Introduction

A new generation of 7T scanner recently obtained 510(k) FDA clearance for brain and knee imaging; however, image non-uniformity remains a major problem for 7T MRI affecting all body areas imaged ^[1]. The problem is particularly acute for clinical muscletoskeletal imaging, particularly in the knee joint, where even subtle changes in signal across the field of view can have a serious deleterious effect on the diagnosis. The main culprit of such non-uniformity is the transmit B₁⁺ inhomogeneity, arising from complex interactions of the radiofrequency (RF) field with the dielectric properties of tissue in vivo. This in turn leads to variability in flip angles achieved in the tissue, and a consequence effect on the measured signal and image contrast. However, a sub-optimal receiver coil element layout can also contribute to this problem, which is rendered problematic at 7T since such non-uniformities cannot be easily correct in post-processing due to the lack of a uniform body transmit RF coil on current 7T scanners. Passive B₁⁺ shimming through the use of high permittivity dielectric pads has been described for brain imaging ^[2-4], but thus far this has not been attempted for knee imaging, nor to address B₁^- receive problems. At 7T (300MHz for ¹H), large displacement currents are generated in high permittivity materials which effect a B₁⁺ field focusing in their vicinity, increasing the flip angles achievable in nearby tissue, with a consequent increase in signal detected from those regions. The aim of this work was to investigate the use of custom-made dielectric pads for routine use in improving image uniformity for clinical 7T knee imaging.

Methods

Six subjects (1 female; mean age = 45y) were recruited on to the IRB-approved study. Dielectric pads were manufactured from high electric permittivity perovskite materials to maximize the magnitude of the displacement currents. Care was taken to avoid any additives which might introduce susceptibility artifacts or excessively load the RF coil. Two types of pad were made, each with 40% vol/vol ratio of perovskite/D₂O and a 0.5% mass fraction of a gelling agent (hydroxyethylcellulose) to reduce separation of the perovskite from the deuterated water over time. The first type of pad used as perovskite 100% CaTiO₃, the second a 50-50 mix of CaTiO₃ and BaTiO₃. The dielectric properties of the resultant slurries were measured using a DAK12 system (Speag, Switzerland). Scanning was performed on a 7T scanner (Terra, Siemens, Germany) using a single-transmit, 28-channel-receiver knee coil (QED, USA). Axial T₂-weighted and coronal proton density-weighted sequences (with and without fat suppression) were acquired (0.37x0.37x2mm³). Pads of varying dimensions were manufactured and tested in volunteer scanning. SNR measurements were performed in tissue regions placed close to and distant from the pads, for a variety of pad configurations (dimensions, compositions).

Results

Relative permittivity/conductivity values of 111.7/0.123 S.m^-1 and 151.8/0.32 S.m^-1 (at 300MHz) were obtained for the Ca- and Ba-based perovskites, respectively. The optimal pad configuration, commensurate with improving image uniformity across all subjects, had dimensions 180x60x10 mm³ and a permittivity of 111.7; the higher permittivity pads over-compensated for the low signal. The 10 mm pad thickness was found to represent an optimal compromise between performance and likelihood to fit the majority of patients (considering the narrow coil). The two optimized pads were located at the 3 and 9 o’clock positions longitudinally along the z-axis, at the join of the anterior and posterior segments of the RF coil. Significant improvements in image uniformity (Figures 1-3) and SNR (Table 1) were found for all subjects scanned with this configuration.

Discussion

Passive shimming using dielectric pads is an effective and easy-to-implement technique and represents a relatively low-tech approach to B₁⁺ shimming in the absence of parallel transmit techniques. The performance is empirical, in the sense that the specific pad configuration (dielectric properties, dimensions) need to be optimized for a given RF coil and anatomical region under investigation. However, the current data demonstrate that consistent improvements are possible with a custom-made dielectric pad configuration. A potential limitation of such pads is a lack of temporal stability if the perovskite material dissociates from the water ^[2]. However, the use of a gel stabilizer resulted in consistent performance for 8 months (and counting).

Conclusion

Passive shimming using dielectric pads of judicious size, permittivity and positioning relative to the anatomy and RF coil were used to successfully improve image uniformity for clinical 7T knee MSK imaging. By increasing B₁⁺ and hence flip angles in their vicinity, dielectric pads made from high permittivity materials increase the measured signal in areas of deficit and improve knee image uniformity at 7T.

Acknowledgements

The authors acknowledge assistance from MR Technologists and study coordinators.

References

[1] Kraff et al., 7T: physics, safety and potential clinical applications, J Magn Reson Imaging 2017; 46: 1573-1589 [2] O’Reilly et al Practical improvements in the design of high permittivity pads for dielectric shimming in neuroimaging at 7T, J Magn Reson 2016; 270: 108-114 [3] Vaidya et al, Improved detection of fMRI activation in the cerebellum at 7T with dielectric pads extending the imaging region of a commercial head coil, J Magn Reson Imaging2017; 48(2): 431-440 [4] Brink et al, High permittivity dielectric pads improve high spatial resolution magnetic resonance imaging of the inner ear at 7T, Invest Radiol 2014; 49(5): 271-277