Good Vibrations: Ultrasound Elastography
In February, France's state-operated agency for the funding of innovation awarded a research-and-development grant worth 8.5 million euros to a European company that wants to validate a new application clinically for the still-nascent modality of ultrasound elastography. In this instance, elastography would be used as a targeting mechanism for the treatment of hyperparathyroidism resulting from a growth on the parathyroid glands.
Douglas O. Chinn, MD
Conventionally, hyperparathyroid cases in France are treated either surgically or with a protracted regimen of drugs, both of which entail risks to the patients and significant costs to the country's national health care system. With the new elastography application, doctors could gain the ability to detect more precisely the lesion site—as well as the extent and degree of necrosis—so that the growth could be bathed in tissue-destroying, high-intensity, focused ultrasound beams. This outpatient-based alternative appears to be simpler, safer, less invasive, and less expensive, leading to the French government's willingness to underwrite clinical investigations of it. Making Strides Meanwhile, the quest for simpler, safer, more efficacious tools for the detection of other types of tumors drives exploration of ultrasound elastography in the United States. This is an ambition fueled by a small (but growing) body of evidence suggesting that lesions can be characterized more rapidly, and with a higher degree of accuracy, when elastography is incorporated into the routine patient-examination process. An important characteristic of tissue is its elasticity. Elasticity can be affected by aging or by inflammation; it can also be affected by malignancy. The ability to visualize changes in that elasticity is thus useful in diagnosing cancer, proponents of elastography say. Hitachi Medical Systems America, Twinsburg, Ohio, is among the ultrasound vendors marketing an elastography application in the United States. The company's signature offering in that segment is known as the Real-time Tissue Elastography (HI-RTE) system. Detection of tumors using HI-RTE is a relatively straightforward process: One simply positions the ultrasound transducer over the tissue targeted for evaluation and gently applies downward pressure to compress it. HI-RTE then measures the amount of strain and calculates relative tissue elasticity, which is displayed as a color overlay on a standard B-mode image. Abnormal tissue is stiffer (or less elastic) than normal tissue; the system gauges the differences and paints the former in blue and the latter in red or green. HI-RTE is basically an add-on module for the company's HI VISION™ 900 ultrasound platform. It uses what Hitachi calls an Extended Combined Autocorrelation Method, or ECAM, algorithm to produce the elastography image. This algorithm performs a 2D correlation in both axial and lateral directions. Hitachi says that this overcomes the problem of sideslip and improves the accuracy of the strain image. At the 2008 RSNA meeting in Chicago, Hitachi demonstrated second-generation advances to HI-RTE. Chiefly, these allow more accurate localization and targeting of lesions in more locations, including thyroid, breast, colorectal, pancreatic, and musculoskeletal structures. In addition to greater accuracy, the showcased improvements also promise faster and easier quantifying of the relative differences in stiffness between a suspicious mass and the surrounding tissue, with the intent being to reduce the need for more burdensome forms of examination. Prostate Imaging Among the more interesting recent applications of elastography is prostate cancer detection. Douglas O. Chinn, MD, a partner in Chinn & Chinn Urology of Arcadia, Calif, has embraced elastography for precisely that purpose. "In my field, there is a paucity of imaging processes that can reliably diagnose prostate cancer," Chinn says. "As a result, we have traditionally been obliged to biopsy tissue in order to make our diagnoses. CT scans, contrast-enhanced MRI, 1.5T and 3T coils, and spectral MRI: They all claim to detect prostate cancer reliably, but the fact of the matter is that they don't. Elastography, however, seems to be different, and it may be more specific." Chinn reports that researchers are now learning that elastography may also increase the ability to identify malignancies in tissues that are not even suspect. "It's conceivable that elastography will improve our watchful waiting with the patients we're following," he says, "and it may also prove useful in helping us demonstrate to patients who want focal therapy that they are better served by having the entire gland treated, because elastography confirms or reveals the presence of other lesions." Chinn began familiarizing himself with elastography at last year's annual meeting of the European Association of Urologists. "I sat in on several lectures dealing with imaging for prostate cancer, and elastography was among the topics," he says. "I also read the papers on elastography that had been presented at this gathering; I found them fascinating." He was particularly intrigued by a paper by Pallwein et al¹ which discussed the role of ultrasound tissue elasticity and target biopsy. Chinn says, "Real-time elastography was used on 383 patients, and only five targeted core biopsies were taken. Then, each of those patients had a repeat standard systematic 10-core biopsy by another urologist, without knowledge of the RTE-targeted biopsy results. RTE-targeted five-core biopsies detected 91% of the cancers, while standard systematic 10-core biopsies detected 76.9% of the cancers. Hence, the authors concluded that RTE is valuable for prostate cancer detection, and with fewer biopsies." Another study that Chinn found helpful was presented by Salomon et al² of Hamburg, Germany, in which 67 patients with biopsy-proven cancer underwent RTE followed by radical prostatectomy. "Suspicious areas were recorded for the apex, midgland, and base areas, and were compared with the findings after radical prostatectomy," Chinn says. "A total of 22 suspicious areas were detected with RTE, and 237 tumor foci were found on the radical-prostatectomy specimen. RTE appeared to be more accurate for apical cancer." Two months after the EAU meeting, Chinn attended the counterpart event held by the American Urologic Association and made a point of visiting the Hitachi exhibit booth. "I told the Hitachi team that I wanted to learn more about elastography and wanted to gain that knowledge by studying with the person who is the most proficient with it," he says, noting that this led to him being introduced to Leo Pallwein, MD, a professor of radiology at the Medical University of Innsbruck, Austria, and Ferdinand Frauscher, MD, director of uroradiology at the same school. Chinn says, "The professors were very open and honest in sharing with me their extensive knowledge about prostate-cancer imaging. I spent three days imaging alongside Frauscher, using elastography, and emerged from the experience with a much greater appreciation of this technology." Operator Dependence Currently, Chinn schedules an elastography exam for every prostate patient he sees. The elastography equipment is located in Chinn's office, but he suggests that many of his urologist colleagues across the nation would probably find it more practical to refer prostate patients to a nearby imaging center where elastography services are offered. "This modality is not for everyone because success with it is so heavily dependent on the skills of the operator," he explains. "It requires someone who is very knowledgeable about elastography and mechanically adroit with it, because how you vary the pressure and the angle of the probe can affect the accuracy of the measurement of elasticity. Consequently, pressure and angle must be consistent throughout the duration of the exam." Chinn says that, from what he has been able to ascertain, elastography is easier to perform on breasts. "In a breast examination, the transducer is perpendicular the entire time," he reports, "whereas with prostate imaging, the transducer must be sharply angled because of it being placed within the narrow confines of the rectum." Chinn speculates that elastography—by dint of its effectiveness and ease of use on breasts—could eventually become the gold standard for the detection of breast cancer. "It certainly seems to hold advantages over mammography, CT, and MRI; inasmuch as it's ultrasound, there's no radiation the patient will be exposed to, the procedure is comparatively inexpensive, it's portable, and throughput could be potentially fast enough to allow more women to be imaged in the span of a day," he says. "All in all, elastography is, in my view, a modality with tremendous potential."