Photoacoustic imaging: Imaging out loud

May 27, 2022 / By Precious Porter, RN, BSN

Imaging methods like x-ray, computerized tomography (CT) and magnetic resonance imaging (MRI) are indispensable in health care, but they have drawbacks. What if there was an imaging procedure without the damaging ionizing radiation of an x-ray or CT, or the danger of a metal implant being affected by the strong magnetic field of an MRI? That technology is here and it is called photoacoustic imaging! 

 Have you ever taken a flashlight or laser pointer and shined it through your palm? You can almost make out shadows from different structures in your hands, like bones or large blood vessels. That’s how photoacoustic imaging works – at least the “photo” or “light” part.  

This idea started with Alexander Graham Bell more than 100 years ago. His experiments demonstrated how intermittent bright lights could heat certain materials to the point that they would expand and produce audible vibratory waves. He also noted that darker colored materials made louder sounds than lighter colored ones. In scientific terms, the amplitude of the generated photoacoustic signal is proportional to the amount of absorbed light. This principle is fundamental to how photoacoustic imaging works.   

The cells, tissues and organs that make up the human body are composed of multiple elements with varying abilities to absorb energy. Hemoglobin in the blood is chemically and structurally different than the calcium that builds your bones. The chemical composition determines how much energy is absorbed through the tissue. Photoacoustic imaging utilizes these differences in composition by tuning a laser light to the wavelength that can be absorbed by the specific tissue or organ you want to examine.   

Here’s what happens: 

• A short pulse, non-ionizing laser is focused on the tissue of interest 
• Chromophores within the tissue absorb the photonic energy 
• The chromophores start to vibrate, causing the tissue to expand and then cool 
• The expansion and contractions produce an oscillating wave of pressure (acoustic wave) 
• Ultrasonic detectors capture these microscopic changes in the tissue 
• An image reconstruction program then converts the signals into 2D or 3D images 

Photoacoustic imaging provides a safer option than most imaging modalities by being non-invasive and not utilizing ionizing radiation. Photoacoustic images are processed in real time, and the level of detail is astounding even without contrast. The available contrasts for photoacoustic imaging allow for a more focused visualization of tumors and other abnormalities that are difficult to locate and see.   

Another advantage of photoacoustic imaging is that it takes up less floor space and is super portable.  Current models usually consist of a variable wavelength laser with a built-in ultrasound transducer (a lot like the handpiece/probe that is available with traditional ultrasound) and a computer/data acquisition system with display. All of which can be wheeled into a patient’s room without a problem.   

Photoacoustic imaging is an amazing innovation, and I can’t wait to see what lies ahead!   

Precious Porter, RN, BSN, clinical data analyst at 3M Health Information Systems.