## 3 Thermographic Techniques for Skin Cancer Diagnosis
### Dynamic Infrared Thermography
#### 3.2.1 Cold Stimuli for IR Imaging  
Different skin cooling methods were investigated and compared in the scientific literature. The results obtained provide useful information when selecting a suitable cold stimulus in terms of the effectiveness of the cooling and the characteristics of the lesion. [PERSON] et al. [31] compared the skin cooling achieved through the spray and contact methods, both theoretically and experimentally, using an in vitro model. They concluded that both methods provide efficient skin cooling and that target depth, cost, and safety should be the deciding factors when choosing the cooling method. [PERSON] and [PERSON] [32] showed that the thermal contrast between skin lesions and healthy skin can be enhanced by inducing the evaporation of water and 75% medical ethanol sprayed on the skin surface. The authors performed both numerical calculations and infrared imaging experiments on a human forearm. They found that the vascular pattern of the forearm became clearly visible several minutes after the induced evaporation. Therefore, according to the author's point of view, induced evaporation is an effective method for enhancing the skin's thermal contrast. For three different skin cooling methods, [PERSON] and [PERSON] [33] performed a computational analysis to properly select the cooling temperature, time, and cooling depth in order to maximize the thermal contrast between a lesion and healthy skin when using dynamic infrared imaging. These authors investigated idealized contact cooling at constant temperature, a water-soaked cotton patch, and convective cooling through a cold air flow or immersion into a liquid. They found that, for the three methods, the cooling effect takes less than 30 s to reach a depth of 4 mm. Also, when a cooling temperature of about 20 \({}^{\circ}\)C is applied, effective skin cooling is achieved; in particular, the cooling effect can reach the lesion depth within 2 min without causing discomfort to the patient. Moreover, [PERSON] and [PERSON] considered cooling durations varying from 5 to 120 s; as a consequence, they studied two thermal recovery behaviors: one for short cooling times (less than 30 s) and another for longer cooling times. In particular, shorter cooling times lead to a cooling penetration depth less than 4 mm, with a maximum thermal contrast appearing within the first few seconds of the thermal recovery phase; while longer cooling durations give a lower maximum appearing 20-45 s after the removal of cold stimulus. The authors concluded that the cooling duration could be further adjusted by considering the characteristics of the lesion. To induce deeper cooling penetration and, therefore, better thermal contrast when using dynamic IR thermography, a constant-temperature active cooling device was numerically investigated by [PERSON] et al. [34]. This set-up involves a metal disk put in direct contact with the lesion and the surrounding tissue; also, a thermoelectric device (Peltier cell) mounted over the disk top provides the cooling effect. The aim was to ensure that the disk temperature was constant as possible during the cooling phase. The authors concluded that to realize faster regulation and more constant cooling temperature, the use of a thin metal disc is preferable to a thick disk. According to the authors, the set-up may be used in any tumor stage as the lesion dimensions do not affect the cooling temperature. Recently, [PERSON] et al. [35] compared different cooling techniques useful in skin cancer diagnosis when using dynamic IR thermography: conductive cooling (aluminum medal, gel pack, and ice), evaporating cooling (alcohol spray), and convective cooling (Vortex tube and Zimmer Crio 6). Skin-mimicking agar phantoms with similar thermalproperties to human skin and resembling both flat skin and an ulcerating skin lesion were used for the experimental tests.