From eye exams to blood tests and surgery: how doctors use light to diagnose disease

This is the next article in our ‘Light and health’ series, where we look at how light affects our physical and mental health in sometimes surprising ways. Read other articles in the series.

---

You’re not feeling well. You’ve had a pounding headache all week, dizzy spells and have vomited up your past few meals.

You visit your GP to get some answers and sit while they shine a light in your eyes, order a blood test and request some medical imaging.

Everything your GP just did relies on light. These are just some of the optical technologies that have had an enormous impact in how we diagnose disease.

1. On-the-spot tests

Point-of-care diagnostics allow doctors to test patients on the spot and get answers in minutes, rather than sending samples to a lab for analysis.

The “flashlight” your GP uses to view the inside of your eye (known as an ophthalmoscope) is a great example. This allows doctors to detect abnormal blood flow in the eye, deformations of the cornea (the outermost clear layer of the eye), or swollen optical discs (a round section at the back of the eye where the nerve link to the brain begins). Swollen discs are a sign of elevated pressure inside your head (or in the worst case, a brain tumour) that could be causing your headaches.

The invention of lasers and LEDs has enabled many other miniaturised technologies to be provided at the bedside or clinic rather than in the lab.

Pulse oximetry is a famous example, where a clip attached to your finger reports how well your blood is oxygenated. It does this by measuring the different responses of oxygenated and de-oxygenated blood to different colours of light.

Pulse oximetry is used at hospitals (and sometimes at home) to monitor your respiratory and heart health. In hospitals, it is also a valuable tool for detecting heart defects in babies.

2. Looking at molecules

Now, back to that blood test. Analysing a small amount of your blood can diagnose many different diseases.

A machine called an automated “full blood count analyser” tests for general markers of your health. This machine directs focused beams of light through blood samples held in small glass tubes. It counts the number of blood cells, determines their specific type, and reports the level of haemoglobin (the protein in red blood cells that distributes oxygen around your body). In minutes, this machine can provide a snapshot of your overall health.

For more specific disease markers, blood serum is separated from the heavier cells by spinning in a rotating instrument called a centrifuge. The serum is then exposed to special chemical stains and enzyme assays that change colour depending on whether specific molecules, which may be the sign of a disease, are present.

These colour changes can’t be detected with the naked eye. However, a light beam from an instrument called a spectrometer can detect tiny amounts of these substances in the blood and determine if the biomarkers for diseases are present, and at what levels.

3. Medical imaging

Let’s re-visit those medical images your GP ordered. The development of fibre-optic technology, made famous for transforming high-speed digital communications (such as the NBN), allows light to get inside the body. The result? High-resolution optical imaging.

A common example is an endoscopewhere fibres with a tiny camera on the end are inserted into the body’s natural openings (such as your mouth or anus) to examine your gut or respiratory tracts.

Surgeons can insert the same technology through tiny cuts to view the inside of the body on a video screen during laparoscopic surgery (also known as keyhole surgery) to diagnose and treat disease.

How about the future?

Progress in nanotechnology and a better understanding of the interactions of light with our tissues are leading to new light-based tools to help diagnose disease. These include:

• nanomaterials (materials on an extremely small scale, many thousands of times smaller than the width of a human hair). These are being used in next-generation sensors and new diagnostic tests

• wearable optical biosensors the size of your fingernail can be included in devices such as watches, contact lenses or finger wraps. These devices allow non-invasive measurements of sweat, tears and saliva, in real time

• AI tools to analyse how blood serum scatters infrared light. This has allowed researchers to build a comprehensive database of scatter patterns to detect any cancer

• a type of non-invasive imaging called optical coherence tomography for more detailed imaging of the eye, heart and skin

• fibre optic technology to deliver a tiny microscope into the body on the tip of a needle.

So the next time you’re at the GP and they perform (or order) some tests, chances are that at least one of those tests depend on light to help diagnose disease.