W. P. Segars
Duke University
Network
Latest external collaboration on country level. Dive into details by clicking on the dots.
Publication
Featured researches published by W. P. Segars.
nuclear science symposium and medical imaging conference | 1998
W. P. Segars; David S. Lalush; Benjamin M. W. Tsui
We develop a realistic computerized heart phantom for use in medical imaging research. This phantom is a hybrid of realistic patient-based phantoms and flexible geometry-based phantoms. The surfaces of heart structures are defined using non-uniform rational B-splines (NURBS), as used in 3D computer graphics. The NURBS primitives define continuous surfaces allowing the phantom to be defined at any resolution. Also, by fitting NURBS to patient data, the phantom is more realistic than those based on solid geometry. An important innovation is the extension of NURBS to the fourth dimension, time, to model heart motion. Points on the surfaces of heart structures were selected from a gated MRI study of a normal patient. Polygon surfaces were fit to the points for each time frame, and smoothed. 3D NURBS surfaces were fit to the smooth polygon surfaces and then a 4D NURBS surface was fit through these surfaces. Each of the principal 4D surfaces (atria, ventricles, inner and outer walls) contains approximately 200 control points, We conclude that 4D NURBS are an efficient and flexible way to describe the heart and other anatomical objects for a realistic phantom.
nuclear science symposium and medical imaging conference | 1999
W. P. Segars; David S. Lalush; Benjamin M. W. Tsui
Respiratory motion can cause artifacts in myocardial SPECT and computed tomography (CT). The authors incorporate models of respiratory mechanics into the current 4D MCAT and into the next generation spline-based MCAT phantoms. In order to simulate respiratory motion in the current MCAT phantom, the geometric solids for the diaphragm, heart, ribs, and lungs were altered through manipulation of parameters defining them. Affine transformations were applied to the control points defining the same respiratory structures in the spline-based MCAT phantom to simulate respiratory motion. The Non-Uniform Rational B-Spline (NURBS) surfaces for the lungs and body outline were constructed in such a way as to be linked to the surrounding ribs. Expansion and contraction of the thoracic cage then coincided with expansion and contraction of the lungs and body. The changes both phantoms underwent were spline-interpolated over time to create time continuous 4D respiratory models. The authors then used the geometry-based and spline-based MCAT phantoms in an initial simulation study of the effects of respiratory motion on myocardial SPECT. The simulated reconstructed images demonstrated distinct artifacts in the inferior region of the myocardium. It is concluded that both respiratory models can be effective tools for researching effects of respiratory motion.
Medical Physics | 2008
W. P. Segars; Mahadevappa Mahesh; Thomas J. Beck; Eric C. Frey; Benjamin Tsui
The authors develop a unique CT simulation tool based on the 4D extended cardiac-torso (XCAT) phantom, a whole-body computer model of the human anatomy and physiology based on NURBS surfaces. Unlike current phantoms in CT based on simple mathematical primitives, the 4D XCAT provides an accurate representation of the complex human anatomy and has the advantage, due to its design, that its organ shapes can be changed to realistically model anatomical variations and patient motion. A disadvantage to the NURBS basis of the XCAT, however, is that the mathematical complexity of the surfaces makes the calculation of line integrals through the phantom difficult. They have to be calculated using iterative procedures; therefore, the calculation of CT projections is much slower than for simpler mathematical phantoms. To overcome this limitation, the authors used efficient ray tracing techniques from computer graphics, to develop a fast analytic projection algorithm to accurately calculate CT projections directly from the surface definition of the XCAT phantom given parameters defining the CT scanner and geometry. Using this tool, realistic high-resolution 3D and 4D projection images can be simulated and reconstructed from the XCAT within a reasonable amount of time. In comparison with other simulators with geometrically defined organs, the XCAT-based algorithm was found to be only three times slower in generating a projection data set of the same anatomical structures using a single 3.2 GHz processor. To overcome this decrease in speed would, therefore, only require running the projection algorithm in parallel over three processors. With the ever decreasing cost of computers and the rise of faster processors and multi-processor systems and clusters, this slowdown is basically inconsequential, especially given the vast improvement the XCAT offers in terms of realism and the ability to generate 3D and 4D data from anatomically diverse patients. As such, the authors conclude that the efficient XCAT-based CT simulator developed in this work will have applications in a broad range of CT imaging research.
ieee nuclear science symposium | 2001
W. P. Segars; Benjamin M. W. Tsui
Respiratory motion can cause artifacts in myocardial single photon emission computed tomography (SPECT) images, which can lead to the misdiagnosis of cardiac diseases. One method to correct for respiratory artifacts is through respiratory gating. We study the effectiveness of respiratory gating through a simulation study using the newly developed four-dimensional (4-D) NURBS-based cardiac-torso (NCAT) phantom. The organ shapes in the 4-D NCAT phantom are formed using nonuniform rational b-splines (NURBS) and are based on detailed human image data. With its basis on actual human data, the 4-D NCAT phantom realistically simulates human anatomy and motions such as the cardiac and respiratory motions. With the 4-D NCAT phantom, we generated 128 phantoms over one respiratory cycle (5 s per cycle) with the diaphragm and heart set to move a total of 4 cm from end-inspiration to end-expiration. The heart was set to beat with a normal contractile motion at a rate of I beat per second resulting in a total of five heart cycles. We divide the respiratory cycle into different numbers of respiratory gates (16, 8, and 4) by summing the phantoms. For each gate, we generate its projection data using an analytical projection algorithm simulating the effects of attenuation, scatter, and detector response. We then reconstruct the projections using an iterative OS-EM algorithm compensating for the three effects. The reconstructed images for each gating method were examined for artifacts due to the respiratory motion during that gate. We found that respiratory artifacts are significantly reduced if the respiratory motion of the heart that occurs during a gating time period is 1 cm or less. We conclude that respiratory gating is an effective method for reducing effects due to respiration. The timing of the respiratory gates for reduced image artifacts is dependent on the extent of the hearts motion during respiration. Index Terms-Biomedical image processing, biomedical nuclear imaging, image analysis, motion compensation, respiratory system, simulation software, single photon emission computed tomography (SPECT).
Proceedings of the IEEE | 2009
W. P. Segars; Benjamin M. W. Tsui
Recent work in the development of computerized phantoms has focused on the creation of ideal ldquohybridrdquo models that seek to combine the realism of a patient-based voxelized phantom with the flexibility of a mathematical or stylized phantom. We have been leading the development of such computerized phantoms for use in medical imaging research. This paper will summarize our developments dating from the original four-dimensional (4-D) mathematical cardiac-torso (MCAT) phantom, a stylized model based on geometric primitives, to the current 4-D extended cardiac-torso (XCAT) and mouse whole-body (MOBY) phantoms, hybrid models of the human and laboratory mouse based on state-of-the-art computer graphics techniques. This paper illustrates the evolution of computerized phantoms toward more accurate models of anatomy and physiology. This evolution was catalyzed through the introduction of nonuniform rational b-spline (NURBS) and subdivision (SD) surfaces, tools widely used in computer graphics, as modeling primitives to define a more ideal hybrid phantom. With NURBS and SD surfaces as a basis, we progressed from a simple geometrically based model of the male torso (MCAT) containing only a handful of structures to detailed, whole-body models of the male and female (XCAT) anatomies (at different ages from newborn to adult), each containing more than 9000 structures. The techniques we applied for modeling the human body were similarly used in the creation of the 4-D MOBY phantom, a whole-body model for the mouse designed for small animal imaging research. From our work, we have found the NURBS and SD surface modeling techniques to be an efficient and flexible way to describe the anatomy and physiology for realistic phantoms. Based on imaging data, the surfaces can accurately model the complex organs and structures in the body, providing a level of realism comparable to that of a voxelized phantom. In addition, they are very flexible. Like stylized models, they can easily be manipulated to model anatomical variations and patient motion. With the vast improvement in realism, the phantoms developed in our lab can be combined with accurate models of the imaging process (SPECT, PET, CT, magnetic resonance imaging, and ultrasound) to generate simulated imaging data close to that from actual human or animal subjects. As such, they can provide vital tools to generate predictive imaging data from many different subjects under various scanning parameters from which to quantitatively evaluate and improve imaging devices and techniques. From the MCAT to XCAT, we will demonstrate how NURBS and SD surface modeling have resulted in a major evolutionary advance in the development of computerized phantoms for imaging research.
IEEE Transactions on Medical Imaging | 2008
Arman Rahmim; Katie Dinelle; Ju-Chieh Cheng; Mikhail Shilov; W. P. Segars; Sarah Lidstone; Stephan Blinder; Olivier Rousset; Hamid Vajihollahi; Benjamin M. W. Tsui; Dean F. Wong; Vesna Sossi
With continuing improvements in spatial resolution of positron emission tomography (PET) scanners, small patient movements during PET imaging become a significant source of resolution degradation. This work develops and investigates a comprehensive formalism for accurate motion-compensated reconstruction which at the same time is very feasible in the context of high-resolution PET. In particular, this paper proposes an effective method to incorporate presence of scattered and random coincidences in the context of motion (which is similarly applicable to various other motion correction schemes). The overall reconstruction framework takes into consideration missing projection data which are not detected due to motion, and additionally, incorporates information from all detected events, including those which fall outside the field-of-view following motion correction. The proposed approach has been extensively validated using phantom experiments as well as realistic simulations of a new mathematical brain phantom developed in this work, and the results for a dynamic patient study are also presented.
Physics in Medicine and Biology | 2005
X Song; W. P. Segars; Yiping P. Du; Benjamin Tsui; Eric C. Frey
Interactions of incident photons with the collimator and detector, including septal penetration, scatter and x-ray fluorescence, are significant sources of image degradation in applications of SPECT including dual isotope imaging and imaging using radioisotopes that emit high- or medium-energy photons. Modelling these interactions using full Monte Carlo (MC) simulations is computationally very demanding. We present a new method based on the use of angular response functions (ARFs). The ARF is a function of the incident photons direction and energy and represents the probability that a photon will either interact with or pass through the collimator, and be detected at the intersection of the photons direction vector and the detection plane in an energy window of interest. The ARFs were pre-computed using full MC simulations of point sources that include propagation through the collimator-detector system. We have implemented the ARF method for use in conjunction with the SimSET/PHG MC code to provide fast modelling of both interactions in the patient and in the collimator-detector system. Validation results in the three cases studied show that there was good agreement between the projections generated using the ARF method and those from previously validated full MC simulations, but with hundred to thousand fold reductions in simulation time.
Medical Physics | 2013
W. P. Segars; Jason Bond; Jack Frush; Sylvia Hon; Chris Eckersley; Cameron H. Williams; Jianqiao Feng; Daniel J. Tward; J. T. Ratnanather; Michael I. Miller; Donald P. Frush; Ehsan Samei
PURPOSE The authors previously developed the 4D extended cardiac-torso (XCAT) phantom for multimodality imaging research. The XCAT consisted of highly detailed whole-body models for the standard male and female adult, including the cardiac and respiratory motions. In this work, the authors extend the XCAT beyond these reference anatomies by developing a series of anatomically variable 4D XCAT adult phantoms for imaging research, the first library of 4D computational phantoms. METHODS The initial anatomy of each phantom was based on chest-abdomen-pelvis computed tomography data from normal patients obtained from the Duke University database. The major organs and structures for each phantom were segmented from the corresponding data and defined using nonuniform rational B-spline surfaces. To complete the body, the authors manually added on the head, arms, and legs using the original XCAT adult male and female anatomies. The structures were scaled to best match the age and anatomy of the patient. A multichannel large deformation diffeomorphic metric mapping algorithm was then used to calculate the transform from the template XCAT phantom (male or female) to the target patient model. The transform was applied to the template XCAT to fill in any unsegmented structures within the target phantom and to implement the 4D cardiac and respiratory models in the new anatomy. Each new phantom was refined by checking for anatomical accuracy via inspection of the models. RESULTS Using these methods, the authors created a series of computerized phantoms with thousands of anatomical structures and modeling cardiac and respiratory motions. The database consists of 58 (35 male and 23 female) anatomically variable phantoms in total. Like the original XCAT, these phantoms can be combined with existing simulation packages to simulate realistic imaging data. Each new phantom contains parameterized models for the anatomy and the cardiac and respiratory motions and can, therefore, serve as a jumping point from which to create an unlimited number of 3D and 4D variations for imaging research. CONCLUSIONS A population of phantoms that includes a range of anatomical variations representative of the public at large is needed to more closely mimic a clinical study or trial. The series of anatomically variable phantoms developed in this work provide a valuable resource for investigating 3D and 4D imaging devices and the effects of anatomy and motion in imaging. Combined with Monte Carlo simulation programs, the phantoms also provide a valuable tool to investigate patient-specific dose and image quality, and optimization for adults undergoing imaging procedures.
IEEE Transactions on Nuclear Science | 2000
Benjamin M. W. Tsui; W. P. Segars; David S. Lalush
In this study, the authors investigate the effects of two patient involuntary motions, namely upward creep (UC) and respiratory (RSP) motion, in myocardial SPECT images. A new realistic torso phantom was developed based on data from the Visual Human Project and using non-uniform rational B-splines (NURBS) modeling. The heart and diaphragm of the phantom move with a linear upward translation to model UC and move in a sinusoidal up and down fashion to model RSP motion. Simulated emission and transmission CT data sets were generated from the phantom using a L-shaped dual-camera SPECT system with a radioactivity distribution modeling that of a Tl-210 study with UC of 2 cm. The effects of attenuation and collimator-detector response are included in the simulation. A patient study with the same extent of UC was used for comparison. Both simulated and patient data were reconstructed with and without correction for attenuation and UC. Similar data sets were generated from the phantom with RSP motion. The simulated reconstructed images demonstrated distinct UC and RSP artifacts in the inferior region of the myocardium. The UC artifact can be greatly reduced with simple UC correction. However, the correction of RSP artifact may require respiratory gating.
The Journal of Nuclear Medicine | 2010
Mary A. Keenan; Michael Stabin; W. P. Segars; Michael J. Fernald
Rodent species are widely used in the testing and approval of new radiopharmaceuticals, necessitating murine phantom models. As more therapy applications are being tested in animal models, calculating accurate dose estimates for the animals themselves becomes important to explain and control potential radiation toxicity or treatment efficacy. Historically, stylized and mathematically based models have been used for establishing doses to small animals. Recently, a series of anatomically realistic human phantoms was developed using body models based on nonuniform rational B-spline. Realistic digital mouse whole-body (MOBY) and rat whole-body (ROBY) phantoms were developed on the basis of the same NURBS technology and were used in this study to facilitate dose calculations in various species of rodents. Methods: Voxel-based versions of scaled MOBY and ROBY models were used with the Vanderbilt multinode computing network (Advanced Computing Center for Research and Education), using geometry and tracking radiation transport codes to calculate specific absorbed fractions (SAFs) with internal photon and electron sources. Photon and electron SAFs were then calculated for relevant organs in all models. Results: The SAF results were compared with values from similar studies found in reference literature. Also, the SAFs were used with standardized decay data to develop dose factors to be used in radiation dose calculations. Representative plots were made of photon electron SAFs, evaluating the traditional assumption that all electron energy is absorbed in the source organs. Conclusion: The organ masses in the MOBY and ROBY models are in reasonable agreement with models presented by other investigators noting that considerable variation can occur between reported masses. Results consistent with those found by other investigators show that absorbed fractions for electrons for organ self-irradiation were significantly less than 1.0 at energies above 0.5 MeV, as expected for many of these small-sized organs, and measurable cross-irradiation was observed for many organ pairs for high-energy electrons (as would be emitted by nuclides such as 32P, 90Y, or 188Re).