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Rotating Linac-MR: B0 Orientation Invariant Quadrature Detection

R Barta1*, V Volotovskyy2, K Wachowicz1, B Fallone1, N De Zanche1, (1) Medical Physics, University of Alberta, Cross Cancer Institute, Edmonton, AB, CA, (2) Magnet Tx, Edmonton, AB, CA,

Presentations

(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose:
To demonstrate a 3-channel co-planar RF-coil array capable of gantry angle independent quadrature reception. This allows flexibility of positioning and orientation in a rotating B0 field linac-MR during multi-field and arc treatments.

Methods:
A stack of two orthogonal butterfly coils and one single-loop coil are positioned co-planar and exactly centered. Each coil is tuned to the resonant frequency of 20.56 MHz (for the 0.5 T Alberta Linac-MR) and decoupled from the other two. Bench measurements of Q-ratio and decoupling are made using a Rhode & Schwarz ZVL Network Analyzer. Imaging measurements are made on the Alberta Linac-MR with a cylindrical phantom. Gradient Echo images are acquired at gantry angles, and hence magnetic field orientations, of 0, 45, and 90 degrees.
Received signals from each channel are combined using root sum of squares weighted by the noise covariance (wRSS) for maximum SNR. Sample regions of the phantom are used to evaluate SNR variation for each channel and for the combined image for various positioning of the planar array.

Results:
The achieved decoupling between any two channels is better than -12 dB. Q of the single loop coil is 100, while the Qs of the horizontal and vertical butterfly coils are 80 and 95 respectively. Clear images of the phantom were acquired with various positioning of the phantom and coil. For a given slice the SNR varied less than 10% across all gantry angles, except for the sagittal slice at 45 degrees.

Conclusion:
The 3-channel array demonstrates 3-D reception by having good image quality regardless of gantry angle. The SNR varied from 5.4 to 16.7 for various slices, but across gantry angles the performance stayed constant. This demonstrates that gantry angle independent quadrature RF reception is possible with a planar array for imaging in a rotating B0 linac-MR.

Download ePoster [PDF]

Funding Support, Disclosures, and Conflict of Interest: Funding received from Natural Sciences and Engineering Research Council (NSERC RGPIN 04844), Alberta Cancer Foundation, Alberta Innovates, and Canadian Institutes of Health Research (Grant No. CIHR MOP 93752). Thank you to Alberta Health Services (AHS) for their continued support. Dr. Fallone, is a cofounder and chairman of MagnetTx Oncology Solutions.

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