There has been a recent push for neuroimaging technological development to enhance our understanding of brain function. One approach is to combine several modalities such as electrophysiology and functional MRI (fMRI) to obtain simultaneous data with high temporal and spatial resolution, providing new insights into brain mechanisms. We could further incorporate targeted stimulation techniques such as optogenetics, and observe stimulus-induced electrophysiological responses and neuromodulation. However, there are several technological challenges involved in merging techniques. For example, placing a metallic neural recording electrode in the region of interest would likely compromise fMRI due to severe susceptibility artifacts, in particular, at high field.

Here we present carbon nanotube (CNT) optrodes that will enable the simultaneous acquisition of local field potentials (LFP) and fMRI BOLD signals in response to optogenetic stimulation. These devices can stimulate photosensitive ion channels by transmitting light through a fiber optic core, and subsequently record the LFP responses from a conductive CNT electrode on their surface. Further, the materials used in probe fabrication introduce minimal susceptibility artifacts in MR images, and therefore should enable simultaneous, co-located fMRI mapping.

CNT optrodes were prepared by bonding polymer-clad optical fibers into ceramic ferrules. The bonded devices were treated with a layer-by-layer nanoscale assembly method to improve adhesion¹. Multi-walled CNTs were oxidized for dispersion in water¹, and then drop cast onto the devices. Terminal leads were attached and the CNT optrodes were insulated with polydimethylsiloxane (PDMS). Finally, device tips were polished for effective optical transmission.

In vitro imaging was performed in a 9.4T Agilent animal scanner with a single loop RF surface coil to examine magnetic susceptibility effects. CNT optrodes were positioned in phantoms (agar 2% wt. in DI water) for scanning with T₁-weighted gradient echo (GEMS) and T₂-weighted fast spin echo (FSEMS) imaging sequences. Magnetic susceptibility effects were quantified by comparing affected image hyper/hypo-intense area to the true device cross-sectional area.

Tungsten optrodes were prepared for electrophysiology comparison by bonding tungsten micro-wire electrodes to optical probes. The protocol was similar to that of Armstrong et al.².

Neural activity was recorded from a mouse expressing CaMKII-ChR2-EYFP. The mouse was anesthetized and a CNT optrode was implanted in the nucleus accumbens, with a tungsten optrode placed in contralateral nucleus accumbens for signal quality comparison. An optical stimulation paradigm with ON and OFF epochs was delivered while electrical activity was recorded at 30 kHz with a Cerebus neural recording system (Blackrock Microsystems, UT). Data was post-processed with custom MATLAB code.

Our previous work to combine neural recording and fMRI in rats (unpublished data) indicates that the BOLD signal can be obtained around implanted CNT electrodes that do not introduce significant susceptibility artifacts. In vitro scans of our first-generation optrodes indicate minimal susceptibility artifacts generated by all materials used except for silver terminal connections (see Fig. 1). Although these artifacts would be located above the skull, their severity could compromise the fMRI BOLD signal. To overcome this issue, we are using carbon fiber wire bonded to the device with a conductive polymer interface of poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) and epoxy for mechanical support. An example second-generation optrode is presented in Fig. 2. As expected, in vitro scans of the carbon fiber wire (Fig. 3) indicate minimal magnetic susceptibility issues³ compared to the silver wire. Whereas a 649% increase in affected area was observed around the silver wire, a modest 55% increase was observed with the carbon wire, suggesting substantially less field distortion. Based on our prior experience and the results of others^3,4, the second-generation optrodes should enable in vivo BOLD signal acquisition around the implanted devices.

Preliminary analysis of bilateral electrophysiology data indicates similar signal quality between the CNT device and the tungsten control, verifying the utility of the CNT optrode for neural recording. Spontaneous LFP activity covering the alpha (8-13 Hz), beta (14-25 Hz)⁵, and higher frequency (25-500 Hz) bands under anesthesia is presented in Fig. 4 for comparison.

Our prototype CNT optrodes are fabricated with atypical materials that reduce susceptibility artifacts in MRI. Initial in vitro scans and in vivo neural recordings indicate the potential to combine fMRI, electrophysiology, and optogenetics. While recently developed carbon fiber optrodes were used to combine the aforementioned techniques⁴, the use of a conductive CNT coating directly on the optical fiber will enable us to pattern multiple recording sites along a single optrode. Combining these modalities will provide a platform to selectively excite or inhibit populations of neurons and subsequently investigate brain function, functional and effective connectivity, and neural-vascular coupling with excellent spatial and temporal resolution.