Prospective real-time slice-by-slice 3D motion correction for EPI using an external optical motion tracking system
The need for motion correction in functional MRI (fMRI) time series has been well recognised. This correction may be performed in two ways: either retrospectively, by image registration after the time series has been fully collected,(1) or prospectively, when the motion parameters are measured during the time series acquisition and the corresponding changes are made to the imaging parameters. Prospective techniques differ in the way they acquire the position information. Navigator techniques(2,3) include additional pulse sequences, capable of object position determination. Image-based methods(4) perform co-registration of the measured volume data to extract motion parameters during the acquisition of the time series. Both these approaches are, however, not without problems. Navigator-based techniques slow the acquisition process considerably and perturb the steady-state because of the additional excitation RF pulses. Image-based methods require volume coverage in order to extract position information and considerable computational effort, which introduces delays. Both navigator and image-based corrections typically update positions of the slice packet once per measured volume, which may not be valid, if motion occurs while the slice packet is being acquired. Here we demonstrate a possibility of slice-by-slice 3D prospective motion correction of EPI time series using an external optical motion tracking system.