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. 2001 Mar;85(3):304–309. doi: 10.1136/bjo.85.3.304

New neuroretinal rim blood flow evaluation method combining Heidelberg retina flowmetry and tomography

C Jonescu-Cuypers 1, H Chung 1, L Kagemann 1, Y Ishii 1, D Zarfati 1, A Harris 1
PMCID: PMC1723875  PMID: 11222335

Abstract

AIM—Accurate Heidelberg retina flowmeter (HRF) measurements require correct manual setting of the HRF photodetector sensitivity. The neuroretinal rim produces a weak signal relative to the peripapillary retina. A newly developed HRF alignment and sensitivity protocol, capable of accurate rim measurement, was investigated.
METHODS—18 eyes of nine healthy volunteers were examined by HRF. Three images of each eye were taken using three different imaging methods. Method 1: a conventional image (optic nerve head centred image with photodetector sensitivity optimised for the strong signal from the peripapillary retina); method 2: the setting of method 1 with photodetector sensitivity optimised for the weak signal from the rim; and method 3: the setting of method 2 with the temporal rim margin tangent to the lateral image border to remove the overpowering signal from the temporal peripapillary retina. The neuroretinal rim was defined by the Heidelberg retina tomograph (HRT). Blood flow and reflectivity values (DC component) in the rim area were compared for the three methods using pointwise analysis. Coefficients of variation of repeated measurements in 12 subjects have been calculated for method 3.
RESULTS—The neuroretinal rim area measured by method 1 had a significantly lower brightness compared with method 2 and 3 (p=0.0002 and p=0.0002, respectively). Method 2 provided proper sensitivity for the weak signals of the rim area based on rim tissue DC values; however, this sensitivity setting was too high for the strong signal from the peripapillary retina. Method 3 avoided the strong peripapillary signal with the proper signal from the rim and provided significantly higher flow values of the rim area at 75 and 90 percentile pixels (p=0.0065 and p=0.0038 respectively) compared with method 2. Interobserver repeatability ranged from 16.85% to 21.96% for the different parameters. 
CONCLUSIONS—Method 3 provides an accurate and reproducible flow measurement of the neuroretinal rim area through proper sensitivity for the weak rim signal, alignment, and removal of the strong temporal signal from the image. This new method is recommended to improve accuracy of blood flow measurement in the neuroretinal rim.



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Figure 1  .

Figure 1  

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Definition of the optic disc margins using HRT images (A). Description of disc and cup area by means of the HRT software, after contour line drawing of the disc margin by an experienced examiner (B). Using a transparency overlay, the optic disc and cup margins, the major vessels as well as a blue shaded rim measurement area are drawn (C) resulting in a optic disc map ready for transposition to the HRF images (D). The HRT transparency overlay is adjusted exactly over the corresponding optic disc and retinal structures and HRF measurements are performed according to method 1 (E). The same design is used for method 2 (F) and method 3 (G).

Figure 2  .

Figure 2  

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The percentage of unacceptably illuminated pixels in the HRF neuroretinal rim measurements (mean (SE)). % bad DC = the percentage of unacceptably illuminated pixels (DC: value <70 or >200) of total pixels. Method 1: a conventional setting which has the optic nerve head in the centre of the image with photodetector sensitivity optimised for the strong signal from the temporal peripapillary retina. Method 2: the setting of method 1 with signal capturing optimised on the rim by increasing sensitivity. Method 3: the setting of method 2 with the temporal rim margin tangent to the lateral image border. *Significant difference between methods 1 and 2 (p=0.0074, Wilcoxon signed rank test). †Significant difference between methods 1 and 3 (p=0.0003, Wilcoxon signed rank test).

Figure 3  .

Figure 3  

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75 Percentile flow in HRF neuroretinal rim measurements (mean (SE)). 75 percentile flow = blood flow values at 75 percentile pixels of blood flow histogram, which is generated from acceptable DC pixels (DC: from 70 to 200). Method 1: a conventional setting which has the optic nerve head in the centre of the image with photodetector sensitivity optimised for the strong signal from the temporal peripapillary retina. Method 2: the setting of method 1 with signal capturing optimised on the rim by increasing sensitivity. Method 3: the setting of method 2 with the temporal rim margin tangent to the lateral image border. *Significant difference between methods 2 and 3 (p=0.0065, Wilcoxon signed rank test).

Selected References

These references are in PubMed. This may not be the complete list of references from this article.

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