Novel Composite Gold-Aluminum Electrode with Application to Neural Recording and Stimulation in Ultrahigh Field Magnetic Resonance Imaging Scanners

Abstract

Traditional electrodes used for neural recording and stimulation generate large regions of signal void (no functional MRI signal) when used in ultrahigh field (UHF) MRI scanners. This is a significant disadvantage when simultaneous neural recording/stimulation and fMRI signal acquisition is desired, for example in understanding the functional mechanisms of deep brain stimulation (DBS). In this work, a novel gold-aluminum microwire neural electrode is presented which overcomes this disadvantage. The gold-aluminum design greatly reduces the magnetic susceptibility difference between the electrode and brain tissue leading to significantly reduced regions of signal void. Gold-aluminum microwire samples are imaged at ultrahigh field 16.4 Tesla and compared with gold-only and aluminum-only microwire samples. First, B₀ field mapping was used to quantify field distortions at 16.4T and compared with analytical computations in an agarose phantom. The gold-aluminum microwire samples generated substantially less field distortion and signal loss in comparison with gold-only and aluminum-only samples at 16.4T using gradient echo imaging and echo planar imaging sequences. Next, the proposed gold-aluminum electrode was used to successfully record local field potential signals from a rat cortex. The newly proposed gold-aluminum microwire electrode exhibits reduced field distortions and signal loss at 16.4T, a finding which translates to MRI scanners of lower magnetic field strengths as well. The design can be easily reproduced for widespread study of DBS using MRI in animal models. Additionally, the use of non-reactive gold and aluminum materials presents an avenue for translation to human implant applications in the future.

Keywords: Gold-aluminum electrodes; Image artifacts; MRI scanners; Matched magnetic susceptibility; Neural electrodes.

Figure 1.

Twisted wire samples for proof-of-concept testing. (a) Side view of gold, gold-aluminum composite, and aluminum twisted wires. (b) Cross section schematic of the samples. Gold wires were 100 μm diameter; aluminum wires 125 μm diameter. (c) Neural electrode formed by twisting insulated 100 μm diameter gold and aluminum wires. Wires were soldered to a nonferrous Omnetics connector. Uninsulated silver wires served as reference and ground. Dental cement was used for mechanical support and to protect soldered connections.

Figure 2.

Comparison of phantom B₀ mapping result at 16.4T (top row) with analytical-based solution (bottom row) for gold, gold-aluminum composite, aluminum twisted wire samples. Paramagnetic aluminum produced the largest distortions in phantom B₀ mapping, followed by diamagnetic gold, and finally gold-aluminum composite samples. The analytical-based solutions match the phantom experimental results. The lower-right subplot shows the analytical-based solution for four platinum wires (100 μm diameter), commonly used in neural electrodes.

Figure 3.

Plots comparing the analytical-based solution and experimental result for B₀ field distortion along the z-direction (top row) and x-direction (bottom row) for a line passing through the center of each of the three samples. Experimental results were aligned with analytical computations, and masked where B₀ distortion computation became inaccurate due to signal loss near the sample.

Figure 4.

Coronal gradient echo multiple slice (GEMS) imaging of a rat implanted with a twisted gold wire sample (G) in the right hemisphere, and a twisted gold-aluminum composite wire sample (C) in the left hemisphere. The gold-aluminum wire sample created a smaller image artifact and was visible in fewer slices in comparison to the gold wire sample. Slices increase from 1 to 8 from caudal to rostral. Slice thickness 500 μm, in plane resolution 94 μm, matrix size 256 by 256.

Figure 5.

Transverse gradient echo multiple slice (GEMS) imaging (top row) and B₀ mapping (bottom row) of a rat implanted with gold (G) and gold-aluminum composite (C) twisted wire samples. The gold wire sample produced larger artifacts and field distortion than the gold-aluminum sample. Slice thickness 500 μm, in plane resolution 94 μm, matrix size 256 by 256.

Figure 6.

Coronal echo planar imaging (EPI) of a rat implanted with gold (G) and gold-aluminum composite (C) twisted wire samples. The gold sample produced larger regions of signal loss in comparison with the gold-aluminum sample. Imaging parameters: 64 by 96 matrix, 8 slices, in-plane resolution 250 μm, slice thickness 500 μm, four shots, TE 6 ms, TR 2 seconds per volume.

Figure 7.

Conductance of neural electrode channels in air and in saline bath at 1 kHz (left). The increase in conductance in saline confirms electrode connection and function prior to in vivo testing. Channels 1 and 4 correspond to gold wires, channels 2 and 3 correspond to aluminum wires. Local field potential (LFP) recorded with the gold-aluminum twisted neural electrode in the rat cortex under varying levels of isoflurane anesthesia (2.0%, 1.5%, and 1.2%, right). The recordings from each of the 4 electrode contacts are superimposed in each subplot. As anesthesia level decreased from 2.0% to 1.2%, cortical LFP transitioned from burst suppression (top panel) to sustained oscillation (bottom panel).

Figure 8.

Field distortion computations around wire samples (top row) and estimated transverse signal decay caused by the presence of the wires (bottom row) neglecting encoding gradient effects. (a) 4 gold wires of 100 μm diameter, (b) 2 gold wires 104 μm diameter and 2 aluminum wires 96 μm diameter, (c) 2 gold and 2 aluminum wires 100 μm diameter, (d) 2 gold wires 100 μm diameter and 2 aluminum wires 125 μm diameter, (e) 4 aluminum wires 125 μm diameter.