Medical Photonics: Using Lasers to Peer into Our Bodies

Photo by Dave Sebele on Unsplash

Light.

Humans have always sought after it’s comforting brightness, and we’ve only just recently learned to capture and control some of its power.

To date, harnessing light has given us solar cells, Xerox, fibre optics, and new systems of space propulsion, but recent advancements have shown amazing promise in the field of medical imaging and diagnostics.

A Brief Overview on Light

We can think of light as massless “particles”, known as photons. These photons travel (propagate) at the speed of light, which is roughly 300,000,000 meters per second in air. Particles has been quoted because quantum mechanics states that light behaves as both particle and wave until openly observed, known as the particle-wave duality theorem of light.

Although these photons have no mass, they carry momentum. This property of light is currently being been used to propel NASA’s future spacecraft with ultra-concentrated lasers reflecting off a “sail”. This is also the reason why we do not sense the billions of photons passing through us every second, and later in this article, we will learn how to use photons passing through the body for diagnostic imaging.

Light can be found in an almost infinite set of energy states, known as its wavelength, and each wavelength lies at a specific point in the Electromagnetic (EM) Spectrum.

A image describing the EM spectrum of light. Different states of energy dictate a different position and color on the spectrum, making some lights visible, and others invisible.

The EM spectrum contains many sectors of wavelengths, with most of the possible energy states of photons being invisible to the eye, such as gamma rays and cosmic rays.

Visible light makes up an overwhelming minority of the EM spectrum; however, we can detect and create images of our surroundings with cameras made specifically to detect certain wavelengths of light through optical filters.

For example, we can see heat energy emitted through infrared waves. What is the reason we can feel a fire’s heat before we can see it? The “heat” that you sense can be thought of as infrared waves coming into contact with your body.

Infrared cameras detect infrared radiation precisely enough to showcase the vein patterns on this person’s hand. Blood is warm, and registers as relatively hotter than its surroundings, giving it a distinct color.

The next time you sense heat, you can attribute a small portion of it to a higher-than-usual concentrated area of infrared waves in your surroundings. This is not always the case, as heat is mostly atomic collisions at higher energy states, known as thermal conduction.

One of the main aspects of photonics is using lasers to our advantage. A LASER (Light Amplification by Stimulated Emission of Radiation) can be seen as a collimator (gatherer) of light. Lasers use reflective chambers with a specific crystal that shoots passing photons through a small slit, which results in an ultra-concentrated beam of pure light.

Medical Imaging

Current medical imaging devices are bulky, expensive, and most importantly, they use rays that fall in the ionizing sector of the EM spectrum (X-rays, Gamma rays). Ionizing radiation is known to cause harm to your DNA with prolonged use, increasing your risk of diseases such as cancer.

Recent discoveries in photonics could now allow for safer scans and diagnostic methods using non-ionizing radiation. This could be a huge breakthrough in imaging technologies if properly implemented into society.

The Process

The most widely known method of constructing medical images with light is known as Diffuse Optical Tomography (DOT).

Despite the difficult name, DOT revolves around basic principles of optics: If you emit photons at a certain substance, you can determine its properties by how much light is reflected, scattered, and transmitted.

The tissue penetration of different wavelengths of light. We see a correlation between higher wavelength of light and higher tissue penetration. This is why CT scans use high wavelength radiation — for high penetration.

You may have used a bare-bones version of DOT if you have tried shining a flashlight through your hand — believe it or not, our bodies are slightly translucent! Our skin, bones, and organs, all let light pass through them, and in different amounts.

The different amounts of light that pass through the object dictate what is known as the optical properties of it. For example, your liver will let less light pass through, on average, compared to your skin. A common example of a precursor to DOT technology is a pulse oximeter, which uses safe light that falls in the near-infrared spectrum to examine your blood’s oxygen saturation.

By using precise photodiodes and CCD arrays to detect the transmitted light on the opposite side of the photon beam (typically a laser), we can reverse-engineer this recorded data and process it to find the locations of different parts of the body.

The optical sensor arrays and the photon source remain opposite to each other at all times, and after multiple spirals around the region of interest, we can construct a coherent image of the body using the same image -reconstruction algorithms that are used in CT and MRI scans.

Not to mention, DOT imaging is extremely precise, with resolutions of roughly 100 nanometres, which gives us a thousandfold improvement compared to the granularity of current imaging tools.

A nanoscale image constructed from photonic imaging techniques. In this image, we see a tumor leaching away oxygen from its surroundings (shown in blue). DOT will be very useful in the field of cancer monitoring and diagnosis.

Challenges

Even with breakthroughs in the field of DOT imaging, researcher face two main challenges, which are penetration and cost.

Penetration:

When light is used in the safe wavelength of the EM spectrum, photons can only penetrate tissue up to a certain extent until sensors cannot detect the transmitted light. To overcome these challenges, researchers are using reflectance-based sensors that are adjacent to the photon source for more accurate readings.

Cost:

To use DOT imaging, or other optical imaging techniques in hospitals would require us to drastically reduce the cost of these systems. DOT setups cost in the range of hundreds of thousands of dollars, and although this is much cheaper than many CT and MRI systems, DOT requires unimaginable amounts of laser calibration, which makes it less feasible as of now.

Despite these barriers, scientists are sure to find a way to overcome them, just as they always have with time. New methods of optical imaging, such as photoacoustic imaging, and TRUE (Time Reversed Ultrasonically Encoded) imaging are advancing at staggering speeds.

With current breakthroughs, it may not be long until your next visit to the doctor may involve a shiny flashlight that instantly identifies disease, checks your glucose levels, or analyzes your genome. That is just a glimpse of the future that photonics can bring us. Thank you for reading.