“Medical Imaging Devices”

What comes to mind when you hear the term “medical imaging device”? Initially, you might think of the X-ray machine that you often encounter during health check-ups, or even the bulky MRI machine. You might also picture the more compact and cute ultrasound machine.

CT, MRI, ultrasound, etc. are already commonplace imaging devices we frequently encounter in everyday life. However, there are next-generation imaging devices that, although not yet seen often, are soon to be commercialized. In this post, we will explore one of these next-generation devices: Optical Coherence Tomography (OCT).

1. What is OCT?

“What is OCT?”

This is a common question from people who encounter OCT for the first time. If we refer to OCT as Optical Coherence Tomography, would it be easier to understand? Not really. Wikipedia provides the following definition of what OCT is.

“Optical coherence tomography (OCT) is a medical imaging technique that captures micrometer-resolution, three-dimensional images from optical scattering media (e.g., biological tissue) using light.” – Wikipedia

In other words, OCT is a medical imaging technique that uses light to image tissue layers with high resolution. By shooting near-infrared light onto tissues like the skin and analyzing the returning light, it acquires information about the tissue. Because of this characteristic, OCT is primarily used in ophthalmology for retinal examinations. Moreover, because of the various advantages of OCT, which will be discussed later, there are active attempts to apply it in dermatology and gastroenterology as well.

What specific advantages make OCT a next-generation imaging technology? Before diving into that, let’s briefly introduce how OCT acquires images. If you aren’t interested, feel free to skip to section 3.

2. How does OCT work?

Before explaining the principle of OCT, let’s go back to 2016. On February 11, 2016, it was reported that the Laser Interferometer Gravitational-Wave Observatory (LIGO) in Louisiana, USA, had successfully detected gravitational waves for the first time. This observation captured the ‘rippling of spacetime’ caused by the collision of two black holes 1.3 billion light-years away.

<Figure 1. View of LIGO. The long arms are the paths traveled by the lasers emitted by LIGO. Each laser travels 1,600 KM inside the arms.>

The term ‘rippling of spacetime’ might sound far-fetched. But think of space as the ocean and ‘ripples’ as waves; the ocean surface rises and falls repeatedly in the direction of the waves. Similarly, when gravitational waves pass through the Earth, space alternately stretches and compresses. Now imagine two beams of light traveling perpendicular to each other in such a scenario.

The light propagating in a direction similar to the gravitational waves will travel through distorted space, resulting in one beam traveling a longer distance than the other. Consequently, the travel time for one beam becomes longer than the other. By measuring and analyzing this time difference, LIGO successfully detected gravitational waves.

The reason for this digression into gravitational waves is that LIGO’s principle of detecting gravitational waves is very similar to the basic principle of OCT. While LIGO analyzes the travel time difference caused by the distance difference between two beams of light, OCT analyzes the travel time difference caused by the speed difference between the two beams.

In OCT, one beam is directed at a mirror in a vacuum, and another beam is directed at biological tissue. The near-infrared light used by OCT penetrates and returns from the tissue (1-12mm deep, depending on the frequency). During this process, the near-infrared light refracts and changes speed when encountering substances like water within the tissue. By comparing and analyzing the differences between the reflected beams, OCT acquires information about the tissue. This is the basic principle of Time-Domain OCT (TD-OCT).

Although useful, TD-OCT has limits, such as prolonged image acquisition time and difficulty in obtaining clear images. However, with the development of Spectral-Domain OCT (SD-OCT) in 2006, the technology has evolved to provide higher resolution images at a much faster speed, opening the doors for its application in various fields.

3. How is OCT different?

So, what are the differences or advantages of OCT technology compared to other imaging technologies?

3.1. Real-time Imaging

The first advantage of OCT is the ability to obtain real-time images, thanks to the significantly increased image acquisition speed. This feature offers several potential benefits, such as facilitating real-time guidance during surgeries and enabling tissue status assessment during endoscopies without the need for biopsies.

3.2. Non-invasiveness

While real-time imaging is not exclusive to OCT, as X-rays and ultrasound can also provide real-time, high-quality images, another standout feature of OCT is its non-invasiveness.

The wavelength of the light used in OCT ranges from 900 nm to 1300 nm, higher than that of visible light. This infrared light does not harm the human body, unlike X-rays. Hence, OCT can be used for long periods in clinical settings without posing risks to patients or medical staff who operate the device frequently.

<Figure 2. Physicians positioning a catheter using real-time X-ray imaging during CARDIAC ABLATION. Real-time imaging is not unique to OCT.[3]>

3.3. High Resolution

<Figure 3. Cross-sectional images of a coronary artery captured by INTRAVASCULAR ULTRASOUND (IVUS, left) and OCT (right).[4]>

Reading this far, one might think of another imaging device with the same advantages as OCT: the ultrasound scanner. However, OCT distinguishes itself from ultrasound scanners with one key difference: higher resolution. Due to the higher frequency of light used in imaging, OCT offers an axial resolution of around 3-16 µm. This high resolution is instrumental in detecting the presence of pathological changes in tissues.

3.4. Others

Additionally, OCT devices are portable. Unlike devices such as X-ray or MRI machines, OCT equipment is smaller. The imaging process involves shooting light through a catheter or endoscope, which adds to its portability.

3.5. Limitations of OCT

OCT is not without its limitations. The most prominent limitation is the depth of axial scanning. The penetration depth of OCT is around 12mm at most, which constrains its observations to surface tissues. This is an inherent limitation of non-invasive imaging techniques using low-frequency light. Hence, unlike X-ray or MRI, which are typically thought of for internal body imaging, OCT is specialized for surface information. However, for detecting superficial lesions in the skin or tissue, a few millimeters is sufficient.

4. Current status and applications of OCT

Given its aforementioned advantages, research is actively underway to apply OCT in various fields. In fact, some areas already extensively use OCT. Let’s explore the specific fields where OCT can be applied.

4.1. Ophthalmology

Ophthalmology is already a domain where OCT is widely used. The ability to observe real-time, high-quality cross-sectional images of thin structures like the retina is the most beneficial in this field. Retinal imaging using OCT is extensively employed to examine glaucoma or the prognosis post-LASIK surgery. The availability of numerous OCT devices tailored for ophthalmologists shows that this field actively integrates OCT into clinical practice.

<Figure 4. Imaging of macular degeneration (in this case, MACULAR MICROCYST) using OCT. Black part in A: lesion. Arrow in B: MICROCYST.>[5]

4.2. Cardiology

The potential uses of OCT in cardiology are also diverse. The most representative application is Intracoronary Optical Coherence Tomography (IOCT), which involves equipping a catheter with OCT devices to obtain high-resolution cross-sectional images of blood vessels. The primary aim of this technique is to scan coronary arteries, visualize implanted stents, or identify micro-vessels using a catheter thinner than 1mm. Recently, combining OCT with other imaging techniques like Fluorescence Molecular Imaging is being developed. However, OCT is not yet as widely used in cardiology as it is in ophthalmology.

4.3. Dermatology

Research in utilizing OCT for dermatological examinations is also actively ongoing. The rising need for non-invasive diagnostic methods propels this area. Recently, OCT technology demonstrated an 87.4% accuracy rate in diagnosing non-melanoma skin cancer (NMSC), emphasizing its definite potential in this domain.[6] However, OCT is less useful in diagnosing pigmented lesions like melanoma due to the characteristics of infrared light, which gets absorbed by pigments. Hence, OCT does not show the same level of effectiveness or potential in melanoma diagnosis as it does in NMSC diagnosis.

Moreover, given OCT uses light, there is also significant interest in combining OCT with other infrared-based examination methods, such as blood glucose or blood flow measurement.

4.4. Gastroenterology

Another application area for OCT is gastroenterology, particularly notable for diseases of the gastrointestinal tract, including tumors.[7] Similar to cardiovascular imaging, OCT’s characteristic of enabling endoscopic imaging allows it to inspect the tissue interior beyond the surface visible to the naked eye. By detecting and removing small lesions uncatchable by the naked eye through real-time cross-sectional imaging, OCT presents its potential in gastroenterology for preventing the progression to more significant diseases.

<Figure 5. Corrosive esophagitis. A: Visual image through endoscopy. B: Histology specimen. C: OCT image showing the lost epithelium (arrow).>[8]

5. Conclusion

So far, we have briefly explored the principles, advantages, and applications of OCT. Given the active efforts to integrate OCT into medical fields, it is predicted that OCT will be utilized in many more areas beyond those described in this article. Although currently more likely encountered in research labs than hospitals, we foresee a future where OCT, alongside other imaging technologies, contributes to people’s health in various fields.

References

[1] B. P. Abbott et al. (LIGO Scientific Collaboration and Virgo Collaboration), “Observation of Gravitational Waves from a Binary Black Hole Merger,” Phys. Rev. Lett. 116, 061102 (2016). [a] https://en.wikipedia.org/wiki/Optical_coherence_tomography

[2] https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=5702

[3] https://www.ottawaheart.ca/the-beat/2013/04/28/cardiac-electrophysiology-repairing-rhythms-heart

[4] Koganti, Sudheer, et al. “Choice of Intracoronary Imaging: When to Use Intravascular Ultrasound or Optical Coherence Tomography.” Interventional Cardiology Review, vol. 11, no. 1, 2016, p. 11.

[5] Zimmermann, H., Oberwahrenbrock, T., Brandt, A., Paul, F. and Dörr, J. (2014). Optical coherence tomography for retinal imaging in multiple sclerosis. Degenerative Neurological and Neuromuscular Disease, p.153.

[6] Olsen J, Themstrup L, Jemec GB. Optical coherence tomography in dermatology. G Ital Dermatol Venereol 2015 October;150(5):603-15.

[7] Gora, Michalina J., et al. “Endoscopic Optical Coherence Tomography: Technologies and Clinical Applications [Invited].” Biomedical Optics Express, vol. 8, no. 5, July 2017, p. 2405., doi:10.1364/boe.8.002405.

[8] Melissa Suter-Benjamin Vakoc-Patrick Yachimski-Milen Shishkov-Gregory Lauwers-Mari Mino-Kenudson-Brett Bouma-Norman Nishioka-Guillermo Tearney – Gastrointestinal Endoscopy – 2008