Melanin

Traditional medical imaging technology has long struggled to obtain clear pictures of patients with Fitzpatrick skin phototypes II to V. This can make certain medical conditions more difficult to diagnose, monitor, and treat. Photoacoustic (PA) imaging using linear array transducers has traditionally adopted epi-photoacoustic imaging, where optical fibers are attached to an ultrasound transducer to illuminate tissue through the skin. While the illumination mode is necessary, it suffers from limitations. Muyinatu Bell, director and founder of the Photoacoustic and Ultrasonics Systems Engineering Lab at Johns Hopkins University, along with colleagues, set out to further understand why this phenomenon occurs and find a way to improve medical imaging for those with darker skin tones. In a newly published study in Photoacoustics , the researchers detailed their findings, including how short-lag spatial coherence (SLSC) beamforming may be the answer. Different skin tones can bias PA target visualization as melanin is an optical absorber within the body which distorts the signal. The PA waves generated due to light absorption at the skin surface can then cause it to act as an ultrasound emitter, transmitting sound waves beyond the skin and creating artifacts in the images, called clutter, according to the researchers. This clutter increases with the amount of melanin content in the skin. “Skin optical absorption coefficients are mainly dictated by melanin content in the epidermis layer,” the researchers wrote. “Therefore, increased epidermal melanin content results in greater skin optical absorption coefficients. It is expected that PA images from darker skinned individuals will suffer from both lower light fluence delivered within the imaging plane and higher levels of clutter artifacts. These limitations are assumed to be the two main contributing factors to compromise the resulting PA image quality obtained with darker skinned individuals, introducing bias within PA imaging based on the epidermal melanin content.” To understand the impact of melanin content on conventional amplitude-based PA images and investigate whether SLSC may reduce clutter, researchers — who collaborated with colleagues in Brazil who had previously used on of Bell’s algorithms — tested the forearms of 18 volunteers with skin tones ranging from light to dark. They used an imaging system consisting of a Nd:YAG laser (Brilliant B; Opotek) and a trifurcated optical fiber bundle (77,536; Newport) attached to a linear array transducer (L14-5/38; Ultrasonix) with a center frequency of 7.2 MHz and 128 elements. The results showed that compared to the conventional PA images, using a signal-to-noise ratio — a scientific measure comparing signal with background noise — improved imaging for all skin tones when using SLSC while performing medical imaging. Of note, this technique was originally used for ultrasounds, but can be applied to PA imaging. Image courtesy of Photoacoustics Referring to an image included in the study, researchers said, “The SLSC PA images in Fig. 4 contain reduced clutter and better visualization of the RA [radial artery] when compared to the amplitude-based PA images. In addition, the SLSC PA images qualitatively have good visualization of the RA and a low level of clutter artifacts, for both the light and darker skinned volunteers. This observation is further supported by the SLSC PA images in Figs. 4(d) to 4(f) improving visualization of a small blood vessel (white arrow) previously masked by clutter in conventional PA images.” While the study wrote that skin tone bias did still exist to some extent in some of the SLSC PA images produced in the study, researchers focused on the bias reduction seen with SLSC compared to amplitude-based PA images, which they deemed as a success. “The quantifiable bias introduced by skin tone variations was successfully mitigated with SLSC… with contributing factors that can be divided into optical considerations, spatial coherence influences, and clutter reduction benefits,” according to the study. “Regarding optical considerations, while both the concentration of melanin absorbers and optical wavelength affect skin PA signal amplitude, melanin concentration appears to be more influential than optical wavelength when considering the impact on skin PA signal coherence.” With these new developments in PA technology, diagnosing, monitoring, and treating health issues in darker skinned individuals may now be more accurate, reducing previously established imaging inequity.

A team found a way to deliver clear pictures of anyone's internal anatomy, no matter their skin tone. Traditional medical imaging works great for people with light skin but has trouble getting clear pictures from patients with darker skin. A Johns Hopkins University-led team found a way to deliver clear pictures of anyone's internal anatomy, no matter their skin tone. In experiments the new imaging technique produced significantly sharper images for all people -- and excelled with darker skin tones. It produced much clearer images of arteries running through the forearms of all participants, compared to standard imaging methods where it was nearly impossible to distinguish the arteries in darker-skinned individuals. "When you're imaging through skin with light, it's kind of like the elephant in the room that there are important biases and challenges for people with darker skin compared to those with lighter skin tones," said co-senior author Muyinatu "Bisi" Bell, the John C. Malone Associate Professor of Electrical and Computer Engineering, Biomedical Engineering, and Computer Science at Johns Hopkins. "Our work demonstrates that equitable imaging technology is possible." The findings are newly published in the journal Photoacoustics. "We show not only there is a problem with current methods but, more importantly, what we can do to reduce this bias," Bell said. The findings advance a 2020 report that showed pulse oximeters, which measure oxygen rates in the blood, have higher error rates in Black patients. "There were patients with darker skin tones who were basically being sent home to die because the sensor wasn't calibrated toward their skin tone," Bell said. Bell's team created a new algorithm to process information from photoacoustic imaging, a method that combines ultrasound and light waves to render medical images. Body tissue absorbing this light expands, producing subtle sound waves that ultrasound devices turn into images of blood vessels, tumors, and other internal structures. But in people with darker skin tones, melanin absorbs more of this light, which yields cluttered or noisy signals for ultrasound machines. The team was able to filter the unwanted signals from images of darker skin, in the way a camera filter sharpens a blurry picture, to provide more accurate details about the location and presence of internal biological structures. The researchers are now working to apply the new findings to breast cancer imaging, since blood vessels can accumulate in and around tumors. Bell believes the work will improve surgical navigation as well as medical diagnostics. "We're aiming to mitigate, and ideally eliminate, bias in imaging technologies by considering a wider diversity of people, whether it's skin tones, breast densities, body mass indexes -- these are currently outliers for standard imaging techniques," Bell said. "Our goal is to maximize the capabilities of our imaging systems for a wider range of our patient population." Other authors include Guilherme S.P. Fernandes, who was a visiting doctoral student at Johns Hopkins from University of Sa?o Paulo, Brazil, as well as Joa?o H. Uliana, Luciano Bachmann, Antonio A.O. Carneiro, and Theo Z. Pavan of University of Sa?o Paulo, Brazil.

Biomedical and genetic engineers have designed a small fluorescent protein that emits and absorbs light that penetrates deep into biological tissue. Tailored to wavelengths in the near-infrared (NIR) spectrum, this protein can help researchers capture deeper, cleaner, more precise biomedical images. Biomedical and genetic engineers at Duke University and the Albert Einstein College of Medicine have designed a small fluorescent protein that emits and absorbs light that penetrates deep into biological tissue. Tailored to wavelengths in the near-infrared (NIR) spectrum, this protein can help researchers capture deeper, cleaner, more precise biomedical images. This work appeared Dec. 1 in the journal Nature Methods. Imaging deep tissues with light is challenging. Visible light is often quickly absorbed and scattered by structures and molecules in the body, preventing researchers from seeing deeper than a millimeter within a tissue. If they do manage to probe further, substances like collagen or melanin often muddy the image, creating the equivalent of background noise through their natural fluorescence. "Biological molecules naturally absorb and emit light in the visible spectrum, which is about 350 to 700 nanometers," said Junjie Yao, assistant professor of biomedical engineering at Duke. "So when using it to image deep tissue, it's like trying to observe the stars in daylight. The signals get flooded out." To wade out from these muddied waters, Yao and his collaborator Vladislav Verkhusha, professor of genetics at Albert Einstein College of Medicine, New York developed a protein that absorbs and emits longer wavelengths of light in the near-infrared (NIR) spectrum. "Tissue is the most transparent in the 700-1300 nanometer window of NIR light," said Yao. "At those wavelengths, light can penetrate deeper into a tissue, and because there is less natural background fluorescence to filter out, we can take longer exposures and capture clearer images." Verkhusha and his lab used a process called directed molecular evolution to engineer their proteins, using photoreceptors normally found in bacteria as the basis for the structure. These photoreceptors are useful for imaging research because they can switch between a silent and active state when hit with a specific wavelength of light. They can bind with biliverdin, a biomolecule that appears in high quantities in mammalian and human tissues. "We studied the structure of the biliverdin to determine how the photoreceptor would best attach to the biomolecule," explained Verkhusha. "After we understood the binding process, we carefully introduced substitutions of key parts of the molecule connecting to the biliverdin to increase the electron binding, which is necessary to obtain the red-shifted fluorescence. Last, we performed random mutagenesis followed by high-throughput screenings so the proteins would evolve and increase their brightness." The brightest found protein, termed miRFP718nano, is easily produced in cells and tissues and emits light just outside of the visible range. But while the NIR activation itself is helpful, what happens after the initial burst of activity holds even more promise for biomedical imaging. "We've seen that the NIR range can be broken up into two main zones," explained Yao. "When NIR light first hit these proteins, they emit light in the first zone, which is about 700-900 nanometers. But as they decay, the wavelength gradually gets longer, like the tail behind a comet. That's when they begin to emit light in the second NIR zone, which is from 900-1300 nanometers." In this second zone, all benefits from using the shorter wavelength, zone one NIR light are enhanced: light can penetrate tissue two times deeper, background fluorescence is significantly dimmed, and image resolution can be two to three times clearer, allowing for detailed images of complex structures. As a proof of concept, Yao and his team at Duke used an imaging technique called short-wavelength infrared (SWIR) to test the efficacy of the new protein. This process sends NIR zone-one light deep into the tissue to activate the fluorescent proteins. As the proteins decay, they emit NIR zone-two light that provides information about the structure and composition of the targeted tissues and cells that can be translated into high-resolution images. After introducing engineered miRFP718nano proteins into their animal models, the team used them to capture images of microbes in the mouse digestive tract, visualize cells in a mouse mammary gland and even track changes to inflammation in a mouse liver. All the images captured were clearer and more detailed than images made using a standard NIR zone-one imaging protein. Yao and Verkhusha are optimistic that their continued partnership will be a boon for their work in both biomedical imaging and protein engineering. As Verkhusha continues to refine and improve the fluorescent proteins and biosensors, Yao is excited to use the new tool to visualize the brain more closely and to potentially track the movement of cancer cells. "This is an exciting new front of our decade-long collaboration because we can use the imaging tools to guide the protein engineering decisions, and we can use the advanced protein engineering to improve the imaging capabilities," Yao said. This work was supported by grants from the US National Institutes of Health (GM122567, EB028143, NS111039, NS115581), the Chan Zuckerberg Initiative (226178), the Cancer Foundation Finland and Magnus Ehrnrooth Foundation, and the NIH Intramural Research Program for the Vaccine Research Center of the National Institute of Allergy and Infectious Diseases.