## 3 Thermographic Techniques for Skin Cancer Diagnosis
### Dynamic Infrared Thermography
#### 3.2.2 Dynamic IR Thermography Using a Cold Provocation
[40] distinguished pigmented BCC and seborrheic keratosis by comparing the thermal recovery patterns of skin lesions and healthy skin recorded after the application of both hot and cold stress supplied with a thermoelectric device. Thermal images were recorded from a group of 37 patients affected by pigmented BCC (22 individuals) and seborrheic keratosis (15 individuals) and enrolled to undergo an original IR imaging procedure. After 5-10 min of acclimatization in a controlled temperature room at 23 \({}^{\circ}\)C and 50 \(\pm\) 5% relative humidity, the skin lesion was initially heated up to 40 \({}^{\circ}\)C; after that, the hot stimulus was immediately removed and the thermal response of the lesion was recorded for five minutes through an FLIRVR A615 IR camera (with an optical resolution of 640 \(\times\) 480 pixels and a noise equivalent temperature difference less than 0.05 \({}^{\circ}\)C at 30 \({}^{\circ}\)C). After 5 min of rest, the same lesion was cooled to 15 \({}^{\circ}\)C, and thermal images were acquired for the other five minutes after removing the cold stimulus. The same procedure was also applied to the healthy skin. The classification method was based on the fact that pigmented BCC showed faster thermal recovery than normal skin, whereas no significant differences between seborrheic keratosis lesions and healthy skin appeared when comparing their thermal responses. In 2015, [PERSON] et al. suggested an analysis method for skin cancer screening using dynamic infrared thermography [41]. About one hundred subjects underwent the suggested method. A cold air flow produced by a Ranque-Hilsch vortex tube was used to cool the subject's skin around the suspected lesion for 30 s. Moreover, the examination room was controlled to be between 20 and 22 \({}^{\circ}\)C. Thermal images were acquired for a total of 2 min during both the cooling and the thermal recovery phases using a long-wave infrared camera with a 320 \(\times\) 256 focal-plane array. A squared infrared marker was used to correct the involuntary movement of the subject using the Harris corner detector algorithm, and an affine transformation matrix was used to map the movements between consecutive frames. The effect of the non-uniform cooling was also considered by judiciously selecting pixels with the same initial temperature. The authors reported that their method allows 95% of the malignant cases to be correctly classified, while more than 83% of the benign lesions would be identified without the need for a biopsy. A group of 30 patients affected by actinic keratosis were periodically monitored after the surgical removal of the lesions by [PERSON] et al. [42], using dynamic IR thermography to evaluate the effectiveness of treatment with Eryfotona(r) in reducing the hyperthermic halo typical of this lesion. The authors employed an FLIR3000 IR camera together with patented equipment to provide the thermal stimulus. In particular, it consisted of an insulated tank containing a mixture of alcohol and water and connected, with a two-way tube, to a rubber balloon, which was put in contact with the skin. The temperature of the mixture can be set from 0 \({}^{\circ}\)C to 40 \({}^{\circ}\)C by means of a heating-cooling system embedded within the tank walls. In their study, the patient's skin was cooled for 20 s until it reached a temperature of 5 \({}^{\circ}\)C. The area of the hyperthermic region and the thermal recovery time, i.e., the time the skin takes to come back to steady-state conditions, were the parameters monitored by the authors. Dynamic IR thermography was also applied to monitor six patients affected by BCC who underwent photodynamic therapy (PDT) [43]. However, in this study carried out by [PERSON] et al., a cold stimulus was not applied; on the contrary, during the photodynamic treatment, the patient's skin was illuminated by a diode laser source.