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M Sehi, JG Flanagan; The Effect of Image Alignment on the blood flow measurement using Heidelberg Retina Flowmeter . Invest. Ophthalmol. Vis. Sci. 2002;43(13):317.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose:To examine the influence of different image alignments on the reliability and repeatability of blood flow values measured using the Heidelberg Retina Flowmeter (HRF). Methods: The sample consisted of 10 normal subjects with the age range of 23-44 years (mean: 29.9 +/- 7.6) (4 females). One eye of each subject was randomly selected. The spherical refractive errors were between -2.50 to +1.75 with cylindrical errors less than -0.75. Heidelberg Retina Tomograph (HRT) imaging was performed to establish the most appropriate location and best focus setting for areas on the temporal side of the neuro-retinal rim. HRF measurements were performed using three different methods of alignment: i. Central: Optic disc positioned at the centre of the image; ii. Nasal: Nasal rim margin tangent to the nasal side of the image; and iii. Temporal: Temporal rim margin tangent to the temporal side of the image. Two sets of high quality images with no saccadic eye movements were included for each method of alignment. A 10x10 pixel measurement voxel was selected using the Central alignment and exactly reproduced for the Nasal and Temporal images. Blood flow values were analysed by repeated measures ANOVA. Intra-class correlation coefficients (ICC) were calculated to assess repeatability. Results: At visit one, there was no significant difference between the Central and Temporal alignments (p=0.33) but there was a significant difference between the Central and Nasal alignments (p=0.003) and Nasal and Temporal alignment (p=0.0003). At visit two, none of the alignments was significantly different. There was a statistically significant difference between the flow values of the Nasal (p< 0.0001) and Temporal (p<0.0001) alignments of visit one compared to visit two. There was no statistically significant difference between the flow values of the Central alignment of visit one and two (p=0.37). The ICCs were 0.83 for the Central, 0.25 for the Nasal and 0.43 for the Temporal alignment.Conclusion: Central alignment provided the only acceptable level of reliability and repeatability for flow measurement of the neuro-retinal rim. It should be noted that these results were obtained taking great care to optimise focus setting and image quality.
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