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A flexible 11-channel coil array for MR-guided high-intensity focused ultrasound (HIFU) studies on rabbit leg muscle at 3 T1Lauterbur Imaging Research Center, Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences, Shenzhen, China, 2High Field Magnetic Resonance Brain Imaging Key Laboratory of Sichuan Province, School of Life Science and Technology, University of Electronic Science and Technology of China, Chengdu, China, Chengdu, China, 3Department of Radiology and Biomedical Imaging, University of California San Francisco, San Francisco, CA, United States, 4UCSF/UC Berkeley Joint Graduate Group in Bioengineering, San Francisco, CA, United States
Synopsis
Signal-to-noise
ratio (SNR) of the radio frequency (RF) receive coil array is a critical factor
affecting the accuracy of temperature measurement in MR-guided high-intensity focused ultrasound (HIFU)
for local heating. In this work, a flexible 11-channel coil array was designed,
constructed and evaluated for MR-guided HIFU studies on rabbit leg muscle at a
3 T MRI scanner. Compared to a commercial available 4-channel flexible coil
array, the dedicated 11-channel coil array provides improved performance in
SNR, parallel imaging capability, and the accuracy of temperature measurement.
Introduction
MR-guided high-intensity focused ultrasound (HIFU) is an efficient noninvasive technique for local heating by using MRI thermal maps1. Based on the proton resonance frequency shift (PRFS), continuous temperature mapping for real-time therapy control can be provided with a good spatial and temporal resolution, if efficient rapid imaging strategy is applied2. To achieve rapid imaging, phase array coils with high parallel imaging capacity are applied. In this work, we have designed and built a flexible 11-channel coil array for MR-guided high-intensity focused ultrasound (HIFU) studies on rabbit leg muscle at a 3 T MRI system (TIM Trio, Siemens, Erlangen, Germany). The coil performance was characterized with amplified SNR and augmented parallel imaging capability, and improved temperature mapping in rabbit studies, in comparison with that of a commercially available 4-channel coil array (Flex Small 4).Methods
The layout and photo of the flexible 11-channel coil are shown in Fig. 1. A single channel loop with a size of 70 mm x 70 mm, and a 62 mm x 62 mm hole in octagon shape was placed on the bottom of phantom or rabbit leg for HIFU path. A flexible 10-channel coil array was covered the top of phantom or rabbit leg. Here, the decoupling methods of employing low input-impedance preamplifiers were applied between the single loop and the 10-channel coil array. For the 10-channel coil array, the overlapped decoupling method was applied in the adjacent elements, and applied with employing low input-impedance preamplifiers or two overlapping self-made inductances for the non-adjacent elements. Each element was tuned to 123.2 MHz and 50 ohm impedance-matched to minimize the noise of the preamp.
The phantom images in the transverse, sagittal and coronal planes were acquired with a 2D GRE sequence (TR/TE=300 ms/10 ms, flip angle=30o, slice thickness=5 mm, Matrix size=128 x 128), and the noise images were acquired with the same sequence when the transmit voltage was set to zero. The results can be applied to compute covariance weighted SNR and SENSE g-factor map3. For PRFS temperature mapping on rabbit leg muscle studies, a 3D GRE sequence was applied to calculate the temperature change4 with parameters: TR/TE=8 ms/4.2 ms, FOV=192 mm x 192 mm, slice thickness=5 mm, slices=16, matrix size=96 x 96, bandwidth=700 Hz/Px, measurement=20.
Results
The SNR distributions of phantom images in the transverse, sagittal and coronal planes are showed in Fig. 2, from which it can be known that the SNR of the 11-channel coil array is about double that of the 4-channel coil in the region of interest (ROI). The inverse g-factor maps with different acceleration factors in the transverse plane was depicted in Fig. 3. Obviously, the 11-channel coil array showed a better parallel imaging capability than that of the 4-channel coil array, particularly at acceleration factor R=2 x 2. The temperature maps and the temperature change that were measured under situations of un-accelerated imaging and accelerated imaging with R=2 x2 are illustrated in Fig. 4. Known from the temperature change, the accuracy of temperature measurement by using the flexible 11-channel coil array shows a much better performance in rapid imaging than that of the 4-channle coil array.Conclusion
A flexible 11-channel coil array for MR-guided high-intensity focused ultrasound (HIFU) studies on rabbit leg muscle at 3 T was designed, constructed and evaluated by phantom studies and in-vivo studies. Compared to a commercial 4-channel coil array, the 11-channel coil system achieves better performance in MR SNR, acceleration capacity and the accuracy of temperature measurement in rapid imaging.Acknowledgements
This work was supported in part by NSFC under Grant No. 61571433, 61801466, 81627901, 81527901; Guangdong Province grants 2014A030312006, and 2014B030301013; Youth Innovation Promotion Association of CAS No. 2017415; city grants JCYJ20170413161314734; NIH U01EB023829, and a Pengcheng Scholar Award.References
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2. C. M. Tempany, E. A. Stewart, N. McDannold, B. J. Quade, F. A. Jolesz, and K. Hynynen. “MR imaging-guided focused ultrasound surgery of uterine leiomyomas: a feasibility study,” Radiology, vol.226, no.3, pp. 897-905, Mar. 2003.
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