BACKGROUND: The current @entity28651 -based attenuation correction (AC) used in combined PET/ @entity28651 systems computes a Dixon attenuation map ( @entity28651 -ACDixon) based on fat and water images derived from in- and opposed-phase MRI. We observed an occasional fat/water inversion in @entity28651 -ACDixon. The aim of our study was to estimate the prevalence of this phenomenon in a large @entity1 cohort and assess the possible bias on PET data. METHODS: PET/MRI was performed on a Siemens Biograph @entity28651 (Siemens AG, Erlangen, Germany). We visually inspected attenuation maps of 283 brain or head/neck (H/N) @entity1 , classified them as non-inverted or inverted, and calculated the fat/water tissue fraction. We selected ten FDG-PET brain @entity1 with non-inverted attenuation maps for further analysis. Tissue inversion was simulated, and PET images were reconstructed using both original and inverted attenuation maps. The FDG-PET images of the ten brain @entity1 were analyzed using 11 concentric annulus regions of 5 mm width placed over a central transaxial image plane traversing PETDixon. RESULTS: Out of the 283 @entity1 , a fat/water inversion in 23 @entity1 (8.1%) was observed. The average fraction of fat in the correct @entity28651 -ACDixon was 13% for brain and 17% for H/N @entity1 . In the inverted cases, we found an average fat fraction of 56% for the brain @entity1 and 41% for the @entity8108 /N @entity1 . The effect of the simulated tissue inversion in the brain studies was clearly seen on AC-PET images. The percent-difference image revealed a radial error where the largest difference was at the ventricles (30%        3%) and smallest at the cortical region (10%        2%). CONCLUSIONS: Tissue inversion in Dixon MRI is well known and can occur when there is an error in the off-resonance correction method. Tissue inversion needs to be considered if, based on Dixon-AC, the construction of normal PET databases is performed or any quantitative physiological parameters are fitted. Visual inspection is needed if Dixon-AC is to be used in clinical routine.

Impact of incorrect tissue classification in Dixon-based XXXX -AC: fat-water tissue inversion.

@entity1 :: ('9606', 'Species') :: ['patients', 'patient'] @entity8108 :: ('D006859', 'Chemical') :: ['H'] @entity28651 :: ('50771(Tax:10090)', 'Gene') :: ['MR', 'mMR']
@entity28651 :: (50771(Tax:10090),Gene) :: ['MR']
