Whole brain P MRSI at 7T with a dual‐tuned receive array

Abstract

Purpose:The design and performance of a novel head coil setup for 31 P spectroscopy at ultra-high field strengths (7T) is presented. The described system supports measurements at both the 1 H and 31 P resonance frequencies.Methods: The novel coil consists of 2, actively detunable, coaxial birdcage coils to give homogeneous transmit, combined with a double resonant 30 channel receive array. This allows for anatomical imaging combined with 31 P acquisitions over the whole head, without changing coils or disturbing the subject. A phosphate buffer phantom and 3 healthy volunteers were scanned with a pulse acquire CSI sequence using both the novel array coil and a conventional transceiver birdcage. Four different methods of combining the array channels were compared at 3 different levels of SNR. Results: The novel coil setup delivers significantly increased 31 P SNR in the peripheral regions of the brain, reaching up to factor 8, while maintaining comparable performance relative to the birdcage in the center. Conclusions: The new system offers the potential to acquire whole brain 31 P MRSI with superior signal relative to the standard options. K E Y W O R D Sdual-tuned, MRSI, 31 P, phosphorus, receive array, spectroscopy | INTRODUCTIONMagnetic resonance spectroscopy of the 31 Phosphorus nucleus can be used to explore several interesting metabolic processes in vivo, including energy metabolism, cell membrane turnover, and intra-and extra-cellular pH. 1-4 However, there are many challenges to acquiring high quality phosphorus spectra, particularly the low sensitivity of the 31 P nucleus relative to 1 H, and the generally low concentrations of the metabolites of interest, which result in low signal-to-noise ratio (SNR). Working at ultra-high field strengths offers a considerable boost to signal strength, for example Rodgers et al found a super-linear increase of 2.8× sensitivity in the heart when moving from 3T to 7T. 5 At the same time, the phosphorus nucleus is not as susceptible to the issues that affect proton MR at UHF, such as B 1 inhomogeneity. In addition, the increased spectral dispersion at UHF allows resolving closely spaced peaks, such as phosphocholine (PC) 766 | ROWLAND et AL.

RF Coil Setup for 31P MRSI in Tongue Cancer in vivo at 7 T

“…Recently, other dedicated 31 P coil setups have been demonstrated for 7 T for brain imaging ( 19 , 20 ). These state-of-the-art head coils have indicated more than a factor 3 in SNR performance gain when comparing their array with the typical volume coil.…”

“…This way, the 31 P setup could be substantially simplified, avoiding the need for adiabatic RF pulses and thus maintaining SAR guidelines within relatively short scan times. While other 31 P head coils (19,20) have been reported in literature, these were optimised for brain imaging.…”

9.4 T double‐tuned 13C/1H human head array using a combination of surface loops and dipole antennas

“…An additional ability to transmit and receive at the higher 1 H frequency is necessary for providing static magnetic field (B 0 ) shimming, spatial localization, and proton decoupling and/or application of polarization techniques in the case of 13 C nuclei. Proton decoupling as well as nuclear Overhauser enhancement can also be beneficial for other X-nuclei such as 31 P. Considering that RF coils require both local transmission and reception at UHF, the designs of DT coils, which often consist of several layers of transmit (Tx) and receive (Rx) resonant elements, [1][2][3][4][5][6] become quite complex. To simplify the DT coil development, usually the major design idea is to preserve high SNR and Tx efficiency (B 1 + /√P, where P is the RF input power) at the X-nucleus frequency, while the performance of the 1 H part of the coil is compromised intentionally.…”

“…There are two major options in designing a DT RF coil (or an array), ie double-tuning the same physical coil structure [7][8][9] (or each element of the array 1,6 ) to two different frequencies simultaneously or arranging two separate ST coils (or coil arrays) resonating at different frequencies close to each other. 1,[3][4][5]10 The first method allows optimization of the coil mainly at one frequency (commonly X-nuclei) while the Tx performance and SNR at the other frequency (commonly 1 H) is degraded.…”

Optimization of high-channel count, switch matrices for multinuclear, high-field MRI

“…In this context, designing a switch matrix suitable for operation with protons and the most common X-nuclei at high-field is desirable. It would, for example, enable the combined operation of a CP mode proton coil with a 32-channel sodium array in a system equipped with 32 receive channels or the connection of more advanced double-tuned arrays as described in [ 38 ]. Note, that switch matrix implementations operating at the proton frequency as well as at one or more X-nucleus frequency has been previously described in [ 39 ].…”