Altres ajuts: This paper is the outcome of the MAGNIMS workshop "Measuring cortical and deep gray matter atrophy in MS", which was kindly supported by the Multiple Sclerosis International Federation (www.msif.org), the Dutch MS Research Foundation (www.msresearch.nl), the journal "Nederlands Tijdschrift voor Geneeskunde" (www.ntvg.nl), the Neuroscience Campus Amsterdam (www.neurosciencecampus-amsterdam.nl), and the VU University medical center. Frederik Barkhof is supported by the National Institute for Health Research University College London Hospitals Biomedical Research Centre.

Atrophy of the brain grey matter (GM) is an accepted and important feature of multiple sclerosis (MS). However, its accurate measurement is hampered by various technical, pathological and physiological factors. As a consequence, it is challenging to investigate the role of GM atrophy in the disease process as well as the effect of treatments that aim to reduce neurodegeneration. In this paper we discuss the most important challenges currently hampering the measurement and interpretation of GM atrophy in MS. The focus is on measurements that are obtained in individual patients rather than on group analysis methods, because of their importance in clinical trials and ultimately in clinical care. We discuss the sources and possible solutions of the current challenges, and provide recommendations to achieve reliable measurement and interpretation of brain GM atrophy in MS.