Rapid Device Localization for Prospective Stereotaxy: Using Computation Instead of Imaging
Miles E. Olsen1, Ethan K. Brodsky1, Jonathan A. Oler2, Marissa K. Riedel2, Eva M. Fekete2, Ned H. Kalin2, and Walter F. Block1

1Medical Physics, University of Wisconsin - Madison, Madison, WI, United States, 2Psychiatry, University of Wisconsin - Madison, Madison, WI, United States


We present a technique for rapidly aiming interventional devices during prospective stereotaxy procedures. Our approach enables accurate computational determination of trajectory guide orientation and the true physical pivot point of frameless stereotaxy guides that mount on the skull.

Historically, these neurosurgical tasks require minutes per iterative cycle consisting of: scan, interpret image, adjust aim, repeat – or no intraoperative imaging at all, relying on preoperative images registered to stereotactic frame coordinates. Our rapid technique (~5 FPS) is closer to the clinician’s preferred responsiveness of optical tracking of devices in the OR (~30 FPS).


The technique of prospective stereotaxy [1] was developed for aiming a class of frameless stereotaxy trajectory guides. Localizing commercial devices in MR-guided neurosurgery with prospective stereotaxy is achieved through iteratively imaging and manipulating the device until it is deemed on target, as in the ClearPoint system (MRI Interventions, Irvine, CA). This process may take several minutes to complete. Compressed sensing improves imaging performance by exploiting sparse image representations, sparse sampling patterns, and novel reconstruction. We use these concepts to compute the orientation of a trajectory guide with a frame rate 25x faster than our previous real-time, Cartesian imaging based approach. We demonstrate our method’s utility in a survival experiment of gene delivery designed to alter behavioral expression of anxiety in 8 non-human primates over several months.


Brain cannulas are inserted with sub-mm accuracy through a trajectory guide fixed to the skull, bypassing the blood-brain-barrier. Preoperative MRI is used to determine a suitable mounting point for the base of the guide such that the target falls within the guide’s range of motion (a 30° cone, Fig. 1).

Intraoperative MRI is used during the procedure to visually identify the target and pivot. Our previous real-time approach [2] then imaged a plane above the brain and perpendicular to the planned trajectory. The operator viewed those cross-sectional images of the stem while moving the guide into alignment with the planned trajectory. Though much faster than iterative approaches, performance is hindered by 1) slow frame rates to generate high resolution images of the guide’s cross section and 2) difficulties visually selecting the true geometric pivot with sub-mm accuracy. Instead of imaging, our new approach acquires sets of 1D projections through the alignment stem at various angles and offsets, as shown in Fig. 2.

Each 1D projection is correlated with the known theoretical projection of the circularly symmetric alignment stem to find its center location along the projection. These computed locations can be configured into an overdetermined system of equations to find the orientation of the guide (blue lines in Fig. 2) and indicate it to the operator. Lines representing device orientation are recorded as the operator moves the guide to three different alignments. All trajectories are constrained to pass through the pivot point, so the 3D intersection of those lines is computed to yield improved pivot point coordinates. The operator then aligns the guide with the target by simply moving the computed point onto the green aim point (Fig. 3). If the trajectory guide is aimed based on an incorrect pivot, the tip of the device may not hit the target, as illustrated in Fig. 4.

We demonstrate the method using a Navigus trajectory guide (Medtronic, Minneapolis, MN) consisting of a pivoting ball joint within a base that can be fixed to the skull over a burr hole [3]. Viral vector containing a genetic payload was precisely delivered to the amygdala using 2 infusions, one anterior and one posterior, in a study of gene therapy to alter expression of anxiety. The technique has been used in 8 primate invivo surgeries to accurately treat two locations within the central amygdala, an obliquely oriented cylinder only 5 mm in length and 2 mm in diameter.

Results and Discussion

Each 1D projection was acquired with 0.4 mm resolution. Sampling 18 angles at 2 offsets was found to produce accurate, repeatable calculation of the device orientations. As each 1D MR experiment required only 5 ms, the device orientation could be calculated at 5 frames/s, a considerable improvement over our 0.2 frames/s performance using Cartesian imaging. The clear visualizaiton of Gd-doped infusate is shown in Fig. 5.

Over 8 subjects, in the N=22 instances of computing a pivot point, the total 3D separation between the manually selected and computed points was 1.62 ± 0.90 mm, and the radial (perpendicular to trajectory) separation was 0.67 ± 0.36 mm.

Our new approach eliminates error-prone manual selection of the pivot. Solving for location with an overdetermined set of equations allows for alignment with theoretical precision greater than the image resolution while reducing the chances for operator error to affect alignment.

Conclusions and Future Work

We have implemented a system for rapidly and computationally identifying the true pivot point and orientation within a trajectory guide. This improves the targeting process by reducing errors that may arise from alignment stem tip uniformity and human interpretation of MR images. The 25x improvement in frame rate eases the aiming process and may be further increased as we experiment with using fewer projections.


The research was supported by the National Research Service Award (NRSA) T32 EB011434. We also acknowledge institutional support from GE Healthcare.


[1] Truwit C and Liu H. Prospective stereotaxy: a novel method of trajectory alignment using real-time image guidance. JMRI. 2001;13(3):452–457.

[2] Grabow et al. Alteration of Molecular Neurochemistry: MRI-guided Delivery of Viral Vectors to the Primate Amygdala. ISMRM. 2014 #0672.

[3] Hall W.; Liu H; Truwit C. Navigus Trajectory Guide. Neurosurgery. 2000;46(2):502.


Preoperative MRI of primate head and 3D model of trajectory guide base (transparent orange). The upper point indicates the coordinates where the burr hole will be drilled and the base installed in the operating room. The lower point is at the target within the left amygdala.

Blue points represent stem/plane intersection computed via radial projections. Repeating in a second plane yields blue lines representing device location and orientation. The apparatus physically constrains trajectories to pass through the true pivot, so multiple orientations are sampled and their intersection computed to yield improved pivot coordinates.

Anatomical planning MRI before aiming. The small dotted line from computed point to aim point represents the necessary motion to align the device along the desired trajectory. The operator views the aiming plane and moves the device until the computed point is on the aim point.

Aiming based on an incorrect pivot is likely to result in missing the target. The device is physically constrained to pass through the true pivot, so it does not take the expected trajectory (dotted line), instead going down along the blue dashed line and missing the target.

MRI volume acquired immediately after infusion of viral vector combined with Gd contrast (total volume = 12 µL). The hyperintense cloud at the target shows the infusate distribution. The plane is near coronal, but slightly oblique so the cannula lies in plane.

Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)