Purpose

Dielectric materials offer beneficial effects to RF coil efficiency, both in the transmit and receive case. Some prior work has characterized benefits to global transmit efficiency with quadrature drive [1,2], and 2 channel pTX [3,4,5]. With quadrature drive, local SAR reduction within the vicinity of the HDC material has been observed in 3T simulations of the head [6]. The goal of this work is to explore the effect of variation of the transmit polarization in several scenarios, possible with a 2 Ch pTX body coil, on the local SAR.

Methods

The interaction of the transmit field polarization and uHDC material was examined in two scenarios. 1a) A spherical phantom (d = 168 mm) filled with agar (σ = 1.9 S/m), surrounded by a plastic frame containing 5 or 7 uHDC blocks (105x85x20 mm, Er = 850). The experimental setup is depicted in Fig. 1. Experimental B₁⁺ mapping was performed with the Bloch-Siegert technique in both quadrature and linear mode, for the case with 7 blocks (Fig 1a) and with no blocks present. Temperature mapping was performed with an elliptical polarization and the PRFS technique, for the case with 5 blocks only (Fig. 1b). A heating period of 2 minutes was utilized. Both experiments were performed on a Siemens 3T PrismaFit. Transmission & reception utilized the 2 Ch pTX body coil. All experiments were calibrated such that the central axial slice achieved a mean value of 15.4 uT, including the heating experiments. In addition, a corresponding version of the setup in Fig 1. was built in xFDTD 7.0 for field simulation. The B & E distributions were simulated on a model of the 2 Ch pTX body coil. The fields were subsequently combined in Matlab to achieve quadrature or linear polarization. 2) The Ella body model (Virtual Family, IT'IS Foundation, Switzerland) was simulated within a model of the 2 Ch pTX body coil, for the case without uHDC material and with either a uHDC block or ring (Er = 850) surrounding the head. The E fields were combined to achieve a near-linear polarization.

Results & Discussion

All SAR values are given in A.U. Figure 2 displays experimental and simulated results for B₁⁺, SAR, and B₁⁺ / sqrt(SAR) acquired with quadrature polarization and 7 blocks within the frame. There is good agreement between measured and simulated B₁⁺ maps. A series of peaks in the B₁⁺ maps are localized near the center of each block. In the corresponding SAR maps, there is a pattern of dips and peaks at the location of each B₁⁺ peak. The inclusion of uHDC material leads to a reshaping of the circumpolar current distribution and SAR in the sphere, such that it is shifted away from the center of the uHDC block. Furthermore, the local transmit efficiency (Fig. 2d) is enhanced greatly at the location of the SAR dip and remains unaffected elsewhere. Figure 3 displays the corresponding results for linear polarization and 7 blocks in the frame. A slight rotation between measured and simulated B₁⁺ maps is due to a 22.5 degree offset in the simulated coil model. An interesting behavior was observed where the null band in the SAR pattern expands at the base of the sphere (Fig. 3c), to coincide with the uHDC blocks at the base of the phantom. Figure 4 displays measured and simulated B₁⁺ and temperature maps from the heating experiment. The B₁⁺ maps show good agreement between experiment and simulation. The temperature maps, while not corresponding in terms of scale, do agree overall in terms of the basic pattern. The temperature map calculation utilized 4 oil phantoms to obtain a frequency drift map, and there is most likely some error in the drift map. Figure 5 displays the results of the human simulation with linear polarization. All cases have been scaled such that a mean of 11.7426 uT is obtained on a central axial slice in the brain. The placement of a uHDC block beneath the head leads to a reshaping of the SAR distribution. The use of a ring leads to a lower SAR distribution compared to baseline, but the overall pattern is similar.

Conclusion

The use of uHDC material can lead to reduced global and local SAR with the typical quadrature polarization. Application of a linearly polarized field in conjunction with discrete uHDC material can lead to a substantial reshaping of the null band in the SAR pattern. This behavior could potentially be exploited for imaging with device implants such as a deep brain stimulator, though this would require the development of an optimized uHDC configuration specific to the application at hand.

Acknowledgements

This work was supported by grants from the NIH and Penn State Hershey Neuroscience Institute.