Motion artifacts in scanning laser polarimetry

PURPOSE: The GDx (Laser Diagnostic Technologies, San Diego, CA) is a scanning laser polarimeter that measures retardation to assess retinal nerve fiber layer thickness in vivo. Eye movements during image acquisition may result in motion artifacts in the GDx image. The aims of this study were to investigate the effect of motion artifacts on the retardation values and to illustrate how motion artifacts can be identified.

DESIGN: Observational case series.

PARTICIPANTS: Thirty-two normal subjects and 28 glaucoma patients participated.

METHODS: We imaged all 60 subjects with the GDx. Images with identified motion artifacts were compared with images without motion artifacts from the same eye and the same session. In 25 cases, the artifact was identified in the superior segment only, and the effect on the superior maximum parameter was calculated. In 26 cases, the artifact was observed in the inferior segment only, and the effect on the inferior maximum parameter was calculated. In nine cases, the artifact was observed superiorly and inferiorly, and the effect on both parameters was calculated. In all 60 cases, the effect on The Number (a summary parameter) was calculated. We also analyzed the groups of glaucoma patients and normal subjects separately.

MAIN OUTCOME MEASURES: Superior maximum parameter, inferior maximum parameter, The Number parameters.

RESULTS: In general, the identified motion artifacts led to an increase in retardation, reflected by an increase in the superior maximum and inferior maximum parameter by 5.9 micro m and 3.4 micro m, respectively (P < 0.001). The Number decreased by 3.4 with motion artifacts (P = 0.001). The variability of this effect was large. In one case, the motion artifact increased retardation by as much as 28.6 micro m. The effect of motion artifacts was greater in glaucoma patients than in normal subjects.

CONCLUSIONS: The identified motion artifacts generally increase retardation values. This increase, however, is highly variable. Therefore, images with such motion artifacts should be viewed with caution or excluded from analysis.