PET Attenuation Map Generation from Virtual Monochromatic Images Obtained with Photon Counting CT Scanners

Attenuation correction (AC) is essential for achieving artefact-free PET/MR images. Many previous studies have primarily relied on conventional CT scanners for AC. Currently, CT-derived PET attenuation maps are based on conversion schemes from conventional CT scanners with data from polyenergetic X-ray spectra and it is known that converting from these CT image data to monoenergetic 511 keV attenuation coefficients unavoidably introduces systematic errors in the attenuation coefficients employed for AC in PET imaging. To address this issue, we utilized spectral CT in order to obtain more accurate attenuation maps. Spectral CT offers advantages over conventional CT scanners by providing the possibility of material decomposition. Spectral CT systems also allow to reconstruct virtual monochromatic images (VMI) from the inherently multiple polyenergetic data sets of CT. VMIs can be used to obtain virtual non-contrast (VNC) images and bone mineral density measurements. In our approach, we plan to use angiographic spectral CT scans with an administration of iodinated contrast agents and to compute VNC images from the obtained dataset. The conversion scheme from monochromatic CT images to linear attenuation coefficients at 511 keV was obtained by acquiring spectral CT data for different tissue types using an electron density phantom developed for spectral CTs and a photon counting (PC) CT scanner.