Using photoacoustic imaging to look deep into the body

A new non-invasive technique has been proposed to examine deep into the body using light. Photoacoustic imaging – which allows us to see inside the body using ultrasound generated when a laser is irradiated onto the living tissue – is attracting attention as a painless imaging device. However, it is still difficult to see the deep tissues with photoacoustic (PA) imaging, so research to improve and apply it to clinical practice is actively being conducted.

To this, a POSTECH research team led by Professor Chulhong Kim and Dr. Byullee Park (Convergence IT Engineering, Department of Electrical Engineering, and Mechanical Engineering at POSTECH) in joint research with a Tianjin University research team led by Professor Yumiao Zhang and Ph.D. candidate Yuanmeng Ding have proposed a new deep-tissue PA imaging method.

The researchers have successfully demonstrated an in vivo imaging depth of up to 5.8cm of intestine and bladder of a rat model using the contrast agent based on the surfactant-stripped semiconducting polymer (SSS) micelles and a laser of 1,064 nm. This is significant in that it is the deepest penetration depth among all PA preclinical studies in the world. A contrast agent is a substance that helps the PA image to be seen more clearly.

PA imaging uses the principle of imaging a sound wave (photoacoustic) signal generated by an instantaneous thermal expansion of tissues absorbing light with an ultrasonic sensor. To put it simply, it is like the principle behind lightening or thunder. Optical imaging methods can only visualize a shallow depth of less than 1mm in thickness, but PA imaging can see up to several centimeters of human tissue.

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Research on PA contrast agents that enable a more detailed view of organs located in deep parts of the body is well underway. However, the short-wavelengths of 650nm to 900nm commonly used so far have limited penetration depth.

To this, the researchers removed the surfactants from the semiconductor polymeric micelles – which strongly absorb light of 1,064nm wavelength – and used them as the PA contrast agent. It was verified that the semiconductor polymer is biologically safe, and the PA image was confirmed at a depth of up to 5.8 cm by injecting a contrast agent into the gastrointestinal (GI) tract and bladder of mice.

Unlike computed tomography (CT) scans that use radiation, the PA imaging method developed by Professor Kim’s team can help diagnose diseases in deep tissue without the risk of exposure. Additionally, the laser of 1,064nm wavelength is relatively inexpensive and compatible with the conventional commercial ultrasonic equipment.

“This study holds great technological value in that it succeeded in imaging the deepest parts of the body with maximum depth of 5.8cm,” explained Professor Chulhong Kim. “This type of pre-clinical study is a must for clinical trials in humans, and will help speed up the application of photoacoustic imaging in the future.”

This study was conducted with the support from the Mid-Career Researcher Program, the Key Research Institutes in Universities program, and the BK21 FOUR Project of the National Research Foundation of Korea. The findings from the study were published in Small on December 1, 2021.