In just a decade, optoacoustic or photoacoustic imaging has become one of the fastest growing areas of biomedical technology, exploding from just a handful of research groups in the late 1990s to more than 400 dedicated scientists and engineers today. Much of the expansion has come since researchers started pairing optoacoustics with the already-existing technology of medical ultrasound and envisioning how it could easily step into the clinic to fight a diversity of diseases.
An established mainstay of the clinic, ultrasound sends acoustic waves into tissue, measures the reflections of those waves, and produces images of anatomical structures. “Optoacoustics adds molecular and functional information that can be used not only in disease detection and diagnostics but also in the monitoring of therapeutic interventions,” says optoacoustic imaging pioneer Alexander Oraevsky. “Researchers can accomplish this in many ways, and that’s why everyone is busy. We can envision just so many different approaches and clinical applications.”
Case Study: Breast Cancer
Several of the field’s research groups are concentrating on breast cancer because it is a major public health concern and ultrasound is well suited to reaching and detecting tumors in breast tissue. When optoacoustics is added, the combined technology provides even more insight into the tumors, says Oraevsky, who is the founder, president, and chief technology officer (CTO) of the Houston, Texas-based TomoWave Laboratories, an adjunct professor in the Biomedical Engineering Department at the University of Houston, and former faculty member at Rice University and the University of Texas (UT) at Galveston.
While ultrasound shows doctors the shape of the tumor and the surrounding morphology, which provides an indication of whether it is malignant or benign, the reliability of such a method of diagnostics is far from 100%, Oraevsky explains. “What optoacoustic images add is functional information about the density of the angiogenesis-related tumor microvasculature and also the blood oxygen saturation, i.e., the ratio of oxyhemoglobin and hemoglobin concentrations,” he describes.
A high density of angiogenesis and a low oxygen saturation in the tumor signify the presence of an aggressively growing tumor, which needs nutrients and oxygen, so the tumor develops its own blood vessel network to satisfy those requirements. By looking for microvascular networks as well as oxygen depletion in the blood, doctors can make a much more reliable diagnosis than with ultrasound alone, he says. “Neither technology is sufficient, but the two together provide a powerful tool to help doctors understand what kind of tumor it is and determine the best course of treatment.”
One of Oraevsky’s current NIH-sponsored projects involves producing three-dimensional (3-D) images of cancerous tumors in the breast (figures above) in collaboration with the group of Prof. Mark Anastasio at Washington University in St. Louis, and the Department of Diagnostic Radiology at the University of Texas MD Anderson Cancer Center in Houston (directed by Prof. Wei Yang) , . To do that, TomoWave engineers designed and produced the Laser Optoacoustic Ultrasonic Imaging System Assembly (LOUISA), capable of providing 3-D full-view optoacoustic tomography, which will be further enabled with the laser-generated ultrasound
In optoacoustics, laser pulses are delivered into the tissue, and differences in the optical absorption generate the acoustic signals, which are measured to produce images of blood distribution in and around the breast tumors. Typical ultrasound involves the use of a transducer probe containing piezoelectric crystals that vibrate when an electrical signal is applied. That reverberation produces ultrasound waves that are transmitted into the tissue, bounced back to be measured, and used to reconstruct an image. Unfortunately, that reverberation also affects the image resolution, so Oraevsky and the research group at TomoWave recently developed laser-generated ultrasound. Here, short laser pulses produce the ultrasound waves but do not generate reverberations, so the resulting image is far clearer with a better contrast and resolution. Together, ultrasound and optoacoustic images produce coregistered functional and anatomical maps of the breast with high sensitivity, high resolution, and minimal artifacts or distortions.
Seno Medical Instruments has acquired the rights for the first core patent by Oraevsky and is presently leading the commercialization of a handheld two-dimensional optoacoustic and ultrasound device. TomoWave is working to bring the next-generation 3-D system to the market. The first clinical prototype of LOUISA-3D is undergoing testing at MD Anderson Cancer Center in Houston, and Oraevsky estimates that a commercial version will enter the regulatory approval process in 2016.
Case Study: Prostate Cancer
Sanjiv “Sam” Gambhir and his research group at Stanford University are blending ultrasound, photoacoustics, and contrast imaging agents in their work to develop a transrectal device and system that is able to differentiate aggressive from nonaggressive prostate tumors and determine which tumors actually need to be removed , . Unlike most other tumors, prostate cancer tumors often grow so slowly that they do not pose any threat to life. While that sounds like great news, current technology, notably ultrasound, is not all that good at definitively distinguishing a slow grower from an aggressive tumor that can be fatal, says Gambhir, professor and chair of radiology at Stanford.
Similar to the approach Oraevsky is using for breast cancer, Gambhir and his research group designed a device that employs both ultrasound and photoacoustics, using photoacoustics to detect oxyhemoglobin and deoxyhemoglobin, which provides the molecular information to help identify a dangerous tumor. Rather than an ultrasound wand that is passed over the body, however, Gambhir’s device is inserted into the rectum. This carried a unique set of engineering requirements, some of which were met through the use of capacitive micromachined ultrasonic transducer (CMUT) arrays developed in Pierre Khuri-Yakub’s Stanford lab. CMUT arrays are ultrasonic transducers based on capacitance changes rather than piezoelectricity. “We were able to adapt the CMUT array technology to a small, flexible, transrectal, photoacoustic instrument and then couple it to the optics and a back-end laser to deliver the light,” Gambhir says, noting that the development of the instrument took them nearly five years.
“We began recruiting patients for a clinical trial of the instrument in November 2014 and are already getting what I think are the first-in-man photoacoustic images of the prostate,” he asserts. An initial pilot trial will help optimize the instrument, and a subsequent, larger clinical trial will answer questions about its limitations, sensitivity, and specificity. “These questions will take some time to answer because we will have to look at normal prostates, prostates with benign disease, and prostates with malignant disease, and then prove that what the instrument sees is what’s actually there at the time of surgery.” He anticipates that the clinical trial will take one to two years to complete.
As that work advances, Gambhir and his research group are busy on the next phase. “If we also inject imaging agents—and we and other research groups have been developing an assortment of photoacoustic imaging agents that absorb light—these agents can latch onto selected cancer targets,” Gambhir says. Light shone on the prostate is absorbed by the agents, which will return sound waves that can be used to identify the aggressiveness of a tumor. While that phase is still in its infancy, he adds, “This technology will fill a unique niche beyond what ultrasound can do in the prostate and hopefully become a significant player in prostate cancer management in the years to come.”
Case Study: Sentinel Lymph Nodes
Photoacoustics, ultrasound, and contrast agents are also being used to find sentinel lymph nodes, the lymphatic system organs believed to be a major conduit through which a metastasizing cancer will spread beyond a primary tumor . This work is under way in the lab of Lihong Wang, Gene K. Beare Distinguished Professor of Biomedical Engineering at Washington University in St. Louis, Missouri (below).
Currently, patients with breast cancer typically go through a procedure to determine the stage of their cancer, a measure of cancer spread. In this procedure, the surgeon injects methylene blue dye or its equivalent, plus some radioactive materials that travel through the body’s lymphatic system and toward the sentinel lymph node. Using a radioactive detector, they can then establish the approximate location of the sentinel lymph node and surgically remove it for a histological analysis to see whether it contains cancer cells. If it does, the cancer has metastasized.
While the procedure works, it has potential complications, such as nerve damage, muscle damage, and edema or swelling of the disturbed area. Wang’s view was that there should be a better approach, so he and his research group began developing a method that utilized the methylene blue dye to pinpoint the sentinel lymph node but eliminated the radioactive materials.
They collaborated with global health-innovation company Philips Research to adapt one of the company’s standard, handheld ultrasound probes to add photoacoustics capabilities (below). The first step was flanking the probe with optical fiber bundles for the delivery of a single laser shot. The next step was adding a separate data-accessing apparatus to tap the suite of raw, multichannel, radio-frequency data collected by the probe. The resulting device is able to access all of the necessary data for photoacoustic imaging in about 100 microseconds. Wang says, “Because it is extremely fast, we minimize motion artifacts altogether.”
The device is now capable of detecting the sentinel lymph node with very high resolution, and once the node is located, the same device is then used to guide a biopsy needle to the node so that cells can be removed and tested for the presence of cancer. This eliminates the need to excise the whole node as well as the possible side effects associated with this procedure.
Wang and his research group have already tested the system’s ability to find the sentinel lymph node and guide the biopsy needle and are now exploring the optimal cytological testing procedure to check for cancer cells. He notes, “If we can prove this works, we would have converted what is currently a surgical procedure into a needle-biopsy procedure that can be done on an outpatient basis.”
Case Study: Atherosclerosis
The applications for photoacoustics go beyond cancer to other health conditions, including work under way in the lab of Stanislav Emelianov, professor of biomedical engineering at UT at Austin, to battle atherosclerosis. Called intravascular imaging, the technology includes an ultrathin, light- and sound-delivering catheter that enters an artery in the groin area and snakes through the blood vessels to reach the coronary artery . Its mission is to find and differentiate atherosclerotic plaques that could lead to a heart attack or stroke.
In intravascular imaging, ultrasound reveals anatomical features of the arteries, such as vessel wall irregularities or reduced lumen (vessel opening) that signify the presence of plaques, while photoacoustic imaging exposes other information that is critical for treatment decisions, according to Emelianov, who is also the founder and director of UT’s Ultrasound Imaging and Therapeutics Research Laboratory.
One vital piece of information is the volume of the so-called lipid pool in the plaque. This assemblage of lipids is covered by a thin cap, which may rupture when weakened by macrophages that are part of the body’s immune response. Lipid pools that make up 40% or more of the overall volume of the plaque are vulnerable to rupture, which causes the formation of a clot, Emelianov says. “So, when physicians and interventional cardiologists detect the plaque, they immediately want to know how thick is the fibrous cap of the plaque, how much lipid it has, and whether macrophages are active there so they can treat the disease in a much more informed and reliable manner.”
To calculate the lipid pool volume, Emelianov’s research group first needed to develop a flexible catheter that not only contained an ultrasound transducer and an optical fiber for photoacoustics but also rotated at 1,800 r/min so it could view the entire lumen. Once they had a working catheter, they took advantage of the very distinct optical absorption properties of lipids, singling them out by irradiating blood vessels at specific wavelengths (1,720 and 1,210 nm). “By comparing the anatomical map of the plaque from the ultrasound with the photoacoustic map of the lipids, we can easily calculate the volume of the lipid pool,” he says.
Unlike lipids, macrophages lack distinctive optical absorption properties and are, therefore, essentially invisible. His lab, which already had a reputation for developing contrast agents, went to work to craft options. Once injected intravenously, macrophages gobble up the new contrast agents “like candy,” he describes. Next, they image the vessels with photoacoustics at wavelengths targeted to the contrast agents and reveal the active macrophages.
In all, the technology provides a full landscape: a view of the plaque, the volume of the lipid pool, and the extent of active macrophages. “When I show the images of all three to clinicians, they get really excited and ask, ‘When can you come to us with this device?’ But it will take time.”
Although Emelianov and his research group have a working catheter and are testing it in animals, human clinical studies are still in the planning stages. He anticipates that the catheter will be available for the clinic in three to five years, with the addition of the contrast agents following in about five to ten years.
Researchers in this field can easily list multiple ways to use combined ultrasound and photoacoustics to combat disease. “We are taking the same instrument we’re developing for prostate cancer and using it transvaginally for ovarian cancer. We’re also using it to image the thyroid gland,” Gambhir says. “In addition, we have a variant of it that is even more miniaturized, so we can use photoacoustics in the intraoperative setting to guide the surgeon.” For the latter, he and his research group are building a toolbox of imaging agents to complement a photoacoustics instrument that would allow surgeons to separate tumor from nontumor cells when removing cancerous tissue from the body. This would not only help surgeons ensure that they are removing all of the cancerous cells but also help them avoid cutting into healthy tissue, he explains. “For this work, we’ve designed the instruments, they’re being tested in animal models now, and we hope to start the first human intraoperative imaging later in 2015.”
At this point, the dominant application for combined photoacoustics and ultrasound is breast cancer, but many other areas are under investigation, Wang notes. In fact, his research group was awarded a grant from the March of Dimes to use photoacoustics to image the function of the cervix as a means to determine whether a woman is susceptible to preterm birth.
The marriage of photoacoustics and ultrasound has great promise, but it is not the only approach. Vladimir Zharov is a researcher at the University of Arkansas for Medical Sciences (UAMS) who has developed a technology using in vivo photoacoustic flow cytometry to scan the blood for circulating tumor cells , , . “In cancer, 90% of cancer deaths are not from the primary tumor. They result from metastasis, from circulating tumor cells (CTCs) that spread from the primary tumor,” says Zharov, director of the Arkansas Nanomedicine Center at UAMS and a biomedical engineering professor in the UAMS Department of Otolaryngology.
Currently, the typical procedure for detecting CTCs is to test a small sample of a patient’s blood. If a CTC is identified from that sample, a positive diagnosis of cancer is made. A patient, however, has 5 L of blood, so it is very possible that the small blood sample may not contain one of the existing CTCs, especially early in disease progression when just a few of them are in the bloodstream. “With this type of sampling, you can miss up to 10,000 CTCs that may be circulating elsewhere in the blood,” Zharov explains. By the time a CTC is discovered through such sampling, the disease has usually already progressed, “and it’s very difficult to treat multiple metastases.”
He and his research group set a goal to detect CTCs at the earliest possible stage when therapies are most effective, and they are doing it with flow cytometry, a technique that sends fluids, such as blood, through a cell-counting, cell-sorting machine.
Zharov’s idea was to augment flow cytometry with photoacoustics so they could envision the cells in vivo through the skin of the hand as the blood coursed past, a technique that can dramatically (up to 10,000-fold) increase diagnostic sensitivity. Nanosecond, high-rate laser pulses continuously pass through the skin to a depth of a few millimeters to pick up the passing CTCs, and a specially created software provides an analysis of the collected data. An ultrafast signal acquisition algorithm increases in vivo blood cancer test sensitivity and simultaneously minimizes physiological artifacts. Together with a newly designed transducer and other innovations, Zharov and his research group developed a clinical prototype to generate real-time information. The prototype can currently scan 1 L of blood in about an hour. The prototype is now about two years into clinical trials with melanoma patients (without a need for labels due to high intrinsic melanin contrast), and based on funding, Zharov expects to bring it to the clinic in next two years.
While cancer and CTCs are an ideal venue for the device, it can also detect other types of circulating cells, microparticles, and protein clusters that are indicative of additional diseases, Zharov says. “Every disease cell releases some markers into the blood, and if we can detect these markers, we can provide early diagnosis of many diseases,” he notes. That includes various pathogens, such as antibiotic-resistant Staphylococcus aureus, malaria, and others; cardiovascular disorders; or sickle cell anemia. He calls this concept of diagnosis “in vivo reading written in blood.” His team plans to develop a universal platform for the early diagnosis of multiple diseases and a portable, hand-worn, watch-like device to predict stroke and heart attack.
Zharov and his research group are also working on contrast agents such as gold and magnetic nanoparticles conjugated with antibodies so that difficult-to-spot cells will stand out for easy identification, further isolation, and molecular analysis. They have already proven this concept by testing the blood of breast cancer patients ex vivo. In a preclinical study, he notes, they also demonstrated that the laser can kill CTCs through nanobubble formation without harmful effects on the surrounding normal blood cells. In addition, new technology offers noninvasive detection of biomarkers in lymphatics, cerebrospinal fluid, and bone vasculature.
He adds, “In vivo flow cytometry is a completely new concept of early disease diagnosis, and the only way we are able to do it in humans is by using photoacoustics.”
- E. K. I. Galanzha and V. P. Zharov, “Circulating tumor cell detection and capture by photoacoustic flow cytometry in vivo and ex vivo,” Cancers, vol. 5, no. 4, pp. 1691–1738, Dec. 2013.
- D. A. Nedosekin, V. V. Verkhusha, A. V. Melerzanov, V. P. Zharov, and E. I. Galanzha, “In vivo photo switchable flow cytometry for direct tracking of single circulating tumor cells,” Chem. Bio., vol. 21, no. 6, pp. 792–801, June 2014.
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These are just a few examples of this burgeoning field. “It really has become one of the most prominent and quickly growing biomedical technologies today,” Oraevsky says.
Wang adds, “Technologies have been commercialized for animal imaging, which is what we call preclinical imaging, by multiple companies. Human applications are the next big step, and they are coming.”
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