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Transcranial focused ultrasound localization in Money at 3T
Yangzi Qiao1, Chao Zou1, Jo Lee1, Xiaojing Long1, Teng Ma1, Changjun Tie1, Xin Liu1, and Hairong Zheng1

Synopsis

Focused ultrasound has been a fast developing technology for non-invasive neuromodulation. Monkey is a good animal model for studying the mechanism of focused ultrasound on neural network. However, the ultrasound beam would be greatly attenuated and distorted by the monkey skull, making the precise focal localization a crucial step. MR-ARFI with little thermal effect was an attracting tool for focus localization. In this study, we investigated the transcranial MR-ARFI in monkey at 3T system. With high performance of the customer designed monkey head coil and the strong gradient system, reliable and repeatable results were obtained.

Introduction

Focused ultrasound neuromodulation has been a hot topic as ultrasound wave can be non-invasively focused into brain [1] . Monkey is one of the most important and widely used non-human primate animal model in neuromodulation research. However, the thickness of monkey skull was around 2mm, which would greatly attenuated and distorted the transmitted ultrasound. It is therefore crucial to precisely localize the transcranial ultrasound beam. MR-ARFI is a powerful focused ultrasound localization method, as it can encode the micro-scale displacement into phase variation with very little thermal effect [2]. The biggest challenges of transcranial MR-ARFI in monkey are the imaging SNR and displacement sensitivity. MR-ARFI in monkey was once reported at 7T system [3]. In this study, we investigated MR-ARFI in monkey at 3T system, which is more clinically common. With the high performance of customer designed monkey head coil and strong gradient system, reliable and repeatable results were obtained.

Materials and methods

All MRI scans were performed on a 3T MR system (uMR790, Shanghai United Imaging Healthcare, Shanghai, China) with the maximal gradient amplitude as strong as 84mT/m. To mitigate the ultrasound attenuation, a customer-designed 300 kHz single element focused ultrasound transducer was used to induce local displacement (Fig. 1(a)). The focal depth of the transducer was 6cm. The axial focal length of the transducer was 3cm. The input electrical power was set to 170W for the ARF generation. The duty cycle of the ultrasound pulse was 2.1%, which would cause very little thermal effect. MR-ARFI data were acquired using a spin echo sequence. The imaging parameters were: TR/TE = 1000ms/45ms, Resolution = 2.0*2.0*5.0mm3, Matrix = 64*64, BW =800Hz/pixel. The amplitude of the repeated bipolar motion encoding gradient (MEG) G = 75mT/m, with duration $\tau$ = 15mT/m. Two measurements with opposite polarity of MEG ( $\phi^{+}$ and $\phi^{-}$) were acquired as a pair to improve displacement sensitivity [4]. The reference MR-ARFI images ($\phi_b^+$ and $\phi_b^-$) were obtained with focused ultrasound off. The displacement was quantified by: $\frac{\left(\phi^{+}-\phi^{-}\right)-\left(\phi_b^+-\phi_b^-\right)}{2\gamma G\tau}$. A customer-designed 8-channel coil (Fig. 1(b)) with the ultrasound transducer inserted in the hole was placed close to the monkey skull (Fig. 1(c)), providing high SNR images in all three orientations [5]. The MR-ARFI experiment with IRB approval was repeated in two monkeys.

Results

Fig.2 shows the MR-ARFI results in two monkeys overlaid on the anatomic image. The yellow line indicated the position of the coronal plane. The peak displacement in Monkey 1 was 0.29μm in transversal plane (Fig.2 (a)) and 0.27μm in coronal plane (Fig.2 (b)). The peak displacement in Monkey 2 was 0.59μm in transversal plane (Fig. 2(c)) and 0.51μm (Fig.2 (d)) in coronal plane. The SNR of the magnitude image in the brain was over 100, making the acquired displacement reliable.

Discussion and conclusion

The repeatable transcranial MR-ARFI results were obtained in two monkeys. The peak displacement in transverse plane and coronal plane demonstrated high accordance. With the same input power, the peak displacement in monkey 2 was nearly twice that in monkey 1. Without phase correction, the little variation in the thickness of monkey skull and ultrasound wave incident position would great affectly the acquired peak displacement. MR-ARFI in monkey was reported at 7T system previously, as high field system permits better SNR. In our study, the customer designed monkey head coil helps acquire high SNR images. More importantly, with the strong gradient system, the displacement sensitivity was greatly increased while the TE was still kept short, making the displacement map from ARFI images reliable. In conclusion, the transcranial MR-ARFI in monkey were successfully implemented at a clinical 3T system.

Acknowledgements

This work was supported by the Key Laboratory for Magnetic Resonance and Multimodality Imaging of Guang-dong Province (No. 2014B030301013), the National Natural Science Foundation (Nos. 81327801, 81527901,11504401)

References

[1] Tufai Y, Yoshihiro A, Pati S, Li MM, Tyler WJ, Ultrasonic neuromodulation by brain stimulation with transcranial ultrasound, Natue protocols, 2011; 6(9): 1453

[2] McDannold N, Maier SE, Magnetic resonance acoustic radiation force imaging, Medical physics, 2008; 35(8): 3748-3758

[3] Jonathan SV, Phipps M, Chaplin VL, Singh A, Yang PF, Newton AT, Gore JC, Chen LM, Caskey CF, Grissom WA, Optical tracking-guided MR-ARFI for targeting focused ultrasound neuromodulation, The ISMRM 25th Annual Meeting & Exhibition Paris, France 2018

[4] Chen J, Watkins R, Pauly KB, Optimization of encoding gradients for MR-ARFI. Magnetic resonance in medicine. 2010; 63(4):1051-1058

[5] Lee J, Yang X, Chen QY, Tie CJ, Long XJ, Li Nan, Zhang XL, Zheng HR, Li Y, Development of 8-channel head array for MRI guided monkey ultrasound stimulation, The ISMRM 25th Annual Meeting & Exhibition, Paris, France, 2018

Figures

Figure. 1 (a) 300kHz focused ultrasound transducer was aligned in the middle of the monkey coil. (b) The coil array structure of 8-channel monkey coil. (c) The photograph of system set-ups

Figure. 2 Transcranial MR-ARFI in 2 monkeys. The yellow line indicates the position of the coronal plane

Proc. Intl. Soc. Mag. Reson. Med. 27 (2019)
3815