It's like the difference between pushing on something to slowly move it and striking it to cause a vibration." "When the laser hits a cell, the energy causes it to heat up a tiny bit and expand instantaneously, creating an ultrasonic wave. "It's basically compressing one second's worth of summer noon sunlight over a fingernail area into a single nanosecond," says Yao. As such, it is safe to use on living tissue repeatedly, the engineers say. Photoacoustic tomography, on the other hand, avoids ionizing radiation altogether and uses only a safe dose of nonionizing energy. Limits to the amount of radiation a subject can tolerate makes X-ray imaging and positron emission tomography (PET) impractical for long-term use. Magnetic resonance imaging (MRI) can also see deep into tissue, but requires a strong magnetic field and often takes seconds to minutes to form an image. Ultrasound waves easily travel through tissue, providing a much more in-depth view, but do not have the ability to discern a tissue's chemical components and therefore do not capture important information that can be conveyed by light-based imaging. ![]() "With this advance, researchers can easily watch as drugs are distributed throughout an animal and track how different organs respond," Yao says, referring to the technique's ability to track individual molecules as they flow through the blood stream. Yao is now an assistant professor of biomedical engineering at Duke University. "Photoacoustic imaging has been highly expected to get real-time whole-body imaging of a small animal with rich functional information," says Junjie Yao, formerly of the Optical Imaging Laboratory. Wang is the first hire of the recently endowed Andrew and Peggy Cherng Department of Medical Engineering at Caltech. "Photoacoustic tomography combines light and sound synergistically for high-resolution imaging of molecular contrast," says Wang, Bren Professor of Medial Engineering and Electrical Engineering. For example, hemoglobin (which defines the color of blood) absorbs more light than the tissue around it, creating a contrast between oxygenated and de-oxygenated blood that makes it possible to take color images of arteries and veins in vivo. Using this technique, medical engineers are able to discern delicate features inside the body because different types of molecules absorb light differently. ![]() It uses extremely short laser bursts that safely cause cells or other light absorbers to emit ultrasound waves, which then travel unimpeded back through the tissue to sensors that translate the signal into an image. Photoacoustic imaging combines the abilities of multiple imaging techniques into one platform. A significant amount of that light is scattered as it travels through tissue, however, so these methods are limited to depths of less than a couple of millimeters. Traditional light-based microscopy provides fast, high-resolution images that retain important functional information based on the wavelengths of light (i.e., colors) the tissue absorbs, reflects, or emits. ![]() ![]() He moved the lab to Caltech in January 2017. Wang conducted this research while the Optical Imaging Laboratory was located at Washington University in St. The paper appears online on May 10 in the journal Nature Biomedical Engineering. In a new paper, the engineers describe for the first time how this hybrid imaging technology can provide a full cross-sectional view of a small animal's internal functions in real time. The technique, dubbed single-impulse panoramic photoacoustic computed tomography (SIP-PACT), uses both light and ultrasound to peer inside living animals. Medical engineers at the Optical Imaging Laboratory led by Caltech's Lihong Wang are now able to take a live look at the inner workings of a small animal with enough resolution to see active organs, flowing blood, circulating melanoma cells, and firing neural networks.
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