5.6.2 Applications of photothermal imaging in the ...
The potential of photothermal imaging for skin cancer detection and，in particular，as an early-stage melanoma diagnostic tool，was already noticed in the early 1960s.Unfortunately，pioneer thermographic studies devoted to the subject showed disappointing results with a high percentage of false-negative outcomes.Those poor results drastically reduced the enthusiasm of the medical community.Nevertheless，they can be explained by several factors.First，potential thermal signals involved in early-stage lesions are small，and highly sensitive infrared imaging devices were not available at the time of the first study.Second，such small temperature differences are usually buried in larger thermal signals originating from the subcutaneous tissue.In such circumstances，small hot or cold spots would be hardly detectable even using current highly sensitive infrared cameras.
In 1995，Di Carlo proposed to overcome those difficulties by performing active thermography measurements.He implemented a photothermal set-up where the skin is stimulated by the contact with a balloon filled with a thermostatic alcohol solution.A few seconds after the removal of the stimulation，melanoma and hyperpigmented basal cell carcinomas exhibited drastic temperature differences compared with the surrounding healthy skin.Several years later，Buzug and his coworkers reproduced Di Carlo's experiment using cold gel packs and a more recent infrared camera.Very recently，Çetingül and Herman at Johns Hopkins University investigated transient thermal signals of melanoma lesions.Using a multilayer heat transfer skin model，they could extract quantitative information from the transient thermal signals.As an example，Figure 5-14 shows a cluster of pigmented lesions located on a shoulder and investigated with photothermal.Whereas the steady state thermogram can not provide any diagnostic information，the melanoma lesion is clearly distinguishable on the infrared image taken few seconds after the removal of the cold stimulation.Çetingül and Hermann claimed that the method was able to detect Clark level Ⅰ（melanoma in-situ）lesions.(https://www.daowen.com)
Bonmarin and Le Gal developed a lock-in thermography-based set-up specifically dedicated to the investigation of skin cancer.In such apparatus，the skin surface temperature is periodically modulated using an airflow，while an infrared camera records the emitted thermal radiation.Compared with the previous set-ups based on pulsed stimulation，lock-in thermography has the ability to suppress lateral heat spreading，leading to sharper thermograms.Figure 5-15 demonstrates the capabilities of lock-in thermography for the accurate detection of skin cancer lesion margins.