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Foot Thermography Background Information


Key Features in Thermal Assessment

Two different colour scales

The Colour Scale

  • The colour scale must be interpreted
  • Absolute scale:
    • a fixed scale with the same range for all thermograms (here: 18℃ to 35℃)
    • advantage: same for all thermograms so different scans can be directly compared
    • disadvantage: contrast of colours may be low if foot has a narrow range of temperatures only
  • Dynamic scale:
    • colours are optimised for temperature range of current image (here 16℃ to 27℃)
    • advantage: high contrast, easy to spot small differences in temperature
    • disadvantage: colours can mislead when comparing scans: same colour ≠ same temperature

It is important to note that a contact making device such as Podium is not thermometric, i.e. it does not measure the absolute temperature of the foot. On contact with the thermal sheet the foot cools (if the sheet is colder than the foot) or warms (if it is warmer). As a result the absolute foot temperature changes. This is often advantageous, e.g. when cooling provides a mild "cold stress" that can serve to highlight areas of inflammation. Relative temperature patterns are thus preserved or emphasised which may be helpful when interpreting the resulting thermograms.

The Histogram

  • Shows the distribution of temperatures in a thermogram
  • Sorts image pixels with the same temperature into "bins"
  • Each bin is represented by a horizontal line
  • The longer the line, the bigger the area of that temperature
  • Two histograms are shown side by side:
    • one for the left foot
    • one for the right foot
    • allows assessment of left/right temperature symmetry (see further below)

As outlined above Podium thermograms are non-thermometric by design. The numbers on the histogram scale are therefore provided as relative indications and do not represent absolute temperature values.

Image of a thermal histogram
The normal foot: average and standard deviation

The Healthy Foot

  • Plantar foot temperature of healthy people is characterised by:
    • medial arch being between 0℃ to 7℃ warmer than the toes
    • left-right (contralateral) mirror-symmetry better than 1.8℃ (>2.2℃ is frequently seen as a warning sign)
    • the 2nd to 5th toe have approximately the same temperature
    • there are no hot spots
    • the average foot temperature is around 26℃ at the toes and 29℃ at the medial arch

The image shows the average plantar foot temperature of 103 healthy subjects who were resting at 22℃ for 10 minutes after taking their socks and shoes off. Reproduced from Kluwe B., Early detection of diabetic foot ulcers using thermal imaging, PhD Thesis, University of South Wales, July 2018

Thermal Variability

  • Healthy feet are thermally symmetrical but different between individuals and over time
  • The lowest variation, i.e. the most stable temperature region, is the medial arch
  • The most variable parts are the toes and the heel

The image shows the variation of temperatures in the 103 healthy individuals within a 1 standard deviation interval. It is noticeable that the temperature at the toes is far more variable between individuals than that of the medial arch. Reproduced from Kluwe B., Early detection of diabetic foot ulcers using thermal imaging, PhD Thesis, University of South Wales, July 2018

Variability in foot temperature
Foot symmetry graph

Thermal Symmetry

  • Healthy feet are thermally highly symmetrical
  • Asymmetry may be an early sign of pathology
  • May be useful to pick up early signs of pathology
  • Additional measure to support clinical assessment
  • Provides an objective indicator of inflammation

The figure shows a histogram of the left-right differences at 33 distinct points in the feet of 103 healthy subjects (resulting in 3,399 data points). The width of the interquartile range (IQR) is 0.9℃, i.e. 50% of all aspects in one foot differ by less than 0.45℃ from those of the other foot, 99% less than 1.8℃. Reproduced from Macdonald A., Petrova N. et al.

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