4089
SNR Comparison of a Receive Array Coil When Mounted on a High-Permittivity Helmet Former vs. Moved Closer to the Head
Giuseppe Carluccio1,2 and Christopher Michael Collins1,2
1Radiology, Center for Advanced Imaging Innovation and Research (CAI2R), New York, NY, United States, 2Radiology, Bernard and Irene Schwartz Center for Biomedical Imaging, New York, NY, United States
1Radiology, Center for Advanced Imaging Innovation and Research (CAI2R), New York, NY, United States, 2Radiology, Bernard and Irene Schwartz Center for Biomedical Imaging, New York, NY, United States
Synopsis
High-permittivity materials (HPM) have provided promising results in terms of higher local SNR, such as when an HPM helmet former is used in combination with a receive coil array. Since the vicinity of the coil affects the SNR and the presence of the HPM helmet prevents placement of the coil closer to the head, in this work we compare the SNR distribution when the receive array coil is mounted on top of an HPM helmet to when it is moved closer to the head. Results show that the array coil with the HPM helmet provides an overall higher SNR.
Introduction
Signal to noise ratio (SNR) in MRI is one of the most common parameters used to determine the quality of a coil because it ultimately affects the quality of the MRI image.High-permittivity materials (HPM) have promised high transmit efficiency and high local SNR, including at the center of the brain, where it is difficult to improve SNR by other means1. Recently, an HPM helmet was designed that, when combined with a receive array, would provide a significant increase in SNR over all the brain and also in the center of the head2. In order to estimate the SNR improvement, the SNR calculations with and without the helmet were performed by keeping the coil in the same location and changing the permittivity of the helmet from $$$ \epsilon = 1$$$ for the case of no helmet to $$$ \epsilon = 115$$$ (as an optimum value for a 7T MRI system) for the Air Helmet and HPM helmet, respectively. However, since the designed helmet has an 8 mm thickness, the coil could, in principle, be positioned closer to the head in the case without the helmet, potentially improving the SNR for the case with no HPM helmet. In this work we compare the SNR improvement obtained with an HPM helmet within a 30-loop receive array compared to a similar array positioned 8 mm closer to the head.
Methods
SNR was computed through numerical simulations for two receive-only head arrays operating at 300 MHz (7T MRI system) loaded with the human body model “Duke”3 with a resolution of 2mm in each direction. The first consists of a 30-loop gapped array positioned outside an 8 mm thick HPM helmet former (Fig. 1). The permittivity of the helmet was set equal to 115. The second consists of the same array that, with a Matlab (The Mathworks ®) script, was projected onto the inner surface of the helmet, thus moving the second coil is 8 mm closer to the head.The electromagnetic simulations were performed with the commercial software xFDTD (Remcom, Inc; State College, PA; USA). The transmitted field was assumed to be uniform for all the cases throughout the Field of View. The electromagnetic fields from each element of the array were computed separately, and the SNR was computed in Matlab with the equation
$$SNR=\sqrt{\Psi'^* R^{-1} \Psi}$$
where ψ is the sensitivity matrix and R the noise correlation matrix.
Results
The SNR distribution along three orthogonal planes is plotted in Fig. 2 for both the simulated cases, $$$SNR_{HPMHelmet}$$$ and $$$SNR_{CloseCoil}$$$. For each voxel the SNR ratio $$$SNR_{Ratio}=SNR_{HPMHelmet}/SNR_{CloseCoil} $$$ is plotted in Fig. 3. Contour lines denote where SNRRatio is equal to 1. As it can be observed in Fig. 3, in most of the locations the presence of the helmet provides an SNR increase. The only locations where the closer array performs better are some near the surface of the head. This could be expected considering the smaller distance of those locations to the coils. On the contrary, in inner locations of the head the array coil in combination with the HPM helmet performs better. Specifically, in the center of the brain the coil with the HPM helmet provides an SNR increase of 23%.Overall, in the brain the average improvement of the SNR when the HPM helmet is used is equal to 14%. The SNR distributions in Fig. 2 looks having smoother contours when the HPM helmet is used, and it is due to the more uniform excitation deriving from the displacement current present in the high-permittivity material, compared to the conductive currents of the close coil in the second case.
Conclusion
Although there are small regions where the close array provides a higher SNR, the use of HPM pads provides greater overall SNR improvement throughout the brain.Acknowledgements
This work was supported by National Institutes of Health grant R01EB021277 and was performed under the rubric of the Center for Advanced Imaging Innovation and Research (CAI2R, www.cai2r.net) at the New York University School of Medicine, which is an NIBIB Biomedical Technology Resource Center (NIH P41 EB017183).References
- S. Rupprecht, C. T. Sica, W. Chen, M. T. Lanagan, Q. X. Yang, “Improvements of transmit efficiency and receive sensitivity with ultrahigh dielectric constant (uHDC) ceramics at 1.5 T and 3 T”, Magnetic resonance in medicine. 2018 May;79(5):2842-51.
- G. Carluccio, G. Haemer, C. M. Collins, “SNR improvement when a High Permittivity Material helmet-shaped former is used with a close-fitting Head Array”, In 2018 International Conference on Electromagnetics in Advanced Applications (ICEAA), 2018 Sep 10 (pp. 304-306). IEEE.
- A. Christ, W. Kainz, E. G. Hahn, K. Honegger, M. Zefferer, E. Neufeld, W. Rascher, R. Janka, W. Bautz, J. Chen, B. Kiefer, “The virtual family—development of surface-based anatomical models of two adults and two children for dosimetric simulations”. Physics in Medicine & Biology, 2010;55: N23–N38
Figures
Figure 1: Geometry with the array coil mounted on top of the helmet (left) and with the array moved closer to the head (right).
Figure 2: SNR distribution for three orthogonal views when the array coil is mounted on top of the HPM helmet (top row) and when it is moved closer to the head (bottom row).
Figure 3: Ratio between the SNR obtained with the array coil outside of the HPM helmet to that obtained with the array coil moved closer to the head.