  Standard dual-energy computed tomography (CT) uses two different X-ray
energies to obtain energy-dependent tissue attenuation information to allow
quantitative material decomposition. The combined use of dual-energy CT and
positron emission tomography (PET) may provide a more comprehensive
characterization of disease states in cancer and many other diseases. However,
the integration of dual-energy CT with PET is not trivial, either requiring
costly hardware upgrade or increasing radiation dose. This paper proposes a
novel dual-energy CT imaging method that is enabled by the already-available
PET data on PET/CT. Instead of using a second X-ray CT scan with a different
energy, this method exploits time-of-flight PET image reconstruction via the
maximum likelihood attenuation and activity (MLAA) algorithm to obtain a 511
keV gamma-ray attenuation image from PET emission data. The high-energy
gamma-ray CT image is then combined with the low-energy X-ray CT of PET/CT to
provide a pair of dual-energy CT images. A major challenge with the standard
MLAA reconstruction is the high noise present in the reconstructed 511 keV
attenuation map, which does not compromise the PET activity reconstruction too
much but significantly affects the performance of the gamma-ray CT for material
decomposition. To overcome the problem, we further propose a kernel MLAA
algorithm to exploit the prior information from the available X-ray CT image.
We conducted a computer simulation to test the concept and algorithm for the
task of material decomposition. The simulation results indicate that this
PET-enabled dual-energy CT method is promising for quantitative material
decomposition. The proposed method can be readily implemented on time-of-flight
PET/CT scanners to enable simultaneous PET and dual-energy CT imaging.
