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The Evolution of Optical Coherence Tomography

Eric Ulmer

Eric Ulmer | May 22, 2019
Product Manager working with customers to provide flexible solutions to new challenges

Contributing Author: Johanna Brito, Marketing Communications Manager

Optical Coherence Tomography (OCT) has come a long way since its development in the 1990’s. With early development focused on retinal imaging, OCT revolutionized diagnosing by providing high definition images. Today, as advances to technology have taken off, OCT has evolved from its original uses in imaging diagnosis in dermatology and retina to aid in surgery with real-time imaging feedback of the tissue layers under surgical procedures. In this post, we’ll talk about OCT through the years and how those advances have impacted the medical market today.

Early History
OCT provides a non-invasive method of using reflected light to create high resolution, cross-sectional and three-dimensional images of tissue and other materials. For decades, technologies have used the reflection of all types of waves to create images, including ultra-sound, radar and MRI’s.  In each case, a wave is transmitted toward an object in space and the object reflects some portion of that wave.  The reflected wave is captured by a sensor that also tracks the time of wave propagation subsequently, associating the reflected wave with a specific point in space.  OCT was developed in the 1990’s as a way of applying light waves to this same technology. In the early days, OCT was applied to Ophthalmology, specifically the imaging of the retina layers of the eye. Its advantage comes from shorter wavelengths of light which in time and space generate higher resolution images.  Though powerful in its imaging resolution, one disadvantage is that the light waves can only penetrate for a short distance into the material or tissue, typically a maximum of 2-3mm.  This same limitation has pushed development in other complementary technologies including endoscopic probes which sparked advancement in OCT for internal body imaging.

Techniques Used
In general, OCT is used to analyze the microstructure in materials and biologic systems. Anything from the anatomy of the tissue, retina, cornea, gastro-intestinal, cardiovascular system to other materials like drug coatings and beyond.

Today, most OCT techniques are based on analyzing the spectral interference between waves reflected from a reference optic and the sample being imaged.  The majority of OCT systems are of type Spectral-Domain OCT (SD-OCT), using a broadband light source.  This light source has a range of wavelengths which are propagating and reflecting from the sample material and reference optic to generate the broadband interference wave patterns.  The interference waves are sent through a grating to separate the wavelengths before entering a spectrometer.  The spectrometer is used to generate the spectral interferogram which represents the spectrum of reflected light intensity at various wavelengths. By applying the Fourier transform to this data, the A-scan is generated.  The A-scan represents the reflected light wavelength and intensity across a depth profile in the sample (conceptually, similar to a core sample).  Multiple A-scans are generated to create a B-scan, or the sliced image of the sample, which is then combined with many other B-scans to create the final 3D volume image of the sample.

A more advanced type of OCT called Swept-Source OCT (SS-OCT) uses a tunable laser source to sweep across multiple, discrete wavelengths to generate the spectral interferogram using a single photodetector.  Since the wavelengths are separated before acquisition, the data processing is simplified and allows for even faster scanning. In addition, SS-OCT has less signal loss at deeper tissue levels which enhances the 3D image.

OCT Throughout the Years
OCT was initially introduced using the Time Domain (TD-OCT) imaging technique, based on a detection technique that uses a low-coherent light source and a changing focus point to generate the A-scan data. By the end of the 1990’s, technology moved to a new technique called Spectral Domain (SD-OCT), which greatly improved sensitivity and allowed a dramatic reduction in scan time by capturing the complete A-scan data relative to a single focus point in the sample.  Currently, the most advanced technique uses a Swept- Source (SS-OCT) input to capture the SD-OCT data resulting in even faster imaging.

As mentioned earlier, the potential for OCT imaging beyond dermatology and retina drove development of endoscopic probe attachments that could access internal anatomy.  Advancements in the last 20 years have seen smaller and higher resolution delivery devices to access tissue in-vivo. This has allowed OCT to be used in ophthalmic procedures, such as cataract surgery, vitro-retinal surgical procedures, intraocular lens modification procedures, and laser-based vascular therapies. What’s more, ophthalmic OCT scans are expanding to the analysis of neurological conditions such as multiple sclerosis and Alzheimer’s disease.

Technology Used in OCT
OCT requires a fast and accurate scanning source for the delivery of the light beam and subsequent acquisition of the reflected wave.  It relies heavily on the galvanometers’ repeatability excellence at high speeds to achieve high imaging quality. An OCT instrument under daily use could see a billion scanning cycles over its lifetime.  That’s why the scanning motors in OCT require premium construction components designed for long life that can maintain the highest quality images.

OCT-based devices come in many configurations from desktop instruments to hand-held devices and surgical device aids.  For this reason, a critical factor for OCT products requires the scanning engine to be compact and flexible in its interface with the optical beam train. Cambridge Technology offers the performance and reliability needed to exceed in OCT applications. Our Galvanometer XY Sets provide a footprint of compact models to ensure integration in small spaces, coupled with reliable, high-performance to achieve high-speed scanning. Learn more about our solution here.