Quantification of patient motion in FDG-PET/CT brain imaging

Objectives

FDG-PET is an important imaging tool in evaluation of neurological disorders, including drug-resistant epilepsy. Head motion can degrade PET image quality; however, the extent in routine examinations remains insufficiently characterized. This retrospective study aimed to quantify the magnitude and pattern of head motion in brain FDG-PET/CT and establish displacement-based scores for describing motion severity.

Materials & Methods

The analysis included 140 patients with focal drug-resistant epilepsy undergoing brain FDG-PET/CT, with 27 examined under general anesthesia as a reference group. PET acquisition comprised five consecutive 2-min frames. Head motion was quantified by rigid PET-to-PET registration and expressed as maximal displacement. Two experienced readers independently graded motion on a four-point ordinal scale. Ordinal logistic regression with repeated 10-fold cross-validation was used to derive displacement thresholds corresponding to visual motion scores.

Results

Patients without anesthesia demonstrated greater maximal displacement than the reference group (median, 2.6 vs. 0.7 mm; p < 0.001). Displacement increased throughout acquisition, peaking in the final frame in 85.7% of examinations. Motion predominantly involved inferior–superior translation and rotation around left–right axis, corresponding to head flexion–extension. Maximal displacement was strongly associated with visual motion grades (Kendall’s τ=0.68–0.70; p < 0.001). Derived thresholds classified displacement >3.25 mm as moderate and >6.4 mm as severe motion. In total, 40.7% of patients without anesthesia demonstrated moderate or severe motion, with severe motion in 12.4%.

Conclusion

Clinically relevant head motion was frequent in brain FDG-PET/CT despite head fixation, predominantly characterized by flexion–extension. Registration-based displacement provided an objective measure closely associated with expert visual assessment.

Key points

• This study investigated the frequency of clinically relevant head motion in brain FDG-PET/CT and whether image-based displacement can provide an objective measure of motion severity.

• Clinically relevant head motion occurred in approximately 40% of brain FDG-PET/CTs, with severe motion in 12.4% of examinations and flexion–extension being the predominant pattern.

• Direct quantitative assessment of head motion from dynamic PET data may improve recognition of clinically relevant motion and support objective decisions on motion correction and repeat acquisition, potentially improving the reliability of brain FDG-PET/CT interpretation.