It has been shown for selected applications at 7T that high permittivity dielectric bags can improve transmit B₁⁺ fields. Improvements have been reported for imaging the cerebellum ¹, inner ear ², and the temporomandibular joint ³, i.e. relative small regions of interest, and all in conjunction with a commonly-used close-fitting 1-channel transmit/32-channel receive head coil (Nova Medical, Wilmington, MA). While reports can also be found for larger cross-sections at 3 Tesla (cardiac ⁴, thighs ⁵), only one study evaluated the use of high permittivity materials for whole brain imaging at 7T ⁶. The aim of this study was therefore to evaluate the effects of different configurations of dielectric pads in imaging upper extremities, especially the shoulder joint, at 7T.

Three healthy male volunteers (BMI: 22/23/26 kg/m2) were included in this study. All subjects were imaged at their right shoulder using a whole-body research system (Magnetom 7T, Siemens Healthcare GmbH, Germany) and an in-house-developed 8-channel transmit/receive shoulder coil (Figure 1). The subjects were imaged in three consecutive scans: without dielectric pads present, using 3 pads made of calcium titanate (CaTiO₃; 175x110x10 mm³, permittivity 110, conductivity 0.11 S/m), and using 3 pads made of barium titanate (BaTiO₃; 175x120x10 mm³, permittivity 286, conductivity 0.44 S/m). The pads were placed around the joint as illustrated in Figure 1. Additionally, one subject was imaged at the upper arm with only two pads: one between arm and trunk and one placed on top (Figure 3). For each configuration a set of measurements was performed: (1) transversal flip angle maps were obtained with the DREAM technique ⁷, and structural imaging with a PD-weighted turbo-spin echo (TSE) sequence (2) in sagittal orientation, and (3) in transversal orientation including fat saturation. Each measurement was obtained twice, (i) with a fixed RF shim, and (ii) using individual RF phase shimming per subject and pad configuration. TSE images were chosen, as they are clinically relevant and more sensitive to B₁⁺ variations than gradient echo images.

Images were evaluated qualitatively by a physicist and a radiologist in consensus regarding overall image quality, homogeneity, and presence of artifacts. Quantitatively, we compared mean flip angle and standard deviations over a large region of interest (ROI) between each configuration, as well as differences in contrast ratios between adjacent tissues in the fat-saturated PD TSE images.

In the qualitative image evaluation no benefit in using dielectric pads was observed. On the contrary, despite individual RF shimming, images appeared more inhomogeneous with very bright regions in the periphery, especially in subcutaneous tissue and the humerus, and low signal in deeper regions within the body. Images appeared more inhomogeneous when using the BaTiO₃ pads compared to the CaTiO₃ pads or without pads (Figures 1-3). The CaTiO₃ pads yielded marginal improvements compared to the situation without pads, with only minimal benefits in depicting the coracobrachialis muscle in one subject, for example (Figure 2). However, no benefits were found for the overall shoulder joint.

Similar results were obtained quantitatively. Although BaTiO₃ pads increased the mean flip angle by 5 to even 40% when a fixed standard RF shim was applied, almost no differences in mean flip angle and standard deviation were measured after individual RF phase shimming (Figure 4). Also, no differences in contrast ratios were observed when dielectric pads were used (Figure 5).

For the investigated configurations, no substantial improvements in imaging the upper extremities at 7T were found from applying high permittivity dielectric pads. This is in line with previous work in neuroimaging at 7T, where a degredation of the transmit uniformity was also shown for increasing permittivity, suggesting an optimum permittivity around ~150. Electrically large pads are known to potentially introduce strong wavelength effects in the B₁⁺ field ^2,8, which was reflected in the structural images when moving to BaTiO₃ pads. Please note, the transmit B₁⁺ field was already rather homogeneous without pads. With these pads, a strong increase in B₁⁺ may be appreciated locally, and especially superficially, which could be of interest in selected applications such as depicting tears of the supraspinatus tendon. The CaTiO₃ pads with lower permittivity did not introduce such strong wavelength effects, however the obtained signal improvements were only marginal and did not justify their application for further studies. The RF coil certainly plays an important role in this comparison, although similar results have been presented in simulation studies for a circular loop array ¹⁰. The results obtained may however not reflect general performance of dielectric pads when other designs for shoulder or body RF transceive arrays are used.