Supplemental transmission method for improved PET attenuation correction on an integrated MR/PET

Although MR image segmentation, combined with information from the PET emission data, has achieved a clinically usable PET attenuation correction (AC) on whole-body MR/PET systems, more accurate PET AC remains one of the main instrumental challenges for quantitative imaging. Incorporating a full conventional PET transmission system in these machines would be difficult, but even a small amount of transmission data might usefully complement the MR-based estimate of the PET attenuation image. In this paper we explore one possible configuration for such a system that uses a small number of fixed line sources placed around the periphery of the patient tunnel. These line sources are implemented using targeted positron beams. The sparse transmission (sTX) data are collected simultaneously with the emission (EM) acquisition. These data, plus a blank scan, are combined with a partially known attenuation image estimate in a modified version of the maximum likelihood for attenuation and activity (MLAA) algorithm, to estimate values of the linear attenuation coefficients (LAC) in unknown regions of the image. This algorithm was tested in two simple phantom experiments. We find that the use of supplemental transmission data can significantly improve the accuracy of the estimated LAC in a truncated region, as well as the estimate of the emitter concentration within the phantom. In the experiments, the bias in the EM+sTX estimate of emitter concentrations was 3–5%, compared to 15–20% with the use of EM-only data.