Technology News: Imaging
Engineers at Duke University (Durham, NC, USA) are developing technology that could allow physicians to use ultrasound waves to visualize the heart’s interior in three dimensions. Compared with conventional sonography, the new procedure greatly improves image quality. “An old-fashioned ultrasound scan only makes a single 2-D slice of the body,” explains biomedical engineering professor Stephen Smith, who heads the project at Duke’s School of Engineering. “With a 3-D scanner, we can scan a four-sided 90° pyramid shape and can display up to five image slices on a television monitor.” Because it can render volume in real time, the technology could be used for locating abnormal tissue responsible for atrial fibrillation, the most common form of arrhythmia. It would also be suitable for applications such as transseptal catheterization, transcatheter closure of atrial septal defects, pacemaker-lead implantation and extraction, and monitoring abdominal aortic-graft deployment.
Another aim of the research is to use ultrasound to produce heat for the selective destruction (ablation) of tissue. Once a tissue sample is selected using the imaging capability of the matrix-array within a catheter, ring transducers would produce a beam for ablating abnormal electrical pathways in cardiac tissue. The research draws on separately performed work done with internal ultrasound probes used for imaging and probes used for tissue ablation.
Although the probes developed in the past 5 years provide better visual guidance for internal surgery than x-rays, they can capture only 2-D images. Advances in cable technology have now enabled the integration of 3-D imaging and ablation capabilities in the same catheter. Smith’s research group is building dual-function imaging-plus-ablation ultrasound probes as small as 3 mm. “We started using very tiny cables, fitting as many as 200 of them into a 3-mm catheter,” he says.
The research may improve on the current ablation technique, which employs radio waves emitted from the end of an electrode probe that touches and then heats selected tissue. After threading the internal probe into the heart through arteries, physicians currently rely on fluoroscopic imaging to help position the device. Since fluoroscopy cannot image soft tissue, it depicts the heart as “a fuzzy background,” according to Smith. Fluoroscopy thus can give physicians “only very gross guidance,” he adds. “The nice thing about ultrasound ablation is that you don’t have to touch the tissue,” he explains. “The sound waves propagate through the blood and can ablate the tissue from a distance of a centimetre or two.”
Using a 10-MHz transducer for ablation, and a 5-MHz, 112-channel array for imaging, the group “takes turns imaging for one instant and then ablating the next,” Smith explains. He acknowledges that further miniaturization and other design work will be necessary to build a device small enough to be inserted through vascular pathways into real hearts for visualization and ablation trials. “We are still working with early prototypes at this stage. I think that this needs roughly four years of development before it could be used in live animals or patient trials.”
