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Impact of incorrect tissue classification in Dixon-based MR-AC: fat-water tissue inversion
Claes Nøhr Ladefoged1, Adam Espe Hansen2, Sune Høgild Keller3
1Department of Clinical Physiology, Nuclear Medicine and PET, Rigshospitalet, University of Copenhagen, Blegdamsvej 9, 2100, Copenhagen, Denmark. claes.noehr.ladefoged@regionh.dk.
Background:
The current MR-based attenuation correction (AC) used in combined PET/MR systems computes a Dixon attenuation map (MR-ACDixon) based on fat and water images derived from in- and opposed-phase MRI. We observed an occasional fat/water inversion in MR-ACDixon. The aim of our study was to estimate the prevalence of this phenomenon in a large patient cohort and assess the possible bias on PET data.
Methods:
PET/MRI was performed on a Siemens Biograph mMR (Siemens AG, Erlangen, Germany). We visually inspected attenuation maps of 283 brain or head/neck (H/N) patients, classified them as non-inverted or inverted, and calculated the fat/water tissue fraction. We selected ten FDG-PET brain patients 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 patients 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 patients, a fat/water inversion in 23 patients (8.1%) was observed. The average fraction of fat in the correct MR-ACDixon was 13% for brain and 17% for H/N patients. In the inverted cases, we found an average fat fraction of 56% for the brain patients and 41% for the H/N patients. 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.
Insights
Fat/water inversion in Dixon MRI occurs in 8.1% of patients, potentially biasing PET data. Visual inspection of attenuation maps is crucial for accurate quantitative PET/MRI analysis.
Area of Science:
- Medical Imaging
- Radiology
- Nuclear Medicine
Background:
- Combined PET/MR systems use MR-based attenuation correction (AC) via Dixon imaging.
- Occasional fat/water inversion in MR-ACDixon maps has been observed.
- The prevalence and impact of this inversion on PET data require investigation.
Purpose of the Study:
- To estimate the prevalence of fat/water inversion in Dixon-based AC in a large patient cohort.
- To assess the potential bias introduced by this inversion on PET data.
Main Methods:
- 283 brain or head/neck (H/N) patients underwent PET/MRI.
- Attenuation maps were visually inspected for fat/water inversion.
- Simulated inversion was used to reconstruct PET images in ten brain patients for analysis.
Main Results:
- Fat/water inversion was observed in 8.1% of patients (23/283).
- Inverted cases showed significantly higher fat fractions (brain: 56%, H/N: 41%) compared to non-inverted cases.
- Simulated inversion caused radial errors in AC-PET images, with largest differences at ventricles (30%) and smallest at cortex (10%).
Conclusions:
- Tissue inversion in Dixon MRI is a known issue that can affect quantitative PET/MRI.
- The findings highlight the need for careful consideration of Dixon-AC inversion when creating PET databases or fitting physiological parameters.
- Visual inspection of attenuation maps is recommended for clinical routine use of Dixon-AC.
