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.

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.< >

The cause of 180-degree diametrical artifactual defects in clinical thallium-201 SPECT imaging was investigated using phantom simulation. This artifact was observed on SPECT images acquired with a "body contour" or "peanut" orbit. It was hypothesized that this artifact was caused by differences in spatial resolution that occur when the heart-to-detector distance changes employing noncircular orbits. To test this hypothesis, a series of planar static images of a normal cylindrical phantom was obtained at varying distances from the camera detector head. From these images, tomographic acquisition files were created that simulated tomographic data acquired with circular orbits and elliptical orbits. The reconstructed phantom short-axis slices showed no artifacts for circular orbits. However, for various elliptical orbits, significant regional nonuniformity, similar to the artifacts noted in patients, was observed. The degree of nonuniformity correlated with the long-short axis ratio of elliptical orbits (r = 0.98). In addition, circular orbits with the phantom in an eccentric position resulted in similar nonuniformities. It is concluded that a noncircular tomographic orbit can create characteristic artifacts on thallium-201 SPECT images. For rotational thallium 201 SPECT, a circular orbit with the heart in the center of rotation should be employed.