Event-by-event attenuation measurement for ACS2-based PET systems

Patient motion during acquisition is known to produce severe image artefacts and to limit the image quality in PET. Moreover, it affects exact quantification of tracer kinetic transport processes. To overcome these limitations, different motion correction methods have already been introduced in the past. They either allow for realigning the images of the individual frames of a dynamic PET study via spatial transformations [1] or for realigning each single coincidence event of a list-mode data stream prior to image reconstruction [2]. In both cases, however, patient motion is only compensated during the emission phase of a PET acquisition. This is the case because PET scanners, with a separate radiation source (e.g. [⁶⁸Ge]) for the transmission measurement, allow for acquiring the attenuation data in histogram-mode only. For an accurate motion correction, however, the patient motion occurring during the several minutes lasting transmission phase also needs to be corrected. This requires the transmission to be processed in list-mode, too. In our study we analysed the hardware and software possibilities and requirements - here of an ACS2-based PET scanner (ECAT Exact HR⁺, Siemens/CTI, Knoxville, Tennessee) - to enable the attenuation measurement to be processed in list-mode. Together with some analysis on motion corrected phantom studies, this should demonstrate the advantages of a fully motion corrected study compared to an emission-corrected study only.