Color shift as a mechanism for depth modulation of skin images

Abstract

To the Editor: Dermatologic structures are present at varying cutaneous levels, prompting investigation of imaging approaches that facilitate depth-resolved skin examinations. Image-based skin assessment is limited by the data present in the files, which are determined by the format (bit depth/color compression) and illuminant characteristics. To address these variables, clinical photographs were acquired in uncompressed RAW format with cross-polarized lighting,1Anderson R.R. Polarized light examination and photography of the skin.Arch Dermatol. 1991; 127: 1000-1005Crossref PubMed Scopus (264) Google Scholar providing both increased color depth (14-bit vs. 8-bit compression in JPEG) and increased light penetration into the tissue, respectively. The resulting dynamic color file can be processed to highlight surface or subsurface structures. Photographs were acquired of anatomic regions of interest using a Nikon (Tokyo, Japan) digital single-lens reflex (DSLR) camera with a 60-mm macro lens; a standard flash; and 2 crossed polarizing filters, oriented at 90° from one another. The resulting files were processed with Adobe (San Jose, CA) software for feature extraction2Evans S. Noorbhai S. Lawson Z. Stacey-Jones S. Carabott R. Contrast enhancement of bite mark images using the grayscale mixer in ACR in Photoshop®.J Forensic Sci. 2013; 58: 804-810Crossref PubMed Scopus (4) Google Scholar (Figs 1 and 2). Specific colors in the RAW file were enhanced by darkening or lightening them in relation to the surrounding tissue, increasing conspicuity.3Braga J.C. Scope A. Klaz I. Mecca P. Spencer P. Marghoob A.A. Melanoma mimicking seborrheic keratosis: an error of perception precluding correct dermoscopic diagnosis.J Am Acad Dermatol. 2008; 58: 875-880Abstract Full Text Full Text PDF PubMed Scopus (33) Google Scholar Orange and red luminance levels are increased or decreased to their maximum or minimum levels, suppressing or enhancing hemoglobin and melanin, respectively.4Shah K.N. Hanlon K. Muntifering M. Visualizing vascular and pigment structures using dermoscopy and targeted luminance adjustments.Dermatol Pract Concept. 2015; 5 (28 p.156-157)Google Scholar Depending on acquisition and tissue variables including skin type, exposure, and subject-to-camera distance, other colors can be targeted to reveal additional data points; for example, reduction in green luminance may be used to target deep vasculature. Visible depth can be modulated to draw out deeper structures by shifting color temperatures and tint toward blue/green or toward yellow/magenta for more superficial structures. Color shifting can be thought of as a correction to the illumination light; therefore, it is the inverse of the expected direction, with warmer light illuminating deep regions and cooler light illuminating superficial structures. The depth assessment can be done continuously, with the dynamic RAW file exhibiting latent data at different locations across the visible spectrum.Fig 2The dorsal aspect of the hands, showing the (A) original, unaltered, cross-polarized photograph on the left, followed by iterations of the same file processed for (B) epidermal pigment, (C) superficial vascularity, and (D) deep structures on the right.View Large Image Figure ViewerDownload Hi-res image Download (PPT) In this manner, vascularity is visualized separately from pigment, and depth is resolved through color shifting. Superficial pigmentation distributed over sun-damaged skin, such as lentigines, is enhanced or suppressed. Dermal structures, such as blood vessels, are separated from epidermal processes, such as telangiectasia. There are commercially available systems, such as Visia (Canfield Scientific) and Siascopy (Astron Clinica), that incorporate algorithms to visualize vascular and pigmentary features, as well as a plethora of research into spectral imaging (see Supplemental References; available via Mendeley at https://doi.org/10.17632/8tygks4y2y.1). We demonstrate a novel method for processing polarized color image data, permitting wide-field visualization of structures at different depths that could help target lesions for further examination with a dermoscope. The flexible RAW file is most beneficial when visualized in real time, with the viewer having the ability to shift between epidermis and dermis, in small increments, such that only the feature of interest is perceived at a given depth. Morphologic data can be identified in the processed versions of the file that were challenging to identify in the original image and to the naked eye. Visualizing features at different depths in the skin can not only help in forming clinical judgement but also in evaluating change and treatment. Both medical and cosmetic dermatologic conditions5Demirli R. Otto P. Viswanathan R. Patwardhan S. Larkey J. RBX® technology overview.2007Google Scholar benefit from improved visual evaluation.