Daniel E. Wessell

Pinhole SPECT of mice using the LumaGEM gamma camera

LumaGEM is a newly developed gamma camera for dedicated, small field of view, high spatial resolution imaging. The system consists of an array of 2/spl times/2/spl times/6 mm/sup 3/ NaI(Tl) pixels coupled to an array of position-sensitive photomultiplier tubes. It has a 125/spl times/125 mm/sup 2/ field of view. A pinhole collimator was used on LumaGEM to acquire SPECT images of mice that had transgenic modifications so as to model various diseases. Pinhole apertures of 1, 2 and 3 mm are interchangeable on the collimator and were used to acquire images. An iterative MLEM algorithm for pinhole SPECT was used to reconstruct the 128 projection images that covered 360/spl deg/ rotation. The reconstruction algorithm is based on a projector and backprojector pair implemented using a ray-tracing algorithm. The crucial reconstruction input parameters are the radius of rotation, center of rotation, and pinhole focal length. Ideal pinhole geometry is assumed, and no correction for attenuation has been made. The preliminary images presented here show detailed uptake in the mice subjects and are a convincing sign that animal SPECT can reach submillimeter spatial resolution and be a valuable tool in the study of diseases and the development of pharmaceuticals in animal models.

Rotating slant-hole single-photon emission-computed tomography

This dissertation presents a detailed investigation of the use of rotating slant-hole (RSH) collimators in single-photon emission-computed tomography(SPECT). We propose RSH SPECT as an alternative imaging technique, which may have advantages over more traditional SPECT techniques, in certain imaging applications. The basic imaging properties of RSH collimators are derived. This includes the derivation of expressions for the size and location of the common volume of view and the geometric resolution and efficiency. In addition, we derive an analytical expression for the geometric response and transfer functions of RSH collimators. The theoretical expressions for the aforementioned imaging characteristics are verified via Monte Carlo simulations and phantom experiments. Next, we present the development of a three-dimensional (3-D) analytic reconstruction algorithm for RSH SPECT. This includes the derivation of both a backprojection of the filtered projections and a filtering of the simple backprojections (BpjF) algorithm. The relationship between the derived 3-D RSH reconstruction filter and a number of other reconstruction filters is discussed. In addition, the implementation of the BpjF reconstruction algorithm is described and its performance is demonstrated via simulations and phantom experiments. We have also implemented an iterative ordered-subsets expectation-maximization (OS-EM) reconstruction algorithm for RSH SPECT. A detailed description of the development of the projector/backprojector employed by the OS-EM algorithm is provided. Simulation and phantom experiments are again used to demonstrate the performance of the projector/backprojector and the iterative OS-EM reconstruction algorithm. Rotating slant-hole SPECT has potential applications in cardiac and breast imaging. Thus, we demonstrate the feasibility, advantages, and disadvantages of using RSH SPECT via a comparison with conventional SPECT in a cardiac and breast imaging phantom experiment. In conclusion, we summarize the results presented and discuss their implications on the current and future state of RSH SPECT research.

Evaluation of rotating slant-hole SPECT mammography using Monte-Carlo simulation methods

Multisegment slant-hole (MSSH) collimators provide much higher detection efficiency at a single view as compared to conventional low-energy high-resolution (LEHR) parallel-hole collimators with the same spatial resolution. The purpose of this study is to evaluate rotating MSSH (RMSSH) SPECT mammography using Monte Carlo simulation methods. We modified the SIMIND Monte Carlo code to generate RMSSH projection data including the effects of attenuation, collimator-detector response and scatter from a realistic 4-D NCAT phantom with attached breasts and lesions placed in the central, medial, and lateral regions of the breast. The projection data were reconstructed using analytic and iterative image reconstruction methods. For comparison, we examine planar scintimammography images obtained by placing an LEHR collimator with a resolution of 1 cm at 15.2 cm, which is the same as that of a four-segment slant-hole (4SSH) collimator at the center of its common volume-of-view, in close proximity to the lateral wall of the breast. Also, we simulated the inclusion of a lead shield that covers the body to keep photons emitted from the other organs from reaching the MSSH collimator. Our results indicated that, depending on the lesion location, the spatial resolution of the RMSSH SPECT images may be comparable or slightly poorer than that of the planar scintimammographic images obtained using a LEHR collimator while the lesion contrast is significantly higher at all lesion locations. Shielding of photon emissions from the other organs substantially improves the RMSSH SPECT image quality. We conclude that the Monte Carlo simulation methods are useful in evaluating the application of RMSSH SPECT to breast imaging as compared to conventional planar scintimammography and that RMSSH SPECT provides higher overall image quality.

Imaging characteristics of scintimammography using parallel-hole and pinhole collimators

The purpose of the study is to investigate the imaging characteristics of scintimammography (SM) using parallel-hole (PR) and pinhole (PN) collimators in a clinical setting. Experimental data were acquired from a phantom that models the breast with small lesions using a low energy high resolution (LEHR) PR and a PN collimator. At close distances, the PN collimator provides better spatial resolution and higher detection efficiency than the PR collimator, at the expense of a smaller field-of-view (FOV). Detection of small breast lesions can be further enhanced by noise smoothing, field uniformity correction, scatter subtraction and resolution recovery filtering. Monte Carlo (MC) simulation data were generated from a realistic 3D MCAT phantom that models the Tc-99m sestamibi uptake and attenuation distributions in an average female patient. The scatter to primary ratio (S/P) decreases from the base to the tip of the breast. For PR collimation, S/P is higher in the left than right breast due to scatter of photons from the heart. For PN collimation, S/P is highest at the base and lowest at the tip of the breast as compared to PR collimation. It is higher in the right than left breast due to contributions from organ uptakes in the body. Results from the study add to understanding of the imaging characteristics of SM using PR and PN collimators and assist in the design of data acquisition and image processing methods to enhance the detection of breast lesions using SM.

Rotating quadrant slant-hole SPECT scintimammography: an initial investigation

The authors investigate the use of rotating quadrant slant-hole (RQSH) collimation in SPECT scintimammography (SM). A RQSH collimator is able to acquire a complete set of projection data for fully 3-D reconstruction of the breast with 90 degrees of collimator rotation at each of 2-3 collimator detector positions on a 90-120 degree arc around the breast. The use of a 90-120 degree arc allows the collimator to remain close to the breast throughout the data acquisition, providing better resolution in the reconstructed image as compared to the conventional parallel-hole collimation SPECT acquisition. In addition, a RQSH collimator for SPECT SM has 2.8-3.5 times the efficiency of a conventional parallel-hole collimator with equivalent spatial resolution. While the field-of-view of each quadrant of the RQSH collimator can encompass a medium sized breast, significant truncation of the body occurs. It is essential that the collimator design parameters and the data acquisition orbit be chosen in such a way as to minimize the effect of truncation on the reconstruction of the breast. Here, the authors investigate the choice of collimator design parameters and data acquisition orbits for RQSH SPECT SM. These initial experiments are performed using the 3-D mathematical cardiac torso (MCAT) phantom with a realistic medium sized breast model. The reconstructions are performed using an iterative ML-EM algorithm and transformation based projector/back-projector pair. The results indicate that while the projection images are severely truncated when using the RQSH collimator, appropriate collimator design parameters and data acquisition orbits can be chosen to limit the effects of truncation on the reconstruction of the breast region.

A 3D filtered back-projection algorithm for SPECT using a multi-segment rotating slant-hole collimator

The purpose of this study is to develop, implement and evaluate a 3D filtered back-projection (FBP) algorithm for SPECT using a multi-segment rotating slant-hole (RSH) collimator. Previously, the authors derived a 3D frequency-domain filter function used in a back-projection filtering (BPF) algorithm for multi-segment RSH SPECT. The 3D orientation of the projection direction from each segment, each collimator rotation angle and each collimator-camera position defines a 2D central section through the center of the 3D BPF reconstruction filter. For FBP, these central sections are determined analytically from the mathematical expression of 3D BPF filter, and are re-scaled and rotated before filtering the corresponding projections to avoid interpolation. The FBP algorithm was implemented to reconstruct simulated and experimentally acquired SPECT data from a four-segment RSH collimator. The reconstructed images were compared to those from BPF. The results of FBP are comparable to BPF, and with reduced low-intensity image artifacts. It is concluded that FBP is a superior alternative to BPF for RSH SPECT. It provides accurate reconstruction in relatively short computing time. In addition, it requires less computer memory, permits on-the-fly image reconstruction during data acquisition, and may facilitate additional processing during reconstruction to reduce truncation artifacts.

Comparison of an analytical and an iterative based collimator-detector response compensation method in SPECT

This study compares two collimator-detector response (CDR) compensation methods in SPECT; an analytical method by Pan et al. (1997), and an iterative method which accurately models the CDR. The spatial resolution recovery is evaluated using simulated data of a point source placed at different distances from the center-of-rotation of the camera. The image noise properties are evaluated using simulated data from a uniform disk phantom. The two methods provide general improvement in spatial resolution but generate reconstructed point source images showing different asymmetric shapes. The analytical method is fast but assumes CDR characteristics that are not fully met in practice. It amplifies high frequency noise drastically. To reduce the noise level, a smoothing filter is used with concurrent degradation in resolution. The iterative method is much slower but achieves improved spatial resolution with lower noise in the reconstructed images. We conclude that the iterative method provides more accurate CDR compensation and lower image noise as compared with the analytical method at a cost of a longer computation time.

Investigation of acquisition and image reconstruction parameters for rotating multi-segment slant-hole SPECT

SPECT using a rotating multi-segment slant-hole (RMSSH) collimator provides a superior alternative for imaging the breast and the heart comparing to conventional SPECT. The aim of this paper is to investigate the effects of the number of camera positions and the number of collimator rotation stops at each camera position on RMSSH SPECT reconstruction image quality, using both Monte-Carlo simulation and physical phantom experiments. Our results demonstrate the importance of sufficient camera positions for complete angle data acquisition and accurate 3D RMSSH SPECT image reconstruction; and for the same total acquisition time, tradeoff exists between the number of collimator rotation stops and the amount of acquisition time at each rotation stop. We conclude that complete angle data acquisition is crucial for accurate 3D RMSSH SPECT image reconstructions, but using inadequate number of camera positions may also result in usable diagnostic information in shorter acquisition time. An optimal trade-off between image resolution, noise and image artifacts is needed to determine the number of collimator rotation stops at each camera position.

A linear shift-variant model for local point response in circular-orbit cone-beam iterative reconstruction

The authors develop and evaluate a spatial-domain model of the local point response arising from iterative reconstruction of circular-orbit cone-beam projections. They proceed from the premise that the missing data in circular-orbit cone-beam is spatially variant. They model the process of circular-orbit cone-beam projection and reconstruction as a linear, shift-variant process, implying that each point in the image space has a different point spread function (PSF). To estimate the local PSF of a given point, the authors determine the surface on which a simple backprojected point response would occur for any point source in the image space. Taking the Fourier transform of this function gives an expression for the missing data region of the Fourier space. For points on the axis of rotation, this space is described by a right circular cone. For points off the axis of rotation, the space is described by a skewed elliptical cone. The authors confirm the model by simulating circular-orbit cone-beam data for individual point sources and reconstructing with an iterative algorithm. Fourier transforms of the estimated point responses show that iterative reconstruction exhibits missing data regions similar to those predicted by the model; however, the authors conclude that further refinements would be required to make the model practical for use in an optimal shift-variant restoration procedure.