A generic PET/MRI respiratory motion correction using a generalized reconstruction by inversion of coupled systems (GRICS) approach

Respiratory motion is a source of artifacts in multimodality imaging such as PET/CT and more recently PET/MRI. These artifacts can take the form of blurring or aliasing of the moving structures in different parts of the image. Solutions proposed to date include retrospective or prospective gating. Although these techniques are routinely used in clinical practice, they have the drawback of increasing overall acquisition duration. More elaborated methods consist in using 4D MRI datasets to extract spatial deformations for respiratory motion correction performed during PET image reconstruction. The drawbacks of such methods include time consuming acquisitions as well as the need for fast 3D MRI acquisitions leading to reduced tissue contrast. The objective of this work is to overcome these limitations by the use of a generalized reconstruction by inversion of coupled systems (GRICS) approach in order to obtain internal MRI structure motion and associated deformation matrices for use in PET respiratory motion correction. The GRICS method consists in formulating parallel MRI in the presence of patient motion as a coupled inverse problem. Its resolution, using a fixed-point method, allows the reconstructed image to be improved using a motion model constructed from the raw MR data, irrespectively of the MR sequence used. This method was tested on two different MR volunteer datasets. It was compared with the use of 4D MR acquisitions, and evaluated within the context of correcting simulated PET image frames for respiratory motion. Our results indicate that using GRICS is an efficient method for PET respiratory motion correction in combined PET/MRI imaging without the need for 4D MR acquisitions and associated specific MR sequences.