Invasive medical procedures, such as those requiring local anesthesia, often involve the risk of nerve damage. During surgery, the surgeon may accidentally cut, stretch, or compress a nerve, especially if the nerve is mistaken for other tissue. This can cause patients to experience long-term symptoms, including sensory and motor problems. Likewise, patients receiving nerve blocks or other types of anesthesia can suffer nerve damage if the needle is not at the correct distance from the target peripheral nerve.
Therefore, researchers have been working hard to develop medical imaging techniques to reduce the risk of nerve damage. For example, ultrasound and magnetic resonance imaging (MRI) can help surgeons pinpoint the location of nerves during surgery. However, distinguishing nerves from surrounding tissue in ultrasound images is challenging, and MRI is expensive and time-consuming.
Johns Hopkins University researchers highlight the potential of multispectral photoacoustic imaging to prevent nerve damage during invasive medical procedures and identify key wavelengths for optimal nerve visualization.
Photoacoustic images of pig ulnar nerve (left) and median nerve (right) were recorded in vivo for the first time. The nerve was illuminated with 1725nm light and superimposed on the confocal ultrasound image. Also shown are the outlines of the nerve and surrounding agarose region of interest (ROI). Source: M. Graham et al., doi10.1117/1.JBO.28.9.097001
TA GPH14Prospects of Photoacoustic Imaging
A promising alternative in this regard is multispectral photoacoustic imaging. As a non-invasive technology, photoacoustic imaging combines light and sound waves to produce detailed images of human tissue and structures. Essentially, the target area is first illuminated with pulsed light, causing it to heat up slightly. This in turn causes the tissue to expand, emitting ultrasound waves that are picked up by the ultrasound detector.
A Johns Hopkins University research team recently conducted a study in which they thoroughly characterized the absorption and photoacoustic characteristics of neural tissue across the entire near-infrared (NIR) spectral range. Their research results were published in the Journal of Biomedical Optics on September 4 and were led by Dr. Muyinatu A. Lediju Bell, associate professor of John C. Malone and director of the PULSE Laboratory at Johns Hopkins University.
One of the main goals of their research is to determine the ideal wavelength for identifying neural tissue in photoacoustic images. The researchers hypothesized that wavelengths of 1630-1850 nanometers, located within the near-infrared-III optical window, would be the optimal wavelength range for nerve visualization because the lipids in neuronal myelin have a characteristic absorption peak in this range.
To test this hypothesis, they performed detailed optical absorption measurements on peripheral nerve samples. They observed an absorption peak at a wavelength of 1210 nanometers, which belongs to the near-infrared-II band. However, this absorption peak is also present in other types of lipids. In contrast, when the contribution of water is subtracted from the absorption spectrum, neural tissue shows a unique peak in the near-infrared-III range at 1725 nm.
Practical testing and impact
Additionally, the researchers performed photoacoustic measurements of peripheral nerves in live pigs using a custom-made imaging device. These experiments further confirmed the hypothesis that using peaks in the near-infrared-III band can effectively differentiate between lipid-rich neural tissue and other types of tissue as well as water-containing or lipid-poor materials.
Bell is satisfied with the results, saying: "Our work is the first to use broad wavelength spectroscopy to characterize the optical absorbance spectrum of fresh pig nerve samples, and is also the first to use multispectral photoacoustic imaging in the near-infrared-III window to demonstrate in vivo visualization of healthy and regenerated pig nerves."
These findings could inspire scientists to further explore the potential of photoacoustic imaging. Additionally, characterization of neural tissue light absorption profiles can help improve neural detection and segmentation techniques when using other optical imaging modalities.
"Our findings highlight the clinical promise of multispectral photoacoustic imaging as an intraoperative technique that can be used to determine the presence of myelinated nerves or prevent nerve damage during medical interventions, and may have implications for other optical-based techniques. Therefore, our contribution successfully establishes a new scientific foundation for the biomedical optics community."