The correspondent of this paper is Professor V. Wang, a famous biomedical optics expert at the University of Washington, USA. Professor Wang is currently the chairman of the International Biomedical Optics Association and a professor of "Changjiang Scholar" at Huazhong University of Science and Technology. Professor Wang has made many achievements in biomedical optical imaging technology, and has published 2 monographs and hundreds of papers in Nature Biotechnology, Physical Review Letters, Physical Review Letters and IEEE Transactions.
Professor Wang and doctors from Washington University School of Medicine applied four photoacoustic imaging techniques in clinic, one of which can observe sentinel lymph nodes, which is of great significance to the occurrence and staging of breast cancer. Another imaging technology can monitor the body's early response to chemotherapy, the third technology can image melanoma, and the last one can observe the digestive tract.
The most exciting thing is that photoacoustic imaging can reveal the oxygen utilization of tissues, because excessive oxygen combustion (called hypermetabolism) is an important symbol of cancer. Professor Wang said that because the early cancer has not spread, it will change the diagnosis of cancer without contrast agent for early warning diagnosis.
The most exciting use of photoacoustic imaging is to detect oxygen metabolism, which is a major sign of cancer, and it will bring us an earlier and more effective diagnosis method. )
Photoacoustic imaging principle
Although we have accepted the gray photos obtained by X-ray imaging, this is only a sparse substitute for the "photos" inside our bodies. However, because photons can only penetrate the soft tissue for about a millimeter, and then they will be scattered out, and it is impossible to analyze their paths and obtain graphics, so they can only accept such pictures.
But scattering does not destroy photons, and these elementary particles can reach a depth of 7 cm (about 3 inches). The method of photoacoustic imaging is to convert deeply absorbed light into sound wave, which is 1000 times lower than light scattering. This can be achieved by irradiating the imaging tissue with nanosecond pulsed laser with a specific light wavelength.
That is to say, when biological tissue is irradiated by broad beam and short pulse laser, the absorber (such as tumor) located in the tissue absorbs pulsed light energy, which leads to temperature rise and expansion, resulting in ultrasonic waves. At this time, the ultrasonic detector located on the tissue surface can receive these emitted ultrasonic waves and reconstruct the image of light energy absorption distribution in the tissue according to the detected photoacoustic signals.
It can be seen that photoacoustic imaging technology detects ultrasonic signals and reflects the difference of light energy absorption, so it can well combine the advantages of optical and ultrasonic imaging technologies. Moreover, because ultrasonic signals are detected, this technology can overcome the shortcomings of pure optical imaging technology in imaging depth and resolution. Moreover, because the image difference of photoacoustic technology comes from the light absorption difference of tissue, it can effectively supplement the defects of pure ultrasound imaging technology in contrast and functionality.
In addition, unlike X-rays, light does not pose any threat to health, and the contrast of photoacoustic imaging is higher than that of X-ray imaging. Color molecular images, including hemoglobin, can be obtained by "endogenous" contrast agents, which change color with the gain and loss of oxygen, melanin and DNA, and DNA is darker in the nucleus than in the cytoplasm.
With the help of "exogenous (introduced)" contrast agents, such as organic dyes or genes that can express color molecules, photoacoustic imaging can also image tissues, such as lymph nodes, which are easily confused with the surrounding environment. Professor Wang also used the reporter gene to encode colored substances, and achieved good results.
Generally speaking, photoacoustic imaging is a nondestructive biological photon imaging method based on the difference of light absorption in biological tissues, which combines the advantages of high contrast of pure optical imaging and high penetration depth of pure ultrasonic imaging, and replaces photon detection in optical imaging with ultrasonic detector, which avoids the influence of optical scattering in principle and can provide high contrast and high resolution tissue images. It provides an important means for studying the structural morphology, physiological characteristics, metabolic function and pathological characteristics of biological tissues, and has a wide application prospect in biomedical clinical diagnosis and imaging of living tissue structure and function.