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A Transceive Inductively Coupled Coil using Dual Wireless Coils for Small Animal Imaging at 15.2T1Department of Health Sciences and Technology, GAIHST, Gachon University, Incheon 21999, Korea, Republic of, 2Center for Neuroscience Imaging Research (CNIR), Institute for Basic Science (IBS), Suwon 16419, Korea, Republic of, 3Department of Biomedical Engineering, Sungkyunkwan University, Suwon 16419, Korea, Republic of
Synopsis
The ultrahigh field 15.2 T Magnetic Resonance Imaging (MRI) is characterized by high magnetic flux density (B1) sensitivity and uniformity. Therefore, it is mainly used for small animal images requiring high resolution. Generally, surface coils are used to obtain high B1 sensitivity to target brain at small animal MRI, but occasionally there is insufficient coverage to cover all of the Region of Interest, so that the desired information may not be obtained. In this study, we propose a coil with wider coverage using inductively coupling by adding dual wireless coils to one channel transceive primary coil at 15.2 T.
Introduction
The ultrahigh field 15.2T Magnetic Resonance Imaging (MRI) offer far better magnetic flux density (B1) sensitivity and uniformity. These advantages allow performing experiments on small animals to acquire high resolution brain images by using surface coils.1 However, a limitation of using a surface coil for whole brain MRI is the imaging depth which is proportionally to the coil diameter. Hence, the field coverage may not be sufficient to cover all parts of the brain.2 In this study, we present one channel transceive inductively coupled coil with a high coverage and sensitivity on the entire head of a small animal at 15.2T than a conventional small animal imaging coil.
Method
The proposed coil in this study consists of one primary coil (PC) and two secondary wireless coils (SC), and the diameter is respectively 20mm and 15mm to fit on the head of the mouse. Only the PC is connected to the source and is drive on transmit/reception mode. The other coils do not have a source and are driven by induced currents. Motion of the SC1 and SC2 produces mutual inductance changes by Faraday's induction law.3 Because it changes the impedance of the PC, wireless coils are fixed to prevent it. The SCs are designed to be 90 degrees on the left and right with respect to the PC. This setup has the advantage of accommodating the whole head of the mouse (Fig. 1). The PC consists of tuning and matching capacitors C1, CTM and CV and the capacitors of the secondary coils are C2 and CV (Fig. 2). The C1 and C2 capacitors had a value of 3.3pF and 10pF respectively. The CV and CTM are variable capacitors that tune and match the primary coil at 647.21 MHz and 50 Ω respectively. On the other hand, if the SCs are precisely tuned to 647.21 MHz, it is difficult to proceed with the fine tune by coupling the PC and SCs. Therefore, we use CV, variable capacitors to deliberately shift the frequency. Then, when the primary coil and the secondary coil were combined, a frequency shift of about 6 MHz occurred, and fine tuning was performed again.4 For the purpose of comparison, a surface coil, which is generally used for small animals, with the same diameter as the PC of the inductively coupled coil is also added in this study. Using these two types of coils, phantom and mouse images were acquired. We performed experiments with a 15.2T Bruker MR scanner (Paravision 6; Bruker Biospec, Billerica, MA) system. The T1 gradient echo sequences (GRE) were used for phantom images (parameters: FOVx = 15mm, FOVy = 15mm, TR = 3000ms, TE = 3.5ms, slice thickness = 0.5mm, filp angle = 60°, matrix = 192 x 192). The T2 spin echo sequences (SE) were used for mouse images (parameters: FOVx = 15mm, FOVy = 15mm, TR = 3000ms, TE = 30ms, slice thickness = 0.5mm, filp angle = 90°, matrix = 256 x 256, average = 10).Result
The signal sensitivity on the phantom was measured with GRE phantom image and a 1-D signal intensity profile (Fig. 3). The signal to noise ratio (SNR) is defined as the ratio of the mean value of the signal to the standard deviation of the noise using a 20 x 20 matrix. Lastly, the SE sequence was utilized to obtain a mouse brain image (Fig. 4).Discussion
The phantom 1-D signal intensity profile of the proposed coil (solid line) shows that the superior region is less sensitive than the surface coil, but has excellent sensitivity on both sides of the phantom. The SNR of the region close to the coil was 0.94 times better than that of the propose coil but the SNR of the L and R region was 2.77 times better. The SE mouse MR image reliably shows enhanced sensitivity on the lateral region of the head, including the bottom of the brain, when using inductively coupled coils.Conclusion
The proposed coil has a wider coverage and higher sensitivity than a common surface coil. If commercial surface coils are optimized with wireless coils, MRI users will have the convenience of getting better volumetric images.Acknowledgements
This work was supported by a grant of the Korea Health Technology R&DProject through the Korea Health Industry Development Institute(KHIDI), funded by the Ministry for Health and Welfare, Korea(HI14C1135) and a grant (HO16C0004) from Osong innovation centerfunded by the ministry of health & welfare, the republic of Korea.References
1. Han, Sohyun, et al. Gradient‐echo and spin‐echo blood oxygenation level–dependent functional MRI at ultrahigh fields of 9.4 and 15.2 Tesla. Magnetic resonance in medicine, 2018; 00: 1-10.
2. Zhang, Xiaoliang; Ugurbil, Kamil; Chen, Wei. Microstrip RF surface coil design for extremely high‐field MRI and spectroscopy. Magnetic Resonance in Medicine: An Official Journal of the International Society for Magnetic Resonance in Medicine, 2001; 46(3): 443-450.
3. Park, Bu S., et al. Improvement of 19 F MR image uniformity in a mouse model of cellular therapy using inductive coupling. Magnetic Resonance Materials in Physics, Biology and Medicine, 2018; 00: 1-9.
4. Nabuurs, Rob JA, et al. High‐field MRI of single histological slices using an inductively coupled, self‐resonant microcoil: application to ex vivo samples of patients with Alzheimer's disease. NMR in Biomedicine, 2011; 24(4): 351-357.
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