Scale Adjustments to Facilitate Two-Dimensional Measurements in OCT Images.
Bottom Line:
The problems encountered with two-dimensional measurements in cases of unequal scales are demonstrated and an estimation of the resulting errors is provided.In this work, we highlight the distortion-related problems in OCT image analysis induced by unequal X and Y scales.Our results demonstrate the need for a proper two-dimensional calibration of OCT data, and we believe that equal scaling will certainly improve the efficiency of OCT image analysis.
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PubMed Central - PubMed
Affiliation: Division of Ocular Neurodegeneration, Institute for Ophthalmic Research, Centre for Ophthalmology, Tuebingen, Germany.
ABSTRACT
Purpose: To address the problem of unequal scales for the measurement of two-dimensional structures in OCT images, and demonstrate the use of intra¬ocular objects of known dimensions in the murine eye for the equal calibration of axes. Methods: The first part of this work describes the mathematical foundation of major distortion effects introduced by X-Y scaling differences. Illustrations were generated with CorelGraph X3 software. The second part bases on image data obtained with a HRA2 Spectralis (Heidelberg Engineering) in SV129 wild-type mice. Subretinally and intravitreally implanted microbeads, alginate capsules with a diameter of 154±5 μm containing GFP-marked mesenchymal stem cells (CellBeads), were used as intraocular objects for calibration. Results: The problems encountered with two-dimensional measurements in cases of unequal scales are demonstrated and an estimation of the resulting errors is provided. Commonly, the Y axis is reliably calibrated using outside standards like histology or manufacturer data. We show here that intraocular objects like dimensionally stable spherical alginate capsules allow for a two-dimensional calibration of the acquired OCT raw images by establishing a relation between X and Y axis data. For our setup, a correction factor of about 3.3 was determined using both epiretinally and subretinally positioned beads (3.350 ± 0.104 and 3.324 ± 0.083, respectively). Conclusions: In this work, we highlight the distortion-related problems in OCT image analysis induced by unequal X and Y scales. As an exemplary case, we provide data for a two-dimensional in vivo OCT image calibration in mice using intraocular alginate capsules. Our results demonstrate the need for a proper two-dimensional calibration of OCT data, and we believe that equal scaling will certainly improve the efficiency of OCT image analysis. No MeSH data available. Related in: MedlinePlus |
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Mentions: By design, OCT data consist of many individual scans along the Y axis, while the X axis is a product of internal post processing. Its scaling involves a number of inferences and normative data that may not fit well to the actual application, both in clinical and experimental work. If X and Y axis are not to the same scale, the resulting images will show distortion. We demonstrate this effect here with the help of two examples of defined geometrical shapes. First, we address the behaviour of a circular shape (Fig 1). We assume that the actual object is a circle with the same diameter along X and Y axes (a/b = 1). It is obvious that the circle becomes elliptical when the scale of the X axis changes, here shown up to a radio of a/b = 5 (Fig 1A). Respective retinal structures like e.g. vessels would feature a highly elliptical cross section depending on the magnitude of this effect. When attempting to measure the diameter of the structure, this is only straightforward at 0° or 90°, i.e. entirely along the X or Y axis. While in case of the (original) circle the diameter is independent of the angle of the section, this is not so in case of an ellipse (Fig 1B). Depending on the angle of the section (α) and the ratio between X and Y scales (a/b), the measured result has to be divided by the relative diameter (dr) to reflect the correct distance. For a derivation of the formula see Appendix 1. A family of curves for different a/b ratios illustrates this behaviour (Fig 1B). |
View Article: PubMed Central - PubMed
Affiliation: Division of Ocular Neurodegeneration, Institute for Ophthalmic Research, Centre for Ophthalmology, Tuebingen, Germany.
Purpose: To address the problem of unequal scales for the measurement of two-dimensional structures in OCT images, and demonstrate the use of intra¬ocular objects of known dimensions in the murine eye for the equal calibration of axes.
Methods: The first part of this work describes the mathematical foundation of major distortion effects introduced by X-Y scaling differences. Illustrations were generated with CorelGraph X3 software. The second part bases on image data obtained with a HRA2 Spectralis (Heidelberg Engineering) in SV129 wild-type mice. Subretinally and intravitreally implanted microbeads, alginate capsules with a diameter of 154±5 μm containing GFP-marked mesenchymal stem cells (CellBeads), were used as intraocular objects for calibration.
Results: The problems encountered with two-dimensional measurements in cases of unequal scales are demonstrated and an estimation of the resulting errors is provided. Commonly, the Y axis is reliably calibrated using outside standards like histology or manufacturer data. We show here that intraocular objects like dimensionally stable spherical alginate capsules allow for a two-dimensional calibration of the acquired OCT raw images by establishing a relation between X and Y axis data. For our setup, a correction factor of about 3.3 was determined using both epiretinally and subretinally positioned beads (3.350 ± 0.104 and 3.324 ± 0.083, respectively).
Conclusions: In this work, we highlight the distortion-related problems in OCT image analysis induced by unequal X and Y scales. As an exemplary case, we provide data for a two-dimensional in vivo OCT image calibration in mice using intraocular alginate capsules. Our results demonstrate the need for a proper two-dimensional calibration of OCT data, and we believe that equal scaling will certainly improve the efficiency of OCT image analysis.
No MeSH data available.