Full activation pattern mapping by simultaneous deep brain stimulation and fMRI with graphene fiber electrodes

Abstract

Simultaneous deep brain stimulation (DBS) and functional magnetic resonance imaging (fMRI) constitutes a powerful tool for elucidating brain functional connectivity, and exploring neuromodulatory mechanisms of DBS therapies. Previous DBS-fMRI studies could not provide full activation pattern maps due to poor MRI compatibility of the DBS electrodes, which caused obstruction of large brain areas on MRI scans. Here, we fabricate graphene fiber (GF) electrodes with high charge-injection-capacity and little-to-no MRI artifact at 9.4T. DBS-fMRI with GF electrodes at the subthalamic nucleus (STN) in Parkinsonian rats reveal robust blood-oxygenation-level-dependent responses along the basal ganglia-thalamocortical network in a frequency-dependent manner, with responses from some regions not previously detectable. This full map indicates that STN-DBS modulates both motor and non-motor pathways, possibly through orthodromic and antidromic signal propagation. With the capability for full, unbiased activation pattern mapping, DBS-fMRI using GF electrodes can provide important insights into DBS therapeutic mechanisms in various neurological disorders.

Fig. 1. GF electrodes characterization.

a A schematic drawing of the DBS–fMRI study using GF bipolar microelectrodes. b A representative SEM image of the axial external surface of a GF fiber. Inset, magnified image of the region in the dashed box. Scale bar, 20 μm; inset, 5 μm. c A typical SEM image of the exposed cross section acting as the active stimulating site of a GF electrode. Inset, magnified image of the region in the dashed box. Scale bar, 20 μm; inset, 5 μm. Experiments were repeated five times (for b) and three times (for c) with similar results. d The picture of a GF bipolar microelectrode assembly. Inset, SEM image of the GF bipolar microelectrode tip, showing two GFs (bright core) with each one insulated with Parylene-C film (dark shell). Scale bar, 1 cm; inset, 100 μm. e, f Impedance modulus and phase of GF and PtIr microelectrodes. g Cyclic voltammetry of GF and PtIr electrodes. The time integral of the negative current shown by the shadow region represents the CSC_c. h CIL of different electrode materials. “AIROF” means activated iridium oxide film. i Stability of GF microelectrodes under continuous overcurrent pulsing at 1 mA current amplitude and 130 Hz frequency (see “Methods” for detailed pulsing parameters). Data represented as mean ± SD in e–i (n = 5 electrodes). Source data are provided as a Source Data file.

Fig. 2. STN–DBS with GF bipolar electrodes alleviates Parkinsonian motor deficits in 6-OHDA-lesioned rats.

a A schematic section showing the placement of the GF bipolar stimulating electrodes at the STN ipsilateral to the 6-OHDA lesion. b–e Quantification of the locomotor activities of the hemi-Parkinsonian rats, including the time spent in mobility (b), time spent in freezing (c), mobile episodes per minute (d), and freezing episodes per minute (e) before (Pre), during (DBS), and after (Post) STN–DBS with GF bipolar electrodes. f Analysis of the average mobile speed of the hemi-Parkinsonian rats before (Pre), during (DBS), and after (Post) STN–DBS with GF bipolar electrodes. g Analysis of the apomorphine-induced contralateral rotation speed (in number of turns per min) before (Pre), during (DBS), and after (Post) STN–DBS with GF bipolar electrodes. h An example of the locomotor activity of a hemi-Parkinsonian rat before (black line, 2 min), during (red line, 2 min), and after (blue line, 2 min) STN–DBS with a GF bipolar electrode. Data from the same animals are connected with lines and distinguished by color in b–g. Data represented as mean ± SEM (n = 6 animals, *p < 0.05; **p < 0.01, ***p < 0.001, two-tailed paired t test). Source data are provided as a Source Data file.

Fig. 3. In vivo assessment of MRI artifact.

a A schematic section showing the placement of the electrodes at the STN of rat brains in MRI artifact studies. Each bipolar electrode was composed of a pair of GFs or PtIr wires (75 μm diameter) insulated with ~5-μm-thick Parylene. Thus, the actual size in the medial–lateral direction was ~170 μm. b, c Representative coronal (left) and horizontal (right) sections of the T₂ MRI images of rat brains implanted with a GF (b) and PtIr (c) bipolar microelectrode, through the position of the implants. d, e Representative three serial coronal scans from rostral (left) to caudal (right) of EPI images from rat brains implanted with a GF (d) and PtIr (e) bipolar microelectrode, with the middle images depicting the electrode implant sites. The numbers in each image denote the relative distance from bregma. f B0 distortion maps observed in rats implanted with a GF (upper) and PtIr (lower) bipolar electrode. Red and blue arrows in b–f point to the GF and PtIr implants, respectively. g MRI artifact size of the GF and PtIr bipolar electrodes. The black dashed line denotes the actual size of the bipolar electrodes. Data represented as mean ± SD (n = 6 electrodes, ***p < 0.001, two-tailed unpaired t test). h SNR of the EPI signal in several brain areas in control rats without any implant, and rats implanted with GF and PtIr bipolar electrodes at the STN. The ROIs of the three tested brain nuclei were defined from single slices, as indicated in the left reference diagrams. The top diagram represents the implantation plane, and the red and blue arrows point to the EPI artifact outline of the GF and PtIr bipolar electrodes, respectively. The numbers below the diagrams denote their relative distance from bregma. Somatosensory cortex (SCx) = orange; thalamus (THL) = green; STN = purple. Data represented as mean ± SD (n = 6 samples, n.s.: not significant; ***p < 0.001, one-way ANOVA tests with Tukey post hoc analysis). Source data are provided as a Source Data file.

Fig. 4. BOLD activation maps evoked by STN–DBS with GF electrodes in PD rats.

Four stimulation frequencies were tested as marked in each panel. The BOLD activation maps are overlaid onto averaged anatomical images. Numbers below slices denote relative distance from bregma (in mm). The same set of distance numbers applies to the slices in b–e. Color bar denotes t-score values obtained by GLM analyses, with a significance threshold of uncorrected p < 0.001. All data are group averaged, n = 24 scans from eight rats. STN subthalamic nucleus, GPe external globus pallidus, GPi internal globus pallidus, THL thalamus, CPu caudate putamen, MCx motor cortex, SCx somatosensory cortex, Cg cingulate cortex.

Fig. 5. BOLD signal time series at selected anatomically defined ROIs evoked by STN–DBS with GF electrodes in PD rats.

Percent BOLD response over time at each ROI is shown for multiple stimulation frequencies (orange, 10 Hz; red, 100 Hz; blue, 130 Hz; gray, 250 Hz). The stimulation epoch is indicated by a gray-shaded band. The solid lines show the average signal, and the shaded regions represent the SEM, n = 24 from eight rats. The bar graphs display the average percent changes in BOLD amplitude during the stimulation period. Data represented as mean ± SEM (n = 24 scans from eight rats, n.s.: not significant; *p < 0.05; **p < 0.01; ***p < 0.001, one-way repeated measures ANOVA tests with Tukey post hoc analysis). The inserts depict representative slice examples for each predefined ROI (note that most ROIs encompassed multiple slices). All ROIs are ipsilateral to the DBS hemisphere. Source data are provided as a Source Data file.

Fig. 6. Correlation between BOLD responses and mobile speed change.

a A schematic section showing the placement of the stimulating electrode at STN. b Scatter plot between regional BOLD responses and averaged mobile speed change under 130 Hz STN–DBS. The averaged mobile speed change is defined as averaged mobile speed under DBS divided by that without DBS (non-DBS), which is the average of pre- and post-DBS values. The Pearson’s correlation coefficient r between mobile speed increase and BOLD responses across rats was calculated for each ROI. Source data are provided as a Source Data file.