  We propose a magnetic resonance (MR)-based method for estimation of
continuous linear attenuation coefficients (LAC) in positron emission
tomography (PET) using a physical compartmental model and ultrashort echo time
(UTE)/multi-echo Dixon (mUTE) acquisitions. Specifically, we propose a
three-dimensional (3D) mUTE sequence to acquire signals from water, fat, and
short-T2 components (e.g., bones) simultaneously in a single acquisition. The
proposed mUTE sequence integrates 3D UTE with multi-echo Dixon acquisitions and
uses sparse radial trajectories to accelerate imaging speed. Errors in the
radial k-space trajectories are measured using a special k-space trajectory
mapping sequence and corrected for image reconstruction. A physical
compartmental model is used to fit the measured multi-echo MR signals to obtain
fractions of water, fat and bone components for each voxel, which are then used
to estimate the continuous LAC map for PET attenuation correction. The
performance of the proposed method was evaluated via phantom and in vivo human
studies, using LACs from Computed Tomography (CT) as reference. Compared to
Dixon- and atlas-based MRAC methods, the proposed method yielded PET images
with higher correlation and similarity in relation to the reference. The
relative absolute errors of PET activity values reconstructed by the proposed
method were below 5% in all of the four lobes (frontal, temporal, parietal,
occipital), cerebellum, whole white matter and gray matter regions across all
subjects (n=6). The proposed mUTE method can generate subject-specific,
continuous LAC map for PET attenuation correction in PET/MR.
