Miniature ultrasound transducers open 3D windows on heart

Duke University engineers have developed a miniature ultrasound transducer capable of generating data for high-quality real-time 3D images, transducers so small they fit in a laparoscope, intracardiac catheter, or transesophageal probe. So far the images have been only of dogs, but Duke's lead engineer on the project believes the step to human application is a small one.

The transducers have the potential to replace the miniature video camera and 2D sonographic systems that now commonly guide minimally invasive procedures, according to Stephen Smith, Ph.D., a professor of biomedical engineering at Duke's Pratt School of Engineering. Current images lack the depth and field-of-view of volumetric renderings.

"Our ultrasound device could allow surgeons to essentially see through the body to the site of interest in 3D, making surgeries easier to perform and, eventually, more precise," Smith said.

Inserted directly into the chest or abdomen through small incisions, the laparoscopic probe would visualize target lesions or a portion of an organ as it appears in 3D and real-time, Smith said. The surgeon would have the option of viewing the tissue simultaneously in three perpendicular cross-sectional slices or volumetrically.

If these options sound familiar, it's because they were the staples of the echocardiography community's first 3D ultrasound system, the Model 1, unveiled at the 1998 American College of Cardiology meeting (DI SCAN 5/15/98, Volumetrics takes 3D ultrasound to new level with debut of Model 1). Model 1 produced simultaneous cross-sectional slices of the beating heart in real-time. Later configurations supported real-time 3D renderings, a feat not replicated commercially until the end of 2002, when Philips released its Live 3D echo capability on the Sonos 7500.

Volumetrics' technology was advanced enough to catch the eye of Philips Medical Solutions, which agreed to buy the company. Philips backed out of the deal later in favor of purchasing Agilent (formerly Hewlett Packard). The company paid dearly for that decision when Volumetrics won a $180 million settlement, which was split among the firm's stakeholders, according to Smith.

Volumetrics exists today, but Smith, a cofounder of the company, and his colleagues, several of whom work in Duke's Pratt School, are oriented more toward proving new concepts than commercializing them. Miniaturization has been their top priority. The transducers that fed data to Model 1 some eight years ago were bulky and awkward to handle. Current 3D transducers still feed data to the Model 1 processor, but they are anything but bulky.

Smith and his collaborators have come up with four iterations of these transducers: one for intracardiac application, one for intravascular, a third for transesophageal echo TEE), and the fourth for a laparoscope.

The TEE and laparoscopic probes each incorporate 500 tiny cables or channels packed into a tube 12 mm in diameter. The cables, about eight times the number found coming from conventional ultrasound systems, supply bandwidth for the data needed to provide high-quality preprocessed volumetric images.

"The scanner produces a 3D moving image instantaneously with no reconstruction," Smith said.

With just a minor incision and local anesthetic, the laparoscopic probe promises to relay information as good as or better than can be obtained with transesophageal echocardiography, which requires general anesthesia. The laparoscopic probe might, therefore, be used to visualize the heart during minimally invasive cardiac surgery, Smith said. It also holds significant potential in the guidance of radiofrequency ablation for atrial fibrillation (AF).

To demonstrate this possible application, the Duke scientists have produced real-time 3D images of a dog's right pulmonary veins, the sites that are targeted for ablation when treating patients for AF. With 3D images, cardiologists working with human patients could steer ablation catheters precisely to the tissue surrounding these veins.

Smith expects the technology behind Duke's miniaturized transducers to fuel the development of ultrasound scanners for applications in the chest and abdomen. The technologies underlying intracardiac, TEE, and laparoscopic applications are ready for clinical trials right now, he said. He predicts that they or similar technologies will be in clinical use within five years.

Volumetrics could commercialize the transducers itself. Or Duke might license certain intellectual properties to others in the industry.

"At this point, we are not sure which direction we will go," Smith said.

For the time being, Volumetrics has a lot on its corporate plate. Several months ago, according to Smith, the company filed litigation alleging patent infringement by GE, Medison, Toshiba, and Siemens.