How can I use ThermoHuman metrics?
If we want to evaluate the skin temperature in humans we can use infrared thermography with different methods.
An experimented thermographer might be able to perform an immediate analysis using the qualitative method, which gives us the possibility to examine the thermal image interpreting the colours. It is fast and very intuitive, but risky, because it is based on the subjective interpretation of the technician that reads colours that might be modified easily using the scale, so quite easy to underestimate or overestimate a colour spot.
On the other hand, we have the quantitative method, based on the radiometric data contained within the pixels of the thermal image, allowing us to carry out a reproducible, reliable and comparable analysis through a software. When using the quantitative method, the main challenge that we face is the variability of the skin temperature due to the influence factors, which forces us not to focus on absolute temperatures (for instance: this knee is 28,5ºC). Among relative temperatures, thermal asymmetry is the most solid and used metric nowadays.
The human body is designed to maintain a balance, in bio-medical sciences this concept is known as homeostasis. Thermoregulation is one of the main systems ruled by this principle. That is why, authors as Uematsu (1988) has showed in asymptomatic normal individuals that “the degree of thermal asymmetry between opposite sides of the body (AT) is very small”, with values under 0.38ºC. The thermal differences between bilateral regions of interest (ROI), with maximum or average temperatures have been shown as a valid method in several studies (Formenti et al. 2018)
That is why we use thermal asymmetries from the first image. In ThermoHuman software, we created a classification scale highlighting with different colours thermal asymmetries above 0.3ºC, so it is very intuitive to spot areas that are not in a thermal balance with a simple glance.
AVERAGE THERMAL ASSYMETRY
Thermal asymmetry (showed as Asymmetry in ThermoHuman) compares the average skin temperature of one ROI with the bilateral one. We recommend using this metric mainly if you are analysing a subject for the first or in the following evaluations. In this example we observe the first evaluation of an athlete, who practices a collective sport that involves high-intensity movements. ThermoHuman avatars show us significant asymmetries (above 0.3ºC) on the left ankle and foot, and on the posterior right thigh area that might be produced by that demand of repeated efforts of his sport.
MAXIMUM THERMAL ASYMMETRY
The analysis of the maximum thermal asymmetries (showed as Asymmetry (Max) in ThermoHuman) is an interesting alternative. It follows the same principle as average thermal asymmetry, but comparing the maximum temperature data within the ROI with its bilateral area. Both metrics (maximum and average thermal asymmetries) are validated and show similar results.
This metric is especially useful when we find pathologies that generate a localized and significant increase of skin temperature (what we know as a hot spot) but that does not affect the entire ROI and might not create an alarm using average thermal asymmetry. In the next example, we see the case of feet affected by papilloma virus (on the right first metatarsal joint). On the left, we observe no significant average thermal asymmetry on that ROI, while if we analyse it with the maximum asymmetry we obtain a very relevant alarm as we see in right avatar.
“Not all asymmetries means injury”, this is one of the quotes that we use the most. Neutralized asymmetry is the metric that helps us to identify better if a thermal asymmetry is relevant or might not.
Although Uematsu (1988) and other authors showed, in some cases, we find thermal asymmetries that are above 0,3ºC (even much more) on areas with no pain, previous injury or affected by influence factors. With just one image, it is hard to differentiate, but if we have the chance of assessing someone frequently we might observe consistent and repeated alarms in some ROIs.
This metric requires several evaluations to establish, as long as there is no pain or injury, a localized an individual threshold based on the historical average of each ROI. That is what we know as building an individualized thermal profile. An example of the usefulness of this metric is to neutralize the asymmetries generated by an asymmetric sport such as tennis or judo where the forearm of the grip will present a hyperthermic asymmetry produced by adaptation to the sport itself (Arnaiz-Lastras et al. 2011)
In the following example, we see the tracking report of a subject with a consistent asymmetry alarm on his left knee. Since he is pain free and not injured, when we choose the neutralized asymmetry option, the alarm on the knee just appears in sixth evaluation, when it is really significant.
COEFFICIENT OF VARIATION
Using only thermal asymmetries (regardless of it is average, maximum or neutralized asymmetry) has two main limitations: firstly, we are not able to detect bilateral issues; and secondly, we mainly focus on the warmer area, but we do not know if it is actually because this ROI is getting warmer or the bilateral is getting colder.
Coefficient of variation is the perfect metric to solve those issues and complement the use of thermal asymmetries. As it also happens with neutralized asymmetry, the coefficient of variation requires several thermograms to build a consistent thermal profile over the time. It basically allows us to analyse the thermal tendency of one ROI regardless of the bilateral ROI and the absolute temperature, it is a metric based on the historical average temperature an standard deviation of assessments done without pain, injury or influence factors.
In the following example we can observe the first tracking report showing the average asymmetry avatars from a soccer player. It is remarkable the thermal asymmetry on the left knee. When we use the coefficient of variation (second image) we can clearly see on the second evaluation that both ankles and knees were getting warmer (regardless of the asymmetry). On the fourth evaluation, the left knee was warmer than the right one (that is what the asymmetry is telling us) and the coefficient of variation complements it showing a hypothermic tendency, that is to say: the left knee is warmer than the right one but getting colder.
This is key factor to understand the thermoregulation behaviour of the different tissues, because the coefficient of variation allows us to understand over the time, if a certain asymmetry is the result of one ROI increasing temperature, the opposite decreasing or even both tendencies as the same time. A very practical example is what happens when a muscle injury occurs: it normally creates an asymmetry, but on the opposite region. Why? Because in case of significant asymmetries, ThermoHuman always highlights the warmer ROI, but it does not mean that the problem is rigth there (take a look of our publication about the importance of hyperthermia an hypothermia). Coefficient of variation might help you to understand that what was happening is that the injured ROI is getting colder, and not the opposite. We can see that on the following right calf muscle injury case, firstly with neutralized asymmetry, then with coefficient of variation (the injury ROI is outlined in purple because it is also painful)
SOFTENED COEFFICIENT OF VARIATION
In some cases, the result of the coefficient of variation can show extreme tendencies. As you can observe in the following tracking report, the coefficient of variation outcomes show extreme cold and warm tendencies from one evaluation to the following one.
This can be the case in subjects that have been analysed over the time with long periods between evaluations or with different external conditions (warmer or colder ambient temperatures) creating global changes on the whole body (as we can see in the example above). To avoid this bias, we use the softened coefficient of variation, which subtracts the general increase or decrease in skin temperatures to emphasize those ROI that have undergone a significant change.
All metrics requiring several evaluations, thus using historical averages (neutralized asymmetry and both coefficient of variations) might show you this alarm on the tracking report:
It means that the values of this session are not used in the historical metrics calculation because it has been indicated that some ROI has pain, injury and/or the presence of influence factors. Therefore you might not have an avatar or the results are based on calculations of other “clean” sessions. You can see an example here:
If you want to include those session in the global calculation just click on the following funnel button:
So you will be able to see those metrics including sessions with pain, injury an/or influence factors.
In conclusion, thermography allows us to use metrics from the first evaluation. In this sense:
- One or few assessments: Use mainly average thermal asymmetry (Asymmetry as it is shown in ThermoHuman) to spot significant imbalances, useful for prevention and diagnosis support
- More than 4 evaluations: Mainly in cases where the focus is prevention, we highly recommend you to start using neutralized asymmetry since it will be more accurate than the single average thermal asymmetry to spot the significant imbalances, in that case with alarms based on the historical thermal profile of this subject.
- More than 4 evaluations: In all application approaches (prevention, diagnosis support, injury monitoring or internal load quantification) both coefficient of variations will help you to understand if, regardless of the existence of asymmetries, there are significant warmer or colder thermal tendencies.
Infrared thermography is a solution that allows us to get relevant information from the first moment. Obviously, the more evaluations we have the better, because besides ThermoHuman will give us the possibility of richer and complementary analysis using all these metrics.
Uematsu, S., Edwin, D. H., Jankel, W. R., Kozikowski, J., & Trattner, M. (1988). Quantification of thermal asymmetry. Part 1: Normal values and reproducibility. J Neurosurg, 69(4), 552-555. doi: 10.3171/jns.1988.69.4.0552
Formenti, D., Ludwig, N., Rossi, A., Trecroci, A., Alberti, G., Gargano, M., . . . Caumo, A. (2018). Is the maximum value in the region of interest a reliable indicator of skin temperature? Infrared Physics & Technology, 94, 299-304. doi: https://doi.org/10.1016/j.infrared.2018.06.017
Arnaiz Lastras, J., Fernández Cuevas, I., Gómez Carmona, P. M., Sillero Quintana, M., García de la Concepción, M. Á., & Piñonosa Cano, S. (2011, 6th-9th july). Pilot study to determinate thermal asymmetries in judokas. Paper presented at the 16th Annual Congress of the European College of Sport Sciences ECSS, Liverpool, United Kingdom.