New tissue phantoms help with imaging-device design

Ashley N. Paddock, ashley.paddock@photonics.com

A novel combination of additives enables gelatin to acquire optical and acoustical properties that accurately match soft tissue in humans. These tissue mimics, or phantoms, provide an accurate proving ground for new photoacoustic and ultrasonic imaging technologies.

Ultrasonic imaging uses high-frequency acoustic pulses to probe the structure of tissues, while photoacoustic imaging uses low-energy laser pulses to create tiny acoustic waves that propagate through tissues. However, certain tissues and materials – fluorescent dyes, blood and nanoparticles used in certain tests – readily absorb the optical wavelengths typically used in photoacoustic imaging. When combined, however, the two imaging techniques could create a more comprehensive picture of soft tissues.

Designing an effective imaging device that harnesses the two technologies at once requires true-to-life phantoms. Scientists led by professor Stanislav Emelianov at The University of Texas have met this need by creating and testing a blend of tiny particles and multicolored dyes that turn gelatin into a realistic surrogate for human tissue.

“Tissue-mimicking phantoms are crucial in the development of a medical imaging modality, and thus their use is becoming increasingly important as photoacoustic and ultrasonic [PAUS] imaging gains greater clinical relevance,” said Jason Cook, a doctoral candidate at the university and lead author of the paper. “We pursued this project to give researchers in the growing PAUS imaging field a set of guidelines to construct tissue-mimicking phantoms with optical and ultrasonic properties.”

To match the acoustical properties, the biomedical engineers added 40-µm silica spheres to the gelatin to help scatter the acoustical signal, which matches the behavior of normal tissue. Next, they used an emulsion of fat to attenuate the acoustical signal. The fat also enhanced optical scattering of the mixture. Finally, to produce optical absorption characteristics similar to those of natural tissues, they added commercial dyes – India ink, Direct Red 81 and Evans blue.

The result was a synthetic human tissue that could prove helpful in clinical applications such as lymph node assessment, vascular imaging and atherosclerotic plaque characterization, Emelianov said.

The paper, which appeared online in the open-access journal Biomedical Optics Express (doi: 10.1364/BOE.2.003193), details the scientists’ investigation of phantom properties at a single optical wavelength; it does not mimic tissue across the optical spectrum.

The researchers’ next step will be to produce phantoms that mimic the optical spectral properties of tissue, Cook said. By achieving this, they could produce tissues for spectroscopic photoacoustic imaging.