  Radiation exposure in positron emission tomography (PET) imaging limits its
usage in the studies of radiation-sensitive populations, e.g., pregnant women,
children, and adults that require longitudinal imaging. Reducing the PET
radiotracer dose or acquisition time reduces photon counts, which can
deteriorate image quality. Recent deep-neural-network (DNN) based methods for
image-to-image translation enable the mapping of low-quality PET images
(acquired using substantially reduced dose), coupled with the associated
magnetic resonance imaging (MRI) images, to high-quality PET images. However,
such DNN methods focus on applications involving test data that match the
statistical characteristics of the training data very closely and give little
attention to evaluating the performance of these DNNs on new
out-of-distribution (OOD) acquisitions. We propose a novel DNN formulation that
models the (i) underlying sinogram-based physics of the PET imaging system and
(ii) the uncertainty in the DNN output through the per-voxel heteroscedasticity
of the residuals between the predicted and the high-quality reference images.
Our sinogram-based uncertainty-aware DNN framework, namely, suDNN, estimates a
standard-dose PET image using multimodal input in the form of (i) a
low-dose/low-count PET image and (ii) the corresponding multi-contrast MRI
images, leading to improved robustness of suDNN to OOD acquisitions. Results on
in vivo simultaneous PET-MRI, and various forms of OOD data in PET-MRI, show
the benefits of suDNN over the current state of the art, quantitatively and
qualitatively.
