Tiny Bubbles, Big Future
- Ultrasound contrast agents use microbubbles to increase the clarity of images. In the U.S, contrast-enhanced ultrasound is used mostly for heart and liver imaging.
- Research at Jefferson is focusing on subharmonics—unique signals produced by microbubbles during imaging. Tuning into subharmonic echoes lets scientists trace the flow of blood through tumors and measure pressure in the heart and liver.
- Techniques developed at Jefferson, entering large-scale clinical trials, could reduce the need for invasive pressure tests in the heart and liver and allow doctors to better identify cancerous and non-cancerous breast masses without biopsy.
Injected into the bloodstream, gas-filled microbubbles—so small that hundreds would fit inside the period at the end of this sentence—create high-contrast ultrasound images that rival the clarity of X-rays, computed tomography scans and magnetic resonance imaging. The simple reason: Bubbles reflect sound waves better than human tissue does.
Now, these little spheres are bouncing ultrasound in surprising new directions.
Researchers from Jefferson’s Division of Ultrasound, led by Flemming Forsberg, PhD, professor of radiology, are at the forefront of investigating unique “subharmonic” echoes emitted by ultrasonic microbubbles. In projects that began in water tanks in the late 1990s and have advanced to large-scale human trials today, Forsberg’s team is harnessing bubble signals to measure blood pressure inside the heart and liver and monitor blood flow in breast cancer tumors.
Their work could reduce the need for anguishing and highly invasive medical tests that tens of thousands of Americans—and more around the globe—face every year.
“Currently, people with liver disease or pulmonary hypertension who need specialized blood-pressure checks must have catheters inserted into the jugular vein, groin or elsewhere and threaded through the heart while they’re sedated. That’s not something you want to repeat very often,” Forsberg says. “Noninvasive tests developed in our lab could allow for regular monitoring of these serious conditions for better treatment decisions. We are also testing subharmonic imaging of suspicious breast masses detected by mammography to determine which are cancerous and which are benign. This could vastly reduce the need for biopsies.”
Subharmonic imaging and pressure estimation are also being studied at Jefferson to gauge the effectiveness of breast, skin and kidney cancer therapies and to look inside gunky plaque in artery walls. “Knowing early on whether a cancer treatment is working would allow doctors to make changes sooner, personalizing cancer therapies for more patients,” Forsberg says. “And understanding more about why some plaque deposits burst, leading to heart attacks or strokes, could lead to new ways to protect the heart and the brain.”
Seeing the Low Notes
In the United States, microbubble-packed ultrasound contrast agents are FDA approved for echocardiograms and, since April 2016, to diagnose certain liver cancers—a development hailed by ultrasound experts across the country. In other parts of the world, these contrast agents are also used clinically to image the kidneys, spleen, breast, prostate, gallbladder, bladder, uterus and peripheral arteries.
“The irony is that Jefferson’s Division of Ultrasound in the Department of Radiology is at the forefront, leading the world in advancements in the use of ultrasound contrast agents even though many of those advances cannot yet be widely used in the United States outside of experimental applications,” notes Barry Goldberg, MD, director emeritus of the Division of Ultrasound. “The work Dr. Forsberg is doing has the potential to save lives in the U.S. and around the world.”
The bubbles’ usefulness was discovered accidentally in the 1960s, when a cardiologist named Claude Joyner, Jr., MD, noticed he got sharper ultrasound images of a patient’s heart after injecting a dye that, it turned out, contained bubbles. Among ultrasound researchers, the story is famous and widely cited. But it was almost two decades before bubbly contrast agents were ready for prime time. “It took until the 1980s to develop small, stable microbubbles with gas cores and outer shells made of proteins or lipids that lasted more than a few seconds in the bloodstream and were the right size to pass through the entire cardiovascular system,” Forsberg notes. “The bubbles used today are about the size of red blood cells or a little smaller. They last up to 10 minutes in the bloodstream, then the shells are metabolized by the liver, and you simply breathe out the gas.”
When bombarded by ultrasound waves, microbubbles react like a plucked violin string. “They vibrate, producing overtones and undertones including signals used in conventional ultrasonography,” Forsberg explains. The bubbles stay inside blood vessels, throwing the borders between the circulatory system and organs into sharp relief. “But microbubbles also produce subharmonic tones at half the frequency of the ultrasound waves. That’s something blood and tissue don’t do,” Forsberg explains. “And the signals change as the pressure on the bubbles changes. That’s the focus of our research.”
That interest has fueled a large and significant body of research in Forsberg’s lab over the past 20 years, with considerable funding from the Department of Defense, the National Institutes of Health, the American Heart Association, ultrasound equipment manufacturers and others. “We were the first to study subharmonic imaging in humans, but it’s important that groups at other institutions have replicated the research,” Forsberg says.
The SHAPE of Things to Come
“Researchers had known about microbubbles’ subharmonic signals for a long time,” Forsberg says. “But the idea of using the subharmonic signals as a pressure marker was totally new. We started working on it in the late 1990s, along with colleagues at Drexel University.”
Early studies in water tanks showed that the bubbles changed shape—and produced weaker or stronger subharmonic signals—under varying amounts of pressure. Lab studies followed, further testing subharmonic-aided pressure estimation (SHAPE). In a 2013 study of 45 people with chronic liver disease who were undergoing conventional pressure tests at Jefferson, lead researcher John Eisenbrey, PhD, a research assistant professor in the Department of Radiology, and others found that SHAPE was highly accurate at estimating blood pressure readings used to gauge liver disease. A larger, 300-person study led by Forsberg, in collaboration with the University of Pennsylvania and the National Institute of Diabetes and Digestive and Kidney Diseases, is recruiting patients.
“Checking portal blood pressure is an important way to know whether liver disease is present and progressing,” Eisenbrey says. “In severe liver disease, high pressure in the portal vein can cause life-threatening bleeding. Some of the people in our first study had their pressure checked every six months by having a catheter inserted into their jugular vein and passed through the heart to the liver. Coming up with a less invasive way to do this could lead to easier, faster and more frequent monitoring of their health.”
Eisenbrey and Forsberg say SHAPE may be ready for commercial use to check portal hypertension in four to five years. “The test uses existing ultrasound equipment, modified with software changes,” explains Eisenbrey, who did postdoctoral work on SHAPE at Jefferson.
Meanwhile, Jaydev Dave, PhD, an assistant professor of radiology at Jefferson, is studying SHAPE to estimate blood pressure in the heart. With grants from the American Heart Association and the National Heart, Lung and Blood Institute, Dave is leading a study that aims to recruit 136 people undergoing cardiac catheterization for heart problems such as heart failure, shortness of breath or to check the health of a transplanted heart.
“We’ll ask them to undergo an ultrasound at the same time and compare the results to see how accurate SHAPE is,” says Dave. “We couldn’t do any of this without the support of Jefferson’s clinicians, many of whom become co-investigators. If we can show that it works, it would be amazing. A cardiac catheterization is highly invasive and takes up most of a day. When we did a small pilot study, people told us they wish it were available now. It’s rewarding knowing there’s a real need.”
Cardiologist Ira Cohen, MD, director of echocardiography at Jefferson, agrees. “The ability to measure these pressures without the need for an invasive procedure would be of great benefit,” he says. “It would assist us immensely in the management of patients with heart disease.”
A New View of Cancer
One in 10 women who have a screening mammogram for breast cancer get a troubling call-back for further testing; many will have a biopsy. Up to 90 percent will find out they don’t have cancer, but they’ll live through days or weeks of worry first. Now, Forsberg and researchers from the University of California San Diego are comparing biopsy results to the results of subharmonic imaging (SHI) of breast abnormalities to find an easier way to find cancers in masses identified via mammogram. Their study will track the flow of blood inside breast masses in 450 women. A small pilot study of 14 women found that noninvasive SHI correctly identified breast cancers up to 90 percent of the time.
“Tumors develop their own system of blood vessels, but the vessels are not like those in normal tissue,” Forsberg says. “They’re twisted and leaky. You can watch the blood move through them using SHI—the scans take about 20 minutes—and see the difference.”
His lab has also used microbubble subharmonics to measure changes in fluid pressure inside breast, skin and kidney cancers during treatment. “In women undergoing neoadjuvant chemotherapy for breast cancer before surgery, pressure dropped in drug responders after the first dose,” Forsberg says. “Typically, women receive 16 doses of this early chemotherapy. If we could tell after the first one whether or not it was working, the drug cocktail could be changed for hopefully better results.”
Extending ultrasound’s reach through subharmonics could have a global impact, Eisenbrey notes. “Ultrasound is extremely useful because it’s far less expensive than other imaging modalities,” he says. “It doesn’t employ ionizing radiation—which could raise cancer risk, especially in children and young adults. And contrast-enhanced ultrasound doesn’t rely on the types of contrast agents used in X-rays, magnetic resonance imaging and computed tomography scans that can result in adverse reactions. Finding new ways to use ultrasound is particularly important for developing countries where health systems cannot always afford advanced imaging technology.”
By Sari Harrar