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SWEPT SOURCE OPTICAL COHERENCE TOMOGRAPH

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SWEPT SOURCE OPTICAL COHERENCE

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SWEPT SOURCE OPTICAL COHERENCE TOMOGRAPH (SS OCT) BY EKENTA UCHECHI ASSUMPTA (15/96751) UCHE UCHECHI KALUNTA (15/96752) A SEMINAR PRESENTED TO THE DEPARTMENT OF OPTOMETRY, FACULTY OF HEALTH SCIENCES ABIA STATE UNIVERSITY, UTURU. DEC, 2021. LECTURERS: PROF. O.U. AMAECHI DR. ANDREW OMAKA
SWEPT SOURCE OCT (SS OCT) INTRODUCTION Swept source optical coherence tomography (SS-OCT) was introduced in clinical practice in 2012. Because of its deeper penetration and faster acquisition time, SS-OCT has the ability to visualize choroid, vitreous, and retinal structures behind dense preretinal hemorrhages. Swept source optical coherence tomography has positively influenced and hugely contributed to the research of the vitreous body. It is the first ophthalmic diagnostic technology to demonstrate the entire structure of the posterior precortical vitreous pocket (PPVP) in vivo.
BACKGROUND INFORMATION. SS-OCT and spectral-domain optical coherence tomography (SD-OCT) are categorized as Fourier domain optical coherence tomography. The light source in SD-OCT is a super-luminescent diode, whereas the light source in SS- OCT is a tunable laser. In SS-OCT, the light source is already divided into a spectrum through a tunable laser, thus a spectroscope is unnecessary in SS-OCT.
Schematic drawing of the swept source optical coherence tomography device, the light source is a tunable laser.
This simplified mechanism contributes to high-speed data acquisition that is twice as fast as that achieved by SD-OCT and it results in a clearer image. The depth of tissue penetration is governed by the wavelength of the light source used. The median wavelength of the light source in SD-OCT is approximately 840 nm. Swept source optical coherence tomography has enabled the visualization of the whole thickness of the choroid and structures beneath a retinal hemorrhage or the retinal pigment epithelium (RPE).
DEFINITION OF TERMS AND CONCEPTS Optical Coherence Tomography (OCT) Optical coherence tomography (OCT) is an imaging technique that uses low-coherence light to capture micrometer-resolution, two- and three-dimensional images from within optical scattering media (e.g., biological tissue). It is a non-invasive diagnostic technique that renders an in vivo cross sectional view of the retina. OCT utilizes a concept known as inferometry to create a cross-sectional map of the retina that is accurate to within at least 10-15 microns, typically employing near-infrared light.
Swept Source OCT. When we refer to swept-source optical coherence tomography (SS- OCT), “swept source” refers to the type of laser incorporated into the device. Instead of the super luminescent diode laser typical of conventional spectral domain OCT (SD-OCT), SS-OCT uses a short- cavity swept laser. Spectral Domain OCT (SD OCT) SD OCT is a better OCT imaging strategy; it is an interferometric technique that provides depth-resolved tissue structure information encoded in the magnitude and delay of the back-scattered light by spectral analysis of the interference fringe pattern.
Time Domain OCT This is an OCT imaging strategy where the depth information of the retina is collected as a function of time by moving the reference mirror. Wavelength Wavelength, distance between corresponding points of two consecutive waves. “Corresponding points” refers to two points or particles in the same phase i.e., points that have completed identical fractions of their periodic motion.
HISTORY OF SWEPT SOURCE OCT During the past decade we have witnessed tremendous development in visualization techniques for the posterior segment of the eye. SS-OCT first became commercially available in 2013, Time-domain optical coherence tomography (TD-OCT) impressed retina specialists but was soon left in the shade by spectral-domain OCT (SD-OCT), which offered higher resolution (1 μm to 3 μm axial resolution with SD-OCT vs 10 μm with TD-OCT) and 3-D imaging possibilities. Retinal imaging in vivo could be correlated with histopathologic findings. It was not long, however, before retina specialists wanted to see deeper into the eye to visualize the choroid, thus the invention of swept source OCT by Spaide.
PRINCIPLES OF SWEPT SOURCE OCT
Basic principle of SS-OCT. SS-OCT uses an interferometer with a narrow band, frequency swept laser and detectors.
Light emitted from the light source is separated using a coupler, with one light beam directed toward the sample and the other directed toward the reference mirror. Light directed toward the sample is backscattered toward a detector in proportion with the difference in the refractive indices of the internal structures. Light directed toward the reference mirror merges and interferes with light returning from the sample. Fringe response versus frequency is detected with a balanced detector. The signal is Fourier transformed, and depth-reflectivity profile is obtained. Cross-sectional image is then reconstructed.
TIME DOMAIN VS. SPECTRAL DOMAIN VS. SWEPT SOURCE OCT From its inception, OCT images were acquired in a time domain fashion. Time domain systems acquire approximately 400 A-scans per second using 6 radial slices oriented 30 degrees apart. Because the slices are 30 degrees apart, care must be taken to avoid missing pathology between the slices. Spectral domain technology, on the other hand, scans approximately 20,000-40,000 A-scans per second. This increased scan rate and number diminishes the likelihood of motion artifact, enhances the resolution and decreases the chance of missing lesions.
Swept source technology uses a wavelength-sweeping laser and dual balanced photo detector, allowing for faster acquisition speeds of 100,000-400,000 A-scans per second. This technology uses longer wavelengths of 1050-1060 nm for deeper tissue penetration without the need for EDI. This wavelength provides an axial resolution of about 5.3 um in tissue compared to the approximately 5 um axial resolution of the standard 800 nm wavelength of commercial spectral domain devices. The enhanced axial resolution along with the faster scanning speeds, which allows for greater image averaging, improves image quality and ability to visualize deeper structures in more detail
USES OF OCT Retina OCT is useful in the diagnosis of many retinal conditions, especially when the media is clear. In general, lesions in the macula are easier to image than lesions in the mid and far periphery. OCT can be particularly helpful in diagnosing: Macular hole, Macular pucker/epiretinal membrane, Vitreomacular traction, Macular edema and exudates, Detachments of the neurosensory retina, Detachments of the retinal pigment epithelium (e.g. central serous retinopathy or age-related macular degeneration), Retinoschisis, Pachychoroid, Choroidal tumors.
Optic nerve OCT is gaining increasing popularity when evaluating optic nerve disorders by accurately and reproducibly evaluate the retinal nerve fiber layer and ganglion cell layer thickness: Glaucoma, Optic neuritis, Non-glaucomatous optic neuropathies, Alzheimer's disease. Anterior segment Anterior segment OCT utilizes higher wavelength light than traditional posterior segment OCT. This higher wavelength light results in greater absorption and less penetration. In this fashion, images of the anterior segment (cornea, anterior chamber, iris and angle) can be visualized.
APPLICATION OF SWEPT SOURCE OCT Applications in glaucoma There are several possible applications of SS-OCT in glaucoma, including for disease detection, identification of novel risk factors and improving the understanding of disease mechanisms. In diagnosis of glaucoma Clinical assessment using multiple parameters, including peripapillary RNFL, ONH, and macular parameters, has proven useful, not only for management and diagnosing glaucoma at various levels of severity, but for evaluating risk in glaucoma suspects.
SWEPT SOURCE OPTICAL COHERENCE TOMOGRAPH
Precortical vitreous pocket observed by SS-OCT Swept source optical coherence tomography has the ability to make the PPVP visible in vivo. Precortical vitreous pocket in children 3years, 6years, 8years
Polypoid choroidal vasculopathy Exudative age-related macular degeneration is a complex lesion consisting of RPE detachment, sub-RPE choroidal neovascularization, polypoid lesions, and exudate or hemorrhages.
Central serous chorioretinopathy (A) Serous retinal detachment with subretinal precipitates. (B) The precipitates emit autofluorescence. (C) Swept source optical coherence tomography shows marked swelling of the outer choroidal vessels (yellow arrowheads). In serous retinal detachment, photoreceptor outer segments are elongated (white arrow). Subretinal fibrin is visible (green arrow). FAF = fundus autofluorescence.
A) The color fundus photograph shows serous retinal detachment in lobular pattern. Swept source optical coherence tomography reveals subretinal fluid (a), intraretinal fluid (b) and layer of the outer segment (c). The choroid is extensively swollen and the chorioscleral border is not visible. CT = central choroidal thickness. Harada disease Harada disease is also posterior uveitis with serous retinal detachment
Retinal arteriolar macroaneurysm (A) Ruptured retinal macroaneurysm in a 79-year-old female. (B) In spectral-domain optical coherence tomography, the inner structure of the hematoma is not visible. (C) In swept source optical coherence tomography, the internal structure of the hematoma is visible.
Macular assessment: Idiopathic macular hole Stage 1 macular hole in a 56-year-old male. (A) At the initial visit, swept source optical coherence tomography shows perifoveal posterior vitreous detachment (PVD) and a foveal cyst. (B) One month later, the perifoveal PVD persists but the foveal cyst has collapsed. (C) Four months later, the vitreous cortex has detached from the macula and the foveal cyst is resolved. P = posterior precortical vitreous pocket. Values 0.7, 1.0 and 1.2 denotes decimal visual acuity.
Diabetic macular edema Diabetic retinopathy in a 45-year-old female. Swept source optical coherence tomography shows perifoveal posterior vitreous detachment with large CME and subhyaloid hemorrhage (yellow arrow). CME = cystoid macular edema; PVD = posterior vitreous detachment. Values 0.5 and 0.3 denotes decimal visual acuity.
DIFFERENCES BETWEEN THE IMAGES ACQUIRED BY SD-OCT AND BY SS-OCT A comparison of the B scan images between spectral-domain optical coherence tomography (SD-OCT) and swept source optical coherence tomography (SS-OCT) in a patient with uveitis.
ADVANTAGES OF SS OCT Compared with SD-OCT, SS-OCT generally has faster image acquisition, which leads to less tradeoff between image size and resolution, and also allows wider scanning ranges longer ranges can be gotten with specific lenses and mosaic programs). In addition, there is no sensitivity roll-off with SS-OCT. Therefore, vitreous and choroid can be imaged well simultaneously. SS-OCT also provides deeper penetration. “SS-OCT can penetrate better through lens opacity, choroid, pigment, blood, and intraocular gas than SD-OCT, and based on two published papers, it appears that SS-OCT will allow better imaging of choroidal neovascularization,”
DISADVANTAGE Higher cost of SS-OCT comes from the light source that is used. “SS OCT requires narrow line width, high speed, frequency swept lasers that are very pricey,” Other disadvantages of SS OCT compared with SD OCT include lower axial resolution, worse signal-to-noise ratio, worse motion artifact, and absence of normative databases, which is important for use in glaucoma imaging. “Worse axial resolution with SS-OCT can be overcome with some image averaging, but in general, the SS-OCT images may not be quite as sharp as what we are used to seeing with SD-OCT. “commercial pressures” may also be contributing to the limited availability of SS-OCT in clinical practice.
LIMITATIONS Because OCT utilizes light waves (unlike ultrasound which uses sound waves) media opacities can interfere with optimal imaging. As a result, the OCT will be limited the setting of vitreous hemorrhage, dense cataract or corneal opacities. As with most diagnostic tests, patient cooperation is a necessity. Patient movement can diminish the quality of the image. The quality of the image is also dependent on the operator of the machine. Early models of OCT relied on the operator to accurately place the image over the desired pathology. Newer technologies, such as eye tracking equipment, limit the likelihood of acquisition error.
CONCLUSION The advent of optical coherence tomography (OCT) revolutionized both clinical assessment and research of vitreoretinal conditions. Since then, extraordinary advances have been made in this imaging technology, including the relatively recent development of swept- source OCT (SS-OCT). SS-OCT enables a fast scan rate and utilizes a tunable swept laser at longer wavelengths than conventional spectral-domain devices. These features enable imaging of larger areas with reduced motion artifact, and a better visualization of the choroidal vasculature, respectively. Building on the principles of OCT, swept-source OCT has also been applied to OCT angiography (SS-OCTA), thus enabling a non-invasive in depth-resolved imaging of the retinal and choroidal microvasculature. Despite their advantages, the widespread use of SS-OCT and SS-OCTA remains relatively limited.

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SWEPT SOURCE OPTICAL COHERENCE TOMOGRAPH

  • 1. SWEPT SOURCE OPTICAL COHERENCE TOMOGRAPH (SS OCT) BY EKENTA UCHECHI ASSUMPTA (15/96751) UCHE UCHECHI KALUNTA (15/96752) A SEMINAR PRESENTED TO THE DEPARTMENT OF OPTOMETRY, FACULTY OF HEALTH SCIENCES ABIA STATE UNIVERSITY, UTURU. DEC, 2021. LECTURERS: PROF. O.U. AMAECHI DR. ANDREW OMAKA
  • 2. SWEPT SOURCE OCT (SS OCT) INTRODUCTION Swept source optical coherence tomography (SS-OCT) was introduced in clinical practice in 2012. Because of its deeper penetration and faster acquisition time, SS-OCT has the ability to visualize choroid, vitreous, and retinal structures behind dense preretinal hemorrhages. Swept source optical coherence tomography has positively influenced and hugely contributed to the research of the vitreous body. It is the first ophthalmic diagnostic technology to demonstrate the entire structure of the posterior precortical vitreous pocket (PPVP) in vivo.
  • 3. BACKGROUND INFORMATION. SS-OCT and spectral-domain optical coherence tomography (SD-OCT) are categorized as Fourier domain optical coherence tomography. The light source in SD-OCT is a super-luminescent diode, whereas the light source in SS- OCT is a tunable laser. In SS-OCT, the light source is already divided into a spectrum through a tunable laser, thus a spectroscope is unnecessary in SS-OCT.
  • 4. Schematic drawing of the swept source optical coherence tomography device, the light source is a tunable laser.
  • 5. This simplified mechanism contributes to high-speed data acquisition that is twice as fast as that achieved by SD-OCT and it results in a clearer image. The depth of tissue penetration is governed by the wavelength of the light source used. The median wavelength of the light source in SD-OCT is approximately 840 nm. Swept source optical coherence tomography has enabled the visualization of the whole thickness of the choroid and structures beneath a retinal hemorrhage or the retinal pigment epithelium (RPE).
  • 6. DEFINITION OF TERMS AND CONCEPTS Optical Coherence Tomography (OCT) Optical coherence tomography (OCT) is an imaging technique that uses low-coherence light to capture micrometer-resolution, two- and three-dimensional images from within optical scattering media (e.g., biological tissue). It is a non-invasive diagnostic technique that renders an in vivo cross sectional view of the retina. OCT utilizes a concept known as inferometry to create a cross-sectional map of the retina that is accurate to within at least 10-15 microns, typically employing near-infrared light.
  • 7. Swept Source OCT. When we refer to swept-source optical coherence tomography (SS- OCT), “swept source” refers to the type of laser incorporated into the device. Instead of the super luminescent diode laser typical of conventional spectral domain OCT (SD-OCT), SS-OCT uses a short- cavity swept laser. Spectral Domain OCT (SD OCT) SD OCT is a better OCT imaging strategy; it is an interferometric technique that provides depth-resolved tissue structure information encoded in the magnitude and delay of the back-scattered light by spectral analysis of the interference fringe pattern.
  • 8. Time Domain OCT This is an OCT imaging strategy where the depth information of the retina is collected as a function of time by moving the reference mirror. Wavelength Wavelength, distance between corresponding points of two consecutive waves. “Corresponding points” refers to two points or particles in the same phase i.e., points that have completed identical fractions of their periodic motion.
  • 9. HISTORY OF SWEPT SOURCE OCT During the past decade we have witnessed tremendous development in visualization techniques for the posterior segment of the eye. SS-OCT first became commercially available in 2013, Time-domain optical coherence tomography (TD-OCT) impressed retina specialists but was soon left in the shade by spectral-domain OCT (SD-OCT), which offered higher resolution (1 μm to 3 μm axial resolution with SD-OCT vs 10 μm with TD-OCT) and 3-D imaging possibilities. Retinal imaging in vivo could be correlated with histopathologic findings. It was not long, however, before retina specialists wanted to see deeper into the eye to visualize the choroid, thus the invention of swept source OCT by Spaide.
  • 10. PRINCIPLES OF SWEPT SOURCE OCT
  • 11. Basic principle of SS-OCT. SS-OCT uses an interferometer with a narrow band, frequency swept laser and detectors.
  • 12. Light emitted from the light source is separated using a coupler, with one light beam directed toward the sample and the other directed toward the reference mirror. Light directed toward the sample is backscattered toward a detector in proportion with the difference in the refractive indices of the internal structures. Light directed toward the reference mirror merges and interferes with light returning from the sample. Fringe response versus frequency is detected with a balanced detector. The signal is Fourier transformed, and depth-reflectivity profile is obtained. Cross-sectional image is then reconstructed.
  • 13. TIME DOMAIN VS. SPECTRAL DOMAIN VS. SWEPT SOURCE OCT From its inception, OCT images were acquired in a time domain fashion. Time domain systems acquire approximately 400 A-scans per second using 6 radial slices oriented 30 degrees apart. Because the slices are 30 degrees apart, care must be taken to avoid missing pathology between the slices. Spectral domain technology, on the other hand, scans approximately 20,000-40,000 A-scans per second. This increased scan rate and number diminishes the likelihood of motion artifact, enhances the resolution and decreases the chance of missing lesions.
  • 14. Swept source technology uses a wavelength-sweeping laser and dual balanced photo detector, allowing for faster acquisition speeds of 100,000-400,000 A-scans per second. This technology uses longer wavelengths of 1050-1060 nm for deeper tissue penetration without the need for EDI. This wavelength provides an axial resolution of about 5.3 um in tissue compared to the approximately 5 um axial resolution of the standard 800 nm wavelength of commercial spectral domain devices. The enhanced axial resolution along with the faster scanning speeds, which allows for greater image averaging, improves image quality and ability to visualize deeper structures in more detail
  • 15. USES OF OCT Retina OCT is useful in the diagnosis of many retinal conditions, especially when the media is clear. In general, lesions in the macula are easier to image than lesions in the mid and far periphery. OCT can be particularly helpful in diagnosing: Macular hole, Macular pucker/epiretinal membrane, Vitreomacular traction, Macular edema and exudates, Detachments of the neurosensory retina, Detachments of the retinal pigment epithelium (e.g. central serous retinopathy or age-related macular degeneration), Retinoschisis, Pachychoroid, Choroidal tumors.
  • 16. Optic nerve OCT is gaining increasing popularity when evaluating optic nerve disorders by accurately and reproducibly evaluate the retinal nerve fiber layer and ganglion cell layer thickness: Glaucoma, Optic neuritis, Non-glaucomatous optic neuropathies, Alzheimer's disease. Anterior segment Anterior segment OCT utilizes higher wavelength light than traditional posterior segment OCT. This higher wavelength light results in greater absorption and less penetration. In this fashion, images of the anterior segment (cornea, anterior chamber, iris and angle) can be visualized.
  • 17. APPLICATION OF SWEPT SOURCE OCT Applications in glaucoma There are several possible applications of SS-OCT in glaucoma, including for disease detection, identification of novel risk factors and improving the understanding of disease mechanisms. In diagnosis of glaucoma Clinical assessment using multiple parameters, including peripapillary RNFL, ONH, and macular parameters, has proven useful, not only for management and diagnosing glaucoma at various levels of severity, but for evaluating risk in glaucoma suspects.
  • 19. Precortical vitreous pocket observed by SS-OCT Swept source optical coherence tomography has the ability to make the PPVP visible in vivo. Precortical vitreous pocket in children 3years, 6years, 8years
  • 20. Polypoid choroidal vasculopathy Exudative age-related macular degeneration is a complex lesion consisting of RPE detachment, sub-RPE choroidal neovascularization, polypoid lesions, and exudate or hemorrhages.
  • 21. Central serous chorioretinopathy (A) Serous retinal detachment with subretinal precipitates. (B) The precipitates emit autofluorescence. (C) Swept source optical coherence tomography shows marked swelling of the outer choroidal vessels (yellow arrowheads). In serous retinal detachment, photoreceptor outer segments are elongated (white arrow). Subretinal fibrin is visible (green arrow). FAF = fundus autofluorescence.
  • 22. A) The color fundus photograph shows serous retinal detachment in lobular pattern. Swept source optical coherence tomography reveals subretinal fluid (a), intraretinal fluid (b) and layer of the outer segment (c). The choroid is extensively swollen and the chorioscleral border is not visible. CT = central choroidal thickness. Harada disease Harada disease is also posterior uveitis with serous retinal detachment
  • 23. Retinal arteriolar macroaneurysm (A) Ruptured retinal macroaneurysm in a 79-year-old female. (B) In spectral-domain optical coherence tomography, the inner structure of the hematoma is not visible. (C) In swept source optical coherence tomography, the internal structure of the hematoma is visible.
  • 24. Macular assessment: Idiopathic macular hole Stage 1 macular hole in a 56-year-old male. (A) At the initial visit, swept source optical coherence tomography shows perifoveal posterior vitreous detachment (PVD) and a foveal cyst. (B) One month later, the perifoveal PVD persists but the foveal cyst has collapsed. (C) Four months later, the vitreous cortex has detached from the macula and the foveal cyst is resolved. P = posterior precortical vitreous pocket. Values 0.7, 1.0 and 1.2 denotes decimal visual acuity.
  • 25. Diabetic macular edema Diabetic retinopathy in a 45-year-old female. Swept source optical coherence tomography shows perifoveal posterior vitreous detachment with large CME and subhyaloid hemorrhage (yellow arrow). CME = cystoid macular edema; PVD = posterior vitreous detachment. Values 0.5 and 0.3 denotes decimal visual acuity.
  • 26. DIFFERENCES BETWEEN THE IMAGES ACQUIRED BY SD-OCT AND BY SS-OCT A comparison of the B scan images between spectral-domain optical coherence tomography (SD-OCT) and swept source optical coherence tomography (SS-OCT) in a patient with uveitis.
  • 27. ADVANTAGES OF SS OCT Compared with SD-OCT, SS-OCT generally has faster image acquisition, which leads to less tradeoff between image size and resolution, and also allows wider scanning ranges longer ranges can be gotten with specific lenses and mosaic programs). In addition, there is no sensitivity roll-off with SS-OCT. Therefore, vitreous and choroid can be imaged well simultaneously. SS-OCT also provides deeper penetration. “SS-OCT can penetrate better through lens opacity, choroid, pigment, blood, and intraocular gas than SD-OCT, and based on two published papers, it appears that SS-OCT will allow better imaging of choroidal neovascularization,”
  • 28. DISADVANTAGE Higher cost of SS-OCT comes from the light source that is used. “SS OCT requires narrow line width, high speed, frequency swept lasers that are very pricey,” Other disadvantages of SS OCT compared with SD OCT include lower axial resolution, worse signal-to-noise ratio, worse motion artifact, and absence of normative databases, which is important for use in glaucoma imaging. “Worse axial resolution with SS-OCT can be overcome with some image averaging, but in general, the SS-OCT images may not be quite as sharp as what we are used to seeing with SD-OCT. “commercial pressures” may also be contributing to the limited availability of SS-OCT in clinical practice.
  • 29. LIMITATIONS Because OCT utilizes light waves (unlike ultrasound which uses sound waves) media opacities can interfere with optimal imaging. As a result, the OCT will be limited the setting of vitreous hemorrhage, dense cataract or corneal opacities. As with most diagnostic tests, patient cooperation is a necessity. Patient movement can diminish the quality of the image. The quality of the image is also dependent on the operator of the machine. Early models of OCT relied on the operator to accurately place the image over the desired pathology. Newer technologies, such as eye tracking equipment, limit the likelihood of acquisition error.
  • 30. CONCLUSION The advent of optical coherence tomography (OCT) revolutionized both clinical assessment and research of vitreoretinal conditions. Since then, extraordinary advances have been made in this imaging technology, including the relatively recent development of swept- source OCT (SS-OCT). SS-OCT enables a fast scan rate and utilizes a tunable swept laser at longer wavelengths than conventional spectral-domain devices. These features enable imaging of larger areas with reduced motion artifact, and a better visualization of the choroidal vasculature, respectively. Building on the principles of OCT, swept-source OCT has also been applied to OCT angiography (SS-OCTA), thus enabling a non-invasive in depth-resolved imaging of the retinal and choroidal microvasculature. Despite their advantages, the widespread use of SS-OCT and SS-OCTA remains relatively limited.