SEEING DEEP IN THE BODY WITH LASERS
X-rays can show bones, but a new technique that combines sound and light visualizes blood vessels deep in the body—and can also indicate the direction and rate of blood flow.
Written by Kayt Sukel
This flow map created using photoacoustic vector tomography shows blood moving faster (lighter color) through a venous valve (dashed lines). Image: Lihong Wang
X-ray imaging shows bones and dense tissue, but not blood vessels.
FRACTURED BONES SHOW UP ON X-RAYS, and internal organs are visible via magnetic resonance imaging.
But it is difficult to assess the health of blood vessels, especially those residing deep in the body, at least before specific damage or symptoms occur. For the most part, physicians have had to rely on invasive exploratory surgery to determine if there is a problem.
Now, however, researchers in Lihong Wang’s laboratory at the California Institute of Technology in Pasadena, have developed a new method to image even the deepest blood vessels in a non-invasive manner.
The new method takes advantage of an existing technology: photoacoustic imaging, where lasers deliver pulses of energy to biological tissues. Those pulses are absorbed by hemoglobin, the iron in red blood cells that help it to carry oxygen, which allows ultrasonic transducers to pick up the resulting vibrations and transform it into an image.
“We’ve been working on photoacoustic imaging for a couple of decades on a variety of different problems,” Wang said. “But we did not think that we could use it to image deep blood vessels. In fact, the whole field of imaging, not just our lab, did not think it was possible because these vessels are so deep in the body and it’s hard to resolve the heterogeneities you naturally see in the blood as it flows.”
The team knew that as the vibrations produced by the illuminated hemoglobin travel from deep within the body, the sound would make it to the skin, where sensors pick it up and then transform it into an on-screen image. Initially, Wang said, the team could only demonstrate the sizes of blood vessels and concentrations of blood with oxygen saturation.
The researchers discovered, however, that as they worked to refine the algorithms used in photoacoustic imaging to image blood flow, they were able to measure vector flow, or the flow rate and direction of the blood flowing through the blood vessels. The team call the new technique photoacoustic vector tomography, or PAVT.
“Being able to see this was really a surprise,” Wang said. “At first, we thought the images might be due to some artifacts. But after doing some in vivo experiments on humans, as well as other tests in a controlled setting, we saw it wasn’t. When we increased the flow speed, the PAVT showed faster flows, too.”
PAVT may one day help measure total concentration of hemoglobin and the metabolic rate of oxygen.
“Being able to see this was really a surprise. At first, we thought the images might be due to some artifacts. But after doing some in vivo experiments on humans, as well as other tests in a controlled setting, we saw it wasn’t.”
Lihong Wang, engineering professor at CalTech.
DEEP IMAGING
The algorithm takes advantage of the fact that red blood cells are distributed in different ways thanks to blood vessels’ natural structures. Wang said, much like the way two rivers may come together into a single body of water yet remain separate even as they flow together, oxygenated and unoxygenated blood also remain somewhat separated as they move through the circulatory system. It’s that heterogeneity that the PAVT system can see.
“You can have these two types of blood coming from smaller veins that then merge in a bigger vein. That means you have different materials within the blood with different properties that are mixed together in a bigger venous vein,” Wang said. “These two blood streams actually mix quite slowly and it’s that heterogeneity that allows us to see it even when the vessels are deep in the body. This also allows us to quantify oxygen consumption, which is an important metabolic parameter that can help physicians.”
PAVT can pick up venous blood flows but not arterial ones.
With this proof of concept, Wang and his colleagues are looking at how such a method could be applied in a clinical setting. For example, they are currently working with physicians to explore how PAVT could be used to diagnose and monitor diabetic neuropathy in the foot.
Wang added there are many other potential applications for this novel imaging approach.
“Our body consumes oxygen just as it consumes glucose. This is a form of metabolism that is important to health,” he said. “Photoacoustics allows us to quantify these kind of parameters, from blood flow, as well as how much oxygen the blood is carrying, or oxygen saturation. PAVT can quantify the total concentration of hemoglobin and the size of the blood vessels to get to the metabolic rate of oxygen. There currently is no other technology that can provide this measurement.”
Kayt Sukel is a technology writer in Houston.
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