Ultrasound catheter looks inside heart

This year's presidential election campaign has brought about increased awareness of heart arrhythmias, or electrical problems. Former Senator Bill Bradley suffers from the condition, which became a health issue in his primary campaign (see sidebar).

While medication can treat arrhythmia, if that option fails then an implanted pacemaker-type device can deliver an appropriate electric current to restore a normal rhythm. A third option is to use a catheter to destroy, via electrical energy, the tissue within the heart causing the arrhythmia, ablating away the pathway of the abnormal signals. But in order to work within the heart, surgeons need to determine precisely the location of the offending tissue, as well as the position of any catheters inserted as part of the detection or ablation process.

By combining proven ultrasound and catheterization technologies, engineers at Acuson Corporation (Mountain View, CA) have developed an intracardiac ultrasound catheter, the AcuNav(TM), that allows surgeons to look at structures within the heart with greater clarity than before. Thus they can more precisely determine any affected area and locate catheters for treatment.

Sound technology. Previously, cardiologists used ultrasound externally to ex- amine heart function and structure in general. Michael Curley, Acuson's program director for AcuNav, says that current catheter-based ultrasound is not ideal to image ablation surgery. These are based on smaller, single-element ultrasound transducers used in probes passed down the esophagus to image the heart. They emit sound radially, which is optimal for imaging circular passages, such as arteries, not the irregular volume of an organ such as the heart, Curley notes.

Development of the AcuNav began as a part-time program in 1991 when doctors from the Mayo Clinic approached the company to produce a catheter-based device to clearly image minimally invasive catheter procedures and much of the surrounding tissue of the heart. A full development since 1996, the FDA granted Acuson clearance to market the device last December.

The small, 64-element piezoelectric transducer used in the AcuNav emits a wavefront sideways. Phased-array beam steering and signal processing, similar to radar systems, produce a sharp image of the heart volume. Curley notes that, "Usually a plastic lens is used to focus an ultrasound transducer to produce a beam in a tight plane. But with a small transducer, that would cause the focus to be too close to the catheter" and the beam would fan out farther away. "We eliminated the need to focus with a proprietary material that keeps the sound in a narrow plane," he adds. "Also, in imaging from the blood vessels, we don't have to penetrate bone or fat to enter softer tissue, which improves image quality." The side-imaging format results in a more intuitive visuali-zation as opposed to a radial format, especially when viewing other devices that are inserted parallel to the catheter during various procedures, says Curley.

The AcuNav features the company's Coherent Image Formation architecture, which uses both ultrasound phase and amplitude data to maximize image resolution. The design also keys on broadband (5-10 MHz) ultrasound transducers, which give both higher frequencies for better resolution as well as the option of lower frequencies for greater tissue penetration. The latter is important if the cardiologist wants to obtain images of the left side of the heart from the AcuNav which is within the right side chambers. Regarding resolution, the surgeons on the development team wanted to make sure all the ridges and folds of tissue in the heart wall could be resolved adequately so that, say, any ablation probe heating in "valley" areas will be sure to remove problematic tissue. The device can also operate at select frequencies for spectral time histories of blood flow (4-5 MHz) or color Doppler flow maps (4, 5, 6, and 7 MHz).

Know your users. The catheter is a 10 "French" (3.3-mm diameter) device, 90 cm (35.4 inches) long. Echocardiologists insert the AcuNav into the body via either the femoral vein in the groin or the jugular vein in the neck to reach the right side chambers of the heart. Curley says that in early development, the engineers built a 15 French (5-mm) device with a bi-directional steerable tip. "The physicians from Mayo Clinic were de- manding," he notes, "They wanted 10 French, four directions of mobility, and the broadband transducers. They set a high bar to enter the market."

Materials were vital in fashioning a useable catheter mechanism. The simple operating mechanism requires only a few movements of two wheels running to pairs of control wires to maneuver the catheter tip in four directions. The doctor then can lock-in any position via a locking ring adjacent to the wheels. The non-homogeneous bending stiffness of the plastic tip also allows up to 160 degrees of steering in each of the four orthogonal directions.

Curley adds, "Steering is a subtle thing. For example, we thought we had it nailed down with originally reducing friction of the control lines used for steering by laminating the lines with a lubricating layer. This design worked well, but in rare instances, we found the friction increased somewhat. Although performance was acceptable, we decided to start over. We ended up changing the control line materials since the original laminated lubricated outer layer could 'bunchup'in the small holes [it passed through] causing binding." The solution turned out to be a lubricated, braided wire.

In the event of voltage surges, the catheter must also be safe electrically for use within the body. Curley highlights that each unit is tested for breakdown voltage up to 3,000V. With little material volume available to "bulk up" the catheter for such protection, he says plastics with high dielectric strength were vital to the design.

The design team also elected to make the portions of the catheter that enter the body disposable, removing long-term issues of performance after repeated sterilizations and liability concerns. This meant driving down the cost as low as possible. Curley notes one portion of the cost cutting involved eliminating a higher cost conventional electrical connector between the catheter and its mounting handle. Substituting a pinless connector, a flexible circuit in the end of the catheter is clamped by contacts in the handle when the catheter is locked into place.

In designing the AcuNav, Curley notes the importance of engineering software tools. "Much of the design work was done with Pro/E from PTC," he says. The company used its proprietary acoustic modeling software and transmission line models to develop the ultrasound system and processing.

In conclusion, James Seward, a cardiologist at the Mayo Clinic and one of the development team says, "This device enables us to clearly see areas of anatomy that govern heart function we have not been able to visualize previously."

Other applications

Medical

Locating cardiac pacemaker leads

Visualizing the septum (membrane) between the right and left sides of the heart to allow penetrating surgeries through the membrane

Imaging minimally invasive devices such as vascular stents, balloon angioplasty systems, and septal closure devices.

Non-medical

Ultrasound position sensors

Low-cost connectors

Remote positioning devices

Bill Bradley is not alone

Nearly two million Americans suffer from atrial fibrillation (AF)-an arrhythmia in which the atria, or upper heart chambers, beat rapidly. They then never adequately fill the lower chambers (ventricles) with blood. The result is reduced flow to the body and also the potential for strokes. The latter comes from blood pooling in the atria in which clots may form and dislodge to travel to the brain. AF produces about 75,000 strokes annually, with numbers rising as the population ages. Another arrhythmia is cardiac arrest due to abnormally fast or chaotic heart rhythms in the lower ventricles that keep the ventricles from adequately pumping blood.

Engineering challenges

Concentration of ultrasound energy-Solved by flat-wavefront generating, side-firing transducer

Clear images-Made possible by proprietary acoustic modeling for signal processing

Cut disposable catheter cost-Developed pinless, flexible-circuit connection

Accurate catheter positioning-Use of simple, four-way steering mechanism