Saxon BMC Biology (2015) 13:62 DOI 10.1186/s12915-015-0171-z

COMMENT Open Access

Over the rainbow

Emma Saxon

Abstract

Shear stress in arteries, which is a measure of the forceexerted by blood flow on the arterial wall, is associatedwith the location of lipid plaques that cause heartdisease. In this study, a mathematical model of shearstress was combined with cross-sectional x-ray imagesof an artery taken using Computed Tomography (CT)scanning, allowing the authors to explore patterns ofshearing stress and shed light on the role of arterialarchitecture in heart disease.

Fig. 1. An example of shear stress patterns in an artery. Shear stressis measured in dyne/cm2, represented on the image by a rainbowscale from dark blue (0) to red (30)

CommentaryRainbow colour scales, which usually run from darkblue (low values) to red (high values), are commonlyused in several different types of biological images:functional magnetic resonance imaging (fMRI) scansshowing activity in the brain, gene expression data inmicroarrays, or in this case, the levels of shear stress inarteries, expressed as unit of force/unit of area (in thiscase, dyne/cm2). One problem with the rainbow scale isthat it can make relatively small differences seem large:the red sections of Fig. 1 appear very different fromthe yellow. However, the colour scale shows that thedifference between yellow and red is actually muchsmaller than it appears visually, in the context of thewhole colour scale range.The opposite can also be true. In Fig. 1, green spans

just under half of the whole colour scale, and dif-ferences between values at the lower and higher ends ofthe middle range are somewhat masked. These dif-ferences could be better represented with a divergingscale, running between two colours only. As long asthese colours are not red and green, a diverging scalewould also be more easy to interpret for red-green

Correspondence: emma.saxon@biomedcentral.comBMC Biology, BioMed Central, 236 Gray’s Inn Road, London WC1X 8HB, UK

© 2015 Saxon. Open Access This article is dInternational License (http://creativecommoreproduction in any medium, provided youlink to the Creative Commons license, andDedication waiver (http://creativecommonsarticle, unless otherwise stated.

colour blind individuals. Michelle Borkin and co-workers at Harvard University recently found that adiverging scale increased accuracy of interpretationrelative to the rainbow scale in the case of arterialshearing images [1], challenging the legitimacy of usingrainbow scales in this field.

Reference1. Borkin MA, Gajos KZ, Peters A, Mitsouras D, Melchionna S, Rybicki FJ, et al.

Evaluation of artery visualizations for heart disease diagnosis. IEEE Trans VisComput Graph. 2011;17:2479–88.

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