Introduction

The advanced technology of in vivo MRS and MRI has provided a wealth of knowledge about brain function, connectivity, neurochemistry and neuroenergetics in the human brain. However, due to the lower cerebral metabolite concentrations, the need for higher sensitivity, spatiotemporal and spectral resolutions is growing for in vivo ³¹P MRS imaging applications even at ultrahigh field to address questions of interest. In addition, the increased RF transmit power at ultrahigh field raises a major safety concern in the human brain research. Recent development of high dielectric constant materials (HDM) incorporated with RF coils has shown significant improvements in the RF transmit field (B₁⁺) and reception field (B₁^-) for MRI applications ^[1-4]. Due to the relatively low Larmor frequency of X-nucleus, the translation of the HDM technique into in vivo X-nucleus application requires an ultrahigh dielectric constant material (uHDM) ^[5] with optimal geometry conformed to a target object. Thus, our aim in this work was to test a proof-of-concept design of a human head-shaped uHDM for improving detection sensitivity of in vivo ³¹P MRS imaging at 7T.

Methods

A bowl-shaped uHDM former (BS-uHDM) was designed and fabricated to conform the human head shape. It was made of low lossy (0.05 loss tangent) PZT (Pb(Zr, Ti)O3) ceramics (effective permittivity: ε_eff = 800, thickness = 0.8 cm, and diameter = 8 cm) (Fig. 1A). ³¹P MRS studies were carried out on a Siemens 7.0T/90 cm human scanner (Magnetom, Germany) using a ³¹P-¹H dual-channel RF head TEM volume coil. All measurements were repeated with or without use of the uHDM former on a spherical phantom (2 liter, 50 mM inorganic phosphate (Pi)) and healthy human subject. Multiple 3D ³¹P CSI data with varied RF pulse powers (voltages) were acquired for estimating B₁⁺ and B₁^- maps based on the Pi resonance in the phantom and phosphocreatine (PCr) resonance in the human brain.

Results

Figures 1 and 3 display the ³¹P CSI spectra of the phantom and in vivo human brain acquired with a reference voltage, respectively. The ³¹P CSI data clearly demonstrate the apparent B₁ improvements in the presence of the BS-uHDM across the entire object, showing a higher signal intensity with a lower reference voltage. Figures 2 and 4 show the quantitative comparisons of the ³¹P B₁ results in the representative central CSI slice for the phantom and in vivo human brain, respectively. In general, the use of the BS-uHDM former in the ³¹P imaging led to overall B₁⁺ and B₁^- enhancement across the entire object. The B₁ ratio map on the phantom shows the significant improvement using the BS-uHDM former reaching up to 60% for both B₁⁺ and B₁^- (Fig. 2C and 2F). In Fig. 4, the BS-uHDM former for in vivo ³¹P human brain also improved the B₁ efficiency, reaching up to 53% for B₁⁺ and 37% for B₁^-, which is consistent with phantom results by considering the loss of detection sensitivity due to the presence of a gap between human skull and brain tissue.

Discussion

The results provide compelling evidence of the uHDM benefits for effectively improving both the B₁⁺ and B₁^- efficiency, consequentially leading to a large increase in ³¹P imaging sensitivity in the human brain at 7T. The degree of the B₁ improvements and their spatial distribution are sensitive to the uHDM design including the ceramic permittivity constant, loss tangent, and geometry for optimizing the performance and maximizing the efficacy ^[6]. The merit of a high-quality uHDM with large coverage further boosts the SNR for X-nuclear MRS imaging applications. These findings open new and exciting opportunities for more research that integrates the MRI/MRS technology with RF engineering and material science.

Conclusion

Our study demonstrates that the novel design of uHDM technology can significantly improve both RF transmitter efficiency and detection sensitivity (i.e., SNR) for human brain application of in vivo ³¹P MRS at ultrahigh field. The uHDM technology provides an important and cost-effective engineering solution for advancing MRI and in vivo MRS techniques at high/ultrahigh field, which will provide enormous benefits for in vivo human applications in particular for brain research. The same concept of the uHDM technology can be extended to other X-nuclear MRS or imaging applications beyond ³¹P and ¹H spins or other organs beyond the brain, as demonstrated in this work, under healthy and diseased condition

Acknowledgements

NIH grants: R01 NS057560, NS070839, MH111447; R24 MH106049, MH106049 S1, S10RR026783, P41 EB015894 and P30 NS057091; the AHC Faculty Research Development (FRD) grant from the University of Minnesota.