Three-dimensional PET emission scan registration and transmission scan synthesis

Attenuation correction in emission tomography using the emission data--A review

The problem of attenuation correction (AC) for quantitative positron emission tomography (PET) had been considered solved to a large extent after the commercial availability of devices combining PET with computed tomography (CT) in 2001; single photon emission computed tomography (SPECT) has seen a similar development. However, stimulated in particular by technical advances toward clinical systems combining PET and magnetic resonance imaging (MRI), research interest in alternative approaches for PET AC has grown substantially in the last years. In this comprehensive literature review, the authors first present theoretical results with relevance to simultaneous reconstruction of attenuation and activity. The authors then look back at the early history of this research area especially in PET; since this history is closely interwoven with that of similar approaches in SPECT, these will also be covered. We then review algorithmic advances in PET, including analytic and iterative algorithms. The analytic approaches are either based on the Helgason-Ludwig data consistency conditions of the Radon transform, or generalizations of John's partial differential equation; with respect to iterative methods, we discuss maximum likelihood reconstruction of attenuation and activity (MLAA), the maximum likelihood attenuation correction factors (MLACF) algorithm, and their offspring. The description of methods is followed by a structured account of applications for simultaneous reconstruction techniques: this discussion covers organ-specific applications, applications specific to PET/MRI, applications using supplemental transmission information, and motion-aware applications. After briefly summarizing SPECT applications, we consider recent developments using emission data other than unscattered photons. In summary, developments using time-of-flight (TOF) PET emission data for AC have shown promising advances and open a wide range of applications. These techniques may both remedy deficiencies of purely MRI-based AC approaches in PET/MRI and improve standalone PET imaging.

Calculation of attenuation factors from combined singles and coincidence emission projections

We have developed a simple method for determining coincidence attenuation-correction factors C (the inverse of the total attenuation factors) from collimated singles (SPECT) and coincidence [positron emission tomography (PET)] projections without transmission data. Attenuation-correction factor estimates are determined for individual lines of response (LOR's) independently. The required data can be acquired using a gamma-camera system with coincidence capabilities. A first-order approximation (R) of C for an LOR is given by the product of the singles count rates, taken at each end of the LOR divided by the square of the coincidence count rate. The method was tested using simulated singles and coincidence projections starting with emission and attenuation maps from patient PET scans. Noise and resolution effects were modeled in separate studies. In the noise-free, high-resolution simulations, a scatter plot of the C values versus the corresponding R values for all LOR's produces a well-defined trajectory with little variance. Values of lnR were reconstructed into good quality attenuation maps that compare favorably with the originals. We conclude that the method works well on ideal data. The introduction of noise results in degraded images. In a simulated patient study, lung and outer body boundaries were visible in images produced with 3.2 x 10(4) coincidence counts.