Photoacoustic imaging scans entire cross-sections of a live mouse in real time. (Credit: Nature Biomedical Engineering, May, 2017)

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.

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 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.

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.