The Q measure is an image quality measure, intended for evaluating computed tomography (CT) scans. It is computed based on peaks detected in the histogram of a CT volume.
For more information on the Q metric, see this publication:
M. Reiter, D. Weiss, C. Gusenbauer, J. Kastner, M. Erler, S. Kasperl: Evaluation of a histogram based image quality measure for X-ray computed tomography. Proceedings of Conference on Industrial Computed Tomography (iCT), Wels, Österreich, 2014, url: ndt.net/?id=15715.