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Editors Selection IGR 20-4
Imaging: GDx and atypical retardation patterns
Comment by Murray Fingeret on:
21528 Atypical retardation pattern: can performance of classification be improved?, Gunvant P; Zheng Y; Toth M et al., Optometry and Vision Science, 2008; 85: 482-488
As the optic nerve and retinal nerve fiber layer (RNFL) are assessed and compared to a normative database, the clinician evaluates basic features of the scan such as the focus, lighting, and centration to assess image quality. Poor quality images should be used with caution when deciding whether a problem exists. Still, images may appear to be within acceptable limits using these parameters and still be unreliable as Gunvant et al. (1055) describe. The authors in this paper assess atypical retardation patterns, a problem that occurs in approximately 15% of GDx variable corneal compensaton (VCC) scans, used to analyze the RNFL. The authors investigate whether other methods of analysis may salvage the data and provide credible results. The authors of this paper evaluate classification performance with Wavelength-Fourier analysis (WFA), Fast-Fourier analysis (FFA), and the standard VCC output in a group that contains both glaucomas and healthy individuals. ARPs are evaluated by the typical scan score (TSS), a quality indicator that calculates the degree of typicality with 100 being excellent and 0 being the worst form. Atypical retinal patterns are due to reduced signal-to-noise ratio. The retardance pattern being measured is not reliable since the diminished signal is lost within the noise. The clinician recognizes ARPs by the appearance of splotchy tie-dye patterns on the RNFL maps since the quality indicators often appear to be adequate. The manufacturer recommends a TSS cut-off of 40 as the point when images should be viewed with caution. In this paper the authors used a criterion of < 80, which may have been used to increase the enrollment. Analysis is also provided for a cut-off of < 60. One issue is that scans with TSS between 60-80 are often useful and not unreliable. The results of the study found that different methods did not improve the classification as compared to the VCC approach. Still, this work is significant and further study is needed because most clinicians have only one imaging device. When unreliable images occur, this device can not be used so alternative analysis methods may allow data to be useful when otherwise it would not be. The manufacturer of the GDx VCC has recognized the significance of this issue and will be moving the GDx default algorithm from VCC to Enhanced Corneal Compensation (ECC) which is felt to reduce the occurrence of ARPs.
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