Guido Boening

Feasibility of stereo-infrared tracking to monitor patient motion during cardiac SPECT imaging

Patient motion during cardiac SPECT imaging can cause diagnostic imaging artifacts. We investigated the feasibility of monitoring patient motion using the Polaris motion-tracking system. This system uses passive infrared reflection from small spheres to provide real-time position data with vendor stated 0.35 mm accuracy and 0.2 mm repeatability. In our configuration, the Polaris system views through the SPECT gantry toward the patients head. List-mode event data were temporally synchronized with motion-tracking data utilizing a modified LabVIEW virtual instrument that we have employed in previous optical motion-tracking investigations. Calibration of SPECT to Polaris coordinates was achieved by determining the transformation matrix necessary to align the position of four reflecting spheres as seen by Polaris, with the location of Tc-99m activity placed inside the sphere mounts as determined in SPECT reconstructions. We have successfully tracked targets placed on volunteers in simulated imaging positions on the table of our SPECT system. We obtained excellent correlation (R/sup 2/>0.998) between the change in location of the targets as measured by our SPECT system and the Polaris. We have also obtained excellent agreement between the recordings of the respiratory motion of four targets attached to an elastic band wrapped around the abdomen of volunteers and from a pneumatic bellows. We used the axial motion of point sources as determined by the Polaris to correct the motion in SPECT image acquisitions yielding virtually identical point source full-width at half-maximum and full-width at tenth-maximum values, and profiled maximum heart wall counts of cardiac phantom images, compared to the reconstructions with no motion.

Use of three-dimensional Gaussian interpolation in the projector/backprojector pair of iterative reconstruction for compensation of known rigid-body motion in SPECT

Due to the extended imaging times employed in single photon emission computed tomography (SPECT) and positron emission tomography (PET), patient motion during imaging is a common clinical occurrence. The fast and accurate correction of the three-dimensional (3-D) translational and rotational patient motion in iterative reconstruction is thus necessary to address this important cause of artifacts. We propose a method of incorporating 3-D Gaussian interpolation in the projector/backprojector pair to facilitate compensation for rigid-body motion in addition to attenuation and distance-dependent blurring. The method works as the interpolation step for moving the current emission voxel estimates and attenuation maps in the global coordinate system to the new patient location in the rotating coordinate system when calculating the expected projection. It also is employed for moving back the backprojection of the ratio of the measured projection to the expected projection and backprojection of the unit value (sensitivity factor) to the original location. MCAT simulations with known six-degree-of-freedom (6DOF) motion were employed to evaluate the accuracy of our method of motion compensation. We also tested the method with acquisitions of the data spectrum anthropomorphic phantom where motion during SPECT acquisition was measured using the Polaris IR motion tracking system. No motion artifacts were seen on the reconstructions with the motion compensation

Study of relative quantification of Tc-99 m with partial volume effect and spillover correction for SPECT oncology imaging

The apparent concentration of activity in structures in nuclear medicine images depends on their size relative to the system spatial resolution. This dependence is called the partial volume effect (PVE). Spillover (SO) or the blurring into a structure of counts originating in nearby structures also alters the apparent concentration of activity. In combination these effects impact the detection of lesions and quantification of activity within structures in the slices. The increased accessibility of dual-modality imaging systems makes available high-resolution anatomical information which is registered with the emission slices and can be used in correcting for the PVE and SO. In this study we investigated the use of the template projection-reconstruction method for correction of the PVE and SO. We examined the impact of correction on visual image quality and the quantification of activity in simulated spheres of varying contrast relative to a uniform background distribution of activity. Our enhancements to the template projection-reconstruction methodology included both an improvement in the matching of the blurring in the reconstructed templates of structures to the actual blurring in the reconstructed slices, and accounting for the fractional presence of structures in SPECT voxels. We determined that such corrections for the PVE and SO can dramatically improve both the visualization and quantification of activity within the source distributions we investigated.

Motion correction for cardiac SPECT using a RBI-ML partial-reconstruction approach

In single photon computed tomography, patient motion can significantly affect image quality. Methods to correct for patient motion rely on available information about the motion or stable algorithms have to be developed to detect and describe object motion. In this work we introduce a reconstruction method that corrects for rigid body motion and we investigate a method to exploit the motion information that might be hidden in the projection data and possibly modify motion descriptions that were retrieved by external motion tracking devices. The search method was based on reconstructing only parts of the projection angles using an RBI-ML partial-reconstruction approach (PRA). It tests a motion description with only one forward projection per detector head instead of a full reconstruction using all projections. A figure of merit based on the per angle likelihood function in projection space was introduced. With simulated MCAT data we could show that the PRA was able to identify the optimum 3D rigid body motion correction with an error of 1 pixel in each cartesian direction. Future work will address 3D translations, multiple motions, detector resolution compensation and will also concentrate on the improvement of the figure of merit.

An investigation of iterative reconstruction strategies for lung lesion detection in SPECT

ROC and localization ROC (LROC) studies assessed whether modeling of the acquisition physics during the reconstruction process could improve detection of SLN in SPECT images. The radiotracer used was Tc-99m-labeled Neotect, and studies were run both with hybrid images and with phantom images. Iterative reconstruction strategies were defined with various combinations of attenuation correction (AC), scatter correction (SC), and resolution correction (RC). Results from both human and model observers indicate that AC degrades lung-lesion detection relative to applying RC alone. Subsequent studies with the model observer suggest that the relative effectiveness of the different iterative strategies will depend on observer knowledge of the mean normal background

The estimation of attenuation maps for cardiac-SPECT using cone-beam imaging of high-energy photons through parallel-hole collimators

The goal of these investigations was to develop a rapid transmission imaging method for estimating truncation-free attenuation maps for cardiac SPECT without mechanical motion of the transmission sources. We conducted both theoretical and experimental investigations of cone-beam transmission imaging. The experimental studies were performed using the two Ba-133 point sources of the Beacon scanning-point source transmission system on our Philips Medical Systems Irix SPECT system. We obtained from the manufacturer the ability to position the point sources at any fixed location axially along the path they move during scanning. We determined experimentally that it was necessary to separate the point-sources axially so that the reconstructed attenuation maps would axially span the heart region. The dual-planar-circular-orbit cone-beam iterative ML-EM reconstruction was used to estimate the attenuation maps. Physical phantom studies and one study with a volunteer have been performed to validate the reconstruction algorithm and evaluate acquisition protocols. We have determined that we can obtain truncation-free attenuation maps covering an axial extent of 16.8 cm with 2 minutes of acquisition time.

Study of relative quantitation of Tc-99m annexin localization in pulmonary nodules using an anthropomorphic phantom

In current clinical oncology practice, it often takes weeks or months of cancer therapy until a response to treatment can be identified by evaluation of tumor size in images. It is hypothesized that changes in relative localization of the apoptosis imaging agent Tc-99m Annexin before and after the administration of chemotherapy may be useful as an early indicator of the success of therapy. The objective of this study was to determine the minimum relative change in tumor localization that could be confidently determined as an increased localization. A modified version of the Data Spectrum Anthropomorphic Torso phantom, in which four spheres could be positioned in the lung region, was filled with organ concentrations of Tc-99m representative of those observed in clinical imaging of Tc-99m Annexin. Five acquisitions of an initial sphere to lung concentration, and at concentrations of 1.1, 1.2, 1.3, and 1.4 times the initial concentration, were acquired at clinically realistic count levels. The acquisitions were reconstructed by filtered backprojection, ordered subset expectation maximization (OSEM) without attenuation compensation (AC), and OSEM with AC. Permutation methodology was used to create multiple region-of-interest count ratios from the five noise realizations at each concentration and between the elevated and initial concentrations. The resulting distributions were approximated by Gaussians, which were then used to estimate the likelihood of Type 1 and Type 2 Errors. It was determined that for the cases investigated, greater than a 20% to 30% or more increase was needed to confidently determine that an increase in localization had occurred depending on sphere size and reconstruction strategy.

Transmission imaging with axially overlapping cone-beams

We have shown that cone-beam transmission imaging of medium-energy photons that penetrate the parallel-hole collimators can be used to rapidly estimate attenuation maps for use in reconstruction of cardiac SPECT images. Such a transmission imaging geometry offers the advantages of eliminating the need to mechanically move the point-sources during imaging, and minimizes cross-talk between emission and transmission imaging. The axial extent over which artifact-free attenuation maps can be reconstructed is limited by the cone-beam geometry and source collimation. We investigated irradiation of a single head by multiple point-sources such that their asymmetric cone-beam fields overlap in the axial direction as a method of extending the axial coverage of the patient. This study reports on testing of a penalized-likelihood algorithm for transmission reconstruction of overlapping cone-beams. This algorithm was evaluated through MCAT simulations and applied to transmission measurements of an anthropomorphic phantom. The experimental work consisted of performing a series of flood and transmission measurements on the anthropomorphic phantom with shifted axial locations of point-sources. We summed the projection data from individual measurements to simulate the projection data for a multiple point-source system. With the proposed penalized-Iikelihood algorithm, the full axial extent (20.5 cm) of the anthropomorphic phantom was reconstructed for the overlapping cone-beam geometry with 2 point-sources per camera head.