New type of ultrasound might lessen need for some biopsies
Tuesday, March 30, 2010
Every year in the United States, more than 5 million biopsies are performed on suspicious tissue from all parts of the body, primarily to detect cancer. Biopsies are most commonly done by taking a tissue sample for lab analysis using a needle, and remain the best test for diagnosing and staging cancer.
But biopsies are not without risk: They can cause infection, major blood vessels can be punctured, untargeted organs can be injured. And they are expensive: from $135 and up for suspicious skin growths to $1,950 for the liver and $4,500 for the prostate. As of now, up to 80 percent of biopsied tissue proves to be benign.
Now, a technique called elastography is being tested that can distinguish between malignant and benign tissue without using a needle or scalpel.
Elastography produces an image of tissue that has been compressed: Healthy tissue compresses more readily than cancerous tissue. The technique is sufficiently promising for the major medical equipment manufacturers -- GE, Siemens, Hitachi, Aloka -- to be investing large sums in the technology.
Elastography is based on ultrasound, which employs the principle that high-frequency sound waves travel through different parts of the body at different speeds. In diagnostic ultrasound, doctors place a hand-held device, called a transducer, on a specific part of the body. The transducer broadcasts sound waves and receives waves reflected back to it by tissue it cannot penetrate. These reflected waves are processed into an image, which is projected onto a monitor.
Unlike X-ray, CT and MRI images, which are produced according to preset protocols, ultrasound images are heavily dependent on the talents of the administering doctor or technician. A judicious twist of the transducer or a subtle increase in finger pressure can turn a blurry, inconclusive image into a definitive diagnosis. Performing an ultrasound scan is much like playing a musical instrument.
Elastography provides two basic add-ons to basic ultrasound: It uses software that produces pictures that enhance the edges and texture of a tumor; and, in advanced elastography systems, it uses sophisticated transducers that further improve the visibility of the tumor and provide data to measure tissue stiffness.
With first-generation elastography machines, dating to around 2000, diagnosticians could measure tumor stiffness by applying hand pressure to the transducer to compress the organ. (In a standard ultrasound exam, there is no application of pressure.) This resulted in significantly variable results from examiner to examiner. In recent months, second-generation systems have been developed in which the sound waves rather than the human hand compress the organ, thereby minimizing the human factor. This second-generation technique is known as ARFI (acoustic radiation force impulse).
Elastograms of the suspected tumor are compared with images taken via standard ultrasound. When malignant tissue is present, it will appear larger in the elastogram than in the standard image. (This is because elastography can detect and project the dense, fibrous tissue that grows around malignant tumors, making the tumor appear larger.)
Tumor shapes are irregular and hard to measure; second-generation elastography provides tools to make more accurate computations than human estimates can provide.
Aiming for accuracy
Since organs at or near the body's surface are easier to image with ultrasound, most research using elastography has focused on the skin, breast and thyroid.
Stamatia V. Destounis, a radiologist at Elizabeth Wende Breast Care, an imaging center in Rochester, N.Y., estimates that breast radiologists will be able to begin passing up biopsies based on elastography within four to five years.