April 28, 2021
How to model an Optical Coherence Tomography system
Optical Coherence Tomography (OCT) is a tomographic imaging system that can produce cross-sectional or three-dimensional images based on light reflected or scattered from the image. Imaging of medical tissue is the most typical application of this system since OCT is safe and high resolution, although the depth to which the light can penetrate is limited to the order of millimeters.
The OCT measurement system relies on a Michelson interferometer such that coherence between the light reflected from the reference and the sample indicates that scattered light originates from a depth in the sample corresponding to the position of the reference mirror.
This article will walk through creating a model of a commercially available OCT in OpticStudio.
Cross-sectional images of the cornea and iris (A) and retinal tissue (B) of a healthy human eye are shown below. Colour changes correspond to changes in the strength of the returned light. This indicates a material change.
A representative OCT system is shown below. The beam should be split evenly into two arms, one of which converges at the sample volume to minimize the area illuminated for a given scan. The source should be a collimated beam of broadband light; the large bandwidth means low coherence and high precision in locating depths that produce coherence.
Depth scans also called axial or A-scans, measure the strength of the reflected light as a function of distance into the sample. Though it varies among types of OCT systems, depth scans are typically performed by the reference mirror such that light returned by the sample corresponds to a specific optical path difference (OPD) between the sample and the reference. Transverse, lateral, or b-scans are performed by rotating the scanning mirror in x or y, thereby translating the probe beam across the area of the sample.
We take our target specifications from those of commercially available OCT systems. The axial resolution, which comes entirely from source characteristics, should be on the order of 5 μm. The transverse resolution, which comes from the beam radius at the sample, should be at 15 μm. Light in the 800 nm range will be used to avoid high absorption in tissue which would limit penetration.
OCT uses interferometry in conjunction with broadband, near-IR light. Wider bandwidths give the best resolution while wavelength selection determines penetration depth in the sample material. For this example, we will use an 840 nm central wavelength, 60 nm FWHM source which provides an axial resolution of 5 μm in the air via:
These spectral properties come from a commercially available superluminescent diode via Superlum possessing a common wavelength for biological imaging and bandwidth for sufficiently high resolution. We will neglect collimating optics and begin with a source beam entering the interferometer.
OpticStudio can define broadband sources in two ways: by defining multiple system wavelengths within the appropriate range or by defining the associated coherence length as a property of the source. Coherence is the requisite source property for OCT, so we will use this method and allow OpticStudio to perform the bandwidth calculation and sampling via:
The object settings are shown:
A better future for medicine and healthcare
We are excited to see both the growth of OCT systems in the medical field and the innovations that will follow in the coming years. As this technology develops, the quality of healthcare will continue to improve, giving all of us a better quality of life.
To learn more on how to Model the base system, a Michelson interferometer, and demonstrate the coherence gate effect which enables depth scanning, Zemax customers can access the entirety of this Knowledgebase article and the entire series on MyZemax.com. Otherwise, please reach out to Zemax Sales to learn more about OpticStudio.
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