Here is an example of an M2 dense MCA sign. The increased area of attenuation in the anterior part of the midline is incidental falcine calcification.
The reference is: Grant Mair, Elena V. Boyd, Francesca M. Chappell, Rüdiger von Kummer, Richard I. Lindley, Peter Sandercock, Joanna M. Wardlaw, IST-3 Collaborative Group. Sensitivity and Specificity of the Hyperdense Artery Sign for Arterial Obstruction in Acute Ischemic Stroke. Stroke. 2015;46:102-107. Originally published December 4, 2014. doi: 10.1161/STROKEAHA.114.007036 http://stroke.ahajournals.org/content/46/1/102.abstract