Minimally invasive implantable NMR microcoils for in vivo metabolic profiling of microliter volumes

International audienceIntroductionThe use of implanted NMR microcoils still remains a relatively unexploited research area, without emerging or significant biomedical applications. The limitations inherent to implanted NMR coils derive obviously from the relatively weak detection sensitivity of NMR, hindering great challenges for nano-volume analyses. In addition, the necessity to preserve tissue during microprobe implantation imposes severe constraints on the geometry and structure of the NMR microcoil. In this study, we present in vitro and in vivo results obtained with innovative and minimally invasive microcoils.MethodsAn example of implantable NMR microprobe is shown in Figure 1. This filar-type architecture is based on the use of twisted copper microwires (diameter of 150 μm). The twisted wires are inserted inside a polyamide tubing (outer diameter of 380 μm). A biocompatible glue is used to seal the polyamide tube, while tuning and matching capacitors are connected to the two sides of the wire. For in vivo experiments, cannulae were stereotaxically positionned the day before the insertion of the NMR microcoils in the brain of male wistar rats. Experiments were performed at 7 T and 17.2 T. NMR spectra were acquired using a PRESS sequence. 3D MRI acquisitions were performed using a ZTE (zero echo time) sequence.Results/DiscussionThe quality factor of the loaded coils was ranging between 100 and 120. The full width at half maximum of water peak were measured to 6 Hz. In vivo results are illustrated in Figure 2 with a PRESS NMR spectrum obtained in the rat brain with a twisted microcoil (a volume coil was used for selective excitation). In this particular example of water-suppressed acquisition (240 averages, 10-minutes acquisition) at 7 Tesla, main peaks of brain metabolites (NAA, glu, gln, pCr, Cr, etc) can be easily identified and quantified. The ZTE MRI image (right side of Figure 1) shows the sensitive detection zone of the microprobe (volume evaluated to 500 nL) extending to about 200 μm away from the wire.ConclusionsThe MRS/MRI results obtained in vitro and in vivo illustrate the relevance of the microcoil design with respect to spectral resolution, detection sensitivity, spatial selectivity and limited invasiveness. Foreseen applications include the investigation of metabolism in microliter volumes in physiological conditions and in diseases with metabolic dysfunctions (tumoral environement, neurodegenerative pathologies, etc)