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Loop Array RF Coil for Vertical Field MRI using Loop/Dipole Parallel RF Coils1Research & Development Group, Hitachi, Ltd., Tokyo, Japan, 2Healthcare Business Unit, Hitachi, Ltd., Tokyo, Japan
Synopsis
To increase the SNR of an RF array coil for vertical field MRI, a loop/dipole parallel RF (LDP) coil was developed. The SNR was increased by forming a solenoid shaped current on an array coil using the LDP coils. The performance of the coils was evaluated using an electromagnetic simulator. The SNR of the LDP coil was 9% better than that of a conventional loop array coil. This technique will contribute to increasing the SNR of the array coil for vertical field MRI.
Introduction
Multi-channel RF-array coil have a high SNR and a low g-factor by high density placement on the surface of the sample 1, 2. In horizontal field MRI, the number of channels has increased rapidly because a coil that provides high sensitivity when installed on the surface of the sample can be used. On the other hand, in vertical field MRI, the number of channels has not increased because the placement of the antenna that can obtain sensitivity is limited.3 Therefore, arranging the coil at high density is not possible, and an array coil with a high SNR cannot be achieved. The sensitivity in the deep region is particularly low, and it becomes a problem. To alleviate this problem, we have developed a loop/dipole parallel RF (LDP) coil for vertical field MRI and evaluated its performance using an electromagnetic simulator.Methods
The basic configuration of a single-channel LDP coil is shown in Fig. 1 (a). In the LDP coil, a loop coil and a dipole antenna were connected in parallel, and both ends of the dipole antenna partially overlap. We aimed to improve the deep region sensitivity by passing a solenoid shaped current to the dipole antenna. The loop coil is arranged along the subject, and the dipole antenna is arranged around the subject where Z is the direction of the magnetic field (B0). To form a solenoid shaped current at the dipole antenna, the resonance frequency of the dipole antenna is adjusted to about 15–35% higher than that of the loop coil. Figure 1 (b, c) shows a sensitivity map of the LDP coil and a conventional same-size loop-array coil arranged at the X-Y plane where sensitivity is low at the vertical magnetic field. The LDP coil can obtain higher sensitivity in the deep region than it can in the conventional loop-coil by the sensitivity of the solenoid shaped current. An element block diagram of a multi-channel (16 ch) array coil using LDP coils is shown in Fig. 2 (a). Each loop coil of the LDP coil was connected to the single dipole antenna via a capacitor. The magnetic coupling of adjacent antennas is removed with a shared capacitor. This antenna is arranged so that the dipole antenna makes two turns around the head, and ch 1 (ch 9) and ch 8 (ch 16) are arranged so that part of the coil partially overlaps in order to magnetically decouple. Furthermore, the top of the head was transformed into a dome shape (Fig. 2 (b)). We numerically calculated the E-fields and sensitivity of the coils using an in-house simulator on the basis of the method of moments.4 A cylinder phantom (L=280 mm, D=160 mm, $$$\sigma$$$= 0.58 S/m, $$$\epsilon _r$$$=89) was used. The coils were tuned/matched to f0 (49 MHz@1.2 T). The SNR of the LDP array-coil and that of the conventional same-size loop-array coil were compared. The SNR was calculated by SNR = $$$\sqrt{\bf{S}^{\ast}\bf{R}^{-1}\bf{S}^{t}}$$$, where $$$\bf{S}$$$ is a vector of each coil sensitivity at the same pixel, and $$$\bf{R}$$$ is the noise correlation matrix calculated from E-fields.5Results and Discussion
The simulation results of the single-channel LDP-coil current-magnitude is shown in Fig. 3 (a). This figure shows that a current similar to a solenoid shape was flowing in addition to the conventional loop-coil current. Figure 3 (b, c) shows the sensitivity maps of the single-channel LDP-coil and the conventional loop-coil that was placed in parallel and perpendicular to the magnetic field. Both channels generated solenoid-coil shaped sensitivity, and a higher sensitivity was obtained in the deep region than in the conventional loop-coil. Fig. 4 (a) shows 16-ch combined SNR maps, and Fig. 4 (b, c) show the Y-axis SNR line profiles of the deep region (central 30-mm circular area) and wide area (central 146-mm circular), respectively. The deep region SNR of the LDP coil was up to 1.08 times higher than that of the conventional loop-coil. On the other hand, the increase in SNR is only 1.03 times higher, and the SNR at both ends of the phantom decreased slightly. These simulation results showed that the signal detection capability of the coil could be gathered in the deep region where it is difficult to obtain the signal by using the LDP coil. This technique is effective for reducing the deep region sensitivity of the loop array-coil.Conclusion
The SNR of the developed LDP coil is 9% better than that of the conventional-loop array-coil. This technique will contribute to increasing the SNR of the array coil for vertical field MRI.Acknowledgements
No acknowledgement found.References
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