Born in the U.S. and incubated in research facilities for more than 15 years, ultrasound elastography emerged with a splash across the Atlantic at the 2006 European Congress of Radiology. Researchers at the March meeting hailed the technique's potential to dramatically reduce benign breast biopsy rates.
In contrast with the 2005 RSNA meeting, where tomosynthesis stole the breast imaging show despite a lack of equipment, the ECR breast sessions were marked by enthusiasm for commercially available elastography. Unlike the potentially revolutionary tomosynthesis, the latter technique is expected to play a more modest, supporting role to mammography and conventional ultrasound.
Elastography refers to the measurement of elastic properties of tissues, based on the well-established principle that malignant tissue is harder than benign tissue. The technique is typically performed with ultrasound, but research with MR is also under way.
The procedure, also known as elasticity, was born in the early 1990s. The University of Texas Medical School holds multiple patents on the technique with ultrasound (see www.elastography.com for details). Images are acquired on high-end ultrasound devices equipped with additional software and hardware. Prototype commercial systems became available to researchers after 2000, and vendors have introduced commercial products in recent years, accelerating interest and development in the field.
Using a research system from Siemens, which is not yet commercially available, British radiologists report-ed results from four years of routine elastography in breast screening. They concluded that the technique could help halve the rate of benign breast biopsies.
"We can decrease the biopsy rate in benign lesions. This has great importance given the increased incidence of screening abnormalities picked up on breast ultrasound," said Dr. William Svensson, a consultant radiologist at Charing Cross Hospital in London.
Some industry sources worry that questions about patents and/or licensing might delay development of the technology. As this edition of Diagnostic Imaging went to press, the potential effect these issues might have on elasticity's future remained unclear.
But Siemens, which plans to release its elastography system in the U.S. later this year, says patent questions are not a limiting issue.
"Patent disputes have not prevented adoption of the technique," said Andy Milkowski, director of radiology at Siemens ultrasound, in an e-mail. "I think that the method just hasn't been good enough to be used in everyday settings, and therefore [major] manufacturers have not provided it on their products. I think once a large manufacturer, like Siemens, puts it on its product, the world will change quickly."
ELASTIC TECHNIQUE
Elastography uses raw ultrasound obtained before and after a slight compression of tissue, typically achieved with a transducer. Compression may also be performed using vibrations in a technique known as sonoelastography. Elastography measures and displays strain, that is, the change in the dimension of tissue elements at various locations in the region of interest.
An estimate of tissue strain is obtained at each point in the field-of-view, and strain values are then displayed as an image. The strain is estimated from minute differences between these two images.
Research has shown that normal tissue and fat have smaller elasticity profiles, while hard areas, such as cancers, are larger than the gray-scale appearance.
"The hypothesis was that the size of cancer is larger in elasticity imaging than B-mode and with benign lesions the reverse is the case," Svensson said.
The British study included women who were referred for routine breast ultrasound for focal breast abnormalities. In cases where abnormalities were identified on ultrasound, women underwent breast strain imaging.
Of 260 lesions identified, 71 were malignant, nine were intermediate with malignant potential, and the majority, 180 lesions, were benign.
British researchers noted that lesions with an elasticity to gray scale ratio of less than .75 were benign. Based on this ratio, all of the malignant lesions would have been identified, aside from a few intermediate lesions. That result equates to a sensitivity of 96%.
In the study, specificity reached just 53%, meaning that if the ratio is applied, half of the benign lesions might not need to be biopsied, Svensson said. In addition to the obvious clinical value, there are also economic advantages.
"Benign biopsies cost us a lot of money," he said. "If we can reduce them, that would bring significant savings for the health service."
In another study presented at the same session, the British team showed that freehand elasticity images depict the extent of breast cancer more accurately. Currently, when conventional imaging techniques are used, the extent is often incorrectly estimated, and repeat excisions are necessary in up to 50% of cases.
Researchers compared elasticity and conventional ultrasound images to the findings at surgery. Gray-scale imaging tended to underestimate lesion size, while elasticity imaging overestimated in some cases.
"Initial results show that elasticity is less likely to underestimate maximum tumor extent and is more reliable than B-mode imaging. We propose that if elasticity images were used to guide surgery, re-excision rates could be reduced," Svensson said.
FRENCH THUMBS UP
In another study presented at the ECR, French researchers presented positive findings for ultrasound elastography in a multicenter prospective study of 345 breast lesions in 314 patients.
Patients were imaged on the Hitachi EUB 8500 Logos ultrasound unit with a technique similar to the one used in the British study. Detected lesions were categorized by size and BI-RADS category.
Researchers used the color classification system developed by Dr. Ei Ueno to score elastography images. For example, elastography images that are completely green (soft) are typically benign with a Ueno score 1, while those that are completely blue (stiff) are thought to be malignant and have a Ueno score 5 (see table).
SENSITIVITY AND SPECIFICITY
For lesions of all sizes, ultrasound elastography achieved sensitivity of 80%, specificity of 93%, positive predictive value of 85.3%, and negative predictive value of 90.3%. Sensitivity was best for lesions less than 5 mm (90%), while specificity was best for lesions over 10 mm (95%).
For lesions in BI-RADS categories 3 and 4, sensitivity was 68% and specificity was 90%.
Researchers also reported 16 false positives with elastography (such as fibrous mastopathy and sclerosis adenosis) and 26 false-negative findings (such as DCIS).
The technique is most useful for lesions in the indeterminate BI-RADS categories and less useful for lesions in BI-RADS category 5, as false negatives might occur in these lesions. In obvious, suspicious nodules on B-mode imaging, elastographic assessment is either of very little use or not needed, said Dr. Anne Tardivon of the Institut Curie in Paris.
Elastography is easy to perform once practitioners are trained, Tardivon said. It provides good visualization and is not time-consuming.
"Elastography does not replace standard B-mode imaging, but clearly, it is a useful complementary tool and may reduce biopsies of benign lesions," she said.
At this time, the modality is being used for breast nodules detected on standard ultrasound studies, she said. Its potential role in evaluating patients without an abnormality on B-mode imaging remains to be evaluated.
KEEPING A LOW PROFILE
The Hitachi system used in the French study became commercially available in the U.S. a few years ago. But the system has not been widely promoted in this country and is being used only at a handful of research facilities.
A Hitachi executive explained that in the U.S., there is a greater tendency to biopsy breast lesions. In order for an imaging study to prove useful, it must provide a very high level of certainty.
Some European countries, such as those in Scandinavia, employ dedicated breast imagers working at high volume, which could result in more accurate diagnoses and a lower rate of unnecessary biopsies. In addition, economic and legal incentives to perform biopsies are lower in some European countries.
In any case, it is important to remember that with breast ultrasound, very seldom will one feature alone prove definitively that a lesion is benign or malignant, said Dr. Richard Ellis, codirector of the Norma Vinger Center for Breast Cancer at the Gunderson Lutheran Medical Center in La Crosse, WI.
Instead, a variety of features guide radiologists as to whether biopsy needs to be performed, he said. Even if elastography indicated a lesion was benign, if another feature looks suspicious, the biopsy might still go ahead.
"Elasticity won't be the only component to guide the radiologist. It will be yet another card in the deck, however, that we have to evaluate solid breast lesions and help show us whether a solid lesion warrants a biopsy or not," Ellis said.
Ms. Hayes is feature editor of Diagnostic Imaging.
Ueno Classification System
Benign
Score 1. Entire area is evenly shaded green, as is surrounding tissue.
Score 2. Lesion area shows a mosaic pattern of green, blue, and red.
Score 3. Central part of the area is blue (stiff), and peripheral part is green (soft).
Malignant
Score 4. Entire area is blue (stiff).
Score 5. Entire area and its surrounding area are blue (stiff).
Provided by A. Tardivon based on the classification system developed by Dr. Ei Ueno.
