DURHAM, N.C. – New and promising ultrasound techniques devised at Duke University's Pratt School of Engineering can "remotely palpate" tissues, detecting and in some cases characterizing breast abnormalities that are deeper and smaller than the 1-centimeter-sized lesions that physicians can detect by feel, said the lead author of a just-released study.
The technology has many other potential clinical applications, such as detecting clogged arteries and deep vein blood clots, Katheryn Nightingale, an assistant research professor of biomedical engineering, noted with her colleagues in the scientific report.
This method, called Acoustic Radiation Force Impulse (ARFI) imaging, is based upon the conventional palpation technique physicians use to characterize breast lesions with their fingers.
But ARFI "is a new radiation force-based imaging method that provides information about the local mechanical properties of tissue," wrote Nightingale and three other authors in the article published in the April 5 issue of Ultrasound in Medicine and Biology.
"It's effectively like putting your fingertips inside of the breast and pushing on a small region of about 1 to 2 millimeters," Nightingale said of ARFI imaging. The project's leader, she has been working on aspects of the technique since 1993 when she began her Ph.D. training at Duke.
The other journal authors are Mary Scott Soo, an associate professor in the Duke Medical Center radiology department; Roger Nightingale, Kathryn Nightingale's husband and an associate research professor of biomedical engineering who helped characterize the mechanical properties of target tissues; and Gregg Trahey, the James L. and Elizabeth M. Vincent Professor of Biomedical Engineering who has been working on the project with Nightingale since its inception.
Other members of the ARFI team include graduate students Mark Palmeri and Deborah Stutz, who are working on simulating the system and evaluating experimental data. The work is supported by the Whitaker Foundation and the National Institute's of Health. The technology, Nightingale said, uses a single ultrasound send-and-receive "transducer" both to emit two different kinds of high frequency sound pulses and to detect the resulting effects on tissues. A computer converts the reflections from the tissue into images.
Of the two kinds of ultrasound pulses, "tracking" beams are used to monitor motion created by the second kind -- high-energy "pushing" beams. The pushing beams actually displace tissue by transferring their momentum to tissues via either absorption or reflection.
Development of the ARFI technique was built on a related and earlier studied phenomenon called Streaming Detection, in which the two kinds of ultrasound waves detect the presence of fluid – such as that in benign breast cysts – by causing the fluid to move.
"With the Streaming Detection method, we can differentiate between a fluid-filled cystic lesion and a solid lesion simply by applying these high-energy pulses and interspersing them with normal pulses," she said.
"If a lesion in the breast is filled with fluid, it is benign and doctors typically leave it alone," said Nightingale. "If the lesion is solid, it can be either benign or malignant, and they generally have to follow it up either with a biopsy or increased visits to monitor the progress of the lesion."
After initially testing the streaming detection idea on "phantom" materials that respond like those in breast tissue, Nightingale's team collaborated with Duke Medical Center radiology researchers to compare results from Streaming Detection with those obtained through normal clinical evaluations. Results were that 14 of 15 patients previously diagnosed as having simple breast cysts were also identified as having fluid-filled cysts by using acoustic streaming. Streaming Detection was also tried on 14 other patients with lesions that physicians were unable to identify as either fluid-filled or solid. "What was exciting about those clinical results was that in four of the 14 indeterminate lesions, we saw streaming," Nightingale said. Those clinical results were published in a 1999 issue of Ultrasound in Medicine and Biology.
The latest Ultrasound in Medicine and Biology article shows the ARFI imaging technique was able to move soft tissues in the body on the order of 10 microns (millionths of a meter) in a way that provided information about various tissue structures' stiffnesses and other mechanical properties.
These differences were revealed both in brightness variations in the ARFI images and in the time it took for tissues to recover following application of the pushing beams. The goal for ARFI imaging in the breast is to differentiate between benign solid lesions and malignant ones in order to reduce the number of biopsies performed on benign breast abnormalities, she said.
ARFI images of human breast, bicep, thyroid and abdominal tissues were all evaluated. While the pushing beams had the potential to produce elevated temperatures in those tissues, the beams' short durations maintained temperature increases at less than 0.2 degrees Celsius, which is well within acceptable guidelines, the article said.
"The intensities and application times of the pushing pulses used for ARFI imaging are safe, and will not result in an increase in risk to the patient over that associated with conventional diagnostic (ultrasound) imaging," the authors wrote.
"Although these findings are preliminary, they present several opportunities for ARFI imaging, and indicate that this method holds considerable clinical promise," their article concluded. Soo is now conducting further clinical studies at Duke Medical Center.
Another potentially promising area is employing ARFI to examine the stiffness of arteries, as well as to detect deep vein clots, or thromboses, Nightingale said.
Doctors currently use conventional palpation to detect such thromboses. But "you have to compress the clots quite a bit in order to evaluate them, and this can cause clots to dislodge, which is dangerous," she added.
Exploring such other uses for ARFI imaging, which Nightingale said might require different ultrasound frequencies and different transducers, is still in the future. "Right now, we're focusing on the breast," she said.
Journal
Ultrasound in Medicine & Biology