Motion correction in simultaneous PET/MR brain imaging using sparsely sampled MR navigators: a clinically feasible tool

Abstract

Background: We present a study performing motion correction (MC) of PET using MR navigators sampled between other protocolled MR sequences during simultaneous PET/MR brain scanning with the purpose of evaluating its clinical feasibility and the potential improvement of image quality.

Findings: Twenty-nine human subjects had a 30-min [(11)C]-PiB PET scan with simultaneous MR including 3D navigators sampled at six time points, which were used to correct the PET image for rigid head motion. Five subjects with motion greater than 4 mm were reconstructed into six frames (one for each navigator) which were averaged to one image after MC. The average maximum motion magnitude observed was 3.9 ± 2.4 mm (1 to 11 mm). Visual evaluation by a nuclear medicine physician of the five subjects' motion corrected rated three of the five images blurred before motion correction, while no images were rated blurred after. The image quality was scored on a scale of 1-5, 5 being best. The score changed from an average of 3.4 before motion correction to 4.0 after. There was no correlation between maximum motion magnitude and rating. Quantitative SUVr scoring did not change markedly with motion correction.

Conclusions: Sparsely sampled navigators can be used for characterization and correction of head motion. A slight, overall decrease in blurring and an increase in image quality with MC was found, but without impact on clinical interpretation. In future studies with noteworthy motion artifacts, our method is an important and simple-to-use tool to have available for motion correction.

Keywords: Clinical tools; Motion correction; Motion quality control; Navigators; PET/MR; Rigid head motion.

Fig. 1

Setup of the PET/MR scan protocol used in this study. The PET framing (F1–F6) was set individually for each subject depending on the acquisition time of the navigators (Nav₁–Nav₆)

Fig. 2

The principle of frame-by-frame motion correction to a common reference. In our study, the reference is the first PET frame (EM₁) and thus there is no A₁-transformation (courtesy of Oline Vinter Olesen, Technical University of Denmark)

Fig. 3

Plots of measured motion magnitude. Fifty-nine navigator volumes were registered to the first navigator (Nav_1,1, reference position). The subject with the largest maximum motion magnitude is shown at the top and the subject with the lowest maximum motion magnitude (1 mm) at the bottom. The red curves are the motion at the 60 mm point representing cortex motion and is the reference point for motion magnitude in our study. The purple curves are the motion magnitude at the scanners center of FOV (cFOV). The individual translational and rotational components of the motion are given in the Additional file 1 (part 2)

Fig. 4

Visual results of motion correction. Subject five with an 11-mm maximum motion magnitude before MC in (a) and after MC in (b). The visual improvement with motion correction is minor, and our PiB images are as such not highly sensitive to motion artifacts (unless motion is unusual severe)