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Purpose  To develop a magnetic resonance (MR)‐based method for estimation of continuous linear attenuation coefficients (LACs) in positron emission tomography (PET) using a physical compartmental model and ultrashort echo time (UTE)/multi‐echo Dixon (mUTE) acquisitions.    Methods  We propose a three‐dimensional (3D) mUTE sequence to acquire signals from water, fat, and short T 2  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.    Results  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, and occipital), cerebellum, whole white matter, and gray matter regions across all subjects (n = 6).    Conclusions  The proposed mUTE method can generate subject‐specific, continuous LAC map for PET attenuation correction in PET/MR.

MR‐based PET attenuation correction using a combined ultrashort echo time/multi‐echo Dixon acquisition