Grant T. Gullberg

Three-dimensional iterative reconstruction algorithms with attenuation and geometric point response correction

A three-dimensional iterative reconstruction algorithm which incorporates models of the geometric point response in the projector-backprojector is presented for parallel, fan, and cone beam geometries. The algorithms have been tested on an IBM 3090-600S supercomputer. The iterative EM reconstruction algorithm is 50 times longer with geometric response and photon attenuation models than without modeling these physical effects. An improvement in image quality in the reconstruction of projection data collected from a single-photon-emission computed tomography (SPECT) imaging system has been observed. Significant improvements in image quality are obtained when the geometric point response and attenuation are appropriately compensated. It is observed that resolution is significantly improved with attenuation correction alone. Using phantom experiments, it is observed that the modeling of the spatial system response imposes a smoothing without loss of resolution. >

Three-dimensional reconstruction in nuclear medicine emission imaging

The study presents application of methods of ascertaining the three-dimensional distribution of isotope concentration or density in nuclear medicine, and differs from previous three-dimensional reconstruction efforts of astrophysics, electron microscopy, and X-ray radiology in that statistically poor measurements and photon attenuation are taken into account by the algorithm. The methods discussed are applicable to photon or heavy ion transmission radiography as well as emission imaging.

Non-uniform attenuation correction using simultaneous transmission and emission converging tomography

Photon attenuation in cardiac single photon emission computed tomography (SPECT) is a major factor contributing to the quantitative inaccuracy and the decrease in sensitivity of lesion detection. A measured map of the attenuation distribution is used in combination with iterative reconstruction algorithms to accurately compensate for the variable attenuation in the chest. The transmission and emission data are acquired simultaneously using a multidetector, fan beam collimated SPECT system with a precisely aligned transmission line source (Tc-99m) at a different energy than the emission source (Tl-201). The contamination of transmission and emission data due to scatter and multiple photopeaks is removed based on measurements from the detectors acquiring only the emission data. The quantitative accuracy of cardiac SPECT is significantly improved using simultaneously acquired transmission and emission data which are obtained in clinically acceptable patient scanning times.<<ETX>>

Emission Computer Assisted Tomography with Single-Photon and Positron Annihilation Photon Emitters

Computed transverse section emission tomography using 99mTc with the Anger camera is compared to positron annihilation coincident detection using a ring of crystals and 68Ga. The single-photon system has a line spread function (LSF) of 9 mm full width at half maximum (FWHM) at the collimator and gives a transverse section reconstruction LSF of 11 mm FWHM with 144 views. The positron ring has a LSF of 6 mm at the center with a transverse section reconstruction LSF of 7.5 mm FWHM. Correction for uniformity of detector response and accurate center of rotation determination is essential in both techniques. The signal-to-noise ratio in a reconstruction is diminished by a factor of 1.2 x (number of resolution elements)1/4 over that expected from the average number of events per resolution element. Attenuation compensation causes more noise to appear in the center than the edge for both modes and an average increase in uncertainty of 30%. The effects of attenuation result in more loss of data for positron coincidence imaging than for single-photon imaging even at energies of 80 keV. For a 20-cm cylinder imaged in transverse section, only 20% of the positron annihilation events are not scattered; however, at 140 keV, 40% of the photons are not scattered. The relative crystal efficiency gives single-photon imaging an advantage of 5. On the other hand, the solid angle advantage of positron photon coincidence imaging is about 100 for the comparisons of this paper. Taking these factors into account, we find positron-computed section imaging has a tenfold increase in sensitivity over multiple-view imaging with the scintillation camera, which gives multiple sections but requires camera or patient rotation.

Unmatched projector/backprojector pairs in an iterative reconstruction algorithm

Computational burden is a major concern when an iterative algorithm is used to reconstruct a three-dimensional (3-D) image with attenuation, detector response, and scatter corrections. Most of the computation time is spent executing the projector and backprojector of an iterative algorithm. Usually, the projector and the backprojector are transposed operators of each other. The projector should model the imaging geometry and physics as accurately as possible. Some researchers have used backprojectors that are computationally less expensive than the projectors to reduce computation time. This paper points out that valid backprojectors should satisfy a condition that the projector/backprojector matrix must not contain negative eigenvalues. This paper also investigates the effects when unmatched projector/backprojector pairs are used.

An attenuated projector-backprojector for iterative SPECT reconstruction

A new ray-driven projector-backprojector which can easily be adapted for hardware implementation is described and simulated in software. The projector-backprojector discretely models the attenuated Radon transform of a source distributed within an attenuating medium as line integrals of discrete pixels, obtained using the standard sampling technique of averaging the emission source or attenuation distribution over small square regions. Attenuation factors are calculated for each pixel during the projection and backprojection operations instead of using precalculated values. The calculation of the factors requires a specification of the attenuation distribution, estimated either from an assumed constant distribution and an approximate body outline or from transmission measurements. The distribution of attenuation coefficients is stored in memory for efficient access during the projection and backprojection operations. The reconstruction of the source distribution is obtained by using a conjugate gradient or SIRT type iterative algorithm which requires one projection and one backprojection operation for each iteration.

Evaluation of cardiac cone-beam single photon emission computed tomography using observer performance experiments and receiver operating characteristic analysis.

RATIONALE AND OBJECTIVES Single photon emission computed tomography (SPECT) with a cone-beam collimator improves the trade-off between detection efficiency and spatial resolution for cardiac imaging. However, acquisitions using orbits where the focus remains in a plane do not provide sufficient data for exact reconstruction. In the current study the authors evaluate the clinical utility of planar-orbit cone-beam SPECT in detecting a simple myocardial defect. METHODS Observer performance experiments compared high-resolution cone-beam with same-resolution parallel-hole and fan-beam collimator designs in myocardial defect detection using a computer-simulated cardiac model. The uptake of Thallium-201 in the myocardium and other tissue organs was modeled by a mathematical three-dimensional upper torso phantom from which physically realistic projections were simulated. Eight observers viewed reconstructed transaxial images from the three collimator designs and indicated the certainty with which they detected a Gaussian-shaped defect at a specified location. RESULTS The area under the receiver operating characteristic curve indicated that the cone-beam design, regardless of slice position, was superior to the fan-beam, which in turn was superior to the parallel-hole design for the specified detection task. CONCLUSIONS The observer study demonstrated that reconstruction artifacts resulting from insufficient data sampling do not hinder obtaining improved diagnostic information from planar-orbit cone-beam cardiac SPECT images compared to conventional cardiac SPECT using parallel-hole and fan-beam collimators.

Three-dimensional SPECT reconstruction of combined cone-beam and fan-beam data

A SPECT system includes three gam camera heads (22a), (22b), (22c) which are mounted to a gantry (20) for rotation about a subject (12). The subject is injected with a source of emission radiation, which emission radiation is received by the camera heads. Camera head (22a) has a fan-beam collimator (24a) mounted on a radiation receiving face and generates fan-beam data indicative of the received emission radiation. The camera heads (22b) and (22c) each have a cone-beam collimator (24b), (24c) mounted respectively on their radiation receiving face and generate cone-beam data indicative of the received emission radiation. A transmission radiation source (26) is mounted opposite the camera head (22a) having the fan-beam collimator (24a). The fan-beam detector head (22a) further receives transmission radiation and generates fan-beam transmission radiation indicative thereof. A transmission data reconstruction processor (50) reconstructs the fan-beam transmission data. An emission data memory (110) separately stores the fan-beam and cone-beam emission data. Attenuation correction processors (78, 86) correct the emission data in accordance with the reconstructed attenuation data. An emission data reconstructor (72) reconstructs the corrected emission data into a corresponding three-dimensional image representation which is selectively displayed on a display (76) in a human-readable form.

Estimation of geometrical parameters and collimator evaluation for cone beam tomography.

A method is presented for estimating the geometrical parameters for a cone beam detector geometry from the coordinates of the centroid of a projected point source sampled over 360 degrees. Nonlinear expressions are derived for the coordinates of the centroids in terms of the geometrical parameters which include: the two-dimensional coordinates of the projection of the center of rotation onto the detector image plane; the focal length; the distance from the focal point to the center of rotation; and the spatial coordinates of the point source itself. Experimental data were obtained using a rotating gamma camera with a symmetrically converging collimator. The Marquardt algorithm was used to estimate the parameters for this particular cone beam geometry. The method was able to estimate the geometrical parameters and evaluate the accuracy of the collimator construction.

Reconstruction and visualization of fiber and laminar structure in the normal human heart from ex vivo diffusion tensor magnetic resonance imaging (DTMRI) data.

Objective:The human heart is composed of a helical network of muscle fibers organized to form sheets that are separated by cleavage planes responsible for the orthotropic mechanical properties of cardiac muscle. The purpose of this study is the reconstruction and visualization of these structures in 3 dimensions. Methods:Anisotropic least square filtering followed by fiber and sheet tracking techniques were applied to diffusion tensor magnetic resonance imaging data of the excised human heart. Fibers were reconstructed using the first eigenvectors of the diffusion tensors. The sheets were reconstructed using the second and third eigenvectors and visualized as surfaces. Results:The fibers are shown to lie in sheets that have transmural structure, which correspond to histologic studies published in the literature. Quantitative measurements show that the sheets as appose to the fibers are organized into laminar orientations without dominant populations. Conclusions:A visualization algorithm was developed to demonstrate the complex 3-dimensional orientation of the fibers and sheets in human myocardium.