Science Highlights

Photoacoustic technique produces ultrasound images of working organs in live mice

Researchers have developed a photoacoustic imaging technique that uses lasers to create detailed ultrasound images in live animals. The method allows for complete internal body scans with enough spatiotemporal resolution to see active organs, circulating cancer cells, and brain function.

Photoacoustic imaging scans entire cross-sections of a live mouse in real time. 1 Although modern imaging techniques such as magnetic resonance imaging (MRI) and positron emission tomography (PET) produce remarkable images of the body, MRI requires heavy shielding of its strong magnetic field and PET uses radiation, making it impractical for longer scan times.

Researchers at the California Institute of Technology have developed photoacoustic tomography, which uses harmless laser pulses and ultrasound waves. The system produces very detailed images that can be used for long scans allowing researchers to study biological processes in real time for extended periods. The work is reported in the May 2017 issue of the journal Nature Biomedical Engineering.1

Behrouz Shabestari, Ph.D., Director of the NIBIB Program in Optical Imaging and Spectroscopy, explains the significance of the work. “Photoacoustic imaging is clearly the safest and most economical approach for longer term real-time imaging of live small animals. The group continues to develop innovative technologies to improve light-based imaging to enhance the system for eventual use in humans.”

The laser light pulses can easily penetrate to deep levels—as far as a few inches—to reach and stimulate the animal’s tissues of interest. The slight increase in heat from the laser stimulation results in the emission of ultrasound waves. The sound waves are then captured and turned into images. The images are extremely sharp because the ultrasound waves travel easily through the surrounding tissue without the scattering that would blur the signal.

Photoacoustic imaging tracks neurons firing in a live mouse’s brain in response to changing oxygen levels.1 Lihong Wang, Ph.D., Departments of Medical and Electrical Engineering at the California Institute of Technology, and lead author of the study, elaborates, “Our imaging technology—single impulse panoramic photoacoustic computed tomography (SIP-PACT) allows us to capture structural, functional, cellular, and molecular small-animal whole-body images with unprecedented speed and quality. Essentially, harmless laser pulses strike tissues—similar to tapping a drum to create vibrations that we then detect as ultrasound wave images.”

Wang explains that SIP-PACT can be used to monitor live animals over long durations, which allows for monitoring of the effects of test drugs without interference from radiation overdose associated with X-ray use in CT scans.

Remarkably, the SIP-PACT system was used to track a wide range of biological processes including whole body and brain cross section scans in real time, and tracking of circulating tumor cells in the blood vessels of the mouse brain. These tumor cells are implicated in metastasis.

The group is very enthusiastic about the capabilities of this system for visualization of biological processes for basic research—eventually moving to more clinically relevant applications.

The work was supported by the National Institute of Biomedical Imaging and Bioengineering through grants EB016986 and EB016963 with additional funding from the National Cancer Institute, and the National Institute of Neurological Disorders and Stroke.

1. Single -impulse panoramic photoacoustic computed tomography of small-animal whole-body dynamics at high spatiotemporal resolution. Lei Li, Liren Zhu, Cheng Ma, Li Lin, Junjie Yao, Lidai Wang, Konstantin Maslov, Ruiying Zhang, Wanyi Chen, Junhui Shi & Lihong Wang. Nature Biomedical Engineering, May, 2017.