A complicating factor of high-field prostate MRI is that, for deeply situated organs, such as the prostate, it becomes increasingly difficult to achieve sufficient B þ 1 excitation in the region of interest as the radiofrequency (RF) is increased with magnetic field strength (3). Different coil concepts have been tested at 7 T, such as an external microstrip array (4), an endorectal loop coil (5), a body coil (6) and a travelling wave concept (7,8).…”

Fast design of local N‐gram‐specific absorption rate–optimized radiofrequency pulses for parallel transmit systems

“…The RF pulse design becomes a challenging optimization problem whose solution (1) must be a set of RF waveforms which fulfill the accuracy criterion of the obtained excitation profile, (2) exhibits minimal local SAR, and (3) can be computed quickly, for clinical feasibility. Methods to optimize the RF pulse with respect to the global SAR have been presented in the past (22)(23)(24)(25)(26) but minimizing the global SAR does not guarantee that the maximum peak N gSAR value (i.e., the local SAR) is below the safety limit (27). For this reason, local SAR minimization methods are required.…”