All patients had idiopathic PD, diagnosed by a neurologist specializing in movement disorders (J.L.O.), and met the standard criteria for STN DBS as described elsewhere. 15
For patients who underwent bilateral electrode implantation, scores on the UPDRS part III were obtained at baseline and at 6–12 months after implantation. The study was approved by the University of California, San Francisco, institutional review board, and written informed consent was obtained from each patient.
Interventional MR Imaging–Guided Implantation
The technical approach has been described previously in the initial 8 implantations,19
but is detailed here to include more recent refinements introduced as the method has evolved. The specialized devices and MRcompatible equipment used are listed in , and MR protocols are shown in . The guidance platform used was the the NexFrame DBA (deep brain access) trajectory guide. This is a single-use item that mounts over a bur hole with 3 bone screws. The device is aimed at a target using a rotate/translate mechanism, maintaining a constant pivot point. The NexFrame can accept 2 possible inserts: an MR-visible alignment stem, or a “multi-lumen insert” containing 5 parallel channels (3-mm separation) for guiding probes into the brain. The basic NexFrame platform is identical to that used for frameless neuronavigation –guided DBS,12
but the 2 inserts are specialized for the interventional MR imaging application.
Equipment used for iMR imaging–guided DBS*
Magnetic resonance imaging pulse sequences for iMR imaging–guided DBS*
Patient Preparation and Positioning
Patients were allowed to take their usual morning dose of antiparkinsonian medications. After premedication with midazolam and fentanyl, general anesthesia was induced with propofol in a room adjacent to the MR imaging suite. Anesthesia was maintained with sevoflurane and intermittent fentanyl and vecuronium boluses. Ventilation was adjusted to maintain end-tidal CO2 between 35 and 40 mm Hg. After placement of a radial artery catheter, the patients’ heads were placed into a carbon fiber headholder designed to mount directly onto the MR imaging gantry. The frontal area was shaved using clippers. An array of 4 flexible surface coils positioned at the sides, top, back, and front of the head was used for MR signal reception ().
Fig. 1 Intraoperative photograph showing the position of the patient’s head in an MR-compatible headholder with placement of radiofrequency surface coils. The connection between the endotracheal tube and ventilator is led through the anterior coil. The (more ...)
Trajectory Planning for Bur Hole Location
Patients were then moved into the bore of the MR imager. An MR imaging–compatible anesthesia machine was used. A landmark was established on the frontal scalp near the presumed coronal suture and advanced to magnet isocenter. A Gd-enhanced volumetric gradient echo MR imaging was obtained (scan parameters in , MR protocol 1) parallel to the line between the AC and PC. On the MR console, approximate anatomical targets were selected bilaterally at a point 12 mm lateral, 3 mm posterior, and 4 mm inferior to the midcommissural point. These targets were used only for trajectory planning, however, because final anatomical target selection was performed in a subsequent step (described below). Single-slice, oblique, parasagittal reformatted images were reconstructed that passed through the approximate targets, but avoided the lateral ventricle. A trajectory that avoided sulci and cortical veins was then selected on the oblique image (). At the point where the trajectory crossed the scalp, a rapidly updating MR fluoroscopy sequence (MR protocol 2, described further below) was prescribed with its center at the intended entry point. The surgeon reached into the bore of the magnet and manually placed an MR-visible pointer at the intended entry point. This was marked with a pen, the patient was moved to the back of the bore, and the skull marked percutaneously by injecting methylene blue through a 22-gauge needle at the scalp entry site.
Fig. 2 Magnetic resonance images showing the method of trajectory planning using reformatted oblique slices passing through the target, angled to exclude the lateral ventricle (MR protocol 1). A: First step. On a coronal plane passing through the target, an (more ...)
Initial Exposure and Mounting of Trajectory Guide
The frontal area was prepared and draped with a bore drape designed to keep the surgical field sterile yet tolerate head movement between the center and back of the bore (a distance of ~ 1 m) (). A pressurized nitrogen tank, electrical power sources for bipolar cautery, one headlight, and one floor light were placed outside the MR imaging room with the regulator hose and electrical cords directed through the waveguide. Monopolar cautery was not used. After making coronally oriented incisions, 14- mm frontal bur holes were drilled with an MR compatible cranial drill. The base rings for the Stimloc lead anchoring device and the NexFrame trajectory guides were mounted over the bur holes. The dura mater was opened bilaterally and the leptomeninges were coagulated. The trajectory guide alignment stems were filled with sterile saline and mounted into the trajectory guides ().
Fig. 3 Intraoperative photographs demonstrating surgical draping and trajectory guides. A: Patient’s head is shown at the back of the MR bore, with a sterile drape. B: Trajectory guides with alignment stems are shown. C: Trajectory guides with multilumen (more ...)
Target Definition and Aiming of Alignment Stem
Patients were moved to reposition the head at the magnet isocenter. Table movement was then disabled and no further patient movement was allowed until the leads were inserted and placement was confirmed on imaging. High-resolution T2-weighted axial MR images were obtained with 2-mm slice thickness, aligned such that 1 slice passed 4 mm inferior to the commissures (MR protocol 3). The brain target was selected on this image (). The intended target was generally very close to the default coordinates of 12 mm lateral, 3 mm posterior, and 4 mm inferior to the midcommissural point. However, small adjustments in the default coordinates were made based on direct visualization of the borders of the STN and red nucleus, so as to place the target within the dorsolateral STN at least 2 mm from the medial, lateral, and posterior borders. Axial and coronal volumetric T2- weighted MR imaging was performed through the pivot points of the alignment stems (MR protocol 4; ). The x, y, and z coordinates of the target and pivot with respect to the MR isocenter were determined by placing a “region of interest” cursor over the desired location. The final x, y, and z coordinates of the pivot were a synthesis of the values on coronal and sagittal views.
Axial MR image used to define the target in the dorsolateral STN (black arrow indicates right STN; MR protocol 3).
Coronal MR image used to define the coordinates of the pivot points for the trajectory guides, prior to aligning the alignment stem (MR protocol 4).
For the first side to be implanted, the x, y, and z coordinates of the target and pivot were used to prescribe the MR fluoroscopy sequence (MR protocol 2). The target and pivot points define a line and the MR scan is prescribed such that it is perpendicular to and centered on this trajectory at a location 9–10-cm superior to the bur hole. The surgeon donned a sterile hood to maintain the sterile field, and reached into the bore of the magnet to manually align the stem to the target line, while viewing the MR fluoroscopy image on an in-room monitor. When the desired alignment was achieved, the NexFrame was locked into place. Rapid, low-resolution, oblique coronal and sagittal images (MR protocol 5) were obtained along the orientation of the stem, and the final anticipated target reconstructed graphically (). Occasionally, the oblique scans predicted a trajectory not perfectly aligned with the intended target. In these cases a new alignment scan was prescribed with its center slightly modified, and manual alignment was again performed by the surgeon. The distance from the target to the relevant level of the trajectory guide (the step-off between thick and thin sections of the alignment stem) was measured on oblique images to allow calculation of the position of the depth stop in the subsequent step. The distance was increased by 4.5 mm so that the sheath and stylet would slightly overshoot the target.
Rapid acquisition oblique coronal (A) and sagittal (B) images passing through the target and pivot point after the trajectory guide has been aligned (MR protocol 5). Black arrows show the predicted trajectory of the DBS lead.
Insertion of Guidance Sheath and Lead
The alignment stem was replaced with a 5-channel multilumen insert, and a ceramic stylet within a plastic peel-away sheath was placed into the center lumen (). A depth stop was placed on the stylet at the appropriate length as described above. The stylet/sheath assembly was advanced into the brain in 2–3 stepwise movements and monitored via inplane MR imaging with oblique sagittal and coronal T2 sequences (MR protocol 6). The alignment and insertion procedure were then repeated for the contralateral side. A high resolution axial T2-weighted image was obtained through the target area to assess sheath/stylet position at the target (MR protocol 3) ().
Fig. 7 A and B: Axial T2-weighted MR images obtained 4 mm below the commissures, showing sheath and ceramic stylet assembly at target (MR protocol 3). Close up (B) showing the stylets in the target region, with the desired targets indicated by the centers of (more ...)
If placement of the peel-away sheath/stylet assembly was found to be inappropriate for either side (defined as distance between intended and actual stylet position of > 2 mm in the axial plane 4 mm below the commissures), a side channel of the NexFrame multilumen insert was considered to provide a parallel track with an offset of 3 mm in a direction perpendicular to the lead trajectory. If an offset of 3 mm could not provide appropriate placement, the sheath and stylet were removed, alignment stem replaced, and the NexFrame trajectory readjusted by repeating the alignment scans.
Two 28-cm DBS leads (Medtronic model 3389–28), were prepared by replacing their standard wire stylet with custom-made, nonferrous titanium wire stylets (supplied by Medtronic, Inc.) so as to allow imaging of the lead with the wire stylets in place and without excessive artifact. On 1 side, the ceramic stylet within the peelaway sheaths was removed and a “bridge” snapped over the multilumen insert. The bridge provided a space between itself and the multilumen insert for the sides of the peel-away sheath, and contained a lead-holding screw. A depth stop was placed on the lead 42.5 mm higher than the depth stop on the ceramic stylet (to account for the extra height of the bridge and lead holder) and the lead was advanced through the sheath to the target. Lead insertion was repeated on the contralateral side. Axial T2- weighted MR imaging was used to confirm lead depth (MR protocol 7).
Closure, Final Imaging, and Implantable Pulse Generator Placement
Patients were moved to position the head at the back of the bore for easier surgical access. The peel-away sheaths were removed. The DBS leads were anchored to the skull with the Stimloc clips. The titanium wire stylets were removed from the leads, and the Stimloc cranial caps set in place. The NexFrame trajectory guides were removed, and the scalp was closed with sutures. Patients were moved back to isocenter for a final high-resolution volumetric T1-weighted MR imaging session (MR protocol 8), to be used to measure the lead tip location and trajectory (). Patients were awakened, allowed to recover in the postanesthesia care unit, monitored overnight in a stepdown unit, and discharged the day after implantation. Lead extenders and a dual channel pulse generator (Medtronic Kinetra) were placed 1–2 weeks later in the standard operating room.
Final lead location as assessed on T1-weighted volumetric MR images. A: Axial image at 4 mm inferior to the commissures. B: Reformatted oblique image in the sagittal plane along the lead trajectory (MR protocol 8).
Measurement of Targeting Errors
Two types of errors were measured. The “radial error” was defined as the scalar distance between the location of the intended target and the actual location of the ceramic stylet in the axial plane 4 mm inferior to the commissures on high resolution axial T2-weighted images. This distance was measured directly on the MR console using the ruler tool. In cases where a second placement of the ceramic stylet was made after the first was advanced to the target and deemed inadequate, the first pass was used to calculate radial error. “Tip error” was defined as the distance between the expected AC-PC coordinates of the lead tip, and the actual AC-PC coordinates. The expected AC-PC coordinates were those of the initial target (at a plane of 4 mm below the commissures), corrected in all dimensions by the planned overshoot of the lead beyond target (4.5 mm), taking into account the double-oblique lead angulation as measured on the final postoperative MR images in Framelink software. The actual AC-PC coordinates of the lead tip were measured using Framelink software as described previously.28
Statistical Analysis and Comparison Data Set
To compare DBS lead placement accuracy measurements between iMR imaging and conventional stereotactic techniques, we used a control data set for 76 STN DBS leads placed using standard frame-based stereotaxy in 44 patients, as we previously described in an earlier study.28
Comparison of mean lead tip errors was performed with unpaired t-tests. We tested iMR imaging errors for correlation with potential error predictors (case order, patient age, side of surgery, sagittal plane trajectory angle, and coronal plane trajectory angle), using the Pearson correlation coefficient r. Statistical significance was set at p < 0.05.