Comparison of RF body coils for MRI at 3  T: a simulation study using parallel transmission on various anatomical targets

Abstract

The performance of multichannel transmit coil layouts and parallel transmission (pTx) radiofrequency (RF) pulse design was evaluated with respect to transmit B1 (B1+) homogeneity and Specific Absorption Rate (SAR) at 3 Tesla for a whole body coil. Five specific coils were modeled and compared: a 32-rung birdcage body coil (driven either in a fixed quadrature mode or a two-channel transmit mode), two single-ring stripline arrays (with either 8 or 16 elements), and two multi-ring stripline arrays (with 2 or 3 identical rings, stacked in the z-axis and each comprising eight azimuthally distributed elements). Three anatomical targets were considered, each defined by a 3D volume representative of a meaningful region of interest (ROI) in routine clinical applications. For a given anatomical target, global or local SAR controlled pTx pulses were designed to homogenize RF excitation within the ROI. At the B1+ homogeneity achieved by the quadrature driven birdcage design, pTx pulses with multichannel transmit coils achieved up to ~8 fold reduction in local and global SAR. When used for imaging head and cervical spine or imaging thoracic spine, the double-ring array outperformed all coils including the single-ring arrays. While the advantage of the double-ring array became much less pronounced for pelvic imaging with a substantially larger ROI, the pTx approach still provided significant gains over the quadrature birdcage coil. For all design scenarios, using the 3-ring array did not necessarily improve the RF performance. Our results suggest that pTx pulses with multichannel transmit coils can reduce local and global SAR substantially for body coils while attaining improved B1+ homogeneity, particularly for a “z-stacked” double-ring design with coil elements arranged on two transaxial rings.

High‐resolution whole‐brain diffusion MRI at 7T using radiofrequency parallel transmission

“…In these studies, we used the commercially available Nova 8‐channel transmit and 32‐channel receive (Nova 8Tx32Rx) head coil (Nova Medical, Inc., MA). More optimal transmit coil geometries exist for SAR reduction and/or homogenization; however, Nova 8Tx32Rx permits a direct comparison to the HCP data because it shares the same 32‐channel receiver as the Nova 1Tx32Rx coil used in the HCP. Our results show that pTx can significantly improve flip‐angle uniformity across the entire brain while simultaneously reducing SAR compared with a single‐transmit configuration (i.e., Nova 1Tx32Rx coil) as used in the 7T HCP protocol.…”

Distributing coil elements in three dimensions enhances parallel transmission multiband RF performance: A simulation study in the human brain at 7 Tesla

“…Similar results have been reported in an array comparison study by Guerin et al (18) in which the authors compared the performance of different 3T body RF arrays designed with loop elements and found that a double-ring 2×8 array can be used to provide better RF performance than a single-ring 1×8 or 1×16 array when designing single-slice pTx spoke pulses for pelvic imaging. We also reported similar results, favoring a double-ring 2×8 RF array when searching for optimum body coil design for 3T MRI (16,35). …”

“…RF coil geometries are intricately linked with pTx pulse design and resulting RF performance. Previous studies with single-band (SB) RF pulses have shown that a multiring RF transmit array, in which the coil elements are azimuthally arranged in two rings along the z-direction (15), can be used to further improve the RF performance of pTx pulses as compared with a single-ring array design (16)(17)(18). In one study, Wu et al (17) demonstrated experimentally that using a double-ring array can improve RF excitation homogeneity across the entire brain at 7 T when designing nonselective kT point pulses (19).…”

Designing parallel transmit head coil arrays based on radiofrequency pulse performance

“…This allows an SAR‐efficient array to be designed for a given application, independent of variables that could change scan‐to‐scan, such as repetition time and flip angle. Finally, although the proposed pTx head array design approach was demonstrated with head‐averaged SAR, it can be extended to regularize local SAR by integrating the local virtual observation points with a reweighting algorithm …”

“…Finally, although the proposed pTx head array design approach was demonstrated with head-averaged SAR, it can be extended to regularize local SAR by integrating the local virtual observation points 37 with a reweighting algorithm. 38 Nevertheless, the array design concept proposed here under the proposed acpTx framework shows a novel direction for pTx array design. This work demonstrates a novel approach to design complex coil geometries that can achieve ideal current patterns 14 with realistic human model geometries.…”