  Patient motion is well-known for degrading image quality during medical
imaging. Especially positron emission tomography (PET) is susceptible to motion
due to its usually long scan times. In hybrid PET/MRI (magnetic resonance
imaging), simultaneously acquired dynamic MRI data can be used to correct for
motion. Usually, MRI model-based motion correction approaches are applied to
the PET data. However, these approaches may fail for non-predictable, irregular
motion. We propose a novel approach for the continuous and real-time tracking
of motion using highly accelerated, dynamic MRI for an accurate motion
estimation. For this purpose, a TurboFLASH sequence is utilized in single-shot
mode with additional exploiting GRAPPA acceleration. Sampling frequency for one
slice is up to 26 ms and 520 ms for one 3D volume of 20 coronal slices.
Principal component analysis and a phase-sensitive resorting of slices is
performed to restore temporal consistency of the volumes. Motion is estimated
from these volumes using hyper-elastic registration. The approach is validated
with the help of a dynamic thorax phantom as well as with eleven healthy
volunteers. Phantom ground-truth data demonstrates that the approach produces
an accurate motion estimation. Volunteer validation proves that the approach is
also valid for different respiratory amplitudes including highly irregular
breathing. The approach could be proved to be promising for a continuous PET
motion correction.
