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

Abstract

Our overall research goal is to devise a robust method of tracking and compensating patient motion by combining an emission data based approach with a visual tracking system (VTS) that provides an independent estimate of motion. Herein, we present the latest hardware configuration of the VTS, a test of the accuracy of motion tracking by it, and our solution for synchronization between the SPECT and the optical acquisitions. The current version of the VTS includes stereo imaging with sets of optical network cameras with attached light sources, 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 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 designed such that 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. The ability of the VTS to track motion was assessed by comparing its results to those of the Polaris infra-red tracking system (Northern Digital Inc. Waterloo, ON, Canada). The difference in the motions assessed by the two systems was generally less than 1mm. Synchronization was assessed in two ways. First, optical cameras were aimed at a digital clock and the elapsed time estimated by the cameras was compared to the actual time shown by the clock in the images. Second, synchronization was 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.

A flexible multicamera visual‐tracking system for detecting and correcting motion‐induced artifacts in cardiac SPECT slices

“…This has ultimately limited the clinical usage of the previous marker-tracking systems we have investigated, which have viewed patients from a single end of the SPECT imaging bed. 18,19,21 We found better, but still less than ideal, operation when two independent tracking systems configured with two cameras each in a fixed geometry viewed patients from each end of the SPECT gantry. 20 We hypothesize that using more cameras which could be flexibly positioned and which acted together to provide stereo-tracking of the markers would further improve the robustness of marker tracking.…”

“…Following the work of others for head motion compensation in SPECT 14 and PET, [15][16][17] our group has been working toward developing a robust method to track and compensate for patient rigid-body ͑RB͒ and respiratory motions in cardiac SPECT. [18][19][20][21] We have investigated employing infrared ͑IR͒ and optical cameras with their own source of illumination to track retroreflective markers on stretchy bands wrapped about the chest and abdomen of patients. These retroreflective markers are imaged with high contrast regardless of room lighting.…”

Digital anthropomorphic phantoms of non-rigid human respiratory and voluntary body motion for investigating motion correction in emission imaging

“…These motions introduce an additional source of blurring and artifacts to the SPECT and PET reconstructions due to inconsistent projection data, and a mismatch between emission data and attenuation maps employed for attenuation correction Alessio et al 2010, McQuaid and Hutton 2008). A number of methods have been developed to detect and correct for body and respiratory motion using emission data (O’Connor et al 1998, Kyme et al 2003, Dawood et al 2008, Gilland et al 2002) or information from external tracking devices (Bloomfield et al 2003, Bruyant et al 2005, Barnes et al 2008, McNamara et al 2009, Buther et al 2009, Mukherjee et al 2009). …”

Respiratory motion correction in gated cardiac SPECT using quaternion‐based, rigid‐body registration

“…8,9 To combat the increased noise levels of respiratory-gated images in emission tomography, motion correction using an estimate of the heart motion between the image frames combined with temporal summing has been proposed. 7,[10][11][12][13][14][15][16][17][18][19] Gating of the respiratory cycle may be accomplished by affixing a piezoelectric elasticized belt around the patient's chest. As the patient breathes, a signal is generated corresponding to the tension in the belt.…”

“…Physical devices have been used that track the motion of markers placed on the patient's chest throughout the acquisition. [10][11][12] These devices offer high precision; however, the motion of deeply embedded structures within the body may be different than that of the surface of the body. Furthermore, implementation of physical tracking methods requires hardware integration and calibration for each scanner in a facility, which may be expensive and require regular quality assurance.…”