A Sensor and a Cradle Turn 2-D to 3-D in Ultrasound Imaging
New method could make 3-D ultrasounds more accessible and less expensive.
Most people have seen ultrasound images, often of babies in the womb. But without long-term specialized training, interpreting the images can be more like reading tea leaves than getting precise information about a patient’s condition.
Now a team from Duke and Stanford universities have devised a simple and inexpensive way to turn a 2-D ultrasound scanner into a 3-D one.
In a standard 2-D ultrasound, a sonographer moves a wand over the area of interest, usually “freehand” rather than following a set pattern. The wand transmits sound waves and interprets the pattern of echoes that come back to reveal images. But while ultrasound has the advantage of not involving ionizing radiation, most scanners don’t provide “volumetric” or 3-D information. They are also usually interpreted by a physician after the exam, rather than being analyzed in real time.
While 3-D ultrasound scanners are available, they’re “just really expensive,” said Joshua Broder, a Duke University School of Medicine associate professor and a co-author on the group’s paper, published this summer in Ultrasonic Imaging. This often confines ultrasound diagnostics to hospitals or large clinics. Some portable scanners are now available, making ultrasound usable in more places, and the Duke-Stanford team wants to further extend the utility of the machines.
To adapt an existing 2-D ultrasound system to 3-D functionality, Broder’s group first restricted the range of motion of the sonographer’s wand by setting it in a small cradle that rests on the patient, limiting the wand's motion so that it can only travel forward and back or around in a circle. However, Broder said, this doesn’t severely restrict the area that can be imaged.
Conventional ultrasound scans don’t provide a way to orient the image to the axes and planes of the human body, so it can be difficult for scan interpreters to know what they are looking at. The top of the liver? The side? Which side? To solve this problem, the group incorporated a microelectromechanical sensor like those in cell phones. These sensors contain a gyroscope, an accelerometer and a magnetometer. The combination of the sensor and the cradle enables the ultrasound device to orient the 2-D images to the axes of the patient’s body. Researchers used an open source program to not only show the actual ultrasound images, but also a small cartoon of the human figure with the image orientation indicated. Broder believes the improved image, clear orientation, and portability of this type of scanner will make diagnosis faster -- for example, in the examining room.
A 3-D ultrasound scan, like a CT image, is constructed from a series of 1-D or 2-D slices by the software. But ultrasound imaging has trouble with movement, which until recently has restricted its utility for heart and lung scans because these organs are always in motion. Only the most expensive 3-D ultrasound machines can handle motion well, and these can cost upward of $200,000.
Broder’s team has not yet solved this problem, but they have shown that their scanner works fast enough to capture images of a baby held in its mother’s arms. The baby may not hold still, but the scanner can produce a useful image within about 10 seconds; if the baby moves, it’s easy to try again, Broder said.
“It is a simple and elegant solution to the problems of freehand volumetric rendering and I would not be surprised if the improvement [in the imaging] as compared to freehand are significant,” said Michael Richards, research assistant professor at the University of Rochester School of Medicine and Dentistry, in New York, in an email.
However, Richards is skeptical that the Broder team’s device will ever be able to capture movement, partly because the clarity of the image is dependent on the patient keeping the device's cradle perfectly still. He further noted that ultrasound scanner manufacturers may not be motivated to either allow adaptations to their systems or make Broder’s team’s version from scratch. They may also object to software changes, as scanner software is usually proprietary, Richards added.
Broder noted that an ultrasound scanner that is faster, cheaper and easier to use is the team’s goal -- not a high-end bells-and-whistles instrument. It may add only about $250 to the cost of a scanner, Broder said.
“We’re always looking for trade-offs in medicine. If we can get enough information at the bedside without radiation, sometimes having enough is what we should be aiming for,” Broder added.