Focus collection on Thermal Imaging in Medicine

Welcome to this focus collection of Physiological Measurement on Thermal Imaging in Medicine.

Thermal imaging had its origins in the Second World War, where scientists were trying to measure infrared signatures for target identification and recognition. In the decades immediately following the war, thermal imaging was almost exclusively confined to military applications. However, as the technology rapidly developed, system miniaturisation followed, and then, through the introduction of un-cooled focal plane detector arrays, the cost of thermal imagers quickly decreased. This resulted in thermal imaging becoming increasingly pervasive in non-destructive testing and other research disciplines, including a rapid uptake in the field of medicine.

Thermal imaging is now routinely used in a variety of clinical and clinically related research settings; for example, it is now utilised in identifying and tracking the course of Raynaud's phenomenon and other vasospastic disorders (Wilkinson et al 2018), and there has been a lot of interest in the widespread deployment of thermal imaging to help prevent diabetic foot ulceration (Machin et al 2017, MacDonald et al 2017).

This focus collection of Physiological Measurement showcases, through ten papers, the diversity of contemporary clinical and clinically related applications for thermal imaging. These papers are a selection based on presentations given at the 14th European Association of Thermology (www.eurothermology.org/) Congress held at the UK's National Physical Laboratory (www.npl.co.uk/), 4–7 July 2018.

The scene is set by the review paper of Machin et al (2019) discussing the importance of standardisation and calibration in the context of clinical thermometry, not just for thermal imaging but for a variety of techniques including more exotic approaches such as magnetic resonance imaging spectroscopy (Babourina-Brooks et al 2015). Blackbody sources that can be used for the calibration and validation of thermal imagers in medicine have been described elsewhere (Machin et al 2009).

Papers by MacDonald et al (2019) 'Between visit variability of thermal imaging of feet in people attending podiatric clinics with diabetic neuropathy at high risk of developing foot ulcers' and Seixas et al (2019) 'Relationship between skin temperature and soft tissue hardness in diabetic patients: an exploratory study' both relate to the deployment of thermal imaging to the feet of diabetic patients, with the aim of reducing and preventing ulceration and hence the many deleterious consequences that follow.

The paper by Pokorná et al (2019) 'Intestinal resection of the porcine model under thermographic monitoring' demonstrates the use of thermal imaging as an essential adjunct to improving the outcome of surgical procedures. This has been powerfully demonstrated, for example, through the improved outcomes in breast reconstruction surgery by Weum, Mercer and de Weerd (2016), after thermal imaging was introduced into the clinical pathway.

There is a growing interest in the deployment of thermal imaging in monitoring and optimising athletic performance and sports science more generally. This was reflected by the three contributions, by da Silva et al (2018) 'Can exercise-induced muscle damage be related to changes in skin temperature?', Pérez-Guarner et al (2019) 'Association between physiological stress and skin temperature response after a half marathon' and Gil-Calvo et al (2019) 'Effects of prefabricated and custom-made foot orthoses on skin temperature of the foot soles after running'.

The use of thermal imaging to monitor the presence of infection is discussed by Benavent Casanova et al (2019) in the 'Application of infrared thermography in diagnosing peripherally inserted central venous catheter infections in children with cancer'. The use of thermal imaging to detect infection is of wider importance. For example, thermal imaging is widely deployed, especially in East Asia, to detect febrile individuals passing through airports (IEC/FDIS 80601-2-59 2017). The objective in this case is to try to prevent the spread of highly infectious diseases such as avian flu in potentially pandemic situations. Hence the paper by Vardasca et al (2019), 'Bilateral assessment of body core temperature through axillar, tympanic and inner canthi thermometers in a young population', is an important corrective showing that reliable body temperature measurement (as opposed to measurement of body temperature change) requires the correct selection of the measurement site. In the case of body temperature, this is generally acknowledged to be the inner canthi when thermal imaging is the method used.

Finally, the paper by Perpetuini et al (2019) on 'Autonomic impairment in Alzheimer's disease is revealed by complexity analysis of functional thermal imaging signals during cognitive tasks' demonstrates the potential of thermal imaging to have a positive impact on individuals affected by chronic life-changing conditions such as Alzheimer's. It also shows that thermal imaging has a very wide diversity of applications, the boundaries of which we have not yet begun to explore.

We, the editors, hope this focus collection will give you a deeper understanding of how thermal imaging can be applied in medicine, and spark fresh ideas about how the technique might positively contribute to your clinical or associated specialism.

Finally, we want to dedicate this focus collection of Physiological Measurement to our long-time mentor and friend Professor Francis Ring, who passed away on 15 July 2019. Francis was a pioneer in thermal imaging in medicine and it is impossible to summarise his many contributions over his long and distinguished career. He had the foresight to introduce thermal imaging into a clinical setting when almost no one else had even heard of the technique, and critically he saw the need for standardisation of both the images and the thermometry to ensure the enduring value of the measurements. Where he led many have followed, and his work has inspired more than one generation of researchers and clinical thermal imaging practitioners. It is characteristic of the man that he was working right up to the end, sharing his knowledge and wisdom with future generations in the book by Ammer and Ring (2019), The Thermal Human Body: A Practical Guide to Thermal Imaging, published only weeks before his passing.