SECTION I: PARKINSON’S DISEASE
4. TARGET PLANNING 1. Image Acquisition
While MRI has superior soft-tissue resolution, CT is less susceptible to the distortional artifacts produced by the inhomogeneities in the magnetic field
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and thus more accurately represents the actual position of intracerebral struc-tures in space. Consequently, we rely on image fusion to allow us to capitalize on the respective advantages of each methodology. Our surgical planning utilizes several MRI sequences (1.5 T Siemens Magnetom Vision, Symphony, or Sonata) and one CT sequence (Siemens Emotion or Plus4). As previously stated, a volumetric T1-weighted axial MRI (MPRAGE) (TR 20, TE 6, Flip angle 30, 256 field of view, 1 mm thick slices 256 256 matrix) is obtained on the day of surgery or up to several days preoperatively without the stereotac-tic frame in place. A coronal T2 sequence (TR 5000, TE 96, Flip angle 180, 300 field of view, 2 mm thick slices 154 256 matrix) is performed from in front of the anterior commissure to behind the posterior commissure. The STN may be directly visualized on this sequence and it serves as an additional method for determining our initial target, as will be described below. A CT scan is then obtained with the patient, frame, and CT fiducial box secured to the table to ensure that the frame is parallel to the axis of scanning. The gantry angle must be kept at 0in order to allow most planning software to construct a volumetric cranial model. The field of view is enlarged so that all of the fiducial rods are visible. Images are taken at 1–2 mm intervals from the base of the frame through the top of the fiducial set. While stereotactic surgery initially relied on contrast ventriculography to visualize the anterior and posterior commissures, the superb visualization of these structures on modern MRI machines renders this technique unnecessary.
4.2. Target and Trajectory
Targeting is performed using several methods. The T2-weighted MRI may be used for direct anatomical targeting of the STN. A defined stereotactic formula in combination with a stereotactic atlas indirectly targets the structure. Lastly, microelectrode recording (MER) locates the nucleus via neurophysiologic mapping.
After CT scanning the patient is transported to the operating room.
The acquired images are uploaded into a planning station via the hospital intranet or from an optical disk. We utilize the Stealth Station Treon Plus (Medtronic Surgical Navigation Technologies, Colorado, U.S.A.). The CT scan serves as the reference examination against the multiple MRI exams.
The CT and the MRI series are fused using automated image fusion soft-ware. The fused images are inspected to confirm that structures on each scan overlie each other. If this fusion is found to be unsatisfactory, corresponding points on each exam may be chosen for a manual point merge. Alterna-tively, the MRI may be fused to the CT scan using the point merge system.
The stereotactic coordinate system is then established by registering the fiducial rods. Ideally, this should be done on the CT images set to bone windows so that artifacts are reduced. The CT scan is utilized due to its
more accurate spatial resolution. Once the rods are identified, the computer can then assign a triplanar set of Cartesian coordinates to any intracranial structure.
The commissures and at least one midline point are then identified and stored. Because stereotactic targeting formulas were all determined with the use of ventriculography, the posterior portion of the AC and the anterior portion of the PC should be selected to correspond with those regions visua-lized on a ventriculogram (Fig. 1). The computer then aligns the images to parallel the intercommissural line. If the Leksell frame is placed perfectly, both the AC and PC will have identical X- and Z-coordinates.
Figure 1 T1-weighted MRI on the sur-gical planning station with the AC and PC marked.
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If redundancy in planning to minimize the chance of error is desired, we advocate a separate manual calculation of the location of the mid-commissural point (MCP). Most CT and MRI scanners provide enough image analysis functions to derive the displacement of AC and PC from frame center. These numbers can easily be transformed into stereotactic coordinates by adding or subtracting them from the x¼ 100, y ¼ 100, and z¼ 100 (frame center). By convention, point displacement to the left, ante-rior, or inferior of frame center are added to 100, while displacement to the right, posterior, or superior of frame center are subtracted from 100.
If no planning station is available, we recommend obtaining all imaging with the frame and performing separate AC/PC calculations on the CT and MRI consoles to provide redundancy.
Standard stereotactic formulas are used to locate the STN in relation to the MCP. For those patients with standard size skulls and no ventri-culomegaly, we target 12.5–13 mm from the midline, 4 mm posterior to the MCP, and 5 mm below the intercommissural line. In older patients with more generous ventricles, the lateral coordinate may need to be shifted several tenths of a millimeter laterally. Some stereotactic systems provide a digitized version of the Schaltenbrand and Wahren Atlas that is morphed to conform to the patient’s caudate head, thalamic height, and brainstem.
This is then overlaid on the formulaic targets to confirm that the target lies within the STN.
Figure 2 shows a T1-weighted MRI with the overlayed atlas and electrode trajectories. A typical trajectory will pass through the anterior thalamus, the zona incerta/fields of Forel, the STN, and the substantial nigra pars reticulata (SNpr). The distinct electrical signatures of each will be used to determine the final target. Figure 3 demonstrates the ability to directly target the STN using T2-weighted MRI.
Entry points are then chosen to provide the ring and arc measurements for targeting. The trajectory taken to the target can be as important as the target itself. This should be approximately 1 cm anterior to the coronal
Figure 2 T1-weighted MRI with the overlayed reformatted Schaltenbrand and Wahren Atlas and bilateral electrode tracks.
suture so the sagittal angle of approach will result in the microelectrode traversing appropriate superficial structures. Similarly, the entry point should be between 2 and 3 cm from the midline in order to avoid the medial bridging veins and avoid a lateral tract in the internal capsule. In addition to the risk of damaging the internal capsule, lateral trajectories that do not pass through the thalamus and zona incerta provide limited MER data. The pre-cise entry point may be refined on the planning console such that the trajec-tory passes through the crown of a gyrus rather than into a sulcus. This avoids inadvertently damaging sulcal or pial vessels which lie on the cortical surface. After selecting preliminary points, the images are then reformatted to the trajectory view, which provides three orthogonal planes positioned with respect to the trajectory rather than the patient’s anatomy. The app-roach then may be traced at millimeter intervals to ensure that no deep sulci are transgressed and that the ventricular ependyma is not scythed. While many groups use a standard entry point for all patients and do not make the effort to intensively refine the trajectory in this manner (Table 1), we feel that this step attempts to minimize the risk of deep hemorrhage.