Introduction

Achieving higher spatiotemporal resolution and detection sensitivity in MRI and MRSI application is of great importance for non-invasively assessing brain structure, function and metabolism. However, the signal-to-noise ratio (SNR) is still limited even at ultrahigh field (UHF). One RF engineering approach that integrated high or ultrahigh dielectric constant (HDC or uHDC) materials with RF coil(s) has proven to improve RF coil transmission (B₁+) and reception (B₁-) fields, thus, SNR.^1-5 Recently, we reported that the tuHDC ceramics made of composite BST (Ba_0.6Sr_0.4TiO₃) material have an extremely high tunibility of relative permittivity (εr) from a few thousands to >15000 when changing a few degree of the ceramic temperature.¹ We found that 14-15 °C is the best ceramic temperature for ¹⁷O MRSI application at 10.5T, resulting an optimal permittivity of ~6000. In this study, we developed new ceramic disks made of mixed Ba_0.6Sr_0.4TiO₃ and BaTiO₃ (BST-BT) materials to achieve an optimal permittivity (εr»6000) at room temperature for 10.5T ¹⁷O MRSI application without the need of temperature controller. The performance and improvement of 10.5T ¹⁷O MRSI using the BST-BT-based ceremics have been rigorously evaluated and quantified.

Methods

Two circular uHDC disks (8-cm diameter, 0.9-cm thickness) with extremely high permittivity (εr»6000, Fig.1A) and a very low dielectric loss, thus a high Q (Fig. 1B, Table 1) at the room temperature (20°C) were made from mixed composite material (Ba_0.6Sr_0.4TiO₃ + 10% w/t BaTiO₃).
¹⁷O MRSI experiments were carried out on a Siemens 10.5T whole-body/88-cm human scanner with a single-loop ¹⁷O RF coil (15cm diameter) operating at 60.6 MHz. 3D ¹⁷O-CSI data were acquired under fully relaxed conditions (TR = 0.2s, 2ms hard pulse, flip angle (FA)=90°, Matrix=9×9×7, spectral bandwidth=30kHz, FOV=10×10×10 cm³, voxel size = 1.1×1.1x1.4 mm³, number of points = 1024). Three conditions were investigated at room temperature: (i) control condition without the uHDC disk, (ii) with one tuHDC disk and (iii) stacking two tuHDC disks together. To quantify the B₁ fields and SNR, we collected multiple ¹⁷O CSI datasets with varied RF pulse voltages from a water phantom (9-cm diameter, 7.2-cm height, 50 mM NaCl). The tuHDC disks were placed between the phantom and the coil plane and Styrofoams were used to replace the disk(s) for keeping the distance between the phantom and coil same in all cases. All results were compared to the control condition. B₁ maps were processed with sinusoidal fitting and interpolated for visualization using standard matlab routines (interp2, makima), and more details can be found in the literature.¹ The SNR value was calculated using the maximum H₂¹⁷O signal at 90° FA divided by the spectral baseline (std) noise.

Results

Self-resonance frequency of each disk was in the range of 82-87 MHz, however, when two disks were stacked together, it dropped to 65 MHz, which was close to the ¹⁷O Larmor frequency (61 MHz). Figure 1 depicts the ceramic temperature dependence of (A) permittivity showing an optimal of εr»6000 at room temperature and a much higher εr at Curie temperature around 8°C; and (B) dielectric loss (tanδ) showing a minimum loss or excellent Q around room temperature. Figure 2 demonstrate representative sinusoidal fittings of the ¹⁷O water signals as a function of pulse voltage in a central voxel to determine B₁+ and SNR values; similarly, the B₁+and SNR maps were generated for the three conditions. Figures 3-4 show the results of B₁+ and SNR maps and their 1D profiles along the depth for demonstrating and quantifying the improvements with one or two BST-BT-based tuHDC disks. Strikingly, we observed approximately 5 times higher B₁+ and SNR near the two-stacked uHDC disks as compared to the control without the disk. For the single disk condition, we also observed moderate improvement of B₁+ and SNR (>50%) compared to the control despite of a large gap between the disk and the imaging object (Figs. 3&4).

Discussion

In this study, we have demonstrated unprecedented improvements on transmission efficiency and reception sensitivity using the newly developed and optimized ceramic disks made of BST-BT composite materials. The magnitude of improvement depends on the disk permittivity and size or thickness. Future research will include RF simulation to elucidate optimal disk design for particular applications of interest with further improvement.^8,9 The same concept of designing optimal HDC/uHDC ceramics via material science and fabrication could be employed to other field strengths or other nuclide applications. One particular interesting and straightforward application of the same BST-BT tuHDC disks is to improve the ¹H MRI/MRSI performance at clinical field strength of 1.5T since the ¹H Larmor frequency at 1.5T is almost identical as the ¹⁷O Larmor frequency at 10.5T.¹

Conclusion

We report the newly developed BST-BT ceramics can provide unprecedented improvement at room temperature for ¹⁷O MRSI applications using a 10.5T human scanner. The novel uHDC-RF coil technology could greatly enhance the spatiotemporal resolution and detection sensitivity for X-nuclear application at UHF and ¹H MRI/MRSI at clinical scanners without temperature controlling. This could reduce the gap between the uHDC-RF coil and the imaging object and make the uHDC-RF coil design, construction less complicated; thus, accerelate the translation.

Acknowledgements

Acknowledgment: This work was supported in part by NIH grants of U01 EB026978, R01 MH111413, R01 CA240953, R24 MH106049, S10 RR026783, P41 EB027061 and P30 NS076408.

References

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