Abstract
13C magnetic resonance spectroscopy is a viable, non-invasive method to study cell metabolism in skeletal muscles. However, MR sensitivity of 13C is inherently low, which can be overcome by applying a higher static magnetic field strength together with radiofrequency coil arrays instead of single loop coils or large volume coils, and 1H decoupling, which leads to a simplified spectral pattern. 1H-decoupled 13C-MRS requires RF coils which support both, 1H and 13C, Larmor frequencies with sufficient electromagnetic isolation between the pathways of the two frequencies. We present the development, evaluation, and first in vivo measurement with a 7 T 3-channel 13C and 4-channel 1H transceiver array optimized for 1H-decoupled 13C-MRS in the posterior human calf muscles. To ensure minimal cross-coupling between 13C and 1H arrays, several strategies were combined: mutual magnetic flux was minimized by coil geometry, two LCC traps were inserted into each 13C element, and band-pass and low-pass filters were integrated along the signal pathways. The developed coil array was successfully tested in phantom and in vivo MR experiments, showing a simplified spectral pattern and increase in signal-to-noise ratio of approximately a factor 2 between non-decoupled and 1H-decoupled spectra in a glucose phantom and the human calf muscle.
Highlights
Magnetic resonance spectroscopy (MRS) is a viable, non-invasive method for the assessment of cell metabolism in skeletal muscle[1,2]; in particular, 13C-MRS can be applied for the investigation of glycogen, triglycerides, and different intermediates of glucose metabolism
A major criterion for the proper functionality of this coil was the electromagnetic isolation between 1H and 13C parts
This was achieved by a combination of the following measures: The number and geometric arrangement of the array elements was chosen in a way to minimize the mutual magnetic flux, and the cross-coupling
Summary
Magnetic resonance spectroscopy (MRS) is a viable, non-invasive method for the assessment of cell metabolism in skeletal muscle[1,2]; in particular, 13C-MRS can be applied for the investigation of glycogen, triglycerides, and different intermediates of glucose metabolism. The decoupling of the coils is achieved by geometrically arrangement, which on the other hand presents a disadvantage of this design: the elements cannot be positioned freely, and the number of elements, and the achievable FOV, is limited To overcome this obstacle, parallel LC or LCC trap circuits, previously used to double tune a single loop coil[22], have been introduced as alternative decoupling method between the 1H and 13C part[23]. These traps suppress current at the higher frequency when inserted in the lower frequency coil, and, in principle, enable free positioning of the elements This method has been successfully applied in a double-quadrature coil for calf muscle studies[24], and in initial work on a 4-channel 1H, and 4-channel 13C array for 13C-MRS in the brain[25]
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