MRI-Guided Stereotactic System for Delivery of Intraspinal Microstimulation

Abstract

Study design: Laboratory/animal-based proof of principle study.

Objective: To validate the accuracy of a magnetic resonance imaging (MRI)-guided stereotactic system for intraspinal electrode targeting and demonstrate the feasibility of such a system for controlling implantation of intraspinal electrodes.

Summary of background data: Intraspinal microstimulation (ISMS) is an emerging preclinical therapy, which has shown promise for the restoration of motor function following spinal cord injury. However, targeting inaccuracy associated with existing electrode implantation techniques remains a major barrier preventing clinical translation of ISMS.

Methods: System accuracy was evaluated using a test phantom comprised of nine target locations. Targeting accuracy was determined by calculating the root mean square error between MRI-generated coordinates and actual frame coordinates required to reach the target positions. System performance was further validated in an anesthetized pig model by performing MRI-guided intraspinal electrode implantation and stimulation followed by computed tomography of electrode location. Finally, system compatibility with a commercially available microelectrode array was demonstrated by implanting the array and applying a selection of stimulation amplitudes that evoked hind limb responses.

Results: The root mean square error between actual frame coordinates and software coordinates, both acquired using the test phantom, was 1.09 ± 0.20 mm. Postoperative computed tomography in the anesthetized pig confirmed spatially accurate electrode placement relative to preoperative MRI. Additionally, MRI-guided delivery of a microwire electrode followed by ISMS evoked repeatable electromyography responses in the biceps femoris muscle. Finally, delivery of a microelectrode array produced repeatable and graded hind limb evoked movements.

Conclusion: We present a novel frame-based stereotactic system for targeting and delivery of intraspinal instrumentation. This system utilizes MRI guidance to account for variations in anatomy between subjects, thereby improving upon existing ISMS electrode implantation techniques.

Level of evidence: N/A.

Figures

Figure 1

Stereotactic intraspinal instrumentation delivery platform. (A) The stabilization platform attaches to the lumbar spine and the stereotactic platform mounts to the stabilization platform for intraspinal delivery. (B and C) The stereotactic platform can be adjusted in the mediolateral (X-axis), rostrocaudal (Z-axis), and dorsoventral (Y-axis). Implantation trajectory can be modified via collar and arc angle adjustment. (D) The electrode guide cannula allows insertion and detachment of electrodes placed within the spinal cord parenchyma. Directional abbreviations: R: rostral, C: caudal, D: dorsal, V: ventral, L: lateral, O: origin.

Figure 2

Magnetic resonance imaging components. (A) Single coil radio frequency-receive only MRI antenna designed for placement within the surgical site. (B) The frame is stabilized using polyether-ether-ketone screws, gimbals, and connection rods that attach the spine to the stabilization platform. The stabilization platform is comprised of titanium rods and polyoxymethylene end braces. The end braces contain mounting points to allow attachment of frame components (i.e., Magnetic resonance imaging fiducial system, stereotactic platform).

Figure 3

System setup in pig lumbar spine. The stabilization platform, imaging coil, and fiducial system are mounted to the lumbar spine.

Figure 4

System setup for delivery of 18-microelectrode array into spinal cord. (A) The microelectrode array was attached to the guide cannula for delivery into the spinal cord. (B) Microelectrode array dimensions and electrode configuration (Left) and approximate implantation of the microelectrode array within the lumbar spinal cord enlargement are shown (Right).

Figure 5

Accuracy testing in phantom. (A) Experimental setup for frame and software coordinates using an imaging phantom containing nine target points. (B-D) Frame coordinates generated by user one (green squares), two (red triangles), and three (black circles), are plotted against software-generated coordinates (blue diamonds) in multiple orientations for visualization purposes along the mediolateral (x), dorsoventral (y), and rostrocaudal (z) axes. Directional abbreviations: R: rostral, C: caudal, D: dorsal, V: ventral, L: lateral, O: origin.

Figure 6

Accuracy testing in an anesthetized pig. (A) An implantation trajectory and final location were chosen (blue line) and stereotactic coordinates were generated using surgical navigation software based on fiducial orientation. (B) Biceps femoris activity was recorded via electromyography during stimulation (2 s stimulus duration, 50 Hz, 250 μs pulse width, 100 μA) at the target intraspinal location. (C) Pre-implantation MRI and (D) post-implantation CT images were fused (E) to confirm electrode delivery into target location.

Figure 7

Muscle activity during intraspinal stimulation. (A) Stimulation (3 s stimulus duration, 50 Hz, 200 μs pulse width, 10 to 50 μA) was applied via microelectrode array contacts 1 (+) and 3 (−). (B) Intramuscular electromyography recordings from the biceps femoris, hamstrings, rectus femoris, and gluteus medius muscles evoked by stimulation (shaded regions) at increasing intensities ranging from 10 to 50 μA.