Catheter Procedures Guided by Small 3D Ultrasound Probe

An ultrasound probe small enough to be carried along on the tip of a catheter can provide clinicians with clearer real-time images of soft tissue without the risks associated with traditional X-ray catheter guidance.

Duke University (Durham, NC, USA) biomedical engineers designed and constructed the innovative ultrasound probe, which is powerful enough to provide detailed, three-dimensional (3D) images. The new device performs like an insect's compound eye, merging images from 108 miniature transducers working together.

Catheter-based procedures involve pushing instruments through blood vessels to perform various tasks, such as clearing arteries or placing stents, typically with the guidance of X-ray images. In a series of proof-of-principle experiments in a water tank using simulated vessels, the engineers used the new ultrasound probe to guide two specific procedures: the placement of a filter within a vessel and the placement of a synthetic patch for aortic aneurysms. The scientists plan to begin tests of the new system in animals within the year.

"There are no technological barriers left to be overcome,” said Dr. Stephen W. Smith, director of the Duke University Ultrasound Transducer Group and senior member of the research team. The researchers published the results of their latest research online in the September 2008 issue of the journal IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control (UFFC).

"While we have shown that the new probe can work for two types of procedures, we believe that results will be more far-reaching,” Dr. Smith said. "There are many catheter-based interventional procedures where 3D-ultrasound guidance could be used, including heart valve replacements and placement of coils in the brain to prevent stroke. Wherever a catheter can go, the probe can go.”

Currently, when maneuvering a catheter through a vessel, physicians rely on X-ray images taken from outside the body and displayed on a monitor to manipulate their instruments. Often, a contrast agent is injected into the bloodstream to highlight the vessel.

"While the images obtained this way are good, some patients experience adverse reactions to the contrast agent,” commented research engineer Dr. Edward Light, first author of the paper and designer of the new probe. "Also, the images gained this way are fleeting. The 3D ultrasound guidance does not use X-ray radiation or contrast agents, and the images are real time and continuous.”

Another advantage of the technology is portability, which is an important issue for patients who are too sick to be transported, since X-rays need to be taken in specially equipped rooms, according to Dr. Light. The 3D-ultrasound machine is on wheels and can be moved easily to a patient's room.

Developments in ultrasound technology have made these latest experiments possible, the researchers reported, by generating detailed, 3D moving images in real time. The Duke laboratory has a long history of modifying traditional 2D ultrasound, similar to that used to image babies in utero, into the more sophisticated 3D scans. After inventing the technique in 1991, the team also has shown its utility in developing specialized catheters and endoscopes for real-time imaging of blood vessels in the heart and brain.

After testing many variations of the design of the probe, also known as a transducer, the engineers came up with a novel approach, lining the front rim of the catheter sheath with 108 miniature transducers. "These tiny transducers work together to create one large transducer, working much like the compound eyes of insects,” Dr. Light explained.

In the first experiment, the new probe successfully guided the placement of a filter in a simulated vena cava, the large vein that carries deoxygenated blood from the lower extremities to the back to the heart. Patients with clots in their leg--known as deep vein thrombosis--who cannot get clot-busting drugs frequently, receive these filters to prevent the clots dislodging and traveling to the heart and lungs.

The second experiment involved the placement of abdominal aorta-aneurysm stent grafts, which are large synthetic "tubes” used to patch weakened areas of the aorta that are at risk of bursting.

"I believe we have shown that 3D ultrasound clearly works in a wide variety of interventional procedures,” Dr. Smith concluded. "We envision a time in the not-too-distant future when this technology becomes standard equipment in various catheter kits.”

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