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Combined volume T/R and surface Rx-only coils for simultaneous brain and spinal cord imaging of squirrel monkey at 9.4 T
Xinqiang Yan1,2, Feng Wang1,2, Ken Wilkens1,2, Daniel Colvin1,2, Li Min Chen1,2, and John C. Gore1,2

1Institute of Imaging Science, Vanderbilt University, Nashville, TN, United States, 2Radiology and Radiological Science, Vanderbilt University, Nashville, TN, United States

Synopsis

Simultaneous imaging of spinal cord and brain potentially provides valuable information about how they work together and visualizes their interactions. However, to date, almost all image studies have investigated these two highly interconnected systems in separations, due mainly to the lack of an adequate imaging hardware. In this work, we designed and fabricated a volume T/R and surface Rx-only coil which can significantly increase the SNR around the spinal cord while simultaneously acquiring whole brain images. It needs only two receive channels and operates in a highly restricted volume, making it suitable for the small animal scanner.

Introduction

Functional and structural MRI play important roles in understanding the relationships between the organization of spinal cord and brain. For example, the processing of innocuous touch and painful stimuli engages circuits within the grey matters of both spinal cord and brain’s somatosensory system. These spinal cord and brain regions interact constantly to execute behavioral demands originated in the brain’s high cognitive areas such as goal-oriented limb movement. Simultaneous imaging of these separate regions potentially provides valuable information about how they work together and visualizes their interactions. However, to date, almost all image studies have investigated these two highly interconnected systems in separations, due mainly to the lack of an adequate imaging hardware. head volume coils used for brain imaging do not perform well for the spine, while surface coil arrays designed for the cord fail to image the cortex. To achieve optimal signal-to-noise-ratio for simultaneous brain and spine imaging, a combined dual coil has been designed that is suited for the smaller bore and lower number of channels typically available on high field animal MRI scanners.

Methods

Figure 1a shows a conventional design typical of human systems using a volume Tx-only coil and a receive-only array1,2. However, due to the limited receive channels and space, it is hard to place such an Rx array around a monkey brain in a smaller bore 9.4T magnet. We therefore propose to use a volume coil for reception and transmission and to add a receive coil focused on the spinal cord, as shown in Figure 1b.

Figures 1c-e shows the dual coil design and how they interface to the spectrometer. The volume coil is based on the classic Alderman-Grant Resonator3 (10-cm length and 8.9-cm diameter) which can cover the whole brain and upper spinal cord of a squirrel monkey. The surface coil is a saddle-shaped loop coil with 3-cm length and 4.5-cm diameter. Figure 1f shows a schematic of the home-built T/R switch circuit. During transmission, the PIN diodes in the T/R switch (D1, D2 and D3 in Figure 2d) are ON and the transmit power goes into the volume coil. Meanwhile, the surface coil is detuned to avoid transmit field distortion (D1 in Figure 1e is ON). During reception, all PIN diodes are OFF and the MR signal will go into both coils. The two coils are carefully positioned to generate orthogonal magnetic fields and thus intrinsically isolated. To avoid cross-talk with the outer environment, a copper foiled tube (separated by 330 pF capacitors) with a diameter of 11 cm was used as shielding.

This design was validated through both EM simulation (using commercial software Ansys HFSS, Canonsburg, PA, USA) and MR experiments (9.4T Varian/Agilent small animal scanner, 12-cm bore size inside the gradients). Low flip angle GRE images with the following parameters were acquired for SNR calculations: FOV=100x100 mm2, matrix= 128x128, TR/TE=200/4 ms. Flip angle =15o, slice number=24, slice thickness/gap=2/0 mm and bandwidth=390.6 Hz/pixel.

Results and Dicussions

Figure 2a shows the simulation results. Figures 2b and 2c show the calculated vector magnetic field in an axial slice from each coil. The two coils have an orthogonal magnetic field (one horizontal and the other vertical) and are expected to be intrinsically isolated. Note the volume coil was designed to be driven linearly to work with the surface coil simultaneously during reception, which leads to ~30% SNR loss in the brain area compared to a quadrature birdcage coil.

Figure 3a shows the coil built for monkey studies. Float trap circuits were used for both coils to suppress the common-mode current. The measured unloaded quality (Q-) factors of the volume coil and surface coil is 76 and 183, respectively, and their Q-factors drop to 34 and 24 when loaded with a saline phantom (4.5-cm diameter, 4g/ml NaCl). Similar to the simulation predictions, the two coils are well decoupled with S21=-27 dB (Figure 3b). Figures 4 and 5 show the measured SNR maps on this water phantom with a surface-only coil, a volume-only coil, and their combination. By using the combined coil, the SNR near the surface coil (corresponding to the spinal cord area) can be boosted by approximate 2.5 times while the SNR in other areas can be maintained.

Conclusion

We designed and fabricated a volume T/R and surface Rx-only coil for simultaneous brain and spinal cord imaging of squirrel monkey which can significantly increase the SNR around the spinal cord while simultaneously acquiring whole brain images. It needs only two receive channels and operates in a highly restricted volume, making it suitable for the small animal scanner.

Acknowledgements

No acknowledgement found.

References

1. Cohen‐Adad, J., et al. "32‐Channel RF coil optimized for brain and cervical spinal cord at 3 T." Magnetic resonance in medicine 66.4 (2011): 1198-1208.

2. Matschl V, Reykowski A, Jahns K, Hergt M, Fischer H. 48 Channel Body / Spine Matrix Coils for 3 Tesla; Proceedings of the 13th Annual Meeting of ISMRM; 2005.p. 952.

3. Alderman, Donald W., and David M. Grant. "An efficient decoupler coil design which reduces heating in conductive samples in superconducting spectrometers." Journal of Magnetic Resonance (1969) 36.3 (1979): 447-451.

Figures

Figure 1 a) Illustration of conventional Tx-only and Rx-only array design for brain+spinal cord imaging. b) Illustration of the novel volume T/R and Rx-only coil design that requires fewer Rx channels and less space. c) Interface with the scanner. d) Diagram of the volume coil based on the classic Alderman-Grant Resonator3. e) Diagram of the saddle-shaped surface coil focusing on spinal cord imaging. f) Schematic of the home-built T/R switch circuit.

Figure 2 a) Simulation model of the volume T/R and Rx-only coil. b-c) Magnetic field (vector) distribution from the volume coil and the surface coil. d-e) Magnetic field (magnitude) distribution from the volume coil and the surface coil. In all cases, the magnetic field is normalized to the same input power.

Figure 3 a) Photograph of the fabricated coil. b) Measured scattering (S-) parameter plots vs. frequency when loaded with the water phantom. It is noted the volume coil and surface coil are well decoupled from each other with S21=-27 dB.

Figure 4 Measured axial SNR maps from GRE images that used only the surface coil during the reception, only the volume coil, and then both coils. In all cases, the volume coil was used for transmission. MR images from the two coils are combined with the root-sum-of-squares (RSOS) method. The SNR was calculated by SI/std(noise)*0.655, where SI is the signal intensity and std(noise) is the standard deviation of background noise. The GRE parameters are: FOV=100x100 mm2, matrix= 128x128, TR/TE=200/4 ms. Flip angle =15o, slice number=24, slice thickness/gap=2/0 mm and bandwidth=390.6 Hz/pixel.

Figure 5 Measured sagittal SNR maps from GRE images that used only the surface coil during the reception, only the volume coil for receive and then both coils. The SNR measurement setup and calculation are same as those in Figure 4.

Proc. Intl. Soc. Mag. Reson. Med. 27 (2019)
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