Use of thermal imaging may facilitate the assessment of wound and wound healing, especially with the intent to address the risk factors discussed earlier. It is important that any proposed methodology for analyzing the thermal image must be standardized.
Typically, when using infrared thermography, the anatomical surfaces of the foot are examined to identify potential hot or cold spots where inflammation or circulatory loss is occurring, respectively. The question of how large or extensive the wound site is is addressed by examining the infrared and visible imagery to determine the shape, area, curvature, and eccentricity characteristics. Identifying the shape will focus on the pattern of the infrared signature to determine if it is round, elliptical, or oval or has a mottled appearance. Describing the base of the wound ulcer in terms of being granular, fibrotic, or necrotic is also helpful. The existence of undermining of the leading edge may indicate an interruption in the skin matrix due to excessive vertical and shear stress forces on the edges.
While this approach provides a general qualitative process for analyzing the thermal images for wounds, the need for an objective parameter (an index based on thermal profile) cannot be overstated. This is especially important when tracking the wound healing over time. The progression of tissue injury or healing can be determined by calculating a TI of the wound based on the thermal profile or thermal signature, as discussed earlier. The processed image can be used to calculate a unitless index, described as follows.
The authors propose a new tool for quantifying wound conditions, which includes the thermal features and wound size, both of which may play a key role in determining the duration of would healing. Because temperature is a surrogate marker for inflammation, the authors term this as WII for feet. The concept of a TI was first proposed by Collins and colleagues23
and was later applied to objective assessment of arthritis in clinical studies.24–26
The proposed TI for wound healing is, however, independent of these prior clinical studies. To the authors’ knowledge, this is a first case report evaluating a TI for wound healing.
is the temperature difference between the ulcer and mean foot temperature, a
is the area of the isotherm (highest or lowest temperature) in the ulcer area, and A
is the area of the wound bed. Area is calculated in terms of the pixels for this analysis. The choice of highest or lowest isotherm must be made at the beginning of the analysis and followed consistently. Currently, these features are manually extracted from the thermal image.
illustrates a typical example for calculation of TI/WII for a wound.
Calculation of TI/WII at baseline for the test subject (63 years, white male).
This assessment removes the subjective comparison of temperature. In
, the average foot temperature of 85.4°F/29.67°C was obtained by recording the temperature at six anatomical sites (metatarsal heads 1–5 and hallux). These sites are manually selected based on the evidence from clinical literature.12, 21
The active wound bed was measured to be 22,387 pixels, while the area of the lowest temperature had a pixel count of 2753. The lowest temperature was 78.4°F/25.78°C. The resultant TI was calculated to be 0.862. The image processing is typically carried out using the thermal imager’s software. Image J Software was used for postprocessing of data and feature extraction from the thermal images.
The following section describes a case report where a single patient with a diabetic foot ulcer on the plantar foot was followed up weekly until healing, and TI was measured as well as correlated with the conventional wound size measurement.