A total of 346 publications were included in this study. Figure 1 shows that the number of publications fluctuated over the past 15 years, but the overall trend was upward. It was noteworthy that the number of increases in publications surged from 2020, culminating in a peak of 33 articles in 2022. The trend of publications indicated that the application of infrared imaging technology in the field of acupuncture has received more attention recently.

From 2008 to 2023, a total of 38 countries made significant contributions to this research field (as shown in Figure 2A and Table 2). The top five countries with the highest publication output were China (169), the United States (73), South Korea (24), Germany (22) and Japan (21), while the remaining countries had fewer than 20 publications. In terms of country centrality, the United States (0.36) was in first place, followed by Germany (0.09) and China (0.08). Subsequently, we constructed cooperative network based on the quantity and correlation of publications in every country (Figures 2B,C). It can be observed from Figure 2C that the United States has a close cooperative relationship with China and England.

As shown in Figure 3 and Table 3, 303 institutions have made contributions in this field. The top three institutions with the most published literatures were Shanghai University of Traditional Chinese Medicine (20), Chinese Academy of Sciences (13), and China Academy of Chinese Medical Sciences (12). Apart from this, the top three institutions with the highest centrality were Harvard University (0.13), Beijing University of Chinese Medicine (0.11), and Medical University of Graz (0.11).

There were 423 authors who participated in the study of infrared imaging technology research on acupuncture (as shown in Figure 4 and Table 4), among whom 7 authors had published more than five articles. Figure 4 was designed to reveal the highly productive authors and the relationships between authors. The node size represents the number of articles published by the author. Litscher, Gerhard (7), and Shen Xueyong (6) were the top two productive researchers, followed by Liu Chi-Feng (5), Xiong Jing (5), and Cheng Ke (5). The figure illustrates the lack of joint publications among prominent writers who have conducted research in the field of acupuncture infrared imaging technology. As the top two productive authors in the field, Litscher, Gerhard and Shen Xueyong both used IRT to study the temperature distribution of different acupuncture points, trying to prove the existence of acupuncture points and the effectiveness of acupuncture from the infrared signature and temperature (Wang et al., 2012; Huang et al., 2013; Zheng et al., 2013). However, Litscher, Gerhard is more concerned about laser acupuncture. He has used IRT to detect the effect of laser acupuncture on acupoint temperature in several experiments (Litscher et al., 2011; Kurath-Koller et al., 2015; Lan et al., 2019). Shen analyzed different types of moxibustion from the perspective of infrared spectroscopy to explore the mechanism of moxibustion (Deng and Shen, 2013). He found that laser moxibustion has a positive effect on cancer-related fatigue and knee osteoarthritis pain (Mao et al., 2016; Li et al., 2020).

A total of 185 journals had published articles related to the application of infrared imaging technology in acupuncture. Table 5 showed the top 10 academic journals that published articles in the field of acupuncture infrared imaging technology. Evidence-Based Complementary and Alternative Medicine (36) was the most productive journal, followed by Journal of Traditional Chinese Medicine (10), Acupuncture in Medicine (7), Lasers in Medical Science (7) and Acupuncture & Electro-Therapeutics Research (6).

The cited journal map is displayed in Figure 5A. Table 6 shows the top 5 cited journals of infrared imaging technology in acupuncture. It is obviously true that Evidence-Based Complementary and Alternative Medicine (124) and the American Journal of Chinese Medicine (0.17) were the top journals in terms of frequency and centrality, respectively. These results indicated that the articles in the field of acupuncture infrared imaging technology in these two journals are highly persuasive and representative.

Using CiteSpace, a dual-map overlay with the citation and cited journals was created (Figure 5B). The left side represents the citing journals and the right side the cited journals. As shown in the map, the green paths illustrate that studies published in “medicine, medical, clinical” journals frequently reference research published in “Molecular, Biology, Genetics” and “Health, Nursing, Medicine.” Orange paths indicate studies published in “Molecular, Biology, Genetics,” “Health, Nursing, Medicine” and “Psychology, Education, social” journals that are cited for research in “Molecular, Biology, Immunology” journals.

The co-cited authors were presented in Table 4. In terms of the number of publications, Litscher G (29) ranked first among the cited authors, followed by Shen XY (21), and Rojas RF (17). Among them, Litscher G has recently focused on laser acupuncture with thermal imaging. And Shen XY was united in his commitment to laser moxibustion. Rojas RF, from the University of Canberra, was dedicated to using fNIRS to study acupuncture analgesia and its relationship with prefrontal cortex (PFC).

Research hotspots and trends in specific fields are usually obtained by analyzing keywords with high centrality and frequency. Figure 6A shows the keyword co-occurrence map. Table 7 lists the top 10 keywords for frequency and centrality. Over the past 15 years, the most frequently occurring keywords in the field of acupuncture infrared imaging technology were “Acupuncture,” “Stimulation,” “Near infrared spectroscopy,” “Blood flow,” and “Nitric oxide.” The five with the highest centrality keywords were “Acupuncture,” “Temperature,” “Model,” “Stimulation” and “Nitric oxide.”

Two (or more) papers cited by one or more papers at the same time is called reference co-citation and is used to describe the degree of correlation between the papers. Co-citation implies a strong correlation between the cited literature and the corresponding research, indicating that the literature typically encompasses high-quality content with substantial impact within a specific research domain.

Table 8 and Figure 7A presented the top five cited references. The study published by Raul Fernandez Rojas in 2019 had the highest co-citation counts, followed by Ghafoor U, (2019) and Shen XY, (2006). Fernandez Rojas et al. (2019) conducted research that identified the anesthetic effect of the Hegu point based on the hemodynamic response as measured by fNIRS. The study published by Ghafoor U and his team in 2019 observed through fNIRS that after acupuncture treatment, the mean hemodynamic response of working memory tasks and functional connectivity in the prefrontal cortex of patients with mild cognitive impairment increased (Ghafoor et al., 2019). Shen XY and his team published in 2006 revealed that thermal action of the traditional moxa stick was more potent than that of indirect moxibustion by using a highly sensitive, infrared-spectrum detection device (Shen et al., 2006).

The fourth article was published in 2017 by Raul Fernandez Rojas, which indicated that potential applications of fNIRS in the development of a physiologically based diagnosis of human pain that would benefit vulnerable patients who cannot self-report pain (Fernandez Rojas et al., 2017). The last article was a randomized controlled trial published in 2016 by Lee-Mei Chi, which identified the effectiveness of cupping therapy (CT) in changes in skin surface temperature (SST) for relieving chronic neck and shoulder pain (NSP) among community residents (8).

Citation burst refers to citation being cited a lot in a short period of time. Through the citation burst, we can focus on the research hotspots of this period and judge the future development trend of this field. Figure 7C presents the top 5 references with strong citation burstiness, four of which had the same results as the highly co-cited references. Two articles published by Raul Fernandez Rojas in 2017 and 2019 received widespread attention starting in 2019 and 2021, respectively, and continue to this day.

To investigate the clusters of a cited-references, the log-likelihood test (LLR) was used to extract clusters from keywords in the literature. A total of 28 clusters are generated. In order to better visualize, we only show the first 12 clustering categories in the timeline view (Figure 7B). The modular Q of all clusters was 0.935, with an average silhouette of 0.9841. This shows that it is a reliable, high-quality cluster graph diagram. From Figure 7B, it is obvious that the topics studied by these two clusters (#0 cortical activation, #1 pathological state) were recent research foci and may also become future research trends.

The “Burst words” represent keywords that are cited frequently over a period, thereby indicating the frontier areas. Figure 6B shows 15 burst keywords sorted by the “begin year.” As displayed, the related investigation started from the keyword “nitric oxide” from 2012 and lasted until 2017. And then, “laser acupuncture” became popular among researchers from 2013 to 2017. Currently, five keywords have become burst ones and last until now: quality, functional near-infrared spectroscopy, diagnosis, system and functional connectivity.

We analyzed the co-occurring keyword maps and found that “Temperature” and “Blood flow” ranked highly. A study on moxibustion found the increase in temperature led to an elevation in blood flow velocity due to the decrease in viscosity (Huang and Sheu, 2013). According to acupuncture theory, the therapeutic mechanism involves the stimulation of specific acupuncture points and meridians. Our study found that infrared imaging technology is often used to explore the physiological specificity and pathological changes of acupoints. Some studies believed that acupoint skin temperature may serve as a useful indicator of the health of nearby tissue and could even influence the choice of acupoints (Reddy et al., 2023). There were many studies using infrared technology to assess differences in acupoint skin temperature between patients and the general population, the temperature difference between the unaffected and affected sides or the temperature difference in the patient’s own treatment (Liu et al., 2020; Li et al., 2021; Reddy et al., 2023; Tu et al., 2023; Wang et al., 2023). An article published in 2012 shows asthma patients had significantly lower overall infrared radiation intensity at the left Taiyuan (LU 9) than that of healthy volunteers which suggested that IRT detection of acupoint temperature has a potential role in the diagnosis of diseases (Zhou et al., 2012). A systematic review suggested the possibility of thermally distinct between healthy and sick states at specific acupoints (Yang et al., 2022). In another study, the infrared thermal image was used to compare the temperature differences of the same acupoints on the bilateral sides to objectively assess the severity of Bell’s palsy (Liu et al., 2011). These findings hold significant implications for enhancing diagnostic accuracy, optimizing treatment point selection, and forecasting disease prognosis within clinical settings.

Nevertheless, due to the types of diseases studied and the limitations of research methods, there is no clear evidence that changes in skin temperature at specific acupoints predict pathological conditions. In the field of acupuncture infrared imaging technology, a large number of high-quality studies are still needed to explore the correlation between point temperature and pathological changes. Interestingly, in some studies, the specificity temperature of acupoint or meridians by infrared exploration is also seen as an exploration of traditional Chinese medicine (Yang et al., 2009; De Souza et al., 2016; Li et al., 2022).

In the co-occurring keyword maps, we found “Near infrared spectroscopy” and “pain” occupied an important position. Several systematic reviews have shown that acupuncture is a suitable treatment for relieving pain and improving quality of life (Manyanga et al., 2014). However, the neural mechanisms by which acupuncture alleviates pain have not been well understood. Pain can affect oxyhemodynamics in the prefrontal and sensory-motor cortices of the brain (Hall et al., 2021). Meanwhile, fNIRS can be used as a non-invasive neuroimaging method to provide long-term measures of cortical hemodynamics by measuring real-time changes of oxygenated (oxy-Hb) and deoxygenated hemoglobin (deoxy-Hb) (Peng et al., 2018). Moreover, fNIRS is superior to other brain imaging techniques such as fMRI in anti-motion interference. Recently, several studies have attempted to explore the therapeutic mechanisms of acupuncture analgesia using fNIRS. Yoshiki Morikawa’s study utilized near infrared spectroscopy (NIRS) and electrocardiography (ECG) to monitor prefrontal hemodynamic activity and autonomic activity. The results suggested that myofascial trigger points (MTrP) compression in the neck area modifies the activity of the autonomic nervous system via the prefrontal cortex to alleviate subjective pain (Morikawa et al., 2017). In the process of treating biliary colic with acupuncture, N. Sun and his team monitored brain neural activity with fNIRS, and correlation analysis will be conducted to investigate the relationship between pain relief and peripheral-cerebral functional changes (Sun et al., 2022).

However, its therapeutic mechanisms are still uncertain, partly because there is no method for providing a nonverbal objective assessment of pain. Therefore, some studies indicate that in the field of acupuncture, fNIRS has great potential for objective pain assessment in the clinical environment (Ding et al., 2016; Peng et al., 2018; Hall et al., 2021; Hu et al., 2021; Du et al., 2023). A systematic review and meta-analysis concluded Pain affects the prefrontal and sensory-motor cortices of the brain and can be measured by using fNIRS (Hall et al., 2021). Recent advance reported a complete suite of automated techniques for the computerized assessment of thermal images are employed to assess pain related thermal dysfunction (Herry and Frize, 2004). The objective assessment of pain by fNIRS helps to optimize the diagnosis of pain and the evaluation of the effectiveness of treatment, which is of great benefit to patients with specific pain, such as children, who are unable to self-report it. In addition, there are studies that suggest the use of fNIRS to predict the occurrence of pain (Hu et al., 2023).

By analyzing the keyword bursts and the main content of the cited paper bursts, we have gained new insights into the future trends in this field. From 2017 to 2023, the main bursts keywords changed to functional near-infrared spectroscopy, quality, diagnosis, system and functional connectivity. The authors of the two strength burst references in recent years were both Raul Fernandez Rojas. Recent advances in fNIRS in acupuncture have focused on the effects of acupuncture interventions on cortical networks.

Functional connectivity refers to the statistical association or correlation between the activities of different brain regions. It reflects the degree of synchrony or coherence in neural activity between distinct brain regions. Functional connectivity analysis can provide valuable information on how the functional structure of the brain changes in response to stimulation. It promotes a deeper understanding of the mechanisms by which acupuncture improves brain function. In recent years, fNIRS has gained more attention by exploring the effects of acupuncture on brain regions and the function of functional connections between brain regions. A study observed the network connections with bilateral PFC as nodes showed significantly increased functional connectivity during acupuncture manipulation compared with controls through fNIRS (Si et al., 2021). Acupuncture is capable for increasing functional connectivity in dorsolateral PFC and primary somatosensory cortex thought fNIRS (Cao et al., 2023). A RCT including 70 patients employed the fNIRS to observe that, compared to patients taking antidepressants alone, depressed patients getting acupuncture in addition to antidepressants report improvements in their depression symptoms as well as greater resting state functional connectivity in the dorsolateral prefrontal cortex (DLPFC). Although it’s yet unknown if acupuncture lowers depression symptoms by altering DLPFC connectivity or if the altered connectivity is only a result of acupuncture’s therapeutic effect., it still offers an unbiased measurement to get beyond the issues with single-blinding in acupuncture-related depression research in the future (Wong et al., 2021).

PFC is the region of the cerebral cortex which covers the anterior portion of the frontal lobe. PFC receives input from all other cortical regions and functions to plan and direct motor, cognitive, affective, and social behavior across time. Research has indicated that acupuncture can be therapeutically applied to a range of neuropsychiatric conditions, such as neuropathic pains, Alzheimer’s disease, and Parkinson’s disease (Liang et al., 2014; Miranda et al., 2015; Jiang et al., 2018). Besides the clinical therapeutic effects, acupuncture is promising for enhancing human cognition and memory, and regulating emotion processing for healthy people (Hui et al., 2005; Zheng et al., 2018). However, the specific neural mechanism of acupuncture on the cerebral cortex remains unclear. Increasing evidence indicates that PFC’s adenosine triphosphate (ATP) level plays a role in the pathogenesis of depression. A study suggested that electroacupuncture (EA) can modulate extracellular ATP levels in the PFC of depressive-like MS rats, contributing to its antidepressant effects (Zheng et al., 2023). Another study using inflammatory bowel disease rat models showed that EA therapy has an alternative clinical use in the relief of visceral discomfort by lowering glutamine toxicity in the PFC (Jiang et al., 2023).

In this review, we discussed the application direction of infrared technology in the field of acupuncture, explored the relationship between acupoints and temperature, and provided potential help for the evaluation of clinical diagnosis and treatment effect of acupuncture. To explore the possibility of objective evaluation of pain with fNIRS and serve acupuncture clinic. In addition, our findings provide a new perspective on the use of fNIRS to explore the effects of acupuncture on functional connectivity between brain regions.