Registration of polarimetric images for in vivo skin diagnostics

The scientific work "Registration of polarimetric images for in vivo skin diagnostics", with the support of the iToBoS project, has been published.

Imaging of the human skin with polarimetric techniques has gained importance in recent years, as the incidence of melanoma, which is the deadliest type of skin cancer, is rising throughout all skin types. Beyond the classical skin cancer screening done by a dermatologist with a dermoscope, new modalities are being implemented for the early detection of melanoma: polarimetric imaging devices such as Stokes or Mueller polarimeters and machine learning-based computer-aided diagnosis systems are among the recently reported approaches. Furthermore, the early detection of melanoma is increasingly profiting from automated registration and analysis. In addition, adaptive optics such as adjustable liquid lenses allow for the implementation of autofocus in imaging systems without moving parts. However, because the up to 36 images necessary for Mueller matrix (MM) determination are typically obtained in acquisition times of 20 to 30 s, depending on the type of the setup, unintended body movements of the patient can become a significant limitation. Patient motion can lead to misalignment of the polarimetric images. Further, it can induce motion blur, in particular because laser power levels have to be reduced due to laser safety and therefore relatively high exposure times are needed to acquire the MMs. In addition, the motion can add up in cases in which a summation process of the polarimetric data is needed to enhance the signal-to-noise ratio (SNR). For example, the registration of scanned polarimetric images for ophthalmology was investigated by Nourrit et al. The above-mentioned problems in the MM acquisition can be partly overcome by hardware upgrades, e.g., camera and lens upgrades. However, such upgrades typically rely on complex and costly components. Therefore, software solutions represent an alternative approach and promise improved results.

In this work, we investigate the need and potential benefits of registering polarimetric images for application in vivo for dermoscopy. Registration of such data is particularly complex because the skin rarely shows sufficient key points with strong contrast (except for mole evaluation). In addition, the contrast of key points usually changes considerably between the polarization states of the lighting and the analyzer. This issue and the possibility of key points moving out of the field of view make it difficult to detect the same set of key points within the up to 36 images typically acquired in MM polarimetry (MMP). To evaluate the potential benefits of this method, we determine the MM and its polarimetric parameters with and without the polarimetric image registration for the case of a printed melanoma phantom, healthy skin with a drop of honey, and a benign nevus, respectively. The instrumentation and registration schemes are described in Sec. 2. The results of registration and polarimetric analysis are presented in Sec. 3. Section 4 concludes the study.


This research was funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany’s Excellence Strategy within the Cluster of Excellence PhoenixD (EXC 2122, Project ID 390833453). This work has been supported by iToBoS (Intelligent Total Body Scanner for Early Detection of Melanoma), project funded by the European Union’s Horizon 2020 research and innovation program, under grant agreement no. 965221

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