Current treatment and diagnosis of pain conditions are dependent on self-reported measures. The objective of this study was to establish the feasibility of determining changes in excitatory neurotransmitter concentrations (glutamate) in the anterior cingulate cortex (ACC) as an objective measure of pain using dynamic single voxel magnetic resonance spectroscopy (MRS). Glutamate levels can accurately be detected with this paradigm, although a general trend in relation to pain was not observed across subjects. This is the first study to report dynamic levels of glutamate in the ACC in relation to pain in healthy individuals using optimized MRS acquisition and processing methods.
Study participants: 5 healthy participants (4F/1M, mean age=24, range:19-45yrs) with no known clinically diagnosed pain condition were studied.
Pain intervention: Each participant received the painful intervention twice, the first time outside of the scanner, on a different day, to avoid novelty effects. The intervention involved the application of topical capsaicin-cream (0.0075%), the active component of chili peppers. After baseline MR data collection, the cream was placed on the inner part of the right forearm and left for 10-minutes, allowing absorption. During this period, a 10-minute dynamic MRS scan was collected (Dynamic1, PRE-HEAT). Then an activated heat-pack (48.7 ± 0.27°C) was placed on the inner forearm for 2 minutes, further activating the capsaicin-cream. This was followed by another 10-minute dynamic scan (Dynamic2, POST-HEAT). The participants remained in the bore of the magnet and provided pain ratings (NRS-11) intermittently (Figure1).
MR Experiments: 3T(Philips Achieva, Best, Netherlands) data was collected with a Transmit-Receive (T/R) head coil including: 3DT1 for voxel placement and tissue segmentation (MPRAGE, sequence 1mm³ isotropic resolution, TE/TR=3.5/7.7ms, shot interval=1800ms, inversion time=808ms, FOV(ap/rl/fh) =256/200/150mm³), 1H-MRS (PRESS, TE/TR/NSA=22/4000/32, ACC voxel dimensions= 30/25/15mm3 =11.2mL, second order shimming, 16-step phase cycle, Figure2A; non-water suppressed spectra also collected for monitoring water signal development and absolute quantification of the metabolites4,5,6,7,8), and T2-weighted images (TE/TR= 90/2000ms FOV(ap/rl/fh)=250/189/36 mm³, reconstructed voxel size= 0.98x0.97x3.00 mm3) were acquired immediately prior to each MRS scan to confirm voxel placement.
Data Analysis: Individual shots were saved and were pre-processed (eddy current correction, frequency alignment) prior to averaging 32 shots together for each 2-minute block, yielding a total of 11 spectra (1 baseline, 5 PRE-HEAT, 5 POST-HEAT). LCModel (v6.3) determined the contribution of glutamate in each 2-minute time block (Figure2B). A Pearson correlation examined the relationship between baseline glutamate and NRS-11 pain intensity scores during the acutely painful stimuli (heat-pack).
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