What is Optical Coherence Tomography?

In the realm of medical imaging, the ability to see inside the body without making a single incision is the gold standard. While X-rays and MRI scans have long dominated this space, a newer technology has revolutionised how we diagnose conditions, particularly within the delicate structures of the eye and heart. This technology is Optical Coherence Tomography, commonly known as OCT.

Often described as "ultrasound with light," OCT provides micron-level resolution images of biological tissue in real-time. It has become arguably the most important diagnostic tool in modern ophthalmology and is rapidly expanding into cardiology, dermatology, and even art conservation.

What is Optical Coherence Tomography?

At its core, Optical Coherence Tomography is a non-invasive imaging test. It uses light waves to take cross-section pictures of your retina—the light-sensitive tissue lining the back of the eye.

With OCT, an ophthalmologist can see each distinctive layer of the retina. This allows them to map and measure their thickness. These measurements help with diagnosis and provide treatment guidance for glaucoma and diseases of the retina, such as age-related macular degeneration (AMD) and diabetic eye disease.

The "Ultrasound" Analogy

To understand OCT, it helps to compare it to a more familiar technology: ultrasound (B-scan).

• Ultrasound sends out sound waves and measures the "echo" (time delay) as they bounce back from internal structures.

• OCT sends out light waves (typically near-infrared light) and measures the reflection of light.

Because light is much faster than sound, measuring the "echo" of light requires incredibly precise timing—far faster than electronic timers can manage. To solve this, OCT relies on a physics principle called interferometry.

Types of OCT

• Time Domain OCT (TD-OCT): The older generation. It mechanically moved the reference mirror to scan depth. It was slower and had lower resolution.

• Spectral Domain OCT (SD-OCT): The current standard in most clinics. It uses a stationary mirror and a spectrometer to analyse light frequencies. It is vastly faster and provides higher definition.

• Swept Source OCT (SS-OCT): The cutting edge. It uses a tuneable laser source to sweep through frequencies. It offers deeper penetration (visualising the choroid behind the retina) and incredible speed.

Key Applications in Ophthalmology

The eye is the perfect subject for OCT because it is transparent, allowing light to pass through easily.

• Glaucoma: OCT measures the thickness of the Retinal Nerve Fibre Layer (RNFL). Thinning of this layer is often the first sign of glaucoma, detectable long before vision loss occurs.

• Age-Related Macular Degeneration (AMD): It detects fluid accumulation (oedema) or drusen (waste deposits) under the retina.

• Diabetic Retinopathy: It is crucial for monitoring diabetic macular oedema, a leading cause of blindness in working-age adults.

• Anterior chamber OCT: Allows high-resolution cross-sectional imaging of the front of the eye which can be helpful in glaucoma diagnosis and in certain corneal diseases.

The Procedure: What to Expect

For a patient, an OCT scan is remarkably simple and quick.

• Preparation: In an eye clinic, you may be given dilating eye drops to widen your pupil, though modern instruments can often scan through an undilated pupil.

• The Scan: You sit in front of the machine and rest your chin on a support to keep your head still. You will be asked to stare at a target light (usually green or blue).

• The Experience: The equipment scans your eye without touching it. The process takes only a few minutes. It is painless and does not involve ionising radiation (unlike X-rays).

• Results: The images are available instantly on the clinician's screen.

Advantages vs. Limitations

The Advantages

• Resolution: OCT offers resolution in the range of 10 micrometres (µm). For context, MRI resolution is typically around 1,000 µm (1 mm). This allows clinicians to see individual layers of cells.

• Safety: It uses non-ionising radiation, making it safe for repeated use, which is essential for monitoring chronic conditions like wet AMD.

• Speed: Modern SD-OCT can acquire over 100,000 axial scans per second.

The Limitations

• Depth of Penetration: Light scatters rapidly in biological tissue. OCT can only "see" about 2–3 mm deep. While this is perfect for the 0.3 mm thick retina, it cannot image deep organs like the liver or brain (unless done intraoperatively).

• Media Opacity: If the light path is blocked—for example, by a dense cataract or bleeding in the eye—the OCT cannot get a clear image.

OCT Angiography (OCT-A)

One of the most exciting recent developments is OCT Angiography (OCT-A). Traditionally, doctors had to inject a fluorescent dye into the patient's vein (fluorescein angiography) to map blood vessels in the eye. This procedure is often used in patients with wet AMD and other microvascular conditions affecting the macula.

OCT-A uses motion contrast to detect blood flow. By taking multiple scans of the same spot in rapid succession, the instrument creates a detailed map of the retinal capillary network without needles or dye.

Conclusion

Optical Coherence Tomography has transformed ophthalmology. It allows for visualisation of the microscopic structure of the eye in real -time, thereby improving diagnosis and clinical outcomes.