Optical Coherence Tomography

Definition and Scope

Optical Coherence Tomography (OCT) draws many parallels to the commonly known ultrasound imaging technique found in a multitude of medical applications. As implied by the name, Optical Coherence Tomography is based on optics instead of sound waves and utilizes interferometry of light to create highly detailed images with superior (axial) image resolution. This allows for resolution as high as single micrometers, all while remaining a non-destructive and non-invasive imagining method.

This means that details, not visible via other imaging techniques, can be examined, and due to this OCT is found in many different applications and industries:

Ophthalmology, where measurements are used to determine early stages of glaucoma, age-related macular degeneration (AMD), or diabetic eye disease, to provide physicians with the highest quality of data to guide their decision on treatment options. It is also used to provide data in other ophthalmologic applications, such as phakic intraocular lens implantation, laser-assisted in-situ keratomileusis (LASIK) enhancement, and deep anterior lamellar keratoplasty (DALK).
Dermatology, as a diagnostic tool for the early stages of skin cancer, without the need for invasive, time-consuming, and costly biopsies.
Cardiology, where it can provide much higher detailed images compared to intravascular ultrasound (IVUS). This means it is possible to differentiate internal and external elastic laminae of coronary arteries and identify lipid-rich plaque with much higher sensitivity and specificity.
Dentistry, with usages including tissue imaging, early caries detection, periodontal disease, and oral cancer diagnosing.
Urology, where cancerous cell clusters can be determined with high specificity and burnt away with a surgical laser, all in the same procedure. Leading to improved patient recovery times, while only having to visit a physician once for screening, classification, and surgery.
Gastroenterology is also a field where OCT essentially is used as an optical biopsy. The technique is among other things used to diagnose gastrointestinal (GI) cancers, colorectal cancers, and Barrett’s esophagus.
Quality Assurance and in-line inspection of multi-layered objects, where it is used to both monitor the qualities of the coatings, layer finishes, and separation. This can lead to both a higher quality product as well as significant cost savings when employed as a process analytic technology (PAT) for process optimization.

Techniques

Optical Coherence Tomography (OCT) can be implemented in several ways. Originally Time-Domain OCT (TD-OCT) was used, where a mirror within a Michelson interferometer system was physically moved with regards to a reference mirror. This method is generally not in use today, as it is impracticable and slow for most applications. The industry has instead moved on to more modern methods: Spectral Domain OCT (SD-OCT), sometimes referred to as Frequency Domain OCT (FD-OCT), and Swept Source OCT (SS-OCT).

SD-OCT utilizes a broadband light source and high-resolution optical spectrometer to measure the interference frequencies from all layers in one snap-shot spectrum, referred to as an A-scan. SS-OCT systems instead rely on a swept laser source to scan through a sequence of wavelengths, while a single photodiode detector measures the amplitude of the resulting interference.

SD-OCT generally provides the highest possible resolution image and signal-to-noise ratio while being less susceptible to motion artifacts than competing OCT techniques. As one measures deeper into an object, a loss of contrast is observed. This artifact is known in the industry as “Roll-Off” and this sets a practical limit to how deep one can measure into tissues or other multi-layered structures.

The Roll-Off is determined by the system’s ability to resolve contrast difference, and the Ibsen Photonics EAGLE spectrometer line-up provides industry-leading roll-off due to its proprietary fused-silica transmission gratings. These allow you to measure deeper with better quality.