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A 4-Channel iPRES-W AIR Coil Array for Simultaneous MR image Acquisition and Wirelessly-Controlled Localized B0 Shimming of the Spinal Cord
Jonathan D. Cuthbertson1,2, Dean Darnell1,2, Robert Stormont3, Fraser Robb3, Allen W. Song1,2, and Trong-Kha Truong1,2

1Brain Imaging Analysis Center, Duke University, Durham, NC, United States, 2Medical Physics Graduate Program, Duke University, Durham, NC, United States, 3GE Healthcare, Aurora, OH, United States

Synopsis

B0 inhomogeneities near air-tissue interfaces can cause distortions, signal loss, and incomplete fat suppression in many applications such as diffusion-weighted imaging of the spinal cord. Here, we develop a 4-channel iPRES-W AIR coil array to perform simultaneous imaging and wirelessly-controlled localized B0 shimming of the cervical spinal cord. In vivo experiments showed a 58.5% reduction in B0 root-mean-square-error (RMSE) after shimming the spinal cord using the iPRES-W AIR coil array, resulting in substantially reduced geometric distortions in diffusion-weighted images, ADC maps, and FA maps

Introduction

B0 inhomogeneities near air-tissue interfaces can cause distortions, signal loss, and incomplete fat suppression in many applications such as diffusion-weighted imaging of the spinal cord. These localized B0 inhomogeneities cannot be effectively corrected for with conventional 2nd/3rd-order spherical harmonic shim coils.

A novel integrated parallel reception, excitation, and shimming (iPRES) coil design was proposed to address this limitation by allowing RF and DC currents to flow on the same coil elements to enable simultaneous image acquisition and localized B0 shimming with a single coil array without reducing the SNR1,2.

Recently, a new integrated RF/wireless coil design that allows RF currents at the Larmor frequency (127 MHz at 3T) and in a wireless communication band (2.4 GHz for WiFi) to flow on the same coil was also proposed to perform simultaneous imaging and wireless data transfer. As a first application, the iPRES and RF/wireless coil designs were combined into an iPRES-W coil design to allow a DC current and both RF currents to flow on the same coil, enabling simultaneous imaging and wirelessly-controlled localized B­0 shimming without any degradation in SNR, wireless data throughput, or B0 shimming performance3. This design was further integrated with the innovative flexible, ultra-lightweight AIR coil technology4 to achieve the same advantages, while providing increased patient comfort and more flexible design opportunities due to the ability to optimize the overlap between coil elements without degrading performance5.

Here, a 4-channel iPRES-W AIR coil array was constructed and applied for the first time in vivo to perform simultaneous diffusion-weighted imaging and wirelessly-controlled localized B0 shimming of the cervical spinal cord.

Methods

First, each element of a 4-channel AIR coil array was modified into an iPRES AIR coil element by adding two DC chokes to allow a DC current to flow on the coil element for shimming and to prevent RF signal loss incurred to the wireless power supply (Fig. 1). Next, a single coil element was further modified into an iPRES-W AIR coil element by adding: a transmission line between the coil element and the WiFi module for data transmission; a 127-MHz band-stop filter between the coil element and the WiFi module to prevent MRI signal loss; and a 2.4-GHz band-stop filter between the coil element and the low-noise amplifier (LNA) to reduce losses in the radiated power. These filters provided -30 dB and -20 dB of isolation between the WiFi module and the LNA at 127 MHz and 2.4 GHz, respectively.

The adjustable DC currents for shimming were supplied from MR-compatible batteries and wirelessly controlled via the digital pulse width-modulated outputs of the WiFi module. Output shim currents could range between ± 1.2 A with a resolution of 2.5 mA and were wirelessly monitored with a feedback loop to the WiFi module.

Basis B0 maps were first acquired on a phantom with 1 A separately applied in each coil element (Fig. 2). A baseline B0 map was then acquired on the cervical spinal cord of a healthy volunteer (Fig. 3a). The optimal shim currents were determined by minimizing the root-mean-square-error (RMSE) between the baseline B0 map and a weighted combination of the basis B0 maps, within an ROI drawn around the spinal cord. The four shim currents were then updated from outside the scanner room via the wireless connection between the iPRES-W AIR coil element and an access point on the scanner room wall.

Sagittal B0 maps, diffusion-weighted echo-planar images (2x2x4 mm, b=600 s/mm2, 25 diffusion directions), and fast spin-echo anatomical images were acquired before and after wirelessly shimming with the iPRES-W AIR coil array.

Results

A representative sagittal slice of the predicted (Fig. 3b) and measured (Fig. 3c) B0 maps after shimming with the iPRES-W AIR coil array shows a drastic reduction of localized B0 inhomogeneities within the rectangular ROI. The improved field homogeneity in turn resulted in substantially reduced geometric distortions, highlighted by the green arrows, in the b = 0 images (Fig. 4b), isoDWI images (Fig. 4d), ADC maps (Fig. 4f), and FA maps (Fig. 4h).

Discussion and Conclusion

These in vivo experiments demonstrate the ability of the iPRES-W AIR coil array to simultaneously perform image acquisition and wirelessly-controlled localized B0 shimming of the spinal cord. The addition of wireless communication within the MRI scanner bore without the need for scanner modifications of additional antenna systems opens up the possibility for applications beyond wirelessly-controlled shimming, including other types of wireless data transfer between the scanner and the coil array.

Acknowledgements

This work was in part supported by grants R21 EB024121, R01 NS075017, and S10 OD 021480 from the National Institutes of Health, by GE Healthcare, and by the Duke-Coulter Translational Partnership.

References

  1. Truong TK et al. NeuroImage 2014; 103;235-40
  2. Darnell D et al. Mag. Reson. Med. 2016; 77(5): 2077-2086
  3. Darnell D et al. Mag. Reson. Med. 2018. doi: 10.1002/mrm.27513
  4. Vasanawala S et al. Proceedings of the ISMRM, April 2017, Honolulu: pg. no. 0755
  5. Cuthbertson J et al. Proceedings of the ISMRM, June 2018, Paris; pg. no. 001
  6. Topfer R et al. Mag. Reson. Med. 2018

Figures

Figure 1. 4-channel iPRES-W AIR coil array for simultaneous MR image acquisition (with RF currents at 127 MHz; red), wireless communication (with RF current at 2.4 GHz; orange), and wirelessly-controlled localized B0 shimming (with DC currents; blue). Pairs of DC chokes between the coil elements and the MR-compatible DC power supply enable RF-isolation for the iPRES AIR design, while band-stop filters between the MRI and WiFi ports ensure isolation from unwanted RF currents for the iPRES-W AIR design. Wireless data to control the shim currents is transferred between the coil array and an access point on the scanner room wall.

Figure 2. Basis B0 maps acquired on a spherical water phantom using a DC current of 1 A per coil element and overlaid on a sagittal anatomical image of the cervical spinal cord.

Figure 3. B0 maps overlaid on a sagittal anatomical image of the cervical spinal cord before (a) and after (b: predicted, c: measured) shimming the rectangular ROI with the iPRES-W AIR coil array. The B0 RMSE was reduced from 56.2 Hz to 23.3 Hz, representing a 58.5% improvement. There was also a good agreement between the predicted and measured B0 RMSE after shimming, with < 4% difference.

Figure 4. Localized B­0 inhomogeneities result in distortions in diffusion-weighted imaging of the cervical spinal cord, including in the b = 0 images (a), isoDWI images (c), ADC maps (e), and FA maps (g), as highlighted by the red arrows and the misalignment with the overlaid contour lines derived from the anatomical images. After shimming the rectangular ROI (in blue) with the iPRES-W AIR coil array, the distortions were substantially reduced (b,d,f,h), as shown by the green arrows.

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