For the analysis of metabolites it can be challenging to obtain enough sample volume for the detection in regular high field NMR spectrometers. Microcoil detection can be used to obtain a higher mass sensitivity than with regular high-field spectrometers and to measure small quantities of metabolites with high sensitivity.¹ However, handling of small sample volumes (≤ 1 µL) is difficult and typically, samples containing metabolites of interest need to be collected and can only be analyzed at a later point of time.² In order to monitor metabolic responses in real-time inside an imaging scanner we have developed a method that combines a sensitive microcoil for detection with a microdialysis for sample extraction. In this way it is possible to measure metabolic responses in vitro and in vivo under continuous flow conditions in real-time without the need of a dedicated spectroscopy magnet.Two microcoil probes were built, one for ¹³C (at B0 = 4.7 T, which corresponds to 50 MHz carbon frequency) and the second one for ¹H detection (at B0 = 7 T, which corresponds to 300 MHz proton frequency). An example probe is depicted in figure 1. In order to optimize for the signal-to-noise ratio, the NMR-coils were designed to have a filling factor of 93%. This was achieved by winding double parallel coils (30 µm or 62.5 µm diameter of the wire) with a length of 2.4 mm around a susceptibility matched polyamide tube (O.D. 780 µm, ID 750 µm). In addition, the coils were placed inside a tube containing fluorinated liquid which is matched to the susceptibility of the copper wire.1 Both detectors were designed as double resonant probes. For the ¹³C resonator the same NMR coil is used for ¹³C and ¹H detection and excitation. In case of the 1H resonator two independent circuits are utilized one that contains a microcoil for 1H excitation and detection and the second one containing a surface coil matched to the ¹³C frequency in the respective magnetic field. The inlet of the microcoil is connected to the outlet of a microdialysis probe, which sits inside a buffer solution containing U87 human glioma cells or inside the cortex of a rat brain. The inlet of the microdialysis is connected to a syringe filled with additional buffer solution which is pumped under continuous flow through the microdialysis probe to collect extracellular metabolites. The cell suspension was infused with ¹³C labeled 3-¹³C-pyruvate (150 mM) and the metabolites measured under continuous flow. ¹H and ¹³C experiments were performed by averaging over 3.5 minutes and 15 minutes respectively on a Bruker Biospec 7 T or 4.7 T.For the carbon probe, the proton channel is only used to shim the field within the sample volume and serves for the decoupling of protons. With the proton probe, shimming is performed directly via the proton channel. The carbon channel is however used to distinguish between resonances that originate from infused ¹³C labeled material or a different source. The sensitivity of the two probes was firstly tested in vitro. For ¹³C labeled lactate a limit of detection (LOD) of 10mM (10 nmol) was achieved within 100 s with the 13C probe. The 1H limit of detection of the methyl group (thee protons) of unlabeled lactate amounts to 1.8 mM (1.8 nmol) in 60 seconds for the 1H probe. In vitro experiments under continuous flow conditions within the timescale of the experiments (3.5 minutes for protons) resulted in a proton LOD of 0.8 mM (800 pmol) of metabolites originating from 3-¹³C pyruvate. Carbon experiments yielded a LOD of 3.3 mM (3.3 nmol) on a 15 minutes timescale under continuous flow. Figure 2 summarizes the results for both experiments. The conversion of pyruvate into alanine, glutamine and lactate is observed under continuous flow in both cases and was confirmed by high resolution NMR at 500 MHz. Furthermore, the performance of the microcoil was tested in vivo on a healthy rat to detect normal brain metabolism, using the same experimental setup. A preliminary essay demonstrated the presence of 8 mM endogenous lactate in the dialysate.We have demonstrated a new approach that combines the use of microcoil detection and a microdialysis to measure extracellular metabolites inside a MRI scanner in real-time. We expect the applications of this approach numerous since it may be readily applied to in vivo human measurements and opens pathways to measure immediate metabolic responses.