The superposition and registration of differing tomographic views is a difficult problem for investigators attempting to correlate brain form (structure), derived from x-ray computed tomography (CT) images, and brain function (metabolism), revealed by nuclear medicine positron emission tomography (PET) images. It is hoped that development of techniques for the accurate correlation of CT structural data with PET metabolic information will enhance our understanding of the processes underlying the generation of PET images. Our approach has three stages: first, practical methods must be discovered for the accurate and reproducible placement of the head within a tomographic scanner's aperture. Second, techniques for monitoring head position during the image acquisition process must be developed to correct for head movement before the image is generated. Third, simplified algorithms must be found for scaling and registering digitized images from different scanners on a digital display subsystem. Precise orientation of the subject's skull within the scanner's aperture is monitored and recorded through the use of a PC-based Polhemus position/orientation measurement subsystem, allowing simultaneous use of two independent sensors. The development of two inexpensive custom-molded oral appliances allows the Polhemus subsystem's sensor to be fixed to the subject's skull. A novel targeting algorithm was derived to provide visual cues related to head position within a scanner's imaging volume to the system operator. Two-sensor software was completed, and extensive evaluation has begun prior to its experimental use with test subjects. An additional position/orientation measurement subsystem has been obtained and evaluated for linearity and for sensitivity to nearby metallic objects, a problem common to all electromagnetic-based tracking systems. This device's utilization of quasi-static fields was designed to increase its immunity to certain types of metal in close proximity. Although performance of this new position measurement system was good, it did not outperform the Polhemus system in the presence of PET scanners.
