Ultrasound probe for imaging and ablating heart tissue in development
Durham, N.C., USA. Engineers from the Pratt School of Engineering, Duke
University, are developing technology that could lead to the use of
ultrasound waves both to visualize the heart's interior in three dimensions
and then selectively destroy heart tissue with heat to correct arrhythmias.
"No one else has developed a way for ultrasound to combine therapy and
imaging in a catheter, let alone 3-D imaging," said Stephen Smith, the
biomedical engineering professor who heads the project at the School of
Smith's group described work that developed initial laboratory prototypes
in two research papers published in October 2005 in the journal IEEE
Transactions on Ultrasonics, Ferroelectronics and Frequency Control and
the journal Ultrasonic Imaging.
In an interview, he said his group's technique may improve on doctors'
most widely used method for destroying — or "ablating" — aberrant tissue
that makes hearts beat irregularly. That current technique employs radio
waves emitted from the end of an electrode probe that touches and
excessively heats tissue selected for destruction.
After threading that internal probe into the heart through arteries,
physicians must now rely on fluoroscopic imaging -- X-ray movies -- to help
point the device. "However, a fluoroscope cannot image soft tissue at all,"
Smith said. "So the heart is just a fuzzy background." Under those
circumstances, fluoroscopy can provide physicians "only a very gross
guidance," he added.
Duke biomedical engineers previously pioneered techniques rendering the
kind of soft tissue internal images that enable fetuses to be seen in the
womb. They have also pioneered the use of ultrasound to create 3-D images of
the heart and other organs.
During the past five years other researchers have followed up by
developing tiny internal ultrasound imaging probes than can provide
physicians better visual guidance than X-rays for internal surgery, Smith
said. But those previous tiny probes acquire only two-dimensional images,
which still have shortcomings for pinpoint tissue ablation, he said.
Meanwhile, other researchers have separately crafted probes using
stronger ultrasound waves to heat internal tissues for ablation rather than
for imaging. But combining ablation with 3-D imaging in one device is new,
"The nice thing about ultrasound ablation is that you don't have to touch
the tissue," Smith said. "The sound waves propagate through the blood and
can ablate the tissue from a distance of a centimeter or two."
His group's new work builds on its previous success at miniaturizing
ultrasound 3-D imaging probes to a dime-sized array of hundreds of
individual ultrasound sound sending and receiving elements, called
transducers. Such probes are small enough to insert inside the esophagus to
render images of the whole heart.
Smith said his team has now built dual-function imaging-plus-ablation
ultrasound probes as small as three millimeters — less than half the size of
a dime. "We started using very tiny cables, fitting as many as two hundred
into a three millimeter catheter," he said. "This advance in cable
technology has allowed us to incorporate both 3-D imaging and ablation in
the same catheter.
"The ability to build these tiny matrix arrays is a technology that we've
developed at Duke for the past decade and now the ultrasound community is
following in our footsteps."
In another paper prepared for an October 2003 IEEE ultrasonics symposium,
Smith and his former graduate student Kenneth Gentry — now a postdoctoral
researcher at the University of Wisconsin — described using a prototype
device to first image and then raise the temperature of a tissue-mimicking
rubber by 25 degrees Fahrenheit. That temperature increase was enough to
ablate real tissue, he said. According to that earlier symposium paper, the
prototype was also ablation tested on a piece of beef muscle and imaging
tested within a fixed sheep heart.
According to Smith, the ablation beam emerging from the same
112-transducer array was 50 times more energetic than the imaging beam. "So
far, we've been taking turns at imaging for an instant and than ablating in
the next instant," he said.
He acknowledged 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've had the design intuition that allows us to do and describe some
preliminary experiments," he said. "But we have not yet made a practical
device that could even be used in a live animal." Other colleagues involved
in the work include Mark Palmeri, another former Pratt biomedical
engineering graduate student who just received his Ph.D., and Nasheer
Sachedina, a former biomedical engineering undergraduate now in medical
school at the University of Miami.