Abstract
Respiratory motion may reduce accuracy in fusion of functional and anatomical images using combined Positron emission tomography / Magnetic resonance (PET/MR) systems. Methodologies for the correction of respiratory motion in PET acquisitions using such systems are mostly based on the use of respiratory synchronized MR acquisitions to derive motion fields. Existing approaches based on tagging acquisitions may introduce artifacts in the MR images, while motion model approaches require the acquisition of training datasets. The objective of this work was to investigate the possibility of generating 4D MR images and associated attenuation maps (AMs) from a single static MR image combined with motion fields obtained from simultaneously acquired 4D non-attenuation corrected (NAC) PET images. Methods: 4D PET/MR datasets were acquired for 11 patients on a simultaneous PET/MR system. The 4D PET datasets were retrospectively binned into 4 motion amplitude frames corresponding to the simultaneously acquired T1-weighted 4D MR images. A T1-weighted 3D MR sequence with Dixon-based fat-water separation was also acquired at end of expiration for PET attenuation correction (AC) purposes. All reconstructed 4D NAC PET images were then elastically registered to the single ‘end-expiratory’ NAC PET image. The derived motion fields were subsequently applied to the ‘end-expiratory’ frame of the acquired 4D MR volume and the AM derived from the Dixon MR images to generate respiratory synchronized MR images and corresponding AMs. Results: The accuracy of the proposed method was assessed by comparing the generated to the acquired images according to metrics such as overall correlation coefficients and distance differences for anatomical landmarks between the generated and acquired MR datasets. High correlation coefficients (0.93±0.03) and small differences (2.69±0.5mm) were obtained. Moreover, small tissue classification differences (2.23±0.68%) between the generated and 4D MR extracted AMs were observed. Conclusion: Our results confirm the feasibility of using 4D NAC PET images for accurate PET AC and respiratory motion correction in PET/MR, without the need for patient specific 4D MR acquisitions.
- Copyright © 2016 by the Society of Nuclear Medicine and Molecular Imaging, Inc.