Optical tools for measuring blood oxygenation
At the level of your entire body or any particular organ, blood oxygenation provides a vital sign of health or disease. It is recognized as the fifth vital sign, after temperature, blood pressure, pulse rate, and respiratory rate. Blood oxygenation happens thanks to hemoglobin, a protein in red blood cells that carries oxygen from your lungs to the rest of your body.
Based on the absorption of light by hemoglobin, optical tools make it possible to detect blood oxygenation and flow. The pulse oximeter is a well-known example. Simple to use, it clips onto a finger (or toe or earlobe) and peers beneath the skin noninvasively to provide continuous, vital information about levels of oxygen in the blood. An essential item for reducing complications in emergency medicine, it’s also routinely used in clinics and at home — screening newborns for heart disease, tracking conditions like sleep apnea, and more.
Beyond pulse oximetry, many newer optical technologies are emerging to detect hemoglobin-based biomarkers and facilitate real-time care for a wide range of debilitating illnesses — from rheumatoid and vascular diseases to neurodegenerative disease and cancer. These methods vary widely — in terms of not only sensitivity and accuracy, but also imaging depth and resolution, as well as physical size and cost — which can make it challenging to select the appropriate tools and understand the relative limitations of different methods.
A group at the University of Cambridge offers a helpful guide to advance the field, with a review of optical tools for noninvasive hemoglobin sensing and imaging, published in the Journal of Biomedical Optics (JBO). Directed by Sarah E. Bohndiek, professor of biomedical physics, the team describes and compares techniques for detecting hemoglobin-based biomarkers, weighing factors that influence their practical application.
Pulse oximetry and beyond
Starting with pulse oximetry, the team explores a variety of methods, indicating their strengths and limitations, as well as clinical uses or directions for research and development. Pulse oximetry typically uses alternating LEDs at two different wavelengths to obtain a ratio of oxygenated and de-oxygenated hemoglobin states, based on the way light is absorbed by body tissue, such as a fingertip. Because of the way the light pulses alternate over time, the measurements are vulnerable to motion. Also, skin pigmentation affects light absorption, resulting in less accurate measurements for patients with darker skin. The authors review the record of research and offer perspective on this evolving technology, as it develops toward better-calibrated and more efficient devices.
Some of the other technologies they explore in this comprehensive work include tools for reflectance imaging of hemoglobin. While light can be transmitted through tissue and detected on the other side by a sensor that measures how much light was absorbed, it can also be reflected so that it bounces back to the light detector. This allows the creation of a kind of 3D map based on the qualities of the detected light, which can also provide vital information beyond oxygenation, such as vascular morphology and disfunction.
Reflectance methods include point-scanning spectroscopy, multispectral imaging, and hyperspectral imaging. These methods find a growing number of clinical uses, from nailfold capillaroscopy for real-time imaging of capillaries and blood flow, to endoscopy and retinal imaging. Because hemoglobin can bind to several other molecules (depending on environment, medications, etc.) and occurs in several structural varieties, numerous options for imaging and sensing are possible.
Where pulse oximetry and reflectance-based imaging fall short, depth-resolved imaging methods can achieve depths beyond a centimeter. Some of these methods are promising for detecting breast cancer as well as other cancer types, plus a wide range of other scenarios where deeper imaging information can contribute to diagnosis — from evaluating inflammation in arthritic joints, to guiding surgical procedures. These technologies can operate on different principles, including photoacoustics (detecting light absorption through sound), diffuse optics, or optical coherence tomography. Each has its strengths and limitations. For instance, whereas the scattering of light in the process of reflectance hinders some imaging techniques, diffuse optical imaging uses that phenomenon to obtain information. Techniques can be used separately or in combinations to obtain complementary results.
Bohndiek and team recognize that the cost and complexity of various technologies, as well as ease of use and interpretability of data, will determine how well they catch on. They highlight new directions for research to overcome present limitations.
According to Brian Pogue, Editor-in-Chief of JBO and Director of the Department of Medical Physics at the University of Wisconsin—Madison, “These techniques are at the very core of what has been the most successful in biomedical optics, pulse oximetry and blood oxygenation measurement. The key to advancing this field is to push for improvements to the approaches and technologies that measure deeper, or measure new physiological parameters. This review does a wonderful job of describing this and pointing out new key directions.”
As these light-based technologies develop, they will alter the landscape of healthcare, by increasing equitable access, enabling self-monitoring, and enhancing the detection and treatment of disease.
Read the Gold Open Access article by M. Taylor-Williams et al, “Noninvasive hemoglobin sensing and imaging: optical tools for disease diagnosis,” J. Biomed. Opt. 27(8), 080901 (2022), doi 10.1117/1.JBO.27.8.080901
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