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Identifying Artifacts and Outliers in Structural Optical Coherence Tomography
C H A P T E R
6
Identifying Artifacts and Outliers in Structural Optical Coherence Tomography Du Tran-Viet
■ Katrina Postell Winter ■ Xi Chen
Suboptimal scans and imaging artifacts, although often unavoidable, can affect the analysis of optical coherence tomography (OCT) images and lead to incorrect interpretation of data. Table 6.11-4 lists the common artifacts seen on OCT images. The table also provides suggestions on how to identify these artifacts and how to avoid the errors to improve the quality of the scans. Some examples of artifacts are shown below.
34
Copyright © 2020 Elsevier Inc. All rights reserved.
TABLE 6.1 ■ Suboptimal Scans and Artifacts from SD-OCT Imaging How to Identify
How to Avoid
Scan clipping on the edges when OCT signal is blocked as a result of lack of pupil dilation or mismatching of eye length and reference arm length such that the OCT beam pivot is not axially aligned within the pupil.
Increase pupil size and/or adjust reference arm to the eye length.1
Out-of-range error
Image is too high or too low in scan window. It may occur if the scanner is too close or too far away from the eye or if the subject moves during imaging. It may also occur when there is ocular pathology (e.g., high myopia, severe edema, vitreous traction, or other pathology).
Adjust working distance accordingly to position area of interest in scan window.
6—IDENTIFYING ARTIFACTS AND OUTLIERS
Artifact Vignetting
35 Continued on following page
TABLE 6.1 ■ Suboptimal Scans and Artifacts from SD-OCT Imaging (Continued) How to Avoid
Mirror artifact
Image crosses zero delay line and results in an inverted image or vitreous pathology superimposed on top of retinal image. It occurs when the scanner is too close to the eye or when opacity is elevated within the vitreous (e.g., asteroid hyalosis).
Adjust working distance accordingly to obtain area of interest in scan window.
Blink
Child blinked resulting in dark or vignetted frames.
Try to reobtain image.
HANDBOOK OF PEDIATRIC RETINAL OCT AND THE EYE-BRAIN CONNECTION
How to Identify
36
Artifact
Images are tilted at an angle. This may affect reflectance of retinal layers, such as Henle fiber layer, which may further affect visualization, and qualitative and quantitative grading. Severely tilted scans may also affect accurate thickness measurements.3
Make lateral pivoting motions to correct the tilt and straighten the scan.
Misalignment
Area of interest (e.g., fovea or optic nerve) is not centered in the retinal view/volume.
Make sliding motion on same plane to center, and obtain image of area of interest.
6—IDENTIFYING ARTIFACTS AND OUTLIERS
Tilt
37 Continued on following page
TABLE 6.1 ■ Suboptimal Scans and Artifacts from SD-OCT Imaging (Continued)
Motion
Child moves during scanning (tracking, active sucking on pacifier, or axial motion with heartbeat when supine) or has saccades or nystagmus.4
Image is less saturated and appears fainter as a result of uncorrected refractive error or misalignment of scan.
How to Avoid Try to reobtain image with less motion. Increase scan speed by reducing number of A-scans and B-scans. Provide an external fixation object (telephone screen or toy).4
Correct imaging system for refractive error and realign.1
HANDBOOK OF PEDIATRIC RETINAL OCT AND THE EYE-BRAIN CONNECTION
Out of focus
How to Identify
38
Artifact
Media opacities including vitreous hemorrhage, cataract, or corneal opacity may block signal to retina. Inner retinal reflective structure (e.g., large retinal vessels or hemorrhage) may shadow deeper structures.
Recognize the blocking and image off axis, if needed.
Scanner artifact
Lens reflectance, oil, dust, mirror alignment, or other particles in the beam path can create artifacts in or diminish the image. Reflections within the system will occur in the same location with the same appearance across different images.
Cannot avoid lens reflectance. For other artifacts, clean the optical pathway of the imaging system.
39
OCT, Optical coherence tomography; SD-OCT, spectral-domain OCT.
6—IDENTIFYING ARTIFACTS AND OUTLIERS
Signal blocking
40
HANDBOOK OF PEDIATRIC RETINAL OCT AND THE EYE-BRAIN CONNECTION
Reference 1. Maldonado RS, Izatt JA, Sarin N, Wallace DK, Freedman S, Cotten CM, Toth CA. Optimizing hand-held spectral domain optical coherence tomography imaging for neonates, infants and children. Invest Ophthalmol Vis Sci. 2010;51(5):2678–2685. 2. Lujan BJ, Roorda A, Knighton RW, Carroll J. Revealing Henle’s fiber layer using spectral domain optical coherence tomography. Invest Ophthalmol Vis Sci. 2011;52(3):1486–1492. 3. Uji A, Abdelfattah NS, Boyer DS, Balasubramanian S, Lei J, Sadda SR. Variability of retinal thickness measurements in tilted or stretched optical coherence measurements. Transl Vis Sci Tech. 2017;6(2):1. 4. Lee H, Proudlock FA, Gottlob I. Is handheld optical coherence tomography reliable in infants and young children with and without nystagmus? Invest Ophthalmol Vis Sci. 2013;54(13):8152–8159.
6
Identifying Artifacts and Outliers in Structural Optical Coherence Tomography Du Tran-Viet
■ Katrina Postell Winter ■ Xi Chen
Suboptimal scans and imaging artifacts, although often unavoidable, can affect the analysis of optical coherence tomography (OCT) images and lead to incorrect interpretation of data. Table 6.11-4 lists the common artifacts seen on OCT images. The table also provides suggestions on how to identify these artifacts and how to avoid the errors to improve the quality of the scans. Some examples of artifacts are shown below.
34
Copyright © 2020 Elsevier Inc. All rights reserved.
TABLE 6.1 ■ Suboptimal Scans and Artifacts from SD-OCT Imaging How to Identify
How to Avoid
Scan clipping on the edges when OCT signal is blocked as a result of lack of pupil dilation or mismatching of eye length and reference arm length such that the OCT beam pivot is not axially aligned within the pupil.
Increase pupil size and/or adjust reference arm to the eye length.1
Out-of-range error
Image is too high or too low in scan window. It may occur if the scanner is too close or too far away from the eye or if the subject moves during imaging. It may also occur when there is ocular pathology (e.g., high myopia, severe edema, vitreous traction, or other pathology).
Adjust working distance accordingly to position area of interest in scan window.
6—IDENTIFYING ARTIFACTS AND OUTLIERS
Artifact Vignetting
35 Continued on following page
TABLE 6.1 ■ Suboptimal Scans and Artifacts from SD-OCT Imaging (Continued) How to Avoid
Mirror artifact
Image crosses zero delay line and results in an inverted image or vitreous pathology superimposed on top of retinal image. It occurs when the scanner is too close to the eye or when opacity is elevated within the vitreous (e.g., asteroid hyalosis).
Adjust working distance accordingly to obtain area of interest in scan window.
Blink
Child blinked resulting in dark or vignetted frames.
Try to reobtain image.
HANDBOOK OF PEDIATRIC RETINAL OCT AND THE EYE-BRAIN CONNECTION
How to Identify
36
Artifact
Images are tilted at an angle. This may affect reflectance of retinal layers, such as Henle fiber layer, which may further affect visualization, and qualitative and quantitative grading. Severely tilted scans may also affect accurate thickness measurements.3
Make lateral pivoting motions to correct the tilt and straighten the scan.
Misalignment
Area of interest (e.g., fovea or optic nerve) is not centered in the retinal view/volume.
Make sliding motion on same plane to center, and obtain image of area of interest.
6—IDENTIFYING ARTIFACTS AND OUTLIERS
Tilt
37 Continued on following page
TABLE 6.1 ■ Suboptimal Scans and Artifacts from SD-OCT Imaging (Continued)
Motion
Child moves during scanning (tracking, active sucking on pacifier, or axial motion with heartbeat when supine) or has saccades or nystagmus.4
Image is less saturated and appears fainter as a result of uncorrected refractive error or misalignment of scan.
How to Avoid Try to reobtain image with less motion. Increase scan speed by reducing number of A-scans and B-scans. Provide an external fixation object (telephone screen or toy).4
Correct imaging system for refractive error and realign.1
HANDBOOK OF PEDIATRIC RETINAL OCT AND THE EYE-BRAIN CONNECTION
Out of focus
How to Identify
38
Artifact
Media opacities including vitreous hemorrhage, cataract, or corneal opacity may block signal to retina. Inner retinal reflective structure (e.g., large retinal vessels or hemorrhage) may shadow deeper structures.
Recognize the blocking and image off axis, if needed.
Scanner artifact
Lens reflectance, oil, dust, mirror alignment, or other particles in the beam path can create artifacts in or diminish the image. Reflections within the system will occur in the same location with the same appearance across different images.
Cannot avoid lens reflectance. For other artifacts, clean the optical pathway of the imaging system.
39
OCT, Optical coherence tomography; SD-OCT, spectral-domain OCT.
6—IDENTIFYING ARTIFACTS AND OUTLIERS
Signal blocking
40
HANDBOOK OF PEDIATRIC RETINAL OCT AND THE EYE-BRAIN CONNECTION
Reference 1. Maldonado RS, Izatt JA, Sarin N, Wallace DK, Freedman S, Cotten CM, Toth CA. Optimizing hand-held spectral domain optical coherence tomography imaging for neonates, infants and children. Invest Ophthalmol Vis Sci. 2010;51(5):2678–2685. 2. Lujan BJ, Roorda A, Knighton RW, Carroll J. Revealing Henle’s fiber layer using spectral domain optical coherence tomography. Invest Ophthalmol Vis Sci. 2011;52(3):1486–1492. 3. Uji A, Abdelfattah NS, Boyer DS, Balasubramanian S, Lei J, Sadda SR. Variability of retinal thickness measurements in tilted or stretched optical coherence measurements. Transl Vis Sci Tech. 2017;6(2):1. 4. Lee H, Proudlock FA, Gottlob I. Is handheld optical coherence tomography reliable in infants and young children with and without nystagmus? Invest Ophthalmol Vis Sci. 2013;54(13):8152–8159.