Surgical resection remains the treatment of choice for
brain tumors with infiltrating margins but is currently limited by visual discrimination between normal and neoplastic marginal tissues during surgery. Imaging modalities such as computed tomography, magnetic resonance, positron emission tomography, and optical techniques can accurately localize
tumor margins. We believe coupling the fine resolution of current imaging techniques with the precise cutting of midinfrared
lasers through image-guided neurosurgery can greatly enhance
tumor margin resection. This paper describes a feasibility study designed to optically track in three-dimensional space the articulated arm delivery of a noncontact ablative
laser beam. To enable optical tracking of the
laser beam focus, infrared-emitting diodes (IREDs) were attached to a handpiece machined for the distal end of the articulated arm of a surgical
carbon dioxide laser. Crosstalk between the ablative
laser beam and the tracking diodes was measured. The geometry of the adapted
laser handpiece was characterized to track an externally attached passive tip and the
laser beam focus. Target localization accuracies were assessed for both instrument points-of-interest and the sources of tracking errors were investigated. Stray infrared
laser light did not affect optical tracking accuracy. The mean target registration errors while optically tracking the
laser handpiece with a passive tip and the
laser beam focus were 1.31+/-0.50 mm and 2.31+/-0.92 mm, respectively, and were equivalent to the errors tracking a 24-IRED pen probe from Northern Digital in a side-by-side comparison. The majority of error during ablation tracking derived from registration accuracy between physical space and the defined space of the ablation phantom and from an inability to freehand align the
laser focus with the target in a consistent manner. While their magnitudes depend on spatial details of the tracking setup (e.g., number and distribution of fiducial points, working distance from the camera, etc.), these errors are inherent to any freehand
laser surgery.