In vivo MR imaging (MRI) and spectroscopic imaging (MRSI) at ultrahigh static magnetic fields (7T and above) is proven to be advantageous due to its intrinsically high sensitivity and improved spectral dispersion [1
C and 23
Na MRSI combined with proton (1
H) imaging is a promising tool to depict metabolism process and intercellular information [11
]. To implement this multinuclear MR technology to ultrahigh field MR for improved sensitivity, a multinuclear RF coil is essential and critical for efficient MR signal excitation and reception of multinuclei involved, ultimately realizing the ultrahigh field advantages and ensuring correct spatial co-registration of signals from different nuclei without changing coils during imaging and spectroscopy examinations.
Birdcage coil is a mature and well-established volume coil structure for in vivo MR applications at relatively low field strengths (1.5T or below) [19
]. Based on this design, a variety of design approaches of double-tuned birdcage coils have been proposed for multinuclear MRI and MRSI studies. These approaches include inserting band-pass/band-stop filters [20
] or LC trap circuit [21
] into a the rungs and/or the end rings of the birdcage coil. A transformer coupled birdcage coil which consists of two coaxial birdcages has been developed as a quadrature double-tuned birdcage [24
], which provides nearly ideal quadrature performance in the low frequency mode. In order to achieve simultaneous, quadrature operation for both low and high frequencies, two four-ring birdcage configurations have been presented [25
]. Another method to double-tuned birdcage coil design is to tune the alternate rungs to alternate frequencies [26
], which can be treated as two imbricate coupled low-pass birdcages.
At ultrahigh magnetic fields, the required high frequency for proton imaging poses considerable technical challenges [2
] in designing RF coils based on the conventional birdcage-coil technology. Despite the cumbersome structure and some degree of design/construction difficulties, RF shielding is a commonly used approach to alleviate the low quality factors (Q-factors) and low B1
efficiency of volume coils due to the high resonance frequency. In the design of multiple-tuned volume coils required for multinuclear MR studies at ultrahigh fields, increased interaction between proton channel and non-proton channels, ultimately degrading the efficiency on MR sensitivity for both proton and non-proton nuclei, makes the design of such multi-modal volume coil more challenging. The design of multiple-tuned multi-modal volume coil becomes even more problematic in the case where Larmor frequencies of nuclei involved, e.g. 1
F and 13
Na, are not separate enough for the volume coil to establish clearly defined multi-modal resonance for each nucleus.
In this work, we explored the feasibility of designing a multinuclear volume coil based on the birdcage-coil design for rat 1H/13C/23Na MR studies at the ultrahigh field of 7T. The proposed multinuclear volume coil was first designed to operate at 7T 1H and 13C frequencies based on the alternative rung double-tuned birdcage coil technology. The third frequency for 23Na is achieved by simply changing the capacitance on each 13C rung of the volume coil. This approach is feasible because Larmor frequency of 13C is close enough to the frequency of 23Na, only some 4 MHz difference at 7T, and therefore the third resonance frequency for 23Na is easy to be reached from 13C frequency by tuning commercial-available trimmer capacitors. The Finite difference time domain method (FDTD) was employed to predict the resonance characteristics and the B1 field distribution. Bench test, phantom MRI and MRSI experiments were carried out to investigate the coil performance. The results demonstrated the proposed strategy for designing multinuclear volume coils for in vivo MR is advantageous in terms of MR sensitivity because it diminishes the signal losses caused by conventional multiple-tuned coils. In addition, it makes the multimodal volume coil design convenient in multinuclear MR applications in which Larmor frequencies in question are not separate enough.