AN ULTRASONIC APPROACH TO LOCALIZATION OF FIDUCIAL MARKERS FOR INTERACTIVE, IMAGE-GUIDED NEUROSURGERY - PART I - PRINCIPLES

Fiducial markers are reference points used in the registration of imag e space(s) with physical (patient) space. As applied to interactive, i mage-guided surgery, the registration of image space with physical spa ce allows the current location of a surgical tool to be indicated on a computer display of patient-specific preoperative images. This intras urgical guidance information is particularly valuable in surgery withi n the brain, where visual feedback is limited. The accuracy of the map ping between physical and image space depends upon the accuracy with w hich the fiducial markers were located in each coordinate system. To e ffect accurate space registration for interactive, image-guided neuros urgery, the use of permanent fiducial markers implanted into the surfa ce of the skull is proposed in this paper. These small cylindrical mar kers are composed of materials that make them visible in the image set s. The challenge lies in locating the subcutaneous markers in physical space, This paper presents an ultrasonic technique for transcutaneous ly detecting the location of these markers. The technique incorporates an algorithm based on detection of characteristic properties of the r eflected A-mode ultrasonic waveform. The results demonstrate that ultr asound is an appropriate technique for accurate transcutaneous marker localization. The companion paper to this article describes an automat ic, enhanced implementation of the marker-localization theory describe d in this article.