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The Future of OCT?Swept SourceTodd J. Purkiss, MD, PhD
River City Retina ClubJuly 16, 2015
Background on OCT
Optical Coherence Tomography
Optical
ReflectedRefracted
Incident
• When a light wave meets the interface of two differing media, it can be absorbed, refracted, and/or reflected
• The reflected light can be gathered and measured by a detector
Coherence• Coherence is in the context of interferometry, a measurement technique based on electromagnetic interference
• Interference is the interaction of waves that are coherent (i.e., correlated) with one another
• OCT is based on low coherence interferometry
Coherence• Light waves from the same source are observed to interfere only when the difference in their optical path length is within the coherence length of the source
Coherence length = λ2/(n · λ)OR
(central wavelength)2/[(refractive index)(wavelength bandwidth)]
• High coherence source = single wavelength
• Low coherence source = broad bandwidth of multiple wavelengths
Coherence• The greater the wavelength bandwidth of the source, the shorter the coherence length
• The shorter the coherence length, the smaller the range of optical path length differences that will produce interference
• The smaller this range, the greater the axial resolution that can be obtained
Tomography
• “Imaging by sectioning”
• Amplitude or ‘A’-scan:• One-dimensional time-amplitude measurement• Time is related to axial distance traveled• Amplitude is related to density of the medium• Allows tissue characterization From Practical Ophthalmology, 5th Ed.
Tomography
• Brightness or ‘B’-scan:• Two-dimensional image consisting of multiple A-scans obtained transversely across a tissue• Brightness of the image is related to density of the medium• Provides information about tissue architecture
From Practical Ophthalmology, 5th Ed.
Time Domain OCT
Adapted from Fujimoto, J. G. Optical coherence tomography for ultrahigh resolution in vivo imaging. Nature Biotechnology. 2003; 21 (11), 1361-1367.
*Axialscanning
Time Domain OCT
From Ko, T. H. et al. Comparison of ultrahigh- and standard-resolution optical coherence tomographyfor imaging macular hole pathology and repair. Ophthalmology. 2004; 111 (11), 2033-2043.
820 nm supraluminescent diode with 25 nm bandwidth:~10 μm axial resolution, 400 A-scans per second
e.g., Zeiss Stratus 3
Time Domain OCT
From Ko, T. H. et al. Comparison of ultrahigh- and standard-resolution optical coherence tomographyfor imaging macular hole pathology and repair. Ophthalmology. 2004; 111 (11), 2033-2043.
800 nm femtosecond TI: sapphire laser with 125 nm bandwidth:~3 μm axial resolution, 400 A-scans per second
e.g., ultrahigh resolution (UHR) OCT
Spectral Domain OCT
Adapted from Fujimoto, J. G. Optical coherence tomography for ultrahigh resolution in vivo imaging. Nature Biotechnology. 2003; 21 (11), 1361-1367.
(amplitude & time information extracted by Fourier transform)
Fixed reference
Spectro-meter
Spectral Domain OCT
820 nm supraluminescent diode with 25 nm bandwidth:5-7 μm axial resolution, 27-40K A-scans per second
e.g., Heidelberg Spectralis, Zeiss Cirrus, Topcon 3D OCT-1000,Reichert Copernicus, OptoVue RTVue, Bioptigen SDOIS
Spectral Domain OCT
Enhanced depth imaging (EDI) can be obtained by moving the peak sensitivity deeper into the tissue, but requires separate scan and loses resolution of more superficial structures
Swept Source OCT
Adapted from Fujimoto, J. G. Optical coherence tomography for ultrahigh resolution in vivo imaging. Nature Biotechnology. 2003; 21 (11), 1361-1367.
(amplitude & time information extracted by Fourier transform)
Fixed reference
Swept Source
Swept Source OCT
1050 nm tunable laser:5 μm axial resolution, 100K A-scans per seconde.g., Topcon Deep Range Imaging (DRI) OCT
*Not currently available for sale in the US
Swept Source OCT
Swept Source OCT
What will be next?
OCT angiography
AngioVue by Optovue, *Not currently available for sale in the US
What will be next?
Adaptive optics
*Research tool, not yet commercially available
Thank you!
