Patient motion during single photon emission computed tomographic (SPECT) acquisition causes inconsistent
projection data and reconstruction artifacts which can significantly affect diagnostic accuracy. We have investigated use
of the Polaris stereo infrared motion-tracking system to track 6-Degrees-of-Freedom (6-DOF) motion of spherical
reflectors (markers) on stretchy bands about the patient's chest and abdomen during cardiac SPECT imaging. The
marker position information, obtained by opposed stereo infrared-camera systems, requires processing to correctly
record tracked markers, and map Polaris co-ordinate data into the SPECT co-ordinate system. One stereo camera views
the markers from the patient's head direction, and the other from the patient's foot direction. The need for opposed
cameras is to overcome anatomical and geometrical limitations which sometimes prevent all markers from being seen
by a single stereo camera. Both sets of marker data are required to compute rotational and translational 6-DOF motion
of the patient which ultimately will be used for SPECT patient-motion corrections. The processing utilizes an algorithm
involving least-squares fitting, to each other, of two 3-D point sets using singular value decomposition (SVD) resulting
in the rotation matrix and translation of the rigid body centroid. We have previously demonstrated the ability to monitor
multiple markers for twelve patients viewing from the foot end, and employed a neural network to separate the periodic
respiratory motion component of marker motion from aperiodic body motion. We plan to initiate routine 6-DOF
tracking of patient motion during SPECT imaging in the future, and are herein evaluating the feasibility of employing
opposed stereo cameras.
Patient motion during SPECT acquisition causes inconsistent projection data and reconstruction artifacts which can significantly affect the diagnostic accuracy of SPECT. The tracking of motion by infrared monitoring spherical reflectors (markers) on the patient's surface can provide 6-Degrees-of-Freedom (6-DOF) motion information capable of providing clinically robust correction. Object rigid-body motion can be described by 3 translational DOF and 3 rotational DOF. Polaris marker position information obtained by stereo infrared cameras requires algorithmic processing to correctly record the tracked markers, and to calibrate and map Polaris co-ordinate data into the SPECT co-ordinate system. Marker data then requires processing to determine the rotational and translational 6-DOF motion to ultimately be used for SPECT image corrections. This processing utilizes an algorithm involving least-squares fitting, to each other, of two 3-D point sets using singular value decomposition (SVD) resulting in the rotation matrix and translation of the rigid body centroid. We have demonstrated the ability to monitor 12 clinical patients as well as 7 markers on 2 elastic belts worn by a volunteer while intentionally moving, and determined the 3 axis Euclidian rotation angles and centroid translations. An anthropomorphic phantom with Tc-99m added to the heart, liver, and body was simultaneously SPECT imaged and motion tracked using 4 rigidly mounted markers. The determined rotation matrix and translation information was used to correct the image resulting in virtually identical "no motion" and "corrected" images. We plan to initiate routine 6-DOF tracking of patient motion during SPECT imaging in the future.
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