Application of machine vision to assess involved surface in patients with psoriasis

Abstract

An objective method is still lacking in the quantitative assessment of psoriasis severity. The purpose of this study was to examine whether computer image analysis (CIA) by a new colour segmentation method can be used as an objective method of estimating involved surface area in patients with psoriasis. Involved surface area in psoriasis was assessed from colour photographs covering the same areas as the psoriasis area and severity index (PASI) scores of 26 patients by human observers from different medical professional groups and by CIA. The reliability of CIA was tested with reference analysis by colouring manually the involved areas in the images with the aid of projected slides. The surface area estimates by the different methods were compared with each other and the resulting effects on the PASI score were assessed. The agreement between the reference analysis and the image analysis was high, especially on trunk areas, but the cylindrical shape of the limbs resulted in some difficulties in assessing involved skin area. The human eye estimates differed from the image analysis ones in almost one-third of the cases and mostly when psoriasis affected under 30% of the skin surface area. Both over- and underestimates emerged, but overestimates were more common. Error estimates had a significant effect on the PASI score. The CIA method seems to be reliable and practicable in estimating the actual surface of psoriasis. The method has the disadvantage of being time-consuming (photographing, processing of pictures) and technically demanding. Further development of this method should make it faster in the future.

How Good Are Clinical Severity and Outcome Measures for Psoriasis?: Quantitative Evaluation in a Systematic Review

“…It is defined as 9% coverage for the head and neck, each arm, anterior and posterior leg as well as the four trunk quadrants respectively, leaving 1% for the genitalia. The BSA can also be estimated by the number of patients' hand areas affected, assuming that one ''handprint'' reflects approximately 1% of BSA (Ramsay and Lawrence, 1991;Long et al, 1992;Savolainen et al, 1997Savolainen et al, , 1998Kreft et al, 2006;Thomas and Finlay, 2007). Validity was only tested with the PGA, which correlated strongly (Langley and Ellis, 2004).…”

Computer-aided measurement of psoriatic lesion area in a multicenter clinical trial—Comparison to physician's estimations

“…Before the therapy and after 8 weeks, the area of the skin lesions was additionally estimated by the objective measurements as follows. The objective measurement is a modification of the method called ''reference analysis'' by Savolainen et al [12]. Each patient was photographed by a physician or a nurse using digital CCD camera Canon PowerShot A60 with the resolution 1600 Â 1200 pixels (each centre/physician used a separate camera).…”

“…Previous studies have indicated wide variations between observer's estimates of involved surface area [4][5][6][7][8]. Computeraided image analysis (CIA) was used by several authors to minimize the variability of psoriatic area estimation [6][7][8][9][10][11][12][13][14]. Both photographs and drawing were used in CIA.…”

“…Distortion due to the curved body sites may be corrected by conversion factor obtained from reference points marked on the skin [11] however previous study show that this error may be neglected [7]. The problem of uneven illumination was minimized by green filter [11] or preferably by color segmentation techniques [12]. In the preliminary study we found that irregularities on the skin (such as wrinkles and scars) introduce false positive result in CIA, whereas the scales in the psoriatic lesions introduce false negative result.…”

“…In the preliminary study we found that irregularities on the skin (such as wrinkles and scars) introduce false positive result in CIA, whereas the scales in the psoriatic lesions introduce false negative result. Most of CIA methods are expensive, time consuming and thus unfeasible in large-scale clinical trials [12,15]. In the present study we optimized a method of psoriatic area measurement by combining (1) the physician's proficiency to accurately determine the edge of the lesion regardless of irreproducible light conditions and (2) the ability of image analysis to accurately measure a defined area.…”