Advancing Chronic Wound Care With Near-Infrared Spectroscopy Imaging: Clinical Applications, Measurement Parameters, and Insights Into Healing Dynamics

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Vascular health is central to wound healing, with oxygen required for almost every step in the healing process.^1,2 Despite the importance of vascular assessment, traditional vascular assessment methods, such as pulse volume recording (PVR), ankle-brachial index (ABI), toe brachial index (TBI), and transcutaneous oxygen pressure (TcPO₂), remain limited in their ability to provide real-time insights into the status of microvascular perfusion and oxygenation at the wound site. For example, ABI has conceptual limitations, such as compromised accuracy in individuals with arterial calcification.³ While PVR and TBI are often preferred in patients with calcified arteries,^4,5 TBI is limited in patients with forefoot amputation, and PVR may lack accuracy in detecting distal arterial disease or in patients with congestive heart failure or low stroke volume. Moreover, PVR, ABI, and TBI evaluate macrocirculation in the larger vessels and do not provide insights into microcirculation.⁶ TcPO₂ measurement is used to evaluate microcirculation; however, it is limited to measuring periwound oxygenation because the electrodes must be placed on intact skin. 

The microvasculature (vessels <100 µm in diameter) is essential for wound healing; it delivers the oxygen and nutrients required for tissue repair.^6,7 Near-infrared spectroscopy (NIRS) imaging offers real-time assessment of tissue oxygenation and microcirculation at wound sites and surrounding areas. This literature review of NIRS imaging in wound care highlights relevant clinical end points and key measurement parameters. The review was conducted by a panel of experts under the auspices of the Wound Care Collaborative Community (WCCC) Tools Work Group (TWG). The objective of the review is to identify, describe, and illustrate NIRS modalities, their measurement parameters, and their clinical applications.

The electromagnetic spectrum encompasses a wide range of wavelengths, including the near-infrared (NIR) region.⁸ The NIR region is adjacent to the visible region and overlaps with the infrared (IR) region, which extends from about 0.6 µm to 1 mm-2 mm (Figure 1).

Spectroscopic techniques encompass a broad range of technologies that leverage spectroscopic principles. These techniques capture interactions between specific portions of the electromagnetic spectrum and the object (eg, human tissue). Spectral imaging combines spectroscopy with imaging, generating datasets consisting of multiple images of the same object, each captured at different wavelengths or spectral bands (range of wavelengths). NIRS imaging is a type of spectral imaging that utilizes the NIR region of the electromagnetic spectrum.

NIR light can penetrate biological tissues. A study utilizing mathematical modeling of human skin demonstrated that a wavelength of 633 nm penetrates to 1.8 mm, 660 nm penetrates to 2.0 mm, 850 nm penetrates to 2.4 mm, and 900 nm penetrates to 2.5 mm.⁸ Thus, NIRS penetration depth is commonly referenced as being approximately 2 mm to 3 mm. 

NIRS measures the tissue absorption and scattering of NIR light based on the fundamental principle that wavelength-dependent signal attenuation (absorption) in an object (eg, skin) is directly proportional to the concentration of chromophores. Multiple chromophores exist in skin, but the main chromophores of skin at the NIR region are oxygenated hemoglobin (HbO₂), deoxygenated hemoglobin (dHb), water, and melanin. Most NIRS oximetry imaging systems employ at least 2 distinct wavelengths, 1 above and 1 below 810 nm, because 810 nm serves as an isobestic point where the absorption spectra of dHb and HbO₂ intersect (Figure 2).⁹

One of the first milestones in the development of NIRS imaging was the introduction of pulse oximetry in the 1970s.¹⁰ Pulse oximeters measure peripheral oxygen saturation (SpO₂), that is, the percentage of oxygen-saturated hemoglobin in the blood, focusing on systemic blood oxygen levels. By employing 2 wavelengths—red (660 nm) and IR (940 nm)—pulse oximeters calculate the proportion of HbO₂ to total hemoglobin (tHb = HbO₂ + dHb) using the Beer-Lambert law. The formula is: SpO₂ = HbO₂ / (HbO₂ + dHb). 

Following pulse oximetry, the development of fiber-based NIRS technologies enabled measurements of tissue oxygen saturation (StO₂) directly beneath the optical fibers, providing a localized assessment of microcirculation oxygenation. Unlike the pulse oximeter’s systemic approach, fiber-based NIRS technologies focus on
local tissue health and perfusion. Fiber-based NIRS measures StO₂ by detecting the absorbance of NIR light by HbO₂ and dHb using the same formula but represented as: StO₂ = HbO₂ / (HbO₂ + dHb). 

Building on the legacy of pulse oximetry and fiber-based NIRS, modern NIRS imaging techniques enable noncontact, real-time microcirculation imaging over larger tissue areas while retaining the foundational principle of measuring StO₂ through hemoglobin NIR absorbance. Among the most notable NIRS imaging techniques are hyperspectral imaging,^11-15 multispectral imaging,¹⁶ and spatial frequency domain imaging,¹⁷ each with unique features and distinct practical and methodological considerations. 

What is common among these techniques is that they all use wavelengths in the visible and/or NIR regions to obtain 2-dimensional maps of StO₂ in the wound bed and surrounding tissue. Table 1 lists key features of these techniques.

The US Food and Drug Administration (FDA) classifies devices using a rules-based approach. For example, 21 CFR Part 870.2700 applies to oximeters, with the product code MUD designated for tissue saturation oximeters.¹⁸ These devices are risk designated Class II, requiring premarket notification, a 510(k) submission. According to FDA identification (§ 870.2700), an oximeter is a device that transmits radiation at specific wavelengths to measure oxygen saturation based on the amount of reflected or scattered radiation.¹⁹

The FDA 510(k) premarket submission process requires submitters to demonstrate that the device is as safe and effective as—meaning substantially equivalent to—a legally marketed predicate device. Figure 3 illustrates the regulatory pathways and predicate devices for several MUD-designated tissue saturation oximeters, including HyperView Hyperspectral Tissue Oxygenation Measurement System (HyperMed Imaging, Inc), SnapshotNIR (Kent Imaging Inc), MIMOSA Imager (Mimosa Diagnostics Inc), and Clarifi Imaging (Modulated Imaging Inc [dba Modulim]).

Typically, NIRS devices emit light at wavelengths ranging from approximately 600 nm to 1000 nm, though this range is not standardized. For all of these devices, as part of FDA clearance, publicly available information typically includes the specific wavelengths (or wavelength range) used. For example, devices may specify individual wavelengths such as 620 nm, 630 nm, 700 nm, 810 nm, 880 nm, and 980 nm, or may provide a range, such as 6 wavelengths between 600 nm and 1000 nm. In NIRS, wavelengths and their specific values are reported because they directly relate to the absorbance and reflectance properties of biological tissues, corresponding to key absorption bands. 

The primary parameters assessed via NIRS include HbO₂, dHb, StO₂, and tHb (Table 2).

Some NIRS imaging systems report HbO₂, dHb, and tHb, and a few can report healing index or some other metrics, but all MUD products must report StO₂. This standardized focus on a single, easily interpretable metric simplifies clinical assessments and supports quicker decision-making.

Percentage area reduction (PAR) and percentage volume reduction measurements are known clinical end points, with the 4-week PAR serving as a prognostic indicator for evaluating the efficacy of treatments.^35-38

While NIRS is not used to measure PAR, it can provide complementary circulatory information that, when integrated with PAR, offers a more comprehensive view of wound healing dynamics. For example, Kounas et al²⁴ evaluated the ability of HbO₂ levels to predict diabetic foot ulcer (DFU) healing in a cohort of 27 patients and observed an inverse correlation between HbO₂ and the percentage of ulcer size reduction. 

Longobardi et al³⁹ conducted a single-
center randomized controlled trial involving 81 adults (aged ≥18 years) with hard-to-heal venous leg ulcers (VLUs). The study evaluated simultaneous measurements of StO₂ and wound dimensions to predict the healing trajectory under protocols using conventional treatment alone or in combination with hyperbaric oxygen therapy. The findings concluded that NIRS analysis of StO₂ and wound area is effective in predicting the healing course of VLUs. 

NIRS imaging offers real-time, objective data on tissue oxygenation and perfusion, making it particularly useful for monitoring wounds of varying etiologies and assessing the efficacy of various treatment modalities. Table 3 provides examples of clinical applications of NIRS in wound care.

Studies show that higher StO₂ levels are associated with better healing potential (Figure 4). Landsman⁵³ found that wounds with an average StO₂ level below 40% across the wound bed rarely achieved closure. Nouvong et al¹³ reported that the mean (SD) StO₂ measured from the dorsal foot was 53% (13%) in DFUs that healed, compared with 47% (12%) in those that did not heal. Similarly, Philimon et al²⁸ found that acute wounds had a mean (SD) StO₂ of 71.38% (7.07%), while chronic wounds had significantly lower levels (56.48% [6.27%]) (P < .05), indicating that chronic wounds, which tend to heal more slowly or fail to heal, are characterized by lower oxygenation. In healthy individuals, Suludere et al⁵⁴ found that mean StO₂ levels ranged from 64% to 73% on the dorsal foot and from 63% to 72% on the plantar foot, providing a baseline for normal tissue oxygenation. Arnold and Marmolejo⁵⁵ educated clinicians on the proper interpretation of point-of-care NIRS imaging in daily wound care practice.

NIRS tissue oxygenation images are typically displayed using a jet color map, in which blue indicates low oxygenation and red signifies high oxygenation. As shown in Figure 4, the gradient transitions from blue/green (low) to yellow (mid-range) to orange/red (high). 

Significant increases in oxygen saturation during the early stages of treatment are also strongly linked to positive response to treatment and improved healing outcomes. In a series of 5 patients with nonhealing wounds, Cole and Woodmansey⁵¹ observed that wounds with a greater than 10% increase in StO₂ within the first week of starting treatment achieved complete healing by 3 to 5 weeks. In contrast, wounds with a smaller increase of 3% to 7% StO₂ within the first week did not heal within 5 weeks. 

NIRS is based on the principles of light-based imaging, and all light-based imaging techniques are susceptible to various light-related artifacts. One such challenge is the effect of skin tone on imaging results. While NIRS imaging is expected to reliably extract physiological parameters for lighter skin tones (Fitzpatrick skin types I-IV),⁵⁶ the high absorption of NIR light by melanin in darker skin can distort NIRS readings.^57,58 In skin with high melanin content, increased absorption of NIR light occurs, reducing the amount of light that penetrates deeper tissues and is reflected back to the sensors. This elevated absorption can result in artificially lower tissue oxygenation measurements. Algorithms can improve NIRS measurements across all skin tones by labeling different skin tones to enable melanin correction in tissue oxygenation maps.^54,59-61 The most common methods for accounting for melanin absorbance include the use of a melanin index, with which the relevant portion of the spectrum is used to estimate melanin content and a corrective factor applied accordingly.⁵⁷ Alternative approaches rely on image-based analysis combined with deep learning or machine learning algorithms to classify skin tone. Whatever approach is used, NIRS devices should be validated across diverse skin tones to confirm that their correction strategies are effective.

Other artifacts arise from motion, whether from the subject or the equipment, introducing unwanted noise or distortions in the spectral data. Movement-related artifacts affect all imaging systems to some degree, prompting the development of advanced mitigation strategies such as motion detection algorithms and post-processing techniques. 

Another group of artifacts arises from geometric factors, such as convex curved surfaces with different depth planes, leading to inaccurate tissue oxygenation measurements on curved or angled surfaces. Curvature correction models have been recently developed to address this issue.⁶² However, the challenge remains that commercial systems rarely disclose the specifics of the techniques and algorithms they use. 

A practical way to mitigate the influence of artifacts (eg, melanin) in real-world applications is to use relative comparisons. This may involve tracking changes over time, such as before and after a procedure (eg, debridement) or between clinic visits, or assessing responses to physiological challenges, such as provocative maneuvers.⁴⁴ Anatomical comparisons may also be used, such as contrasting the affected area with the contralateral side (eg, left leg vs right leg) or with adjacent healthy tissue, or by applying site-specific indices such as the plantar-palmar index.⁶³

Artifacts caused by motion or geometric factors cannot typically be corrected by the end user in practical settings. However, they are often visually distinctive, allowing users to recognize and address them by simply retaking the image. This underscores the importance of proper training and the user learning curve in acquiring reliable images.

The FDA 2025 draft guidance document titled “Pulse Oximeters for Medical Purposes: Non-Clinical and Clinical Performance Testing, Labeling, and Premarket Submission Recommendations” provides recommendations for nonclinical and clinical performance testing to support premarket submissions for pulse oximeters intended for medical purposes.⁶⁴ However, the draft guidance does not address oximeters under the product code MUD. The authors of the present review believe there is a need for greater standardization in this area. Standardized performance information from manufacturers would benefit both clinicians and payors. 

NIRS imaging is emerging as a valuable tool in advancing wound care, with significant potential for future development. A major innovation on the horizon is integrating artificial intelligence (AI) with advanced medical imaging to create predictive wound healing models as a decision support tool.^65-67 Combination of NIRS and AI offers clinicians real-time insights into healing potential, enabling faster and more accurate treatment decisions.⁶⁸

Innovative spatiotemporal NIRS-based approaches, which are capable of mapping oxygenation flow patterns over time and space, hold promise for a deeper understanding of wound dynamics.⁶⁹ Spatiotemporal NIR-based imaging methods can map oxygenation flow patterns around DFUs, identifying areas in need of urgent care.

The miniaturization of NIRS technology into smartphone-based imaging devices for point-of-care applications represents another exciting advancement.^70-72 Such portable systems could bring real-time monitoring to bedside care, improving accessibility and efficiency, especially in underserved or remote settings. Furthermore, combining NIRS imaging with other diagnostic methods, such as thermal or fluorescence imaging, can enhance wound healing monitoring. For example, smartphone thermography has been used to assess wound healing in second-degree burns by detecting improved blood flow to the burn site, demonstrating high validity in evaluating both healing potential and progression.⁷³ In another study, tissue oxygenation and thermal maps were used together to assess the healing status of DFUs.⁷⁰ This integrated approach allowed for the differentiation between healing and nonhealing DFUs by providing complementary physiological data.

An interesting future direction for NIRS could be its application in the early detection, prevention, or treatment of pressure ulcers/injuries. Wywialowski⁷⁴ highlighted tissue perfusion as a key underlying factor in the development and treatment of pressure ulcers. Andersen et al⁷⁵ conducted a study involving 15 patients (16 wounds), including 3 pressure injuries, using serial imaging to track wound changes over time. Wahab and Lapucha⁴⁰ discussed the clinical applications of NIRS in chronic pressure injuries, demonstrating that NIRS is a valuable adjunct for wound diagnosis and for selecting the appropriate advanced treatment modality. Bates-
Jensen et al⁷⁶ and Wyss et al⁷⁷ investigated the effects of external pressure on tissue oxygenation. 

The wound biome, comprising the microbiome and its biochemical interactions, is also critical to advancing chronic wound management.⁷⁸ NIRS imaging can complement biochemical analysis by providing real-time, noninvasive data on tissue oxygenation and perfusion, contextualizing microbial and metabolic activity. Emerging research also suggests that NIRS could theoretically identify microbes via spectral signatures, potentially enabling a single system to assess physiological and microbial traits.⁷⁹ Integrating NIRS-derived StO₂ with proteomic or metabolomic profiling can yield a holistic view of wound healing, enhancing diagnostic precision and personalized treatment.

Although several NIRS imaging systems are available on the US market, detailed pricing information is rarely publicly disclosed. Predicate devices such as T.Ox (ViOptix Inc) have been reported to cost around $25 000, with additional per-use expenses of approximately $1450 for disposable probes.⁸⁰ In contrast, modern NIRS imaging systems are designed without the need for disposable components,⁴⁶ eliminating ongoing consumable costs and potentially reducing overall long-term expenditures. Looking ahead, cost optimization remains an important factor for enabling future broader adoption of NIRS imaging in both clinical and research settings.

As NIRS technology advances, it is poised to play an even more integral role in early detection, monitoring, and guiding treatment decisions, thus significantly enhancing care pathways for those at risk of chronic wounds. Thus, the authors of the current review, as participating members of the FDA-approved WCCC TWG, will follow up this review with surveys to manufacturers of MUD FDA-listed devices used by wound practitioners to further elucidate their capabilities for use in research and clinical practice, as well as other adoption-related metrics. 

This literature review of NIRS imaging in chronic wound care is limited by its reliance on published sources, which may not fully reflect the most recent technological advancements or unpublished methodologies. To help address these gaps, the authors of this review are conducting follow-up surveys with manufacturers of FDA-listed MUD devices to further clarify system capabilities. Additionally, the nonsystemic nature of this review presents inherent limitations. Its primary focus is on describing NIRS modalities, measurement parameters, and clinical applications, and it lacks the level of critical appraisal found in systematic reviews.

This comprehensive review underscores the value of integrating NIRS imaging into routine clinical practice, with NIRS imaging offering noninvasive, real-time evaluation of tissue oxygenation and perfusion. By combining traditional wound assessment metrics, such as PAR, with the advanced capabilities of NIRS real-time imaging, health care providers can personalize care using enhanced diagnostic accuracy, optimized treatment strategies, and patient engagement to ultimately improve patient outcomes. As the prevalence of chronic wounds continues to rise, the adoption of innovative tools like NIRS imaging will play an essential role in addressing this growing public health burden.