Abstract
In order to study metabolic processes in animal models of diseases and in patients, microdialysis probes have evolved as powerful tools that are minimally invasive. However, analyses of microdialysate, performed remotely, do not provide real-time monitoring of microdialysate composition. Microdialysate solutions can theoretically be analyzed online inside a preclicinal or clinical MRI scanner using MRS techniques. Due to low NMR sensitivity, acquisitions of real-time NMR spectra on very small solution volumes (μL) with low metabolite concentrations (mM range) represent a major issue. To address this challenge we introduce the approach of combining a microdialysis probe with a custom-built magnetic resonance microprobe that allows for online metabolic analysis (1H and 13C) with high sensitivity under continuous flow conditions. This system is mounted inside an MRI scanner and allows performing simultaneously MRI experiments and rapid MRS metabolic analysis of the microdialysate. The feasibility of this approach is demonstrated by analyzing extracellular brain cancer cells (glioma) in vitro and brain metabolites in an animal model in vivo. We expect that our approach is readily translatable into clinical settings and can be used for a better and precise understanding of diseases linked to metabolic dysfunction.
Highlights
The proton and carbon-13 microcoils are respectively interfaced and positioned inside magnetic resonance imaging (MRI) scanners operating at B0 = 7.0 T and B0 = 4.7 T (Fig. 1A,B), respectively
The limits of detection (LOD) for the probes, corresponding to a signal-to-noise ratio (SNR) of 3, are 1.8 nmol in 60 s for the methyl protons of lactate and 10 nmol in 100 s for the carbon atoms in an enriched 3-13C lactate sample
The 1H channel can be utilized for proton decoupling in 13C experiments to further enhance the sensitivity, this being useful for a 13C-enriched dialysate solution
Summary
Micro-structured NMR/MRS probes have been fabricated in a variety of different geometries[15,16,17,18,19,20,21,22,23,24,25,26,27,28] and have partly been applied to continuous flow investigations[29,30] with solenoidal coils being the most sensitive to date[15,16]. The same experiment was performed at B0 = 7 T with the 1H probe (Fig. 2B) by averaging over less than 4 minutes, and resulted in a LOD of 0.9 mM (900 pmol in 1 μL) for three protons of the methyl group of the 13C-enriched metabolites lactate, alanine and pyruvate (linewidth of 20.0 Hz) and an LOD of 1.4 mM for the two glutamine protons Based on these LODs, single proton species are expected to be detectable if the concentration exceeds 2.7 mM (2.7 nmol in 1 μL). Further increases in resolution and sensitivity can be expected by utilizing higher order shims, by increasing the microcoil volume or by building microcoils for MRI scanners operating at higher magnetic fields The application of this combined microprobe and microdialysis systems is limited to the brain study, but it can be implemented in other organs or living tissues. As microdialysis probes are being used in vivo in humans[4] and microcoil probes are being utilized for human exploration[18], we further expect that the MR approach presented here could be applied in clinical settings
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