4D attenuation map generation in PET/MR imaging using 4D PET derived motion fields

Respiratory motion can potentially reduce accuracy in functional and anatomical images fusion from combined PET/MR systems. Methodologies for the correction of respiratory motion in PET acquisitions in combined PET/MR systems are almost exclusively based on the use of respiratory synchronized MRI acquisitions to derive motion fields. The acquisition of 4D MR datasets is however associated with a compromise between image quality and acquisition duration limiting its use in clinical practice. The objective of this work was therefore to generate 4D MR images and associated attenuation maps from a single static MR image and motion fields obtained from simultaneously acquired 4D non-attenuation corrected (NAC) PET images. 4D PET/MRI datasets were acquired for five patients on the SIEMENS mMR PET/MR system. The 4D PET acquired datasets were retrospectively binned into 4 motion amplitude frames corresponding to the simultaneously acquired 4D gradient echo MR images. A single MR Dixon scan was also acquired at end of expiration for PET AC purposes. All the reconstructed 4D NAC PET images were then registered using an elastic registration algorithm to the single `end of expiration´ NAC PET image. The derived motion fields were subsequently applied to the `end expiration´ 3D gradient echo MR volume and its corresponding attenuation map to generate respiratory synchronized MR images and corresponding attenuation maps. The accuracy of the proposed method was assessed by comparing the generated to the original images. Good diaphragm profile matching, high correlation coefficients (0.91± 0.03) and small errors (<;3mm) were noticed between the generated and acquired 4D MRI series. Moreover, small classification errors between the generated and 4D MR extracted attenuation maps. As a conclusion, these results may allow for accurate PET AC and respiratory motion correction in PET/MR, without the need for patient specific 4D MR acquisitions.