PMC8452437_01
(a) Skin area affected with psoriasis. (b) Enhanced lesion representation by a* color channel of lab space. (c) Segmented lesions (yellow spots) superimposed on the initial image to visualize the result of morphological processing. (d) Lesions' centroids generating Voronoi tessellation.
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fig1
An afebrile, 27-year-old male patient, in otherwise good general condition, presented with a rash of scattered, up to 1.0 cm large psoriasiform lesions that progressively affected his torso (shown in Figure 1) and the extremities during the past two weeks. For the detection of psoriasis lesions, RGB clinical images (shown in Figure 1(a)) were converted into the L*a*b* color space. The high-peak intensity areas ("spots") in the a*color channel (shown in Figure 1(b)), which correspond to psoriasis lesions, were extracted by serially applying the h-maxima transform and the regional maxima morphological operations (shown in Figure 1(c)). To quantify the spatial distribution of the psoriasis lesions, Voronoi diagrams were generated with generator points the centroids of above intensity spots (shown in Figure 1(d)).  Figure 2 exemplifies this procedure in the case of a tattoo (arbitrary designated tattoo #1). Skin areas without a tattoo, like the front of the torso (Figure 1), are characterized by a rather homogeneous, unimodal size distribution of the Voronoi patches centered on psoriasis lesions (CV = 0.46, Table 1).
Psoriasis and tattoo #1. (a) Clinical picture. (b) Retained a* color channel of L*a*b* color space: high-peak intensity areas mark psoriasis lesions and low-level intensity areas correspond to tattooed skin segments. (c) Applying h-maxima transform followed by regional maxima, the lesional skin spots (patches) were extracted and are displayed in yellow pseudocolor. (d) Equivalently, the tattoo area was extracted by applying h-minima transform, followed by regional minima and is shown in yellow pseudocolor to enhance visualization. (e) The extracted psoriasis skin lesions' centroids (panel (C) applied to generate Voronoi diagrams (tessellation) of the tattoo area. Voronoi polygons with and without overlapping with the tattoo area (yellow border) are displayed in red and green pseudocolors, respectively.
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fig2
Remarkably, the rash spared almost completely the sites of his three tattoos (lower leg, forearm, and shoulder; larger diameters 15-25 cm, shown in Figures 2 and 3). The tattoos (two of them monochrome black and the third black contours on whitish background; Figures 2 and 3) were performed on different occasions over the last 3 years (the last one about 9 months before the flare-up) in the same tattoo studio. To quantify the spatial distribution of the psoriasis lesions, Voronoi diagrams were generated with generator points the centroids of above intensity spots (shown in Figure 1(d)).  Figure 2 exemplifies this procedure in the case of a tattoo (arbitrary designated tattoo #1). However, the presence of a tattoo introduces a higher variation in the size distributions of the corresponding Voronoi patches in its neighborhood (CV = 0.60-1.23; shown in Figures 2 and 3; Table 1).
Tattoo #2 (panels (a) and (b)) and tattoo #3 (panels (c) and (d)). (a)-(c) Clinical pictures. (b)-(d) Voronoi patches intersecting (red) or not intersecting (green) tattooed skin areas (yellow contoured).
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fig3
Remarkably, the rash spared almost completely the sites of his three tattoos (lower leg, forearm, and shoulder; larger diameters 15-25 cm, shown in Figures 2 and 3). The tattoos (two of them monochrome black and the third black contours on whitish background; Figures 2 and 3) were performed on different occasions over the last 3 years (the last one about 9 months before the flare-up) in the same tattoo studio. However, the presence of a tattoo introduces a higher variation in the size distributions of the corresponding Voronoi patches in its neighborhood (CV = 0.60-1.23; shown in Figures 2 and 3; Table 1).