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Technique for the three-dimensional localization of implanted fiducial markers

Registration of image space and physical space lies at the heart of any interactive, image- guided neurosurgery system. We have developed a localization technique that enables permanently implanted fiducial markers to be used for the registration of these spaces. Permanently implanted markers are desirable for surgical follow-up, monitoring of therapy efficacy, fractionated stereotactic radiosurgery and improved patient comfort over stereotactic frames. Bone-implanted extrinsic fiducial markers represent a potential long-term mechanism for registration. The major challenge to using implanted markers is the localization of the markers in physical space after implantation. We have developed and tested an A-mode ultrasound technique for determining the location of small cylindrical markers (3.7 mm in diameter, 3 mm in height). Accuracy tests were conducted on a phantom of a patient's head. The accuracy of the system was characterized by comparing the location of a marker analogue with an optically tracked pointer and with the ultrasound localization. Analyzing the phantom in several orientations revealed a mean system accuracy of 0.5 mm with a +/- 0.1 mm 95% confidence interval. These initial tests indicate that transcutaneous localization of implanted fiducial markers is possible with a high degree of accuracy. The results of these experiments will help in determining a final marker design.