Proton MRS on sub‐microliter volume in rat brain using implantable NMR microcoils

Abstract

The use of miniaturized NMR receiver coils is an effective approach for improving detection sensitivity in studies using MRS and MRI. By optimizing the filling factor (the fraction of the coil occupied by the sample), and by increasing the RF magnetic field produced per unit current, the sensitivity gain offered by NMR microcoils is particularly interesting when small volumes or regions of interest are investigated. For in vivo studies, millimetric or sub-millimetric microcoils can be deployed in tissues to access regions of interest located at a certain depth. In this study, the implementation and application of a tissue-implantable NMR microcoil with a detection volume of 850 nL is described. The RF magnetic field generated by the microcoil was evaluated using a finite element method simulation and experimentally determined by high spatial resolution MRI acquisitions. The performance of the microcoil in terms of spectral resolution and limit of detection was measured at 7 T in vitro and in vivo in rodent brains. These performances were compared with those of a conventional external detection coil. Proton MR spectra were acquired in the cortex of rat brain. The concentrations of main metabolites were quantified and compared with reference values from the literature. In vitro and in vivo results obtained with the implantable microcoil showed a gain in sensitivity greater than 50 compared with detection using an external coil. In vivo proton spectra of diagnostic value were obtained from brain regions of a few hundred nanoliters. The similarities between spectra obtained with the implanted microcoil and those obtained with the external NMR coil highlight the minimally invasive nature of the coil implantation procedure. These implantable microcoils represent new tools for probing tissue metabolism in very small healthy or diseased regions using MRS.

Implantable theranostic device for in vivo real-time NMR evaluation of drug impact in brain tumors

“…The NMR detection volume reported here for the implanted device was found to be similar to those measured previously 14 , 15 , of the order of 1 μL, when evaluated by FEM modeling or measured by MRI. Comparison of the detection sensitivity obtained with the microcoil of the implanted device and a commercial surface coil designed for rat brain confirmed the importance of miniaturization to achieve sufficient sensitivity in the vicinity of the capillary used for CED.…”

“…We have previously reported results obtained with brain-implanted microcoils of comparable size in healthy and tumor-bearing rats 14 , 15 . In these studies, the concentrations, relative to tCr, of tCho, glutamine, glutamate, lactate, mI, tNAA and taurine obtained in the brain of healthy animals with the implanted microcoil were similar to those reported in the literature using standard extracorporeal NMR coils.…”

Implantable theranostic device for in vivo real-time NMR evaluation of drug impact: application to brain tumors

“…A microcoil configuration close to the implanted microcoil of the device was previously investigated 12,13 . The results reported on healthy animals and tumor-bearing animals demonstrated the feasibility of the approach and the key role played by the miniaturization of the coil for enhancing the detection sensitivity on very small volume.…”

“…One solution to this sensitivity limitation is to design a dedicated NMR coil whose dimensions correspond to the volume to be examined, and to position it as close as possible to the region of interest. Reducing the size of the RF coil advantageously translates into an increase in coil sensitivity per unit volume, while positioning it in close proximity to the region of interest optimizes detection sensitivity in this region [9][10][11][12][13] .…”

Project-Oriented RF Coil Comparison and Optimization for Preclinical, Single-Voxel MR Spectroscopy of the Rat Visual Cortex at 9.4 T

“…Nevertheless, in principle, using multi-channel Rx arrays may also be advantageous in MRS since a more homogeneous Tx field and highly sensitive Rx signal might be achieved [44]. If practically feasible, using implanted micro-coils [45], [46], [47] and cryocooled coils [48], [49], [50], [51], [52] would also be beneficial to enhance SNR in very small and specific areas.…”