Can you imagine pointing a flash light toward a cave and hearing the echo from within? No? The analogy appears absurd, but photoacoustic effect is real. It describes the occurrence of sound, or more accurately sound waves caused by the absorption of photonic energy. Photoacoustic tomography (PAT) is an imaging method that applies such phenomenon to examine anatomic structures and even physiological functions in biomedical research.
You might wonder how this imaging modality is compared to the ones that are already used routinely in the clinics. To generate images, X-ray, CT, SPECT (single photon emission computed tomography) and PET (positron emission tomography) all rely on a source of ionizing radiation, either external or internalized one. MRI (magnetic resonance imaging) requires strong magnetic field and radio waves, the monetary and time cost could easily add up. Compared to ultrasonic imaging, PAT has better optical contrasts and is free of speckle – noise caused by sound wave scattering. Finally, unlike other purely optical-based imaging techniques that are under development, PAT can penetrate deeper and sustain high spatial resolution.
Related reading: Photoacoustic tomography
Now the PAT family has a new member, which is designed to take the technique to a whole new level. According to an article recently published on Nature Biomedical Engineering, a new technique called single impulse panoramic-photoacoustic computed tomography (SIP-PACT) was developed by a team of researchers from Duke University and Caltech. It did a lot to improve upon the current PAT, and achieved several outstanding qualities: high spatiotemporal resolution, deep tissue penetration, multiple contrasts, and full-view fidelity.
The imaging device consists of a short-pulse laser as light source and a ring-shaped detector made from 512 ultrasonic transducer elements. To investigate the oxygenation status of hemoglobin and hemodynamics through the body, the authors used a head-focused and a whole-body tomographic slicing illumination scheme respectively. Thanks to the excellent performance of SIP-PACT, the joint research team not only captured real time images/videos of hemodynamic of small animals with exquisite detail of anatomic structures and physiological changes, but also tracked down circulating melanoma cells in the rat brains without any labeling.
“This penetration range enables functional imaging of whole bodies of small animals. It is expected to enable all kinds of biological studies in small animals and to accelerate drug discovery,” said the corresponding author of the paper Lihong Wang, who is a PAT pioneer of Caltech.
Article title image: an SIP-PACT cross section image of a rat's torso. Credit: Lihong Wang