Joyeeta Mitra Mukherjee

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

Patient motion is inevitable in SPECT and PET due to the lengthy period of time patients are imaged. The authors hypothesized that the use of external-tracking devices which provide additional information on patient motion independent of SPECT data could be employed to provide a more robust correction than obtainable from data-driven methods. Therefore, the authors investigated the Vicon MX visual-tracking system (VTS) which utilizes near-infrared (NIR) cameras to stereo-image small retroreflective markers on stretchy bands wrapped about the chest and abdomen of patients during cardiac SPECT. The chest markers are used to provide an estimate of the rigid-body (RB) motion of the heart. The abdomen markers are used to provide a signal used to bin list-mode acquisitions as part of correction of respiratory motion of the heart. The system is flexible in that the layout of the cameras can be designed to facilitate marker viewing. The system also automatically adapts marker tracking to employ all of the cameras visualizing a marker at any instant, with visualization by any two being sufficient for stereo-tracking. Herein the ability of this VTS to track motion with submillimeter and subdegree accuracy is established through studies comparing the motion of Tc-99m containing markers as assessed via stereo-tracking and from SPECT reconstructions. The temporal synchronization between motion-tracking data and timing marks embedded in list-mode SPECT acquisitions is shown to agree within 100 ms. In addition, motion artifacts were considerably reduced in reconstructed SPECT slices of an anthropomorphic phantom by employing within iterative reconstruction the motion-tracking information from markers attached to the phantom. The authors assessed the number and placement of NIR cameras required for robust motion tracking of markers during clinical imaging in 77 SPECT patients. They determined that they were able to track without loss during the entire period of SPECT and transmission imaging at least three of the four markers on the chest and one on the abdomen bands 94% and 92% of the time, respectively. The ability of the VTS to correct motion clinically is illustrated for ten patients who volunteered to undergo repeat-rest imaging with the original-rest SPECT study serving as the standard against which to compare the success of correction. Comparison of short-axis slices shows that VTS-based motion correction provides better agreement with the original-rest-imaging slices than either no correction or the vendor-supplied software for motion correction on, our SPECT system. Comparison of polar maps shows that VTS-based motion-correction results in less numerical difference on average in the segments of the polar maps between the original-rest study and the second-rest study than the other two strategies. The difference was statistically significant for the comparison between VTS-based and clinical vendor-supplied software correction. Taken together, these findings suggest that VTS-based motion correction is superior to either no-motion correction or the vendor-supplied software the authors investigated in clinical practice.

Estimation of Rigid-Body and Respiratory Motion of the Heart From Marker-Tracking Data for SPECT Motion Correction

Motion of patients undergoing cardiac SPECT perfusion imaging causes artifacts in the acquired images which may lead to difficulty in interpretation. Our work investigates a technique of obtaining patient motion estimates from retro-reflective markers on stretchy bands wrapped around the chest and abdomen of patients being imaged clinically. Motion signals obtained from the markers consist of at least two components, body motion (BM) and periodic motion (PM) due to respiration. We present a method for separating these components from the motion-tracking data of each marker, and then report a method for combining the BM estimated from chest markers to estimate the 6-degree-of-freedom (6-DOF) rigid-body motion (RBM) of the heart. Motion studies of volunteers and patients are used to evaluate the methods. Illustrative examples of the motion of the heart due to patient body movement and respiration (upward creep) are presented and compared to estimates of the motion of the heart obtained directly from SPECT data. Our motion-tracking method is seen to give reasonable agreement with the motion-estimates from the SPECT data while being considerably less noisy.

MRI investigation of the linkage between respiratory motion of the heart and markers on patient's abdomen and chest: Implications for respiratory amplitude binning list-mode PET and SPECT studies

Respiratory motion of the heart impacts the diagnostic accuracy of myocardial-perfusion emission-imaging studies. Amplitude binning has come to be the method of choice for binning list-mode based acquisitions for correction of respiratory motion in PET and SPECT. In some subjects respiratory motion exhibits hysteretic behavior similar to damped non-linear cyclic systems. The detection and correction of hysteresis between the signals from surface movement of the patients body used in binning and the motion of the heart within the chest remains an open area for investigation. This study reports our investigation in nine volunteers of the combined MRI tracking of the internal respiratory motion of the heart using Navigators with stereo-tracking of markers on the volunteers chest and abdomen by a visual-tracking system (VTS). The respiratory motion signals from the internal organs and the external markers were evaluated for hysteretic behavior analyzing the temporal correspondence of the signals. In general, a strong, positive correlation between the external marker motion (AP direction) and the internal heart motion (SI direction) during respiration was observed. The average ± standard deviation in the Spearmans ranked correlation coefficient (ρ) over the nine volunteer studied was 0.92±0.1 between the external abdomen marker and the internal heart, and 0.87±0.2 between the external chest marker and the internal heart. However despite the good correlation on average for the nine volunteers, in three studies a poor correlation was observed due to hysteretic behavior between inspiration and expiration for either the chest marker and the internal motion of the heart, or the abdominal marker and the motion of the heart. In all cases we observed a good correlation of at least either the abdomen or the chest with the heart. Based on this result, we propose the use of marker motion from both the chest and abdomen regions when estimating the internal heart motion to detect and address hysteresis when binning list-mode emission data.

Quantitative Study of Rigid-Body and Respiratory Motion of Patients Undergoing Stress and Rest Cardiac SPECT Imaging

We report patient motion in 110 Tl-201 cardiac perfusion SPECT studies in 66 patients. The imaging consisted of emission followed by sequential transmission imaging during which motion tracking with a visual tracking system (VTS) was performed. We investigated the extent, time, and frequency of respiratory and rigid-body motion in these patients. We also determined whether the motion occurred gradually or in sudden jumps, whether it was sustained, and if it occurred along one or more axes predominantly. We then studied the differences in respiratory and body motion (BM), if any, between stress versus rest imaging groups, male versus female subjects, and exercise versus pharmacological stress groups. We found that 23% of the studies had sustained motion (> 4 min.) of between 3-6 mm, and 5% had sustained motion larger than 6 mm during emission imaging. In terms of respiratory motion, 13% showed a downward trend of the respiratory baseline of more than 6 mm during emission imaging. Also, in 9% of the studies, the average position of patients was displaced by more than 3 mm between emission and transmission imaging phases. Both of these motions may lead to misalignment of the attenuation map. In hypothesis testing of grouped studies, it was determined that stress and rest imaging did not show any significant differences in body motion but did in respiratory motion associated with a change in respiration following stress. Exercise-stress studies showed a larger extent of respiratory motion than the pharmacologically induced stress studies. Significant differences in body and respiratory motion of male and female groups were also observed. A visual assessment of the reconstructed slices in the studies with measured motion was made to investigate the impact of the motion. Illustrative example studies are included.

A Comparison of Cost Functions for Data-Driven Motion Estimation in Myocardial Perfusion SPECT Imaging

In myocardial perfusion SPECT imaging patient motion during acquisition causes severe artifacts in about 5% of studies. Motion estimation strategies commonly used are a) data-driven, where the motion may be determined by registration and checking consistency with the SPECT acquisition data, and b) external surrogate-based, where the motion is obtained from a dedicated motion-tracking system. In this paper a data-driven strategy similar to a 2D-3D registration scheme with multiple views is investigated, using a partially reconstructed heart for the 3D model. The partially-reconstructed heart has inaccuracies due to limited angle artifacts resulting from using only a part of the SPECT projections acquired while the patient maintained the same pose. The goal of this paper is to compare the performance of different cost-functions in quantifying consistency with the SPECT projection data in a registration-based scheme for motion estimation as the image-quality of the 3D model degrades. Six intensity-based metrics- Mean-squared difference (MSD), Mutual information (MI), Normalized Mutual information NMI), Pattern intensity (PI), normalized cross-correlation (NCC) and Entropy of the difference (EDI) were studied. Quantitative and qualitative analysis of the performance is reported using Monte-Carlo simulations of a realistic heart phantom including degradation factors such as attenuation, scatter and collimator blurring. Further the image quality of motion-corrected images using data-driven motion estimates was compared to that obtained using the external motion-tracking system in acquisitions of anthropomorphic phantoms and patient studies in a real clinical setting. Pattern intensity and Normalized Mutual Information cost functions were observed to have the best performance in terms of lowest average position error and stability with degradation of image quality of the partial reconstruction in simulations and anthropomorphic phantom acquisitions. In patient studies, Normalized Mutual Information based data-driven estimates yielded comparable image quality to that obtained using external motion tracking.

An evaluation of data-driven motion estimation in comparison to the usage of external-surrogates in cardiac SPECT imaging

Motion estimation methods in single photon emission computed tomography (SPECT) can be classified into methods which depend on just the emission data (data-driven), or those that use some other source of information such as an external surrogate. The surrogate-based methods estimate the motion exhibited externally which may not correlate exactly with the movement of organs inside the body. The accuracy of data-driven strategies on the other hand is affected by the type and timing of motion occurrence during acquisition, the source distribution, and various degrading factors such as attenuation, scatter, and system spatial resolution. The goal of this paper is to investigate the performance of two data-driven motion estimation schemes based on the rigid-body registration of projections of motion-transformed source distributions to the acquired projection data for cardiac SPECT studies. Comparison is also made of six intensity based registration metrics to an external surrogate-based method. In the data-driven schemes, a partially reconstructed heart is used as the initial source distribution. The partially-reconstructed heart has inaccuracies due to limited angle artifacts resulting from using only a part of the SPECT projections acquired while the patient maintained the same pose. The performance of different cost functions in quantifying consistency with the SPECT projection data in the data-driven schemes was compared for clinically realistic patient motion occurring as discrete pose changes, one or two times during acquisition. The six intensity-based metrics studied were mean-squared difference, mutual information, normalized mutual information (NMI), pattern intensity (PI), normalized cross-correlation and entropy of the difference. Quantitative and qualitative analysis of the performance is reported using Monte-Carlo simulations of a realistic heart phantom including degradation factors such as attenuation, scatter and system spatial resolution. Further the visual appearance of motion-corrected images using data-driven motion estimates was compared to that obtained using the external motion-tracking system in patient studies. Pattern intensity and normalized mutual information cost functions were observed to have the best performance in terms of lowest average position error and stability with degradation of image quality of the partial reconstruction in simulations. In all patients, the visual quality of PI-based estimation was either significantly better or comparable to NMI-based estimation. Best visual quality was obtained with PI-based estimation in one of the five patient studies, and with external-surrogate based correction in three out of five patients. In the remaining patient study there was little motion and all methods yielded similar visual image quality.

Design of a combined fan and multi-pinhole collimator combination for clinical I-123 DaTscan imaging on dual-headed SPECT systems

For the recently FDA approved Parkinsons Disease (PD) SPECT imaging agent I-123 labeled DaTscan the volume of interest (VOl) is the interior portion of the brain. However imaging of the occipital lobe is also required with PD for calculation of the striatal binding ratio (SBR), a parameter of significance in early diagnosis, differentiation of PD from other disorders with similar clinical presentations, and monitoring progression. Thus we propose the usage of a combination of a multi-pinhole (MPH) collimator on one head of the SPECT system and a fan-beam on the other. The MPH would be designed to provide high resolution and sensitivity imaging of the interior portion of the brain. The fan-beam collimator would provide lower resolution but complete sampling of the brain addressing data sufficiency and allowing a volume-of-interest to be defined over the occipital lobe for calculation of SBRs. Herein we analyze 20 clinical DaTscan studies to provide information on the VOl, and then design a MPH collimator to image this VOl. Using standard collimator equations we determine a system spatial resolution for the MPH of 4.4 mm which is comparable to that of clinical PET systems, and significantly smaller than that of fan-beam collimators employed in SPECT. The combined sensitivity of the apertures of the MPH was larger than that of an ultra-high resolution fan-beam (LEUHRF) collimator, but smaller than that of a high resolution fan-beam collimator (LEHRF). On the basis of these early results we propose the exploration of further improvements in design, and the development of combined MPH and fan-beam reconstruction.

Combined respiratory and rigid body motion compensation in cardiac perfusion SPECT using a visual tracking system

We report on the validation of our combined respiratory and rigid body motion compensation strategy through acquisitions of the Data Spectrum anthropomorphic phantom, and investigation of clinical efficacy and robustness in 25 cardiac perfusion patient studies, employing a visual tracking system (VTS). The heart and liver was filled with a 2∶1 concentration of Tc-99m and two sets of SPECT data were acquired. Each set of SPECT data consisted of a rest-perfusion baseline frame-mode emission acquisition, a Beacon (Philips, Cleveland, OH) transmission acquisition, and a list-mode emission acquisition. Respiratory motion was simulated during the list-mode acquisitions using the Quasar (Modus Medical Devices Inc. ON, Canada). Rigid-body motion was introduced in one of the two list-mode acquisitions by rotating the phantom around the x-axis and translating the phantom in x, y, and z. Patient volunteers with written consent were similarly acquired and asked to execute some predefined body motion during the list-mode acquisition. Motion tracking was performed using 6 near infrared Vicon cameras in combination with 7 retro-reflective markers, 5 placed on the chests of both patient volunteers and phantom, 2 placed on the abdomen of patient volunteers, and 2 placed on the vertical motion stage of the Quasar to simulate abdominal phantom motion. Processing steps included, down sampling VTS positional data to 10 Hz (100 ms) and synchronized with 100 ms SPECT frames, separating rigid body and respiratory motion and estimating 6 DOF rigid body motion, amplitude bin 100 ms frames into respiratory projection sets, reconstruct with rigid body motion compensation respiratory projection sets, estimated respiratory motion employing intensity based estimation, combine rigid body and respiratory motion, and reconstruct with combined compensation. We showed for both phantom and patient acquisitions that combined respiratory and rigid body motion compensation improve the visual appearance of slices.

Quantitative study of rigid-body and respiratory motion of patients undergoing stress and rest cardiac SPECT imaging

In our previous work we investigated the technique of obtaining patient motion estimates from retro-reflective markers on stretchy bands wrapped around the chest and abdomen of patients undergoing cardiac SPECT perfusion imaging [1]. Motion signals obtained from the markers were separated into two components, body motion (BM) and respiratory motion (RM). In this paper we studied the frequency of occurrence, the time and the extent of estimated body motion of patients, and the extent of respiratory motion. Additionally, studies were grouped by Stress or Rest imaging to determine statistically significant differences in respiratory and body motion. Based on 80 studies, with 40 pairs of Stress and Rest motion data, we found 39% of studies containing 3-6 mm and 10% containing more than 6 mm of body motion during emission imaging. About 26% of the studies contained body motion more than 3 mm during transmission imaging. In 21% of the studies, large respiratory baseline drift of more than 6 mm was observed. Stress and Rest studies showed significant differences in body motion and respiratory motion, all of which were associated with change in respiration due to stress.

Fast shift-variant resolution compensation within iterative reconstruction for fan-beam collimator

In this paper we have modeled the spatially-varying resolution of a fan-beam collimator as a function of both distance and lateral position across the imaging plane within iterative reconstruction. We demonstrate that an incremental blurring technique can be used to more efficiently model such non-stationary resolution than the straightforward non-stationary convolution methodology with a gain of 1.5X in speed and 99% less memory usage. This method can therefore be used for efficient variable resolution compensation in 3D for collimator geometries in which the detector response function varies across the imaging plane as well as with distance from the face of the collimator†.