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Integrated ΔB0/Rx coil array for improved spinal cord imaging at 3T
Ryan Topfer1, Alexandru Foias1, Nibardo Lopez Rios1, Angel Chauffray1,2, Grégoire Germain1, Nick Arango3, Lawrence L. Wald4,5, Jason P. Stockmann4,5, and Julien Cohen-Adad1,6

1NeuroPoly Lab, Institute of Biomedical Engineering, Polytechnique Montreal, Montreal, QC, Canada, 2École polytechnique fédérale de Lausanne, Lausanne, Switzerland, 3Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA, United States, 4A. A. Martinos Center for Biomedical Imaging, Massachusetts General Hospital, Charlestown, MA, United States, 5Harvard Medical School, Boston, MA, United States, 6Functional Neuroimaging Unit, CRIUGM, Université de Montréal, Montreal, QC, Canada

Synopsis

Spinal cord imaging is hampered by static and respiration-induced dynamic variations of B0, causing serious issues for EPI-based protocols and spectroscopy. Integrated ΔB0/Rx arrays could provide an effective means to further compensate small-scale variations, complementary to the existing spherical harmonic coils. Here, we designed, built and tested in a human subject an 8-channel integrated ΔB0/Rx coil for the cervical spinal cord. Results showed improvements for correcting static field variations as well as respiratory-induced variations. Future studies will investigate dynamic and real time shimming applications for the spinal cord.

Introduction

Spinal cord imaging is hampered by static and respiration-induced dynamic variations of ΔB0 which cause significant issues for EPI-based protocols and spectroscopy.1,2 Integrated ΔB0/Rx arrays, combining RF reception and ΔB0 shim functionality are a promising solution due to their high efficiency, low inductance, cost-efficient current drivers, and the capability to correct small-scale variation in ΔB0.3 Moreover, these integrated coils provide similar SNR to RF-only coils at both 3T and 7T. Based on previous simulations,4 we designed, built and tested an 8-channel RF/shim coil for the cervical spinal cord.

Methods

Design: An integrated ΔB0/Rx coil array was constructed featuring 8 x 55 mm diameter loops of 16-AWG copper mounted on a close-fitted holder adapted to the neck region (Figure 1). The elements were disposed in a 3-3-2 configuration being geometrically decoupled to reduce interactions between neighboring elements. In addition, low input impedance preamps (MPB-127R73-90, Hi-Q.A. Inc., ON, Canada) were closely mounted to the loops to improve the decoupling between non adjacent elements. 1uH toroidal chokes are used to bridge the tuning capacitors on each RF loop in combination with 1000 pF capacitors to block the DC in the RF feed. Shim currents are controlled using a current driver board5 and a microcontroller. The desired values are updated from the microcontroller via SPI interface to an octal DAC (AD5628), which drives 8 push-pull amplifiers. A current sense resistor, included in the feedback loop of the amplifiers, was used to measure the actual DC current values circulating in the coils. The voltage drops were read with two 4-channel ADCs (ADS1015) connected on the I2C interface of the microcontroller. RF common mode currents during transmission phase are reduced in the DC feed cables by using high impedance RF chokes and parallel resonant LC circuits at the output of the current driver board.


Calibration: Shim reference maps in Hz/A were acquired on a small phantom (2L distilled water doped with 7.5 g NISO4 X 6H2O + 10 g NACL) in a manner similar to [6]: for each channel, field maps were acquired with ± 400 mA circulating in the shim; the difference between the two field maps was divided by the difference in current (800 mA), followed by low-pass filtering to reduce the effect of noise (Figure 2).

In vivo experiment: GRE field maps of a single subject were acquired separately under inspired and expired breath-hold conditions with acquisition and phase processing parameters similar to those described in a previous work [6]. A shim volume of interest (VOI) was designated, encompassing the inferior portion of the cervical spinal cord and the region posterior to it. Using the reference maps, optimal shim currents were calculated independently for the two breath-holds, limiting the max absolute current per channel to 400 mA. Field maps were reacquired for each of the two breath-holds upon setting the shims.

The effect of the optimized shims was further explored on a 2D single-shot GRE-EPI using the following parameters: TE=16 ms, volume TR=500 ms; flip angle 50°; bandwidth=1630 Hz/pixel; R=2 acceleration factor using GRAPPA; partial Fourier=6/8; spatial resolution=1.5x1.5 mm2 in-plane, with 3 sagittal slices of thickness 3.0 mm centered on the spinal cord, for an effective FOV=192x192x9 mm3. EPI were acquired for both breath-holds, in both shim-off and shim-optimized states.

Results and Discussion

Over the shim VOI, application of the ΔB0/Rx array reduced the standard deviation of the field from its original values of 53.6 and 52.9 Hz (inspired and expired, respectively) down to 32.1 and 28.6 Hz, amounting to respective improvements in field homogeneity of 40% and 46% (Figure 3). Figure 4 shows that EPI voxel shift over the same region, as calculated from the scaled absolute field maps, was reduced from 2.71 ± 2.1 mm and 2.5 ± 2.1 mm (inspired and expired) down to 1.8 ± 1.2 and 1.4 ± 1.3 mm, with the maximum shift for the breath-holds reduced from an initial 13.1 mm to 8.0 mm. The mean-normalized percent-difference of the EPI (ΔEPIavg = 200% x | EPIin – EPIex|/ (EPIin + EPIex) ) was reduced 12.5% from an original 23.3 ± 28.7 % to 20.4 ± 27.9 % (median ± standard deviation).

Though limited to a single subject, this preliminary proof-of-concept demonstrate the dual potential of the integrated ΔB0/Rx design for high-quality signal reception along with ΔB0 correction for both static and respiration-induced ΔB0 distortions. Forthcoming experiments will include the characterization of Rx SNR and, once a heat-sink is incorporated to allow for shim currents > 400 mA, real-time shimming using a set of respiratory bellows.

Acknowledgements

Study funded by the Canada Research Chair in Quantitative Magnetic Resonance Imaging (JCA), the Canadian Institute of Health Research [CIHR FDN-143263], the Canada Foundation for Innovation [32454, 34824], the Fonds de Recherche du Québec - Santé [28826], the Fonds de Recherche du Québec - Nature et Technologies [2015-PR-182754], the Natural Sciences and Engineering Research Council of Canada [435897-2013] and the Quebec BioImaging Network. The authors thank the Unité de Neuroimagerie Fonctionnelle (University of Montreal) for the support.

References

1. Verma, T. & Cohen-Adad, J. Effect of respiration on the B0 field in the human spinal cord at 3T. Magn Reson Med 72, 1629–36 (2014).

2. Stroman, P. W. et al. The current state-of-the-art of spinal cord imaging: methods. Neuroimage 84, 1070–81 (2014).

3. Stockmann, J. et al. An Integrated 32ch RF-Shim Array Coil for Improved B0 Shimming of the Brain at 7 Tesla. in Proceedings of the 25th Annual Meeting of ISMRM, Honolulu, 2017. Abstract 967.

4. Germain, G. et al. Optimization of geometry for combined RF/shim coil arrays for the spinal cord. in Proceedings of the 24th Annual Meeting of ISMRM, Singapore, 2016. Abstract 1154.

5. Arango, N., Stockmann, J. P., Witzel, T., Wald, L. & White, J. Open-source, low-cost, flexible, current feedback-controlled driver circuit for local B0 shim coils and other applications. in Proceedings of the 24th Annual Meeting of ISMRM, Singapore, 2016. Abstract 1157.

6. Topfer, R. et al. A 24-channel shim array for the human spinal cord: Design, evaluation, and application. Magn Reson Med 76, 1604–1611 (2016).

Figures

Figure 1: ΔB0/Rx coil. a: Back view showing RF & DC feed connections. b: Bottom view. c: Control setup in the equipment room: the current driver board, fed by a laboratory power supply, is operated via microcontroller. d: Schematic of a hybrid loop. Toroidal chokes Lsc pass shim current in and out of the loop and bridge the capacitor C1 (tuning capacitor). Capacitor C2 blocks shim current from escaping into the RF. The detuning circuit is composed of inductor L, capacitor C4 and PIN diode D. Capacitor C3 and C5 transforms the coil impedance to the input impedance required by the preamplifier.

Figure 2: Interpolated ΔB0 shim reference field maps in Hz/A. Shown is the central coronal slice corresponding to the experimental results (Figures 3 and 4).

Figure 3: Example of respiration-induced ΔB0 field shifts in the cervical spine of one subject. The subject was asked to perform breath-holds in inspired (top row) and expired (bottom row) breath-hold conditions for the duration of the GRE field mapping scan (10 s). This process was repeated upon activation of the 8ch shim, optimized over the cervical spine shim VOI indicated on the expired magnitude image (bottom left). Red horizontal arrows point to a region of decreased field distortion under shimming.

Figure 4: Example of EPI geometric distortion due to field inhomogeneity. The top panel shows EPI and the corresponding voxel shift contour maps (scaled from the field maps). The bottom panel (2x zoom) shows the mean-normalized percent-difference of the EPI images between the two breath-hold conditions (ΔEPIavg), with the red arrows pointing a region of improved consistency via shimming.

Proc. Intl. Soc. Mag. Reson. Med. 26 (2018)
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