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Artifacts in optical coherence tomography topographic maps
Artifacts in Optical Coherence Tomography Topographic Maps MICHAEL R. HEE, MD PHD
O
VER THE LAST FEW YEARS, OPTICAL COHERENCE
tomography (OCT) has become an indispensable technique for high-resolution cross-sectional imaging of the macula and optic nerve. One of the most useful applications of OCT is its ability to sensitively and reproducibly monitor retinal thickness. Although originally developed as a technique to evaluate diabetic macular edema,1 in many institutions the OCT topographic map is becoming a de facto standard for evaluating other macular conditions and therapies for age-related macular degeneration. In this issue, Ray and associates examine artifacts which may appear in OCT topographic maps.2 All imaging systems are subject to artifacts. Fundus photography and fluorescein angiography, for example, depend on the training of the photographer, the quality of the media, pupil dilation, and patient eye motion. Ultrasound A and B scans can be affected by improper alignment or excessive pressure of the probe on the cornea and intraocular air or silicone oil.3,4 Eye motion, vitreous opacities, and other artifacts are known to affect scanning laser polarimetry.5-8 Measurements of macular thickness with the retinal thickness analyzer may be adversely affected after cataract surgery.9 Ray and associates characterize artifacts arising from one particular OCT scanning protocol known as the fast macular thickness map. In this protocol, six scans are rapidly obtained in a radial spoke pattern. A computer algorithm identifies the inner and outer retinal boundaries, and a topographic map of macular thickness is created. The authors analyzed artifacts in fast macular thickness maps obtained over a 3-week period in 171 eyes. Broadly, the artifacts could be divided into two categories—those that related to a misidentification of the retinal surfaces by the computer software versus those that reflected a deficient scan acquisition. Clinical experience tells us that OCT provides useful information that helps guide therapy. Indeed, the authors themselves note that OCT has supplanted both angiography and fundus photography as the most commonly ordered retinal imaging test at their institution. Therefore, it is somewhat surprising that over 40% of all fast macular thickness maps contained a recognizable artifact. Clearly, Accepted for publication Aug 25, 2004. From Pacific Eye Specialists, Dale City, California. Inquiries to Michael R. Hee, MD, Pacific Eye Specialists, 1850 Sullivan Avenue, Suite 540, Daly City, CA 94015; fax: 650-755-2107; e-mail: [email protected]
154
©
2005 BY
this is an issue that should be recognized and addressed by care in acquisition and review of images. The authors’ analysis of artifacts focuses on the fast macular thickness map scanning protocol not on individual cross-sectional scans. This difference is important for three reasons. First, only one of the six cross-sectional scans comprising the topographic map needs to have a boundary detection abnormality to create an artifact in the map. Therefore, the frequency of artifacts occurring in the topographic map is naturally much higher than the frequency of artifacts occurring in an individual scan. Second, the retinal thickness maps rely on a computer algorithm to segment the inner and outer boundaries of the retina. These algorithms may fail to identify the boundaries if the retinal architecture is distorted due to severe pathology, even though the individual OCT scans comprising the topographic map correctly reveal the true retinal structure. The enhanced resolution offered by OCT compared with other imaging modalities provides images displaying more complex internal features which, ironically, require more sophisticated boundary detection algorithms. As a reference, many other imaging modalities do not even attempt quantitative boundary detection. Tumor thickness on ultrasound B-scan, for instance, is usually measured with manually placed computer calipers. In Heidelberg retinal tomography assessment of the optic nerve, the boundaries of the nerve must be outlined by the examiner. The original macular thickness algorithms2 (upon which the current algorithms are based) were designed to determine the inner and outer retinal boundaries in diabetic macular edema, a condition leading to retinal thickening in which intraretinal fluid accumulation and hard exudates occur but the retinal pigment epithelium and inner limiting membrane remain intact. Ray and associates demonstrate that software-related artifacts may occur when these boundaries are not well defined. With a macular hole, in particular, the absence of foveal retinal tissue may not be correctly interpreted by the algorithms. With exudative AMD, especially following photodynamic therapy, the outer retinal boundary may be poorly defined and obscured by choroidal neovascularization or a disciform scar. A review of the individual OCT scan, however, shows that the crosssectional anatomy is displayed accurately without artifact. Third, obtaining scans in an automated sequence, such as in the fast macular map protocol, increases the duration of scanning and the possibility of eye movement or a blink creating an artifact. These artifacts virtually can be elimi-
ELSEVIER INC. ALL
RIGHTS RESERVED.
0002-9394/05/$30.00 doi:10.1016/j.ajopht.2004.08.066
nated if the six cross-sectional images used to create a topographic map are obtained individually by the examiner, such as in the standard macular map protocol. Then, individual scans that have a low signal-to-noise, or those affected by patient eye movement, can easily be reacquired. Poor scan acquisition, responsible for an artifact in approximately 11% of the topographic maps, depends on the skill of the OCT examiner. The examiners of Ray and associates were clearly experienced in terms of the number of scans they had performed; however, the presence of “out of register” artifacts suggests that they were not aware that OCT scans displaced anteriorly in the image frame can lead to failure of the boundary detection algorithms, even when the scan is otherwise normal. Similarly, “cut edge” artifacts often occur upon initiation of scanning and may be reduced by disregarding the first scan. A “degraded image” appeared in an additional 11% of the topographic maps. The decreased signal-to-noise in these images was also possibly secondary to poor scan acquisition (e.g., maladjustment of the OCT focus). Interestingly, because the near infrared OCT probe beam has better penetration through media opacities compared with visible light, the presence of a cataract was not found to increase the frequency of scan artifacts. Failure to center the topographic map directly on the fovea depends on both the examiner and the patient. When patients have relatively good central acuity, a fixation light may be provided in the eye being scanned and the examiner can easily adjust the position of the scan so that the scanning beam intersects the fixation light. However, in patients with poor vision in one eye, fixation is provided by the contralateral eye. In these cases, or when eccentric fixation is present, identification of the center of the fovea from the video funduscopic image can be difficult, especially when foveal pathology is present. What can be done to address the issue of artifacts in the OCT macular thickness maps? First, examiners need simply to be aware of the presence of artifacts. OCT images that are clearly “out of register,” have decreased signal-to-noise, or manifest signal “dropouts” because of patient eye motion or blinking should not be saved. The situation is similar to ultrasound B-scan, where a continuously acquired image is not “frozen” and saved until the probe has been adequately adjusted and the desired image is obtained. Second, interpretation of OCT topographic maps should always include examination of the cross-sectional OCT scans. The individual scans provide useful qualitative information (for example, is the edema diffuse or cystic?) not apparent with just a measurement of retinal thickness. A misidentification of the retinal surfaces is readily apparent in the cross-sectional image, because the boundaries determined by the computer algorithm are automatically overlayed on each tomogram. Manually placed computer calipers to measure retinal thickness may always be used, analogous to ultrasound B-scan, in any case where the computer algorithm fails. VOL. 139, NO. 1
Third, new technology is on the horizon. Improvements in the image segmentation algorithms are being developed. Stratus OCT software version 4.0, recently released, automatically identifies scans in the fast macular thickness map that are likely to have artifacts because of poor scan acquisition or problematic pathology and provides a framework for the examiner to easily reacquire those images. Spectral domain OCT is a new technology that effectively acquires an entire A-scan simultaneously and promises image acquisition at 50 to 100 times the current speed with equal or improved spatial resolution.10,11 When commercially available, spectral OCT will potentially allow complete three-dimensional scanning of the macula and dramatically reduce the effect of patient eye motion and blinking in current macular maps. As with any imaging modality, however, some artifacts will always remain. Ultimately, as discussed herein, it is increased recognition of these artifacts by both the OCT technician and the physician that will lead to improved image acquisition and interpretation, thereby enhancing patient care.
REFERENCES 1. Hee MR, Puliafito CA, Duker JS, et al. Topography of diabetic macular edema with optical coherence tomography. Ophthalmology 1998;105:360 –370. 2. Ray R, Stinnett SS, Jaffe GJ. Evaluation of image artifact produced by optical coherence tomography of retinal pathology. Am J Ophthalmol 2005;1: 3. Grigera DE, Zambrano A, Cazon GP, Cavanagh E, Girado SG. Ultrasound biomicroscopy in silicone oil-filled eyes. Retina 2000;20:524 –531. 4. Ishikawa H, Inazumi K, Liebmann JM, Ritch R. Inadvertent corneal indentation can cause artifactitious widening of the iridocorneal angle on ultrasound biomicroscopy. Ophthalmic Surg Lasers 2000;31:342–345. 5. Colen TP, Lemij HG. Motion artifacts in scanning laser polarimetry. Ophthalmology 2002;109:1568 –1572. 6. Pons ME, Rothman RF, Ozden RG, Liebmann JM, Ritch R. Vitreous opacities affect scanning laser polarimetry measurements. Am J Ophthalmol 2001;131:511–513. 7. Kogure S, Chiba T, Kinoshita T, Kowa H, Tsukahara S. Effects of artefacts on scanning laser polarimetry of retinal nerve fibre layer thickness measurement. Br J Ophthalmol 2000;84:1013–1017. 8. Hoh ST, Greenfield DS, Liebmann JM, et al. Factors affecting image acquisition during scanning laser polarimetry. Ophthalmic Surg Lasers 1998;29:545–551. 9. Cohen KL, Patel SB, Ray N. Retinal thickness measurement after phacoemulsification. J Cataract Refract Surg 2004;30: 1501–1506. 10. Nassif N, Cense B, Park BH, et al. In vivo human retinal imaging by ultrahigh-speed spectral domain optical coherence tomography. Opt Lett 2004;29:480 – 482. 11. Wojtkowski M, Srinivasan VJ, Ko TH, Fujimoto JG, Kowalczyk A, Duker JS. Ultrahigh-resolution, high-speed, Fourier domain optical coherence tomography and methods for dispersion compensation. Opt Express 2004;12:2404 –2422.
EDITORIAL
155
O
VER THE LAST FEW YEARS, OPTICAL COHERENCE
tomography (OCT) has become an indispensable technique for high-resolution cross-sectional imaging of the macula and optic nerve. One of the most useful applications of OCT is its ability to sensitively and reproducibly monitor retinal thickness. Although originally developed as a technique to evaluate diabetic macular edema,1 in many institutions the OCT topographic map is becoming a de facto standard for evaluating other macular conditions and therapies for age-related macular degeneration. In this issue, Ray and associates examine artifacts which may appear in OCT topographic maps.2 All imaging systems are subject to artifacts. Fundus photography and fluorescein angiography, for example, depend on the training of the photographer, the quality of the media, pupil dilation, and patient eye motion. Ultrasound A and B scans can be affected by improper alignment or excessive pressure of the probe on the cornea and intraocular air or silicone oil.3,4 Eye motion, vitreous opacities, and other artifacts are known to affect scanning laser polarimetry.5-8 Measurements of macular thickness with the retinal thickness analyzer may be adversely affected after cataract surgery.9 Ray and associates characterize artifacts arising from one particular OCT scanning protocol known as the fast macular thickness map. In this protocol, six scans are rapidly obtained in a radial spoke pattern. A computer algorithm identifies the inner and outer retinal boundaries, and a topographic map of macular thickness is created. The authors analyzed artifacts in fast macular thickness maps obtained over a 3-week period in 171 eyes. Broadly, the artifacts could be divided into two categories—those that related to a misidentification of the retinal surfaces by the computer software versus those that reflected a deficient scan acquisition. Clinical experience tells us that OCT provides useful information that helps guide therapy. Indeed, the authors themselves note that OCT has supplanted both angiography and fundus photography as the most commonly ordered retinal imaging test at their institution. Therefore, it is somewhat surprising that over 40% of all fast macular thickness maps contained a recognizable artifact. Clearly, Accepted for publication Aug 25, 2004. From Pacific Eye Specialists, Dale City, California. Inquiries to Michael R. Hee, MD, Pacific Eye Specialists, 1850 Sullivan Avenue, Suite 540, Daly City, CA 94015; fax: 650-755-2107; e-mail: [email protected]
154
©
2005 BY
this is an issue that should be recognized and addressed by care in acquisition and review of images. The authors’ analysis of artifacts focuses on the fast macular thickness map scanning protocol not on individual cross-sectional scans. This difference is important for three reasons. First, only one of the six cross-sectional scans comprising the topographic map needs to have a boundary detection abnormality to create an artifact in the map. Therefore, the frequency of artifacts occurring in the topographic map is naturally much higher than the frequency of artifacts occurring in an individual scan. Second, the retinal thickness maps rely on a computer algorithm to segment the inner and outer boundaries of the retina. These algorithms may fail to identify the boundaries if the retinal architecture is distorted due to severe pathology, even though the individual OCT scans comprising the topographic map correctly reveal the true retinal structure. The enhanced resolution offered by OCT compared with other imaging modalities provides images displaying more complex internal features which, ironically, require more sophisticated boundary detection algorithms. As a reference, many other imaging modalities do not even attempt quantitative boundary detection. Tumor thickness on ultrasound B-scan, for instance, is usually measured with manually placed computer calipers. In Heidelberg retinal tomography assessment of the optic nerve, the boundaries of the nerve must be outlined by the examiner. The original macular thickness algorithms2 (upon which the current algorithms are based) were designed to determine the inner and outer retinal boundaries in diabetic macular edema, a condition leading to retinal thickening in which intraretinal fluid accumulation and hard exudates occur but the retinal pigment epithelium and inner limiting membrane remain intact. Ray and associates demonstrate that software-related artifacts may occur when these boundaries are not well defined. With a macular hole, in particular, the absence of foveal retinal tissue may not be correctly interpreted by the algorithms. With exudative AMD, especially following photodynamic therapy, the outer retinal boundary may be poorly defined and obscured by choroidal neovascularization or a disciform scar. A review of the individual OCT scan, however, shows that the crosssectional anatomy is displayed accurately without artifact. Third, obtaining scans in an automated sequence, such as in the fast macular map protocol, increases the duration of scanning and the possibility of eye movement or a blink creating an artifact. These artifacts virtually can be elimi-
ELSEVIER INC. ALL
RIGHTS RESERVED.
0002-9394/05/$30.00 doi:10.1016/j.ajopht.2004.08.066
nated if the six cross-sectional images used to create a topographic map are obtained individually by the examiner, such as in the standard macular map protocol. Then, individual scans that have a low signal-to-noise, or those affected by patient eye movement, can easily be reacquired. Poor scan acquisition, responsible for an artifact in approximately 11% of the topographic maps, depends on the skill of the OCT examiner. The examiners of Ray and associates were clearly experienced in terms of the number of scans they had performed; however, the presence of “out of register” artifacts suggests that they were not aware that OCT scans displaced anteriorly in the image frame can lead to failure of the boundary detection algorithms, even when the scan is otherwise normal. Similarly, “cut edge” artifacts often occur upon initiation of scanning and may be reduced by disregarding the first scan. A “degraded image” appeared in an additional 11% of the topographic maps. The decreased signal-to-noise in these images was also possibly secondary to poor scan acquisition (e.g., maladjustment of the OCT focus). Interestingly, because the near infrared OCT probe beam has better penetration through media opacities compared with visible light, the presence of a cataract was not found to increase the frequency of scan artifacts. Failure to center the topographic map directly on the fovea depends on both the examiner and the patient. When patients have relatively good central acuity, a fixation light may be provided in the eye being scanned and the examiner can easily adjust the position of the scan so that the scanning beam intersects the fixation light. However, in patients with poor vision in one eye, fixation is provided by the contralateral eye. In these cases, or when eccentric fixation is present, identification of the center of the fovea from the video funduscopic image can be difficult, especially when foveal pathology is present. What can be done to address the issue of artifacts in the OCT macular thickness maps? First, examiners need simply to be aware of the presence of artifacts. OCT images that are clearly “out of register,” have decreased signal-to-noise, or manifest signal “dropouts” because of patient eye motion or blinking should not be saved. The situation is similar to ultrasound B-scan, where a continuously acquired image is not “frozen” and saved until the probe has been adequately adjusted and the desired image is obtained. Second, interpretation of OCT topographic maps should always include examination of the cross-sectional OCT scans. The individual scans provide useful qualitative information (for example, is the edema diffuse or cystic?) not apparent with just a measurement of retinal thickness. A misidentification of the retinal surfaces is readily apparent in the cross-sectional image, because the boundaries determined by the computer algorithm are automatically overlayed on each tomogram. Manually placed computer calipers to measure retinal thickness may always be used, analogous to ultrasound B-scan, in any case where the computer algorithm fails. VOL. 139, NO. 1
Third, new technology is on the horizon. Improvements in the image segmentation algorithms are being developed. Stratus OCT software version 4.0, recently released, automatically identifies scans in the fast macular thickness map that are likely to have artifacts because of poor scan acquisition or problematic pathology and provides a framework for the examiner to easily reacquire those images. Spectral domain OCT is a new technology that effectively acquires an entire A-scan simultaneously and promises image acquisition at 50 to 100 times the current speed with equal or improved spatial resolution.10,11 When commercially available, spectral OCT will potentially allow complete three-dimensional scanning of the macula and dramatically reduce the effect of patient eye motion and blinking in current macular maps. As with any imaging modality, however, some artifacts will always remain. Ultimately, as discussed herein, it is increased recognition of these artifacts by both the OCT technician and the physician that will lead to improved image acquisition and interpretation, thereby enhancing patient care.
REFERENCES 1. Hee MR, Puliafito CA, Duker JS, et al. Topography of diabetic macular edema with optical coherence tomography. Ophthalmology 1998;105:360 –370. 2. Ray R, Stinnett SS, Jaffe GJ. Evaluation of image artifact produced by optical coherence tomography of retinal pathology. Am J Ophthalmol 2005;1: 3. Grigera DE, Zambrano A, Cazon GP, Cavanagh E, Girado SG. Ultrasound biomicroscopy in silicone oil-filled eyes. Retina 2000;20:524 –531. 4. Ishikawa H, Inazumi K, Liebmann JM, Ritch R. Inadvertent corneal indentation can cause artifactitious widening of the iridocorneal angle on ultrasound biomicroscopy. Ophthalmic Surg Lasers 2000;31:342–345. 5. Colen TP, Lemij HG. Motion artifacts in scanning laser polarimetry. Ophthalmology 2002;109:1568 –1572. 6. Pons ME, Rothman RF, Ozden RG, Liebmann JM, Ritch R. Vitreous opacities affect scanning laser polarimetry measurements. Am J Ophthalmol 2001;131:511–513. 7. Kogure S, Chiba T, Kinoshita T, Kowa H, Tsukahara S. Effects of artefacts on scanning laser polarimetry of retinal nerve fibre layer thickness measurement. Br J Ophthalmol 2000;84:1013–1017. 8. Hoh ST, Greenfield DS, Liebmann JM, et al. Factors affecting image acquisition during scanning laser polarimetry. Ophthalmic Surg Lasers 1998;29:545–551. 9. Cohen KL, Patel SB, Ray N. Retinal thickness measurement after phacoemulsification. J Cataract Refract Surg 2004;30: 1501–1506. 10. Nassif N, Cense B, Park BH, et al. In vivo human retinal imaging by ultrahigh-speed spectral domain optical coherence tomography. Opt Lett 2004;29:480 – 482. 11. Wojtkowski M, Srinivasan VJ, Ko TH, Fujimoto JG, Kowalczyk A, Duker JS. Ultrahigh-resolution, high-speed, Fourier domain optical coherence tomography and methods for dispersion compensation. Opt Express 2004;12:2404 –2422.
EDITORIAL
155