Validation of vessel-based registration for correction of brain shift.

The displacement and deformation of brain tissue is a major source of error in image-guided neurosurgery systems. We have designed and implemented a method to detect and correct brain shift using pre-operative MR images and intraoperative Doppler ultrasound data and present its validation with both real and simulated data. The algorithm uses segmented vessels from both modalities, and estimates the deformation using a modified version of the iterative closest point (ICP) algorithm. We use the least trimmed squares (LTS) to reduce the number of outliers in the point matching procedure. These points are used to drive a thin-plate spline transform to achieve non-linear registration. Validation was completed in two parts. First, the technique was tested and validated using realistic simulations where the results were compared to the known deformation. The registration technique recovered 75% of the deformation in the region of interest accounting for deformations as large as 20 mm. Second, we performed a PVA-cryogel phantom study where both MR and ultrasound images of the phantom were obtained for three different deformations. The registration results based on MR data were used as a gold standard to evaluate the performance of the ultrasound based registration. On average, deformations of 7.5 mm magnitude were corrected to within 1.6 mm for the ultrasound based registration and 1.07 mm for the MR based registration.

Clinical validation of vessel-based registration for correction of brain-shift.

In this paper, we have tested and validated a vessel-based registration technique for correction of brain-shift using retrospective clinical data from five patients: three patients with brain tumors, one patient with an aneurysm and one patient with an arteriovenous malformation. The algorithm uses vessel centerlines extracted from segmented pre-operative MRA data and intra-operative power Doppler ultrasound images to compute first a linear fit and then a thin-plate spline transform in order to achieve non-linear registration. The method was validated using (i) homologous landmarks identified in the original data, (ii) selected vessels, excluded from the fitting procedure and (iii) manually segmented, non-vascular structures. The tracking of homologous landmarks show that we are able to correct the deformation to within 1.25 mm, and the validation using excluded vessels and anatomical structures show an accuracy of 1mm. Pre-processing of the data can be completed in 30 s per dataset, and registrations can be performed in less than 30s. This makes the technique well suited for intra-operative use.

A realistic phantom for brain-shift simulations.

Validation of techniques that characterize and correct for brain shift for image guided surgery requires a realistic anthropomorphic phantom for use as a gold standard. The purpose of this study was to determine the characteristics of a deformable brain phantom made of polyvinyl alcohol cryogel (PVAc). The phantom was made of three layers of PVAc with inserted plastic tubes to simulate blood vessels. A catheter with an inflatable balloon was placed under the phantom in order to deform it in a nonlinear manner. The reproducibility of the elastic deformation was evaluated using MR imaging and surface measurements. Our experiments show that the phantom is well suited for MR and ultrasound imaging (B-mode and Doppler) with sub-millimeter reproducibility for the deformations.

Validation of model-guided placement of external ventricular drains.

PURPOSE: Inaccurate placement of external ventricular drains (EVDs) is a common issue in cerebrospinal diversion procedures. The conventional freehand technique results in a high fraction of sub-optimally placed catheters, and the use of image guidance can improve these results. The purpose of this paper is the validation of the use of an average model for guidance of EVD procedures. METHODS: Three neurosurgeons have tested the model-based technique on three normal volunteers, and we have compared the model-based technique to the freehand technique and neuronavigation based on volunteer-specific images. RESULTS: Our results show that the surgeons perform significantly better when using the model-based technique than when using the freehand technique. CONCLUSIONS: Our results suggest that the use of an average model may improve the accuracy of catheter placements. However, further refinement of the method and testing in a clinical setting is required.

A new system for 3D ultrasound-guided placement of cerebral ventricle catheters.

PURPOSE: We present a new system for 3D ultrasound-guided placement of cerebral ventricle catheters. The system has been developed with the aim to provide accurate ultrasound-based guidance with only minimal changes to the current surgical technique and workflow. METHODS: The system consists of a pre-calibrated navigation adapter for the catheter and a reference frame attached to a standard surgical retractor in addition to an ultrasound-based navigation system with a probe that fits on top of a standard burr hole. RESULTS: The accuracy of the pre-calibrated system has been evaluated, and our measurements indicate that the accuracy of the pre-calibrated system is better than 3 mm. We also present a clinical case. CONCLUSIONS: The navigation accuracy is considered sufficient for clinical use, and initial clinical tests are promising. Further testing will be necessary to fully evaluate the performance of the system in a clinical setting.