Here we expand the application of MR-guided stereotaxy to the pig brain to deliver and monitor the distribution of an infusion agent. The size of the swine brain allows for imaging and neurosurgical interventions that are translational. We use the pig model for MR-guided stereotactic injection in a 3T MRI suite.
We visually report the implementation of the stereotaxic procedure in pigs and describe the adaptations of the MRI suite to accommodate the pig, visualize the procedure both in video and concurrent MR imaging to evaluate infusate distribution. Animal positioning. Place a subject in the MRI table in preparation for the MRI scan.
Raise the torso with towels and foam pads. The goal is for the head to fall slightly downwards with neck flexed and the snout nearly touching the table. The MRI head holder pins were anchored on the bilateral zygoma to keep the head affixed to the MR table.
Once set, the MRI table is moved into the bore of the scanner until the subject's head reaches the end of the bore. Planning surgical insertion with MRI-assisted visualization guidance. Prepare the area in a sterile fashion.
Place the fiducial planning grid on the subject's scalp by affixing the adhesive side of the grid over the patient's head, centered around the location of where the burr hole will be. Perform the MRI scout scan with the grid set in place. Adjust the suggested trajectory, including the desired entry and target points, by manually dragging the projected entry and target points in the software to avoid blood vessels and minimize peel and sulcal transgressions.
Once the desired trajectory is identified based on the surgeon's preference, run the MR guidance software to find the entry point on the grid. Securing the stereotactic frame and adjusting alignment iteratively through software projection. Assemble the stereotactic frame around the desired entry point coordinates on the grid by first securing the base with six bone-anchored screws and four offset screws.
Secure the six bone-anchored screws to the skull over the grid through the scalp. The six anchor screws are used to stabilize the stereotactic frame and avoid any movements during drilling. Secure the four offset screws located at the base of the tower through the skin, anchored on the skull.
They act as a counterforce to tighten the center bone screws by lifting the frame base to the center screws and to stabilize the base. Once the stereotactic frame base is secure, continue with frame assembly. Perform the high-resolution, T1-weighted, MP-RAGE MRI scan, an option in the MRI software, with the frame set in place to capture the frame fiducials and confirm the trajectory.
Confirm the desired projected cannula insertion trajectory with the software. Perform the pitch, roll, and XY adjustments by turning the thumbwheels, as indicated by the output adjustment parameters in the software. Using the MR guidance software, measure the thickness of the skull of the desired trajectory and the total distance to the brain.
Drilling and inserting the cannula for infusion. Use an iodine scrub before performing the incision to prevent infection. Make a three-centimeter incision on the scalp using a scalpel under the stereotactic frame.
Set up the frame for drill insertion by performing the adjustments prior to creating the access hole. Remove and replace the center guide tube with the one that fits a 3.4-millimeter drill bit for drilling. Ensure that an assistant is present to hold the frame in place while the surgeon drills with the manual drill to add additional stability to the frame.
Set up the frame for the second drill insertion to widen the burr hole and avoid boney collisions that may alter the trajectory. Set up the drill with the 4.5-millimeter drill bit. Replace the center guide to it with the one that fits this bigger drill bit.
Create a 4.5-millimeter burr hole. Pierce the dura with a sharp stylet. Insert the pre-primed, frame-compatible infusion cannula.
Please ensure the cannula has a consistent neutral or positive back pressure to eliminate air bubbles being introduced. The software provides a specific depth to the planned target. Measure the depth on the stereotactic frame-compatible infusion cannula, and use the cannula-associated depth stop.
Monitoring the infusion with repeated MRI scans. Start the infusion of the desired agent as a co-infusion with a gadolinium-based contrast agent. Perform an MRI scan at regular time intervals to monitor the infusion and volume of distribution of the cannula-inserted agent in the brain, which can be inferred due to the co-infusion of gadolinium.
A hyperintense area around the cannula tip indicates the presence of the gadolinium-based contrast agent. Once infusion ends, stop the pump. Let the cannula stay in the brain for five minutes following termination of the infusion prior to removing the cannula.
The pig position in the MRI scanner provides optimal access for the surgeon to operate and clearance for the stereotactic frame and infusion cannula. The MRI-guided visualization allows for precise planning and insertion of a cannula to the brain. The stereotactic frame is scanned in the software, and it is adjusted to effectively reach the desired location.
Iterative interoperative MRI scans after the cannula infusion show how the infusion is delivered to the brain tissue. The stereotactic frame allows for precise and controlled infusion into pig brain models. With this protocol, we established that parameters such as the rate of infusion or accuracy of the cannula insertion can be changed in real time or paused as dictated by the intraprocedural imaging.
The real-time MR imaging system allows for accurate determination of volume of distribution. Pigs, as large animal models for infusions tracked in real-time MRI, present the possibility of the study of drug delivery to the brain with cell delivery and other agents of translational value. The MR-guided visualization provides real-time guidance for access to the pig brain, cannula insertion, and monitoring of the infusion agent.
The drilling process, tissue deformation, and/or disruption of white matter tracts have been reported to contribute to difficulties and agent delivery to the brain. Iterative MRI scans during the planning and cannula insertion provide the capability for small adjustments.
