Conventional B-mode image (left) and matched ARFI displacement image (right) of a fresh, ex vivo breast tissue sample. Correlation between structures in the matched images is good, and the ARFI image provides more contrast between these structures. The transducer was located at the top of the images -- positive displacement is in the direction of wave propagation, toward the bottom of the page. The displacement scale for the ARFI image is microns. (Provided by K. Nightingale)

The power of sound waves should never be underestimated. Focused beams can push tissue around and possibly characterize suspicious masses, according to researchers at Duke University.

Led by Kathryn Nightingale, Ph.D., an assistant research professor of biomedical engineering, the Duke team used acoustic radiation force impulse (ARFI) imaging to apply localized pressure to small amounts of tissue. After applying the focused ultrasound, they were able to measure the amount of tissue displacement using ultrasonic correlation methods.

"ARFI imaging provides information about the mechanical properties of tissue, both elastic and viscous," Nightingale said. "No diagnostic imaging system available today provides this type of information."

Using a modified scanner that allows user control of acoustic beam sequences and intensities, Nightingale and colleagues generated standard diagnostic B-mode pulse tracking beams and pushing beams that were similar to those employed in Doppler imaging but had longer pulse lengths and were applied at a 15% duty cycle.

The study, published in the April issue of Ultrasound in Medicine and Biology, reported that tissue displacements were inversely proportional to the stiffness of the tissue and that peak displacement magnitudes ranged from 5 to 13 microns in vivo.

ARFI imaging revealed a stiffer region surrounded by a noisier, softer region in the breast tissue examined. Core biopsy later revealed an infected lymph node. Researchers noted that transient response to ARFI excitation in the breast varies with tissue structure.

"To date, we have performed this method in the breast, thyroid, abdomen, quadriceps, and biceps in vivo," Nightingale said. "We are also performing studies ex vivo in the liver, kidney, and prostate. Our current implementation is most successful at depths up to 3 cm, but we are investigating the use of different frequencies/pulsing sequences to interrogate deeper regions of interest."

Addressing concerns about potential temperature increases resulting from the technique's pushing beams, the researchers reported a mechanical index (MI) less than the U.S. FDA limit of 1.9. Nightingale admitted that while the technique showed promising results, testing was still in its early stages.

"The findings are very preliminary," she said. "We are aware of many shortcomings in our current methods that will be rectified by system hardware and software modifications, improving our frame rate, power efficiency, and sensitivity. Our ongoing studies include evaluation of ARFI imaging in soft-tissue lesion detection and classification, vessel stiffness measurements, and muscle viability studies."

-- By Merlina Trevino

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