Pathological classification system of GM lesions in MS. GM lesion classification system as proposed by Bo et al,. 2003. Type 1 lesions (A) extend through both white and gray matter. Type 2 lesions (B) are intracortical, having no contact with white matter or with the surface of the brain. Type 3 lesions (C) extend inward from the surface of the brain. Type 4 lesions (D) extend through the whole width of the cortex without reaching into white matter. Reproduced from Geurts and Barkhof  with permission from Elsevier.