This article presents a dual-tuned (DT) radiofrequency (RF) coil for signal acquisition of 2 nuclei, namely, hydrogen ( 1 H) and sodium ( 23 Na), in the ultra-high magnetic field of a 7-T magnetic resonance imaging (MRI) system. The doublelayered dual-tuned (DLDT) coil comprises a 2-loop coil configuration per singlepair geometry, with the 1 H and 23 Na coils being located on the outside and inside, respectively. The 1 H and 23 Na single-pair elements are tuned to resonance frequencies of 297.20 and 78.61 MHz, respectively. The single-pair geometry of the DLDT coil is extended to an 8-pair configuration to cover the human head, and the operation mode is transmission/reception (Tx/Rx). The 8-pair DLDT Tx/Rx coil array is designed with a non-overlapped single pair between the 1 H coil elements for geometric decoupling, and capacitive decoupling is implemented to minimize the mutual inductance coupling. The 2 resonance frequencies are fed through a single RF port to a common matching board, and each frequency is selected using the voltages at both ends of a PIN diode. Through use of the PIN diode in the DLDT coil configuration, with a voltage drop at both ends, different resonance frequencies can be selected for each coil element in accordance with the diode ON/OFF state. The experiments conducted showed that the proposed DLDT coil is effective in acquiring signals of 1 H and 23 Na in the MRI system.
K E Y W O R D Sdouble-layered dual-tuned (DLDT) coil, magnetic resonance imaging (MRI), multinuclear, PIN diode
The purpose of this study was to develop a new double-layer coupled (DLC) surface radiofrequency (RF) coil using a combination of single-layer planar (SLP) and single-layer circular (SLC) coils, for enhancement of magnetic flux (B1 ) sensitivity and RF penetration in 7 T rat-body magnetic resonance imaging (MRI). The proposed DLC surface coil was fabricated according to an electromagnetic (EM) simulation and validated based on the B1 distribution and bench measurements. The DLC coil performance was quantitatively evaluated based on the signal-to-noise ratio (S/N) and coil-response signal intensity curves in phantom and in vivo rat-body images. In the computational EM calculation and 7 T in vivo experimental results, the DLC surface coil clearly showed an increased S/N and higher RF transmit (B1 (+) ) profiles, compared to those of the SLP and SLC coils. While all surface coils displayed a rapid decrease in the MR signal from the near-coil region to the subject, the results reveal that the DLC coil concept may be used to provide sufficient RF penetration and high S/N and degrees of freedom for use in partial body imaging for 7 T ultra-high-field small-animal MRI.
Radio wave propagation of three types of birdcage transmit/receive radiofrequency (RF) coils (lowpass filter, highpass filter, and bandpass filter configurations) was analysed for mouse body magnetic resonance imaging at main magnetic fields of 1.5, 3.0, 4.7, 7.0, 9.4, and 11.7 T (Larmor frequencies of 63.87, 127.74, 200, 300, 400, and 500 MHz) in terms of magnetic (|B 1 |) field sensitivity and homogeneity. The observed radio wave propagation in the central axial |B 1 | field, as influenced by the biological subject, was calculated numerically in the finite-difference time-domain method and compared with assessment criteria using the mean value and standard deviation. The results for different Larmor frequencies RF shielding cage configurations, and biological subject loading versus unloading were compared and are discussed in detail.
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