A robust visual tracking system for patient motion detection in SPECT: hardware solutions

The goal of our project is to devise a robust method to track and compensate patient motion by combining an emission data based approach with a visual tracking system (VTS) that provides an independent estimate of motion. In a previous study, we used the Polaris infra-red system (NDI Inc., Ontario, Canada) as the gold standard to show that the VTS can accurately track motion without having the limitations of the Polaris. In the present work, we present the latest hardware configuration of the VTS and our solution for temporal synchronization between the SPECT and the optical acquisitions. The current version of the VTS includes stereo imaging with sets of optical network cameras with lighting, a SPECT/VTS calibration phantom, a black stretchable garment with reflective spheres to track chest motion, and a computer to control the cameras. The computer also stores the JPEG files generated by the optical cameras with timing synchronization to the list-mode acquisition of events on our SPECT system. Five Axis PTZ 2130 network cameras (Axis Communications AB, Lund, Sweden) were used to track motion of spheres with a highly retro-reflective coating using stereo methods. The calibration phantom is comprised of seven reflective spheres, and radioactivity can be added to the tip of the mounts holding the spheres. This phantom is used to determine the transformation to be applied to convert the motion detected by the VTS into the SPECT coordinates system. Synchronization is assessed in two ways. First, optical cameras are aimed at a digital clock and the elapsed time estimated by the cameras is compared to the actual time shown by the clock in the images. Synchronization is also assessed by moving a radioactive and reflective sphere three times during concurrent VTS and SPECT acquisitions and comparing the time at which motion occurred in the optical and SPECT images. The results show that optical and SPECT images stay synchronized within a 150 ms range. The 100Mbit network load is less than 10%, and the computer's CPU load is between 15 and 25%; thus, the VTS can be improved by adding more cameras or by increasing the image size and/or resolution while keeping an acquisition rate of 30 images per second per camera