Hugh T. Morgan

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>>

Review of convergent beam tomography in single photon emission computed tomography

Investigation of convergent-beam single photon emission computed tomography (SPECT) is actively being pursued to evaluate its clinical potentials. Fan-beam, cone-beam, pin-hole and astigmatic collimators are being used with rotating gamma cameras having large crystal areas, to increase the sensitivity for emission and transmission computed tomography of small organs such as the thyroid, brain or heart. With new multi-detector SPECT systems, convergent-beam geometry offers the ability to simultaneously obtain emission and transmission data necessary to quantify uptake of radiopharmaceutical distributions in the heart. The development of convergent-beam geometry in SPECT requires the integration of hardware and software. In considering hardware, the optimum detector system for cone-beam tomography is a system that satisfies the data sufficiency condition for which the scanning trajectory intersects any plane passing through the reconstructed region of interest. However, the major development of algorithms has been for the data insufficient case of single planar orbit acquisitions. The development of these algorithms have made possible the preliminary evaluation of this technology and the imaging of brain and heart are showing significant potential for the clinical application of cone-beam tomography. Presently, significant research activity is pursuing the development of algorithms for data acquisitions that satisfy the data sufficiency condition and that can be implemented easily and inexpensively on clinical SPECT systems.

The design and performance of a simultaneous transmission and emission tomography system

A commercial three-detector single-photon emission computed tomography (SPECT) system that enables simultaneous acquisition of transmission and emission data without increasing patient scanning time has been designed and manufactured. This system produces a reconstructed attenuation coefficient distribution that can be used to correct for photon attenuation in the emission reconstruction. The three detectors with fan-beam collimators are mounted to the gantry in a triangular arrangement. A transmission line source assembly was mounted at the focal line of one of the detectors and controlled to move in synchrony with the opposing fan-beam collimator. Data from transmission and emission sources at different energies were acquired in one detector, while the other two simultaneously acquired emission data. A transmission source of /sup 153/Gd was used with /sup 99m/Tc-labeled radiopharmaceuticals, and /sup 57/Co was used with /sup 201/Tl. Algorithms were developed to subtract crosstalk between transmission and emission energy windows in all three detectors. A transmission maximum-likelihood iterative algorithm was used to reconstruct the attenuation distribution, which was used in combination with an iterative maximum-likelihood expectation-maximization algorithm to compensate for the attenuation of the projection of the emission distribution. The results in phantom studies displayed greater uniformity of activity with attenuation-corrected reconstruction. This was demonstrated visually and quantitatively by using anterior-to-inferior ratios close to one and low spatial %rms error as a measure of improved uniformity.

Performance evaluation of a transmission reconstruction algorithm with simultaneous transmission-emission SPECT system in a presence of data truncation

A simultaneous transmission-emission SPECT system (STEP) was developed on a three-detector gamma camera (Picker Prism 3000) equipped with fan-beam collimators (65 cm focal length) and a transmission line source. With this system, fan-beam geometry can cause transmission projection data to be truncated. An iterative transmission reconstruction algorithm was formulated to determine the distribution of attenuation coefficients from the system of linear equations for only measured projections. In this paper we evaluated this algorithm using phantom data with varying degree of data truncation. The results showed that with up to 30% truncation, differences in partial attenuation integrals in the non-truncated region were statistically not significant (p<0.05). Also, a study was performed to determine the minimal number of iterations necessary to obtain quantitatively accurate results. It was shown that partial attenuation integrals were not significantly different (p<0.05) when 9 to 100 iterations were performed. We conclude that the described transmission reconstruction algorithm using nine iterations is quantitatively accurate and is able to correct for the truncation of the data.<<ETX>>