Thermoacoustic CT Under Development for Breast Imaging

SAN ANTONIO—Measurement of radiowave absorption could offer a new noninvasive approach to the identification and diagnosis of breast cancer, investigators involved in an ongoing evaluation of the technology reported at the 22nd Annual San Antonio Breast Cancer Symposium.

Called thermoacoustic computed tomography (TCT), the technology takes advantage of the fact that the dielectric properties of cancer differ from those of surrounding normal tissues, Kathy Miller, MD, an oncologist at Indiana University, Indianapolis, said at a symposium poster session.

Thermoacoustic CT employs computerized analysis of ultra-high-frequency (UHF) radiowaves to distinguish breast cancer, which exhibits increased UHF absorption, from normal tissue. As UHF energy is absorbed, tissue heats and expands, creating a pressure wave that propagates through the tissue and that can be detected as sound, Dr. Miller explained.

A prototype TCT device consists of a hemispherical bowl containing 64 transducers mounted in a spiral array. The bowl is mounted in a shaft that rotates 360 degrees on its axis. A water-filled cylindrical waveguide, mounted beneath a cushioned examining table, provides radiowave illumination to the breast. A patient lies prone on the table, positioning one breast into an examining tank.

Radiofrequency Energy Pulses

Radiofrequency energy pulses are supplied by a 25 kW generator at an average repetition rate of 500 Hz. Immediately following each pulse, the output of the 64 transducers is captured for computer analysis. After completion of an acquisition series of 2,048 pulses, the bowl is rotated 1.4 degrees, and the series is repeated. The process is repeated 256 times during a total acquisition that lasts less than 10 minutes. The pulse sequences are assembled by computer into a tomographic image.

After validation of the technology in preclinical studies involving excised pig kidneys and a mouse xenograft model, the TCT device underwent an initial clinical evaluation of five patients with documented breast cancer. Three patients were imaged at diagnosis, and two patients had TCT imaging after completion of chemotherapy that resulted in complete remission.

Dr. Miller reported that TCT revealed areas of thermoacoustic contrast enhancement corresponding to palpable tumor in the three patients who had diagnostic scans. In the two treated patients, TCT yielded normal thermoacoustic images that confirmed the pathologic complete remissions.

“This is the first study proving that we can image the breast with a thermoacous-tic technique and that we can distinguish cancerous lesions from normal surrounding tissue,” Dr. Miller said.

Second Prototype

The investigators, in conjunction with Optisonics, Inc. (Indianapolis), have begun work on a second prototype device that should offer improved spatial resolution and decreased scan acquisition time, she said. Additionally, planning has begun for a clinical trial of TCT for evaluation of response to neoadjuvant chemotherapy.

“The neoadjuvant setting is really the ideal way to evaluate a new imaging technique,” Dr. Miller said. “The patients will all have known tumor and will be imaged with more traditional methods at the same time that they undergo thermo-acoustic imaging.”

Preclinical evaluation also continues in an effort to provide more insight into the physiologic aspects of cancer that contribute to the characteristics of TCT images.