Infrared Thermography in Wound Care, Surgery, and Sports Medicine: A Review

Abstract

For many years, the role of thermometry was limited to systemic (core body temperature) measurements (e.g., pulmonary catheter) or its approximation using skin/mucosa (e.g., axillary, oral, or rectal) temperature measurements. With recent advances in material science and technology, thermal measurements went beyond core body temperature measurements and found their way in many medical specialties. The article consists of two primary parts. In the first part we overviewed current clinical thermal measurement technologies across two dimensions: (a) direct vs. indirect and (b) single-point vs. multiple-point temperature measurements. In the second part, we focus primarily on clinical applications in wound care, surgery, and sports medicine. The primary focus here is the thermographic imaging modality. However, other thermal modalities are included where relevant for these clinical applications. The literature review identified two primary use scenarios for thermographic imaging: inflammation-based and perfusion-based. These scenarios rely on local (topical) temperature measurements, which are different from systemic (core body temperature) measurements. Quantifying these types of diseases benefits from thermographic imaging of an area in contrast to single-point measurements. The wide adoption of the technology would be accelerated by larger studies supporting the clinical utility of thermography.

Keywords: infection; inflammation; perfusion; surgery; thermography.

Conflict of interest statement

JR-G is employed by Swift Medical Inc., as Director, Clinical Research and Validations. RB is employed by Swift Medical Inc., as Chief Medical Officer. RF is employed by Swift Medical Inc., as Director of Clinical Services – Canada. GS is employed by Swift Medical Inc., as VP Strategic Innovations. The remaining author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Copyright © 2022 Ramirez-GarciaLuna, Bartlett, Arriaga-Caballero, Fraser and Saiko.

Figures

FIGURE 1

Reflectivity and thermal imaging. The human skin behaves almost like a blackbody with emissivity ranges close to 100%. Thus, this tissue is ideally suited for infrared thermal imaging because the surrounding infrared radiation is absorbed and the output that the camera images and measures correspond almost entirely to the radiation the skin is emitting. In contrast when imaging a reflective surface such as a mirror with an index close to 0%, it is practically impossible to differentiate the radiation emitted by the object and the surrounding radiation scattered by it. In the image above, it can be seen how the skin thermal output (Sp1) can also be seen in a mirror, and how its measurement (Sp2) differs from the output of the surrounding structures (Sp3). Also, note how glasses, despite being transparent for visible light are not under IR. Images, courtesy of JR-G, were acquired using a FLIR One Pro camera.

FIGURE 2

Dependence of the emissivity on the viewing angle for several indexes of refraction. Index of refraction n = 1.5 (red solid line), n = 2 (dashed blue line). As the angle of viewing increases, the emissivity of the surface is reduced. Thus, for images acquired obliquely, the lower emissivity can lead to a reduction in the apparent temperature of the object in the thermogram.

FIGURE 3

Comparison of the thermographic pattern of two subjects with and without diabetes. The left foot of a healthy subject (A,A1) and a patient with diabetes mellitus and vascular complications (B,B1) were imaged. Striking differences in the absolute temperature distribution pattern of the foot (A,B) can be observed. These differences are magnified when the images are scaled to present temperature gradients (A1,B1). While the healthy control presents a more or less uniform temperature pattern in the hind and mid-foot, the patient with diabetes shows extensive temperature variability that is correlated to microangiopathy and neuropathy. Note that the first toe of the diabetes patient and its medial side show signs of significant vascular compromise (arrows). Images, courtesy of JR-G, were acquired using the Skin and Wound mobile app paired to a Swift Vision camera (Swift Medical Inc., Toronto, ON, Canada).

FIGURE 4

Thermographic assessment of a patient with Raynaud’s phenomenon. Clinical photographs (A,D) and infrared thermal images (B,E) of the hand and feet of a patient with Raynaud’s phenomenon (left) and a healthy subject (right) were obtained. Color gradients of the heat maps are shown in the far-right side of (B,E). Purple/blue indicates a lower temperature, while yellow/white indicates higher temperatures. Analysis of the temperature distribution across the different structures are presented in (C,F). The patient’s temperature was significantly lower to the control and exhibited a higher degree of variation, as demonstrated by wider confidence intervals. The solid line represents the mean temperature, and the shaded area, its 95% CI. Images, courtesy of JR-G, were acquired using the Skin and Wound mobile app (Swift Medical Inc., Toronto, ON, Canada) paired to a FLIR One Pro camera. Analysis of the images was performed in Swift’s dashboard.

FIGURE 5

Pressure ulcer thermographic assessment. Infrared thermography was used to assess the severity of a sacral pressure ulcer in a bedridden patient. The wound’s temperature (crosshairs) was found to be very similar to the surrounding healthy skin (asterisk). Thermal gradients close to 0°C are highly suggestive of wounds that will re-epithelize by themselves in a relatively short term. Images, courtesy of Dr. Mario A. Martinez-Jimenez, were acquired using a FLIR One Pro camera.

FIGURE 6

Deep tissue injury thermographic assessment. Infrared thermography was used to assess the severity of deep tissue injury in a patient with a healing pressure ulcer. Thermography confirmed an extensive area of devitalized tissue underneath the wound’s eschar. In addition, it also allowed the identification of significant peri-wound inflammation and more devitalized tissue (arrow) which was not apparent at clinical inspection alone. Images, courtesy of JR-G, were acquired using the Skin and Wound mobile app (Swift Medical Inc., Toronto, ON, Canada) paired to a FLIR One Pro mobile camera.

FIGURE 7

Assessment of inflammation in a patient with hidradenitis suppurativa. Infrared thermography was used to assess the severity and area of inflammation in a patient with hidradenitis suppurativa. Assessment of the area of inflammation on regular clinical photographs (left) is challenging in patients with darker skin tones, which leads to subjective patient severity scoring. In contrast, because infrared thermography images (right) are impervious to skin color, hotspots (asterisk) can easily be used to map the extent of inflammatory changes. Previous research has demonstrated that the area of inflammation and its temperature gradient highly correlate with current clinical scores, offering a powerful insight into the underlying condition. Furthermore, thermography is able to identify open wounds (arrows) and tunneling in the dermis (arrowheads) that may not be evident to clinical inspection alone. Images, courtesy of Dr. Sheila C. Wang and JR-G, were acquired using the Skin and Wound mobile app (Swift Medical Inc., Toronto, ON, Canada) paired to a FLIR One Pro mobile camera.

FIGURE 8

Assessment of a surgical site infection. Infrared thermography was used to monitor the extent of a surgical site infection following the external reduction of an open tibial fracture. The thermogram shows an area of extensive inflammation with the maximum thermal signal in the region of interest (ROI) bounded by the box. Images, courtesy of JR-G, were acquired using the Skin and Wound mobile app (Swift Medical Inc., Toronto, ON, Canada) paired to FLIR One Pro camera.

FIGURE 9

Assessment of free flap survival. Infrared thermography was used to assess survival of free tissue flaps (arrows) 2-weeks post-surgery. Restoration of the initial negative temperature gradient between uninjured tissue and the flap has been found to be highly predictive of flap perfusion and survival (A). In contrast, failing flaps show coldspots that represent areas of malperfusion (B). Interestingly, despite the clinical images of both patients showing dermal necrosis of the flaps, the thermal images show striking differences in tissue perfusion. Images, courtesy of Dr. Mario A. Martinez-Jimenez, were acquired using a FLIR One Pro mobile camera.

FIGURE 10

Thermographic imaging of a patient after endovascular revascularization. Thermograms 24 h post-surgery of a patient with severe peripheral artery disease (PAD) who underwent endovascular revascularization show a positive gradient of temperature of 4.2°C compared to the non-revascularized limb. The hotspots (arrows) are highly predictive of a successful treatment. In this patient, reperfusion was confirmed angiographically during surgery and by Doppler ultrasound at follow-up. Images, courtesy of JA-C, were acquired using a FLIR One Pro mobile camera.