Photoacoustic Imaging for Cancer
Jan. 28, 2010 – Photoacoustic imaging combines light and sound to create detailed pictures of tiny structures in the body without the use of high-energy X-ray beams, which can be damaging. Unlike traditional radiology techniques, it also provides functional information about tissues and cells, with the ability to show blood flow and oxygen saturation. In this edition of the Breakthroughs in Cancer Research podcast series, Lihong Wang, PhD, explains how photoacoustic imaging works and how it could revolutionize the way doctors detect and monitor cancer. Wang, the Gene K. Beare Distiguished Professor of Biomedical Engineering at Washington University, is a world leader in the field of optical imaging.
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TRANSCRIPT OF AUDIO FILE
On this edition of Breakthroughs in Cancer Research, we'll be talking about a new way of creating images or scans of the body's tissues and organs that relies on converting light to sound. The technique is called photoacoustic imaging, and it is causing a lot of excitement because it offers advantages for seeing even tiny structures very clearly and in a very safe way.
Host: Thanks for downloading this podcast from the Siteman Cancer Center at Barnes-Jewish Hospital and Washington University School of Medicine in St. Louis. I'm Gwen Ericson, and I'm talking with Lihong Wang, PhD, director of the Optical Imaging Laboratory at Washington University. Dr. Wang is also the Gene K. Beare Distiguished Professor of Biomedical Engineering. His laboratory is dedicated to developing new biomedical imaging technologies that combine optical imaging, or light-based imaging, with ultrasound imaging. Dr. Wang, thanks so much for talking with me today.
Wang: Thank you for having me.
Ericson: I think the first thing we need to know is what is photoacoustic imaging?
Wang: We are essentially listening to optical structure instead of looking at optical structure. In fact, Alexander Graham Bell reported photoacoustics as a physical phenomenon over 100 years ago. He had this notion of a photophone instead of a telephone. He wanted to encode sound into a light beam then propagate the light beam in free space, and the light beam is received and converted back into sound again. And so this was probably the earliest optical communication before fibers were invented.
Ericson: How can light be converted to sound?
Wang: When light is absorbed, it generates a temperature rise. It doesn't have to be a very high temperature rise – typically millidegrees – and that temperature rise will cause thermoelastic expansion. Very much like when you heat up a balloon, the balloon will expand. The temperature rise will cause ultrasound wave emission. We detect the ultrasound signal outside the tissue, and from there on, it's a mathematical problem. We simply use a computer to reconstruct an image.
Ericson: Why are scientists so interested in developing optical, or light-based, imaging techniques?
Wang: First of all, it's very safe to use light because the photon energy is only a couple of electron volts. In comparison, X-ray imaging uses thousands of electron volts. PET imaging uses very high photon energy as well. We can provide very high intrinsic contrast, so we don't have to inject contrast agents into the body to image biological tissue. Most contrast agents have various degrees of side effects. And so if we can do away with contrast agents, we can provide a safer modality for imaging.
Ericson: I understand that in addition to shapes or structures, you can also see molecular signatures, like the difference between blood that is carrying oxygen and blood that isn't. Tell me more about that.
Wang: By imaging the intrinsic contrast in the body, such as the oxy- and deoxy- forms of hemoglobin, we can provide you with functional imaging. And so we are taking advantage of the different colors of oxy- and deoxyhemoglobin to quantify the concentrations of both forms. So we image whether a piece of tissue is dead or alive. If you use X-ray imaging, you can't tell if tissue is alive or not. By using optics, we can quantify the concentration of hemoglobin, we can quantify the oxygen saturation of hemoglobin, and we can also quantify blood flow by using the Doppler effect. You might have heard of optical Doppler, or you might have heard of ultrasonic Doppler. Here we're looking at photoacoustic Doppler as a combination of light and sound.
Ericson: How is photoacoustic imaging better than some other methods for cancer diagnosis?
Wang: Let me use breast cancer detection as an example. Currently there's mammography that uses X-ray contrast, primarily detection of microcalcification for early breast cancer detection. And we're trying to use nonionizing radiation because optics and sound are both very safe to use. So there's no potential of causing cancer at all. The optical contrast of a tumor tends to be very high. You're talking about more than 100 percent sometimes. Where using X-ray, the soft-tissue contrast is typically very low. You're talking about more or less 1 percent. As a result, sometimes we can see breast tumors better than X-ray can. We're looking at functional contrast in addition to structural information. The hope is that this modality can provide you with more sensitive detection of breast cancer. And the same can be translated into prostate cancer imaging.
Ericson: And you also have projects investigating detection of melanoma, don't you? How is photoacoustic imaging used in melanoma research?
Wang: Most melanomas have an increased melanin concentration. And melanin is a great optical absorber. We can image melanin extremely well using photoacoustic tomography because it's dark-colored. Anything you can see with the naked eye, we can potentially detect with photoacoustic tomography, except that we can do it better with higher resolution. So here we're not only detecting melanoma structure, but we're also detecting the potential function as well because we can see the melanin, we can detect its depth and we can monitor functional information such as the oxygenation of hemoglobin to see if there is tumor hypoxia. Tumor hypoxia means the tumor has lower oxygen content because a tumor draws a lot of oxygen out of the blood. It's a signature of a tumor.
Ericson: What about metastatis, when cancer cells such as melanoma cells break away from the main tumor and spread through the body?
Wang: You want to detect metastasis early. When melanoma cells circulate in the bloodstream or in the lymph vessels, we can, again, using the melanin as a contrast, try to detect it while the concentration of melanoma cells are still low. You want to catch the melanoma early.
Ericson: And I hear that you are also developing ways to monitor response to chemotherapy. Tell me about that.
Wang: We have an active grant working on early prediction of chemotherapy response. The current practice for chemotherapy is that the entire treatment may last about eight weeks. We can't really tell the response very well using standard imaging techniques. And as a result, sometimes the patient goes through the better part of the course only to find out this is not the right drug for this patient. Any chemotherapy has a systemic side effect. So that can be on the one hand very costly and on the other hand very painful – especially when it's not efficacious to this patient. So we have to add functional imaging capability to monitor this response. And by using photoacoustics, we can provide functional parameters such as the oxygen saturation. By adding the functional parameters, the hope is that we will be able to predict. Our goal is to detect the response within the first two weeks of chemotherapy. If this is the right drug, we should continue the course. If it's not, we should change course.
Ericson: And your group was one of the first to begin development of photoacoustic imaging. When was that?
Wang: In 2003, our lab published the first functional brain imaging in small animals, and that really excited the whole field. The field has more than quadrupled in size since then. So there are multiple groups in the world exploring different potential applications while they develop the technologies further. You're talking about full-body small-animal imaging all the way down to capillary-level imaging through photoacoustic microscopy. And there are preclinical applications as well as clinical applications. We can study drug screening to discover new drugs. We can study brain functions. We can image gene activities through reporter genes. We can detect molecular features specific to cancer. We can also miniaturize our device, making it small enough to be used for monitoring of the gastrointestinal tract and colon cancer.
Ericson: How soon can we walk into our doctors' offices and get photoacoustic tomography?
Wang: We have started doing clinical experiments, with more coming soon. Unfortunately, this is a long developing process. The good news is that both large companies and small companies are commercializing photoacoustic tomography in various forms.
Ericson: Dr. Wang, thank you for joining us today. It was very interesting.
Wang: My pleasure.
Ericson: If you are interested in finding out about photoacoustic tomography and the work of Dr. Wang and his lab, you can go to the lab’s Web site. It's oilab.seas.wustl.edu. That's the address for the optical imaging laboratory in the School of Engineering and Applied Science at Washington University in St. Louis. Again that's oilab.seas.wustl.edu.