November 12, 2004 —
Three-dimensional ultrasonic imaging systems became an indispensable diagnostic
tool in obstetrics the minute they were introduced. Now researchers at USC’s Viterbi
School of Engineering are delving into 3-D ultrasonic scanners to detect breast,
liver and kidney cancer.
The key to this technology’s commercial success depends on advances in the design
of “2-D array ultrasonic transducers,” which are the ultrasound source and which
act as both the receivers and “ear pieces” of the system, says Jesse T. Yen, a
USC biomedical engineer and designer. He has built the largest transducer around
— an array composed of over 65,500 individual elements, 169 transmitter elements
and 1,024 receiver elements — giving it four times the imaging sensitivity over
his previous arrays. The instrument operates at five Megahertz and can produce
3-D volumes rather than 2-D slices of a targeted organ.
“This technology promises to give physicians a much better diagnostic tool for
spotting lesions in the breast, as well as for detecting carotid arteries or disease
in other organs, such as the liver,” Yen says.
“The arrays are able to focus ultrasound beams in three dimensions simultaneously,
which gives doctors a 3-D view of the target area, rather than a two-dimensional
plane, like you see using other imaging techniques, such as x-rays or magnetic
resonance imaging.”
Unsurpassed diagnostic tool
Three-dimensional ultrasonic imaging is already an unsurpassed diagnostic tool
in prenatal care. The imaging technique is relatively inexpensive, non-invasive. It does not use harmful ionizing radiation and it allows obstetricians to detect
fetal heart defects, spina bifida (curvature of the backbone), cleft palate and
other prenatal deformities.
Yen says one of the first commercially available 3-D systems based on 2-D arrays
like his, is the GE Voluson, which is ideally suited for fetal imaging, while
the SONOS 7500 from Philips is used mainly for cardiac applications..
Ultrasound transducers come in different shapes and sizes, and are used to scan
different parts of the body, Yen says. Because of the very high frequencies they
emit, pulsed ultrashort sound waves can be aimed at specific targets, such as
lesions the size of an apple seed in the breast or liver.
“As transducers emit ultrasound in rapid pulses, the waves travel through the
fluid and tissues of the body and are either reflected, refracted or absorbed,”
says Yen. “Only the reflected sound waves are processed into images that can
be displayed on an oscilloscope screen or a video monitor.”
Sound waves propagate at a speed of approximately 1,540 meters per second in
soft tissues. Yen explains that the thickness, size and location of various soft
tissue structures, such as subcutaneous layers of skin, muscle and tissue masses
can be determined based on their distance from the transducer. The variations
in the acoustic impedance of the tissue being targeted will determine the strength
and shape of the reflected sound wave and give physicians clues about its origin
and composition.
Higher frequency ultrasound waves are better at resolving small structures, but
they are not able to penetrate very deeply into soft tissues, so Yen’s arrays
are best at spotting lesions about 2 centimeters (1 inch) below the skin. Conversely,
a transducer emitting lower frequencies will provide greater depth of penetration
but won’t be able to produce images that are as sharp. Eventually, Yen hopes
to develop a transducer array that operates at 10 Megahertz.
Narrowing the sound beam
Focusing and aperture control technology is often used to narrow the sound beam
along its entire path to achieve maximum resolution laterally, or parallel to
the transducer face.
Voluson uses a traditional 1-D array and an automatic electronically
controlled motor to scan the imager up and down over the body. Images are “stacked
up” to create a 3-D volume. Yen’s 2-D arrays do not have any moving parts,. Instead,
they can focus the ultrasound beam in two lateral dimensions simultaneously.
“My transducer is a 2-D array of elements that will image in both the X and Y
planes,” he says. “The way it works is simple: I launch sound out into the body,
which we refer to as the ‘volume’ of space in which the sound wave is propagating,
but then in receive mode, I can focus the ultrasound beam in these two dimensions,
right down to a single point.”
The images are more accurate because the array has not moved up and down over
the body and because he is focusing in two lateral dimensions instead of the single
dimension of a 1-D array, he says.
Yen built his 65,500-element array at Duke University, where he received his
Ph.D. in electrical engineering in 2003, and is continuing the work in collaboration with Kirk Shung, professor of biomedical
engineering in the Viterbi School. He uses flexible printed circuit boards, which
are found in desktop computers, laptops, stereos and other standard electronic
equipment. The flex boards are bonded with a ceramic material called PZT (lead
zirconate titanate), which emits sound when electrically excited and converts
received echoes into electrical signals.
Cross configuration
The array of electronic elements sits in the center of the flex circuit, and
the ends of the flex have solder pads. It looks like a large cross. The cross
configuration fits into the volumetric scanner, so that a connection can be made.
The array area is 38.4 by 38.4 millimeters. Yen routed 169 transmitters
along four “wings” of the cross-shaped array, so that the north, west, south and
east wings have 48, 48, 37 and 36 transmitters, respectively. Each wing also contains
gold pads for 256 receivers for a total of 1024 receivers.
Using artificial, tissue-mimicking phantoms, he has been able to detect
cysts two centimeters (one inch) in diameter, as well as smaller cysts measuring
one centimeter and half a centimeter (0.2-inch diameter).
“The images showed marked improvement over the previous array,” Yen
says. “I’ve also been able to see real-time images of an in vivo hepatic blood vessels using this array technology.”
The new imaging technology is bound to improve current cancer screening
techniques and help revolutionize prenatal care by allowing physicians to visualize
and detect fetal heart defects and other inherited deformities in time to correct
them with surgery.
“The ability to visualize organs in multiple planes, in real-time motion
will give physicians an incredible advantage in finding and treating disease,”
Yen says. “The technology isn’t far from becoming standard. Give it, maybe, three
to five years.”
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Yen’s work on 3-D ultrasonic devices was featured on the cover of the
February 2004 issue of IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control and he presented his findings at the Aug. 24-27 international IEEE conference on ultrasonics in
Montréal, Canada.
--Diane Ainsworth
