D.R. Gilland

Simultaneous compensation for attenuation, scatter and detector response for SPECT reconstruction in three dimensions

A three-dimensional reconstruction method for simultaneous compensation of attenuation, scatter and distance-dependent detector response for single photon emission computed tomography is described and tested by experimental studies. The method determines the attenuation factors recursively along each projection ray starting at the intersected source voxel closest to the detector. The method substracts the scatter energy window data from the primary energy window data for scatter compensation. The detector response is modelled to be spatially invariant at a constant distance from the detector. The method convolves source distribution with the modelled response function to compensate for the smoothed by use of a non-uniform entropy prior to searching for the maximum a posteriori probability solution. The method was tested using projections acquired from a chest phantom by a three-headed detector system with parallel hole collimators. An improvement was shown in image noise, recognition of object sizes and shapes, and quantification of concentration ratios.

A 3D model of non-uniform attenuation and detector response for efficient iterative reconstruction in SPECT

A 3D physical model for iterative reconstruction in SPECT has been developed and applied to experimental data. The model incorporates non-uniform attenuation using reconstructed transmission CT data and distance-dependent detector response based on response function measurements over a range of distances from the detector. The 3D model has been implemented in a computationally efficient manner with practical memory requirements. The features of the model that provide efficiency are described including a new region-dependent reconstruction (RDR) technique. With RDR, filtered backprojection is used to reconstruct areas of the image of minimal clinical importance, and the result is used to supplement the iterative reconstruction of the clinically important areas of the image. The 3D model was incorporated into the maximum likelihood-expectation maximization (ML-EM) reconstruction algorithm and tested in three phantom studies--a point source, a uniform cylinder, and an anthropomorphic thorax--and a patient 9Tc(m) sestamibi study. Reconstructed images with the 3D method exhibited excellent noise and resolution characteristics. With the sestamibi data, the RDR technique produced essentially the conventional ML-EM estimate in the cardiac region with substantial time savings.

Four-dimensional superquadric-based cardiac phantom for Monte Carlo simulation of radiological imaging systems

A four-dimensional (x,y,z,t) composite superquadric-based object model of the human heart for Monte Carlo simulation of radiological imaging systems has been developed. The phantom models the real temporal geometric conditions of a beating heart for frame rates up to 32 per cardiac cycle. Phantom objects are described by boolean combinations of superquadric ellipsoid sections. Moving spherical coordinate systems are chosen to model wall movement whereby points of the ventricle and atria walls are assumed to move towards a moving center-of-gravity point. Due to the non-static coordinate systems, the atrial/ventricular valve plane of the mathematical heart phantom moves up and down along the left ventricular long axis resulting in reciprocal emptying and filling of atria and ventricles. Compared to the base movement, the epicardial apex as well as the superior atria area are almost fixed in space. Since geometric parameters of the objects are directly applied on intersection calculations of the photon ray with object boundaries during Monte Carlo simulation, no phantom discretization artifacts are involved.

Long focal length, asymmetric fan beam collimation for transmission acquisition with a triple camera SPECT system

A system design has been proposed for fast sequential SPECT/transmission CT on a three-headed SPECT camera. The design consists of a long focal length (114 cm) asymmetric fan beam collimator and a stationary transmission line source. The advantages of this system include: increased field-of-view, high sensitivity to the transmission line source, low sensitivity to radionuclide within the patient (reduced cross-talk effects), high spatial resolution, easy transition between SPECT and transmission acquisition, and no moving line source. An iterative ML-EM reconstruction algorithm for transmission data was implemented and adapted to the proposed acquisition geometry. Evaluations were performed using Monte Carlo simulated data. With a 40 cm detector width, transmission reconstructions of a 38/spl times/26 cm elliptical body were effectively artifact-free. Reconstructions of a 46/spl times/31 cm body contained only minor truncation artifacts that did not substantially affect the attenuation compensated SPECT image. Contamination of the transmission data from radionuclide within the patient can be substantially reduced with this system by increasing the resolution of the fan beam collimator. We conclude that long focal length, asymmetric fan beam collimation, combined with iterative reconstruction, offers a promising approach for fast sequential SPECT/TCT acquisition on a three-headed SPECT camera.

The effect of truncation reduction in fan beam transmission for attenuation correction of cardiac SPECT

A limitation of fan beam transmission imaging using a 40 cm field-of-view scintillation camera is the data truncation that occurs when imaging medium to large-sized patients. With filtered backprojection, truncation may cause bright rings in the reconstructed image. The primary objective of this study is to evaluate a method to extrapolate the truncated transmission data under clinically relevant count density conditions. The method involves obtaining the patient contour by processing the scatter and photopeak emission data, filling the contour with the attenuation coefficient for soft tissue, reprojecting the contour image, extrapolating the truncated projection set with the projections. A long focal length (114 cm) fan collimator is used on one head of a triple camera SPECT system to acquire transmission data. The two remaining detectors are equipped with low energy, ultra high resolution parallel hole collimators. A large thorax phantom (38 cm/spl times/26 cm) and patient data are used to evaluate the method. For SPECT image reconstruction, non-uniform attenuation correction is performed with a truncated attenuation map, an extrapolated attenuation map and the untruncated attenuation map. The SPECT results indicate that image uniformity changes very little using any of the three different attenuation maps when a long focal length fan beam collimator is used for transmission data acquisition. Truncation artifacts that are apparent in the transmission image can be substantially reduced for objects up to 40 cm wide.

Evaluation of a pinhole collimator for I-131 SPECT head imaging

The performance of a pinhole collimator for I-131 SPECT of the head was evaluated. The evaluation included planar and SPECT spatial resolution, sensitivity in air and in water, septal penetration, reconstructed image quality, and activity quantitation within a simple phantom that models tumor uptake in the head. The pinhole collimator was compared to medium and high energy parallel hole collimators. The pinhole collimator demonstrated improved resolution/sensitivity tradeoff compared with the parallel hole collimators over the range of distances relevant to head imaging. In planar point source images the pinhole collimator showed reduced penetration effects although in reconstructed images penetration effects were not apparent for either pinhole or high energy parallel hole collimators. Comparable activity quantitation accuracy was observed with all collimators. The accuracy was dependent on the segmentation threshold and calibration procedure. These results indicate that the pinhole collimator can provide improved performance conventional parallel hole collimators for I-131 imaging in the head.

Quantitation of 211At in small volumes for evaluation of targeted radiotherapy in animal models

We have evaluated SPECT and two planar imaging methods, geometric mean (GM) and buildup factor (BF), for their potential to quantitate in vivo 211At distributions in rat spinal subarachnoid spaces using phantom studies. The use of medium-energy collimators and the small diameter (3 mm) of the subarachnoid space complicate quantitation. Net activities from distributions in various backgrounds were obtained using a large region of interest with background subtraction. Results showed quantitation accuracy within 10% for SPECT and BF in low backgrounds increasing to 25% at higher background levels while GM errors ranged from 20 to 45%. We have also obtained images of [211At]astatide distributions, administered intrathecally, in rats.

Quantitative SPECT imaging with indium-111

The potential of SPECT (single photon emission computed tomography) to quantify the distribution of indium-111 was investigated in an experimental phantom study. Nonuniform attenuation compensation using acquired transmission data was compared to uniform compensation based on reconstructed quantitative accuracy and noise. The reconstructed transmission data provided the attenuation map for the nonuniform compensation. Results showed that nonuniform attenuation compensation improved image quality, quantitative accuracy, and noise compared to uniform compensation. Noise increased with a decrease in counts in the nonuniform attenuation map but remained substantially below the uniform compensation level. The noise effect was observed with both Chang and ML-EM (maximum-likelihood expectation-maximization) reconstruction methods. Independent reconstruction of the 172- and 247-keV emission data was compared to the reconstruction of the combined 172- and 247-keV projection data. Improved quantitative accuracy and image noise resulted when both In-111 emission energies were used. However, independent reconstruction of the two energies did not substantially improve accuracy or noise compared with the reconstruction of the combined data. >

A Hierarchical Feature Based Deformation Model Applied to 4D Cardiac SPECT Data

In this paper we describe a statistical model for the observation of labeled points in gated cardiac single photon emission computed tomography (SPECT) images. The model has two major parts: one based on shape correspondence between the image for evaluation and a reference image, and a second based on the match in image features. While the statistical deformation model is applicable to a broad range of image objects, the addition of a contraction mechanism to the baseline model provides particularly convincing results in gated cardiac SPECT. The model is applied to clinical data and provides marked improvement in the quality of summary images for the time series. Estimates of heart deformation and contraction parameters are also obtained.

A direct measurement of skull attenuation for quantitative SPECT

The attenuation of 140-keV photons is measured in three empty skulls by placing a /sup 99m/Tc line source inside each one and acquiring projection data. These projections are compared to projections of the line source alone in order to determine the transmission through each point in the skull surrounding the line source. The effective skull thickness is calculated for each point using an assumed dense bone attenuation coefficient. The relative attenuation for this thickness of bone is compared to that of an equivalent amount of soft tissue to evaluate the increased attenuation of photons in brain single photon emission computed tomography (SPECT) relative to a uniform soft tissue approximation. For the skull regions surrounding most of the brain, the effective bone thickness varies considerably, but is generally less than 6 mm, resulting in a relative attenuation increase of less than 6%. >