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Design of 5-channel On-coil Shimming Coil for Rat Brain MRI Guided High Intensity Focused Ultrasound: a preliminary study1Lauterbur Imaging Research Center, Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences, Shenzhen City, People's Republic of China, 2Department of Radiology and Biomedical Imaging, University of California San Francisco, CA, United States, 3UCSF/UC Berkeley Joint Graduate Group in Bioengineering, San Francisco, CA, United States
Synopsis
Signal-to-noise ratio (SNR) and homogeneity of the static magnetic field B0 are critically important in achieving accurate temperature measurement for MRI guided HIFU. In this work, a 5-channel shimming coil on the top of a 3-channel RF coil array at 3T has been fabricated to increase the B0 homogeneity and the SNR in region of interest. Phantom studies have demonstrated the the capability of the proposed shimming coil to improve B0 homogeneity and SNR of the images, which will lead to accurate temperature mapping.
Introduction
MRI guided high intensity focused ultrasound (HIFU) has great potentials for image-guided thermal therapy due to MRI’s unique capability of providing accurate and real-time target localization and temperature monitoring1,2. MR signal-to-noise ratio (SNR) and homogeneity of the static magnetic field B0 are critically important in achieving accurate signal localization and temperature measurement. Due to the off-center position of rat in the HIFU system, the B0 homogeneity need to be improved to achieve accurate temperature mapping. In this work, we designed and constructed a 5-channel shimming coil on the top of a 3-channel RF coil array at 3T3, to increase the B0 homogeneity and the SNR in rat brain.Methods
In order to minimize negative impact to the imaging performance of the 3-channel RF coil array, the shimming coil was made and placed outside the RF coil array as shown in Figure 1. A 6 cm diameter hole was opened on the coil former to deliver the ultrasound wave and also ensure the accuracy of HIFU4,5. One of the five shimming coil was built around the hole as shown in Figure 2. The other four coils were made in ellipse-shape placed on the surface of the coil former with the dimensions of 7.5cm and 5cm. Since the requirement of DC wire for the shimming coils, a 14cm height support well was built to frame up the wire, which saves the space for animal experiments. An user interface was custom-developed to control the power supply with -5A ~ 5A DC current range and 1mA accuracy of the shimming coils, The interface is capable of fine adjusting the current for each shimming coil channel.
Phantom study: In phantom imaging experiment, the B0 distribution was measured by using a 2D dual echo GRE sequence with following parameters: TR=15ms, TE1=2.66ms, TE2=5.45ms, TE3=8.24ms, Bandwidth=780Hz/pixel, FOV=160×160mm2, Slice thickness=1.25mm, Acquisition matrix=128×128 , Flip angle=10˚, Slice number=104, The B0 distribution was then calculated by subtraction of the unwrapped phase maps of the two echoes. The B0 distribution was measured when shimming coil was turned off and on respectively.
SNR is measured by using a 2D GRE sequence (TR=300ms,TE=10ms, Bandwidth=130Hz/pixel, FOV=150×150mm2, Slice thickness=3mm, Acquisition matrix = 256×256 ,Flip angle=60˚,Slice number=1, Measurement=1), and the noise map was obtained by setting transmit power to zero. The SNR of the images is calculated according to reference6. Transverse plane and sagittal plane was acquired to measure both SNR and B0 homogeneity in this study.
Results
As shown in Figure.4, the B0 homogeneity (standard deviation/mean) in the region of interest in transverse and sagittal planes are improved with from 1.21 to 0.72 and from 0.82 to 0.68 respectively when shimming coils turn on., at At the same time, SNR and homogeneity is higher improved around 5% and 17% respectively.as well. Since the accuracy of temperature mapping is proportion to SNR, the accuracy of temperature mapping is improved accordingly.Conclusion
In this work, a 5-channel shimming coil was fabricated to improve B0 homogeneity and accuracy of temperature mapping for MRI guided HIFU system. Preliminary phantom study has demonstrated the capability of the proposed shimming coil to improve B0 homogeneity and SNR of the images, which will lead to accurate temperature mapping. Further work will be focused on optimizing algorithm of shimming current in the coils to achieve best shimming condition and in-vivo studies.Acknowledgements
This work is supported in part by national key R&D programno. 2016YFC0100100, national grants no. 51307171, 61571433,61401450, 81470077 and 2013CB733800/2013CB733803, provincialgrants no. 2015B020214006 and 2014A030310200, city grant no.KQJSCX20160301143250, CYJ20140417113430589,JSGG20141020103440414 and internal grant no. 201314.References
[1] Ryan Alkins, et al., Cancer Research 2013, 73(6): 1892-1899. [2] Xuegang Xin, et al., Magn. Reson. Eng.2010, 37B(4): 237-244. [3] Xiao Chen, et al.,ISMRM2016 p.2118.[4] Mihaela Rata, et al., Phys. Med. Biol. 2008, 53: 6549-6567. [5]Costas D Arvanitis, et al., Phys. Med. Biol. 2013,58: 4749-4761.[6] Keil B, et al., J Magn Reson 2013
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