Traditional Chinese physiotherapy, primarily represented by acupuncture and cupping, has been employed for thousands of years to treat musculoskeletal diseases and relieve pain symptoms. Acupuncture is popular worldwide owing to its remarkable curative efficacy and safety. However, the theory of traditional Chinese medicine fails to provide substantial scientific evidence to clarify the mechanisms underlying acupuncture treatment. Moreover, most acupuncture therapies lack effective scientific assessment during treatment, failing to ensure their efficacy and safety. Optical imaging can be used to propagate light in cells and tissues, combine various molecular probes to image organs, and safely obtain robust biological information. Optical imaging is optimal for observing the vascular structures of biological tissues, as well as for monitoring local hemodynamic changes. Optical imaging techniques employed to examine acupuncture primarily include laser Doppler blood perfusion imaging (LDPI), laser speckle imaging (LSI), near-infrared spectroscopy (NIRS), and photoacoustic imaging (PAI). Considerable progress has been made with regard to measuring hemodynamic effects, brain response, therapeutic mechanisms, and the curative effect of acupuncture. However, a systematic summary of these findings is lacking. This review helps readers in the field of traditional Chinese medicine to establish a comprehensive understanding of diverse optical imaging techniques and outline their recent advancements in acupuncture research.
This review briefly introduces the characteristics of different types of optical imaging and their progress in assessing acupuncture. In addition, their limitations and development directions are summarized.
LDPI and LSI use non-contact data acquisition, with advantages such as non-invasiveness and rapid scanning and eliminating certain hidden dangers generated by contact imaging systems. Measuring the improvement in blood circulation at specific sites using LDPI and LSI can assess the efficacy of acupuncture and reveal its therapeutic mechanism. LDPI and LSI can be employed to measure blood flow perfusion of specific regions in real-time, observe changes in internal organs during acupuncture, and verify the correlation between meridians and internal organs. However, the shortcoming of LDPI and LSI should be noted. Owing to the limited penetration depth, research on acupoints and internal organs is only performed on small animals. To determine the microcirculation effect of acupuncture on human viscera, LDPI and LSI should be combined with nuclear medical imaging technologies such as PET.
NIRS probes can be easily attached to the skin surface owing to their small size. Currently, NIRS is employed to monitor real-time oxygen levels in the muscle and brain during acupuncture. Owing to its economic advantage, convenience, and safety, functional NIRS (fNIRS) is suitable for examining the changes in hemodynamic parameters in the target area in clinical practice, aiding therapists in effectively evaluating the treatment effects. In recent years, fNIRS has gradually been established as an important supplement to traditional brain functional imaging technologies [such as functional magnetic resonance imaging (fMRI)]. fNIRS has been primarily applied to verify the specificity of acupoints in specific brain regions. However, owing to the skull, human respiration, heartbeat, and other factors, the signal quality of fNIRS has failed to reach the ideal state, accompanied by delayed changes in blood oxygen signals in different regions; this presents a considerable challenge for examining the acupuncture-mediated brain effects using fNIRS. Future developments in fNIRS will focus on improving the filtering algorithm, suppressing physiological interference and random noise, and improving signal delay.
PAI combines the advantages of optical and acoustic imaging, easily surpassing the 1 mm penetration depth limit of traditional optical imaging and allowing simultaneous high-resolution and high-contrast imaging. PAI can also monitor blood volume, hemoglobin concentration, blood oxygen saturation, and other tissue indicators while determining structural images. PAI is primarily used to observe the sensitization of acupuncture points, changes in cerebral blood flow perfusion, and cerebral vascular morphology during and after acupuncture. In addition, acupuncture can be used to assist various optical probes, thereby improving the sensitivity and contrast of PAI in the brain. However, traditional PAI requires a couplant to achieve imaging, which is unsuitable for acupuncture. In the future, with the development of air-coupled PAI technology, non-contact PAI will overcome the limitations of traditional PAI and play a role in clinical research assessing acupuncture.
Herein, we summarized the characteristics of different optical imaging methods and their application scope (Table 1). Although optical imaging has facilitated the elucidation of underlying mechanisms and efficacy of traditional Chinese acupuncture therapy, some limitations are known to persist. As the meridian system of the human body is markedly complex, most current studies only select certain formal acupoints. Additional investigations are required to examine changes induced by simultaneously stimulating multiple acupoints and verify the specificity of each acupoint in the human body. In future research, scientists should continue exploiting optical imaging in combination with other imaging methods, such as fMRI or PET, to examine the acupuncture-mediated brain effects, as well as effects on energy metabolism and receptor expression in various regions of the body. This strategy would further reveal the therapeutic mechanism of acupuncture and establish complete guidance, which would benefit a large patient population.
