Jianying Li

Pinhole collimation for ultra-high-resolution, small-field-of-view SPECT

The objective of this investigation was to evaluate small-field-of-view, ultra-high-resolution pinhole collimation for a rotating-camera SPECT system that could be used to image small laboratory animals. Pinhole collimation offers distinct advantages over conventional parallel-hole collimation when used to image small objects. Since geometric sensitivity increases markedly for points close to the pinhole, small-diameter and high-magnification pinhole geometries may be useful for selected imaging tasks when used with large-field-of-view scintillation cameras. The use of large magnifications can minimize the loss of system resolution caused by the intrinsic resolution of the scintillation camera. A pinhole collimator has been designed and built that can be mounted on one of the scintillation cameras of a triple-head SPECT system. Three pinhole inserts with approximate aperture diameters of 0.6, 1.2 and 2.0 mm have been built and can be mounted individually on the collimator housing. When a ramp filter is used with a three-dimensional (3D) filtered backprojection (FBP) algorithm, the three apertures have in-plane SPECT spatial resolutions (FWHM) at 4 cm of 1.5, 1.9 and 2.8 mm, respectively. In-air point source sensitivities at 4 cm from the apertures are 0.9, 2.6 and 5.7 counts s(-1) microCi(-1) (24, 70 and 154 counts s(-1) MBq(-1)) for the 0.6, 1.2 and 2.0 mm apertures, respectively. In vitro image quality was evaluated with a micro-cold-rod phantom and a micro-Defrise phantom using both the 3D FBP algorithm and a 3D maximum likelihood-expectation maximization (ML-EM) algorithm. In vivo image quality was evaluated using two (315 and 325 g) rats. Ultra-high-resolution pinhole SPECT is an inexpensive and simple approach for imaging small animals that can be used with existing rotating-camera SPECT system.

Determination of both mechanical and electronic shifts in cone beam SPECT

The difference between the displacement of the centre of rotation (mechanical shift, MS) and the electronic centring misalignment (electronic shift, ES) in cone beam SPECT is evaluated. A method is proposed to determine both MS and ES using the centroid of a projected point source sampled over 360 degrees and the Marquardt non-linear fitting algorithm. Both shifts are characterized by two orthogonal components. This method is verified using Monte Carlo simulated point source data with different combinations of mechanical and electronic shifts. Both shifts can be determined correctly. We have also applied the proposed method to our cone beam SPECT system to determine both shifts as well as the focal length. The determined ES parameters are then used to correct the projections and the MS parameters are incorporated into a reconstruction algorithm. The point source images are reconstructed and the image resolutions with and without the shift corrections are measured. The experimental results demonstrate that the image resolution is improved after shift corrections. The experimental results also indicate that the shift parameters determined in the same experiment with the point source located at different places are consistent but change from time to time, suggesting that calibration of the system is needed on a periodic basis.

Implementation of an accelerated iterative algorithm for cone-beam SPECT

In this paper we describe the implementation of an accelerated iterative reconstruction algorithm (AIRA) for cone-beam (CB) projections using a single circular orbit in single-photon-emission computed tomography (SPECT). This algorithm is a modified maximum-likelihood-expectation-maximization (ML-EM) algorithm and several approaches have been used to accelerate the reconstruction process. These approaches include: (i) the use of ordered subsets; (ii) the use of active areas and volumes; and (iii) the storing in memory of the transition vector for a given ray (during the forward projection step). This algorithm, which compensates for collimator geometric sensitivity variation as a function of position and makes uniform attenuation corrections has been evaluated using experimentally acquired phantom data. The results demonstrate a two-orders-of-magnitude decrease of the computational time of this algorithm over the conventional ML-EM algorithm with similar convergence properties.

Breast tumour imaging using incomplete circular orbit pinhole SPET: A phantom study

Improvements in 99Tcm-sestamibi breast lesion visualization using single photon emission tomography (SPET) may help define the clinical role of this technique alongside X-ray mammography in the diagnosis and management of breast cancer. Pinhole SPET offers the advantages of high resolution and sensitivity when compared to conventional parallel-beam collimation for sources located near the pinhole aperture. In this work, the potential of incomplete (180 degrees) circular orbit (ICO) SPET with pinhole collimation is investigated as a means to visualize small (6.4 and 9.6 mm diameter) spherical simulated tumours, at clinical count densities and tumour-to-background ratios, in a breast phantom. ICO pinhole SPET is compared to complete circular orbit (CCO) pinhole SPET for reference, and planar breast imaging (scintimammography) using parallel-beam and pinhole collimators. A prototype box-shaped pinhole collimator with a 4 mm diameter circular aperture was used to acquire projections of an 890 ml breast phantom both in isolation and mounted on a cylinder filled with a mixture of 99Tcm-pertechnetate and water. A heart phantom containing 99Tcm activity in the myocardium was placed in the cylinder. Simulated tumours containing 99Tcm were placed in the breast phantom and scanned at clinically relevant count densities and scan times with tumour-to-normal tissue concentration ratios of 5.0:1 (9.6 mm sphere) and 7.7:1 (6.4 mm sphere). Phantom data were reconstructed using pinhole filtered backprojection (FBP) and maximum likelihood-expectation maximization (ML-EM). The tumours were not visualized with scintimammography, in which lesion contrast and signal-to-noise were estimated from region of interest analysis to be < 2% and 0.01, respectively. Average (over lesion size and scan time) contrast and signal-to-noise in the ICO (CCO) SPET images were 33% and 1.72 (34% and 1.3), respectively. These values indicate that ICO pinhole SPET has the potential to improve visualization of small (< 10 mm) breast tumours when compared with scintimammography, which may be beneficial for the early classification of cancers of the breast.

Fan-beam reconstruction algorithm for a spatially varying focal length collimator

Fan-beam collimators are used in single-photon-emission computed tomography (SPECT) to improve the sensitivity for imaging of small organs. The disadvantage of fan-beam collimation is the truncation of projection data surrounding the organ of interest or, in those cases of imaging large patients, of the organ itself, producing reconstruction artifacts. A spatially varying focal length fan-beam collimator has been proposed to eliminate the truncation problem and to maintain good sensitivity for the organ of interest. The collimator is constructed so that the shortest focal lengths are located at the center of the collimator and the longest focal length is located at the periphery. The focal length is assumed to increase monotonically toward the edge of the collimator. A reconstruction algorithm for this type of fan-beam collimation, expressed as an infinite series of convolutions followed by one backprojection, is presented. Simulations show that only a small number of N terms in the series is needed to obtain high-quality reconstructions. Computer simulations showed that if the focal length function is smooth, the reconstructions are free of artifacts.

Lead and tungsten pinhole inserts for I-131 SPECT tumor imaging: experimental measurements and photon transport simulations

The potential use of lead and tungsten pinhole inserts for high-resolution SPECT imaging of intratumor activity in I-131 radioimmunotherapy was investigated using experimental point source measurements and photon transport simulations. I-131 imaging is challenging because the primary photon emission is at 364 keV and penetration through the insert near the pinhole aperture is significant. Point source response functions (PSRFs) for lead (Pb) and tungsten (W) pinhole inserts were measured experimentally. These response functions were simulated using a photon transport computer code that modeled the primary emission at 364 keV and secondary emissions at 284, 637, and 723 keV. Scatter within the pinhole insert, camera shielding, and scintillation crystal was modeled. There was good agreement between the experimental and simulated PSRFs. Simulated point source response functions for geometrically identical Pb and W pinhole inserts were narrower for the W insert due to reduced penetration. SPECT pinhole imaging with these inserts was simulated for 3-cm-diameter tumors with a central core and 3-5-mm-thick shells. For one set of simulations there was no core activity, and for a second set the shell:core activity concentration ratio was 5:1. In both cases, the tumor shells were better resolved with the W insect. As a result, shell:core activity ratios were more accurate and contrast was improved with the use of the W pinhole insert. This study suggests that W inserts have potential advantages over Pb inserts for high-resolution I-131 pinhole imaging.

Pinhole SPECT for imaging In-111 in the head

With the development of targeted radiotherapy techniques, quantitation of radionuclides that emit high-energy photons (>140 keV) by gamma camera scintigraphy has become increasingly important in external imaging applications. The radionuclide In-111 (171 and 245 keV) has been used experimentally with monoclonal antibodies and receptor specific pharmaceuticals to obtain pre-treatment information for various types of brain tumors. Conventional protocols for imaging In-111 utilize parallel-hole collimators designed for medium energy (ME) applications. The performance of ME collimators suffers from decreased spatial resolution and/or sensitivity. Septal penetration can also lead to image degradation. Pinhole collimation can offer improved spatial resolution and/or sensitivity compared with ME collimators when imaging In-111 in objects the size of the head or smaller, especially when restricting the field-of-view to regions near the central plane. Simulation and experimental phantom studies have been used to investigate pinhole SPECT for imaging In-111 in the head. Chang attenuation and dual-window scatter subtraction compensation methods have been evaluated for potential accuracy in pinhole geometry. Results have shown improved image quality with pinhole collimation with a /spl les/15% quantitative accuracy in phantom studies. We demonstrate that pinhole SPECT is a viable alternative to ME collimator imaging of In-111 in objects the size of the head. >

Maximum likelihood reconstruction for pinhole SPECT with a displaced center-of-rotation

The authors describe the implementation of a maximum likelihood (ML) algorithm using expectation maximization (EM) for pinhole SPECT with a displaced center-of-rotation. A ray-tracing technique is used in implementing the ML-EM algorithm. The proposed ML-EM algorithm is able to correct the center of rotation displacement which can be characterized by two orthogonal components. The algorithm is tested using experimentally acquired data, and the results demonstrate that the pinhole ML-EM algorithm is able to correct artifacts associated with the center-of-rotation displacement.

A filtered-backprojection algorithm for fan-beam SPECT which corrects for patient motion

In this study we have derived a filtered-backprojection (FBP) reconstruction algorithm for fan-beam SPECT that eliminates artifacts associated with patient motion during brain imaging. It is assumed that the patient motion is known or can be measured separately, and that the head is rigid. The in-plane translation and rotation were studied and were incorporated into the proposed FBP reconstruction algorithm. Angular-dependent translation/rotation parameters were used to correct for multiple motions that could occur during a scan. The proposed FBP reconstruction algorithm was evaluated using Monte Carlo simulated phantom data, including a two-point-source phantom and the Hoffman brain phantom. Projections with translation, rotation and their combinations were generated, and images reconstructed using the proposed algorithm and the standard FBP algorithm were compared. Artifacts were observed in images without the motion correction, but the artifacts were eliminated or greatly reduced using the proposed reconstruction algorithm.

SPECT reconstruction of combined cone beam and parallel hole collimation with experimental data

Three methods for combining parallel and cone beam (P&CB) SPECT data using modified maximum-likelihood expectation-maximization (ML-EM) algorithms are presented. The first method applies both parallel and cone beam data sets to reconstruct a single intermediate image after each iteration using the ML-EM algorithm. The other two are iterative methods that combine the intermediate parallel beam and cone beam source estimates to enhance the uniformity of images. These two methods are ad hoc methods. These combined collimation methods are qualitatively evaluated using experimental data. Attenuation compensation is performed by including the effects of attenuation in the transition matrix as a multiplicative factor. The results indicate that the combined P&CB approaches suppress artifacts caused by truncated projections and corrects for th distortions of the CB-only images. >