Three-dimensional ultrasound probes poised to advance minimally invasive surgery

Three-dimensional ultrasound probes built by researchers at Duke's Pratt School of Engineering have imaged the beating hearts of dogs. The engineers said their demonstration showed that the probes could give surgeons a better view during human endoscopic surgeries in which operations are performed through tiny "keyhole" incisions.

If the probes prove beneficial in human testing, the advance might lead to more precise and safer endoscopic surgeries, said the Duke engineers. The researchers reported their advance in the latest issue of the journal Ultrasonic Imaging, which was issued in late March 2006, but dated July 2005. The research was supported by the National Institutes of Health and the National Science Foundation.

"Surgeons now use optical endoscopes or two-dimensional ultrasound when conducting minimally invasive surgery," said lead engineer Stephen Smith, a professor of biomedical engineering at the Pratt School. Optical endoscopes are thin tubes with a tiny video camera that surgeons can insert directly into the abdomen or chest through small incisions.

"With our scanner, doctors could see the target lesion or a portion of an organ in a real-time three-dimensional scan," Smith said. "They would have the option of viewing the tissue in three perpendicular cross-sectional slices simultaneously or in the same way a camera would see it -- except that a camera can't see through blood and tissue."

The technology has yet to be tested in human patients, but its success in dogs makes it ready for clinical trials, according to the researchers.

Endoscopic surgical methods have the advantage of reduced postoperative pain and a faster recovery. However, the two-dimensional ultrasound imaging now available offers surgeons only a limited view, which can impede their depth perception and make such procedures difficult to master.

"Our ultrasound device could really advance the use of minimally invasive surgery," Smith said. "By allowing surgeons to essentially see through the body to the site of interest in three dimensions, the scanner could make such surgeries easier to perform and eventually more precise." Such surgeries also might be cheaper and less traumatic as they could be performed in less time and, in some cases, without the need of general anesthesia, he said.

Duke developed the first 3D ultrasound scanner in 1987 for imaging the heart from outside the body. As technology enabled ever smaller ultrasound arrays, the researchers engineered probes that could fit inside catheters threaded through blood vessels to image the vasculature and heart from the inside out.

The current advance relies on 500 tiny cables and sensors packed into a tube 12 millimeters in diameter -- the size required to fit into surgical instruments, called trocars, that surgeons use to allow easy exchange of laparoscopic tools. By comparison, most two-dimensional ultrasound probes use just 64 cables.

"It's a feat of technology and craftsmanship to build these devices," Smith said. "More cables translate into better image quality. The scanners achieve a 3D moving image instantaneously, with no reconstruction."

Each cable carries electrical signals from the scanner to the sensors at the tip of the tube, which in turn send pulses of acoustic waves into the surrounding tissue, Smith explained. The sensors then pick up the returning echoes and relay them back to the scanner where they produce an image of the moving tissue or organ. The scanner uses parallel processing to listen to echoes of each pulse in 16 directions at once.

The laparoscopic ultrasound probes have so far been applied only to heart imaging, in which they may be particularly useful for monitoring heart function during minimally invasive cardiac surgery, Smith said. Current methods often monitor the heart with a 2D ultrasound endoscope probe down the throat, a method that requires general anesthesia.

"If physicians instead used the laparoscopic ultrasonography imager, they could monitor function for hours through a tiny incision -- possibly without anesthesia," Smith said. "That would be a big step forward."

The 3D ultrasound probes also might help guide physicians during cardiac ablation therapy, he added. In such procedures, cardiologists use catheters to burn specific locations on the surface of the heart in patients with atrial fibrillation, a disorder characterized by an abnormal heart rhythm.

In order to demonstrate this possible use, the researchers produced real-time 3D images of a dog's right pulmonary veins -- sites that are targeted in treating atrial fibrillation.

Similar 3D ultrasound devices also hold promise for minimally invasive abdominal and brain surgery applications, Smith said.

Collaborators on the study include research and development engineer Edward Light, assistant professor of pediatrics Salim Idriss, Pratt undergraduate Kathryn Sullivan and associate professor of biomedical engineering Patrick Wolf, all of Duke.