The common teaching is that restricted diffusion on MRI indicates stroke—acute cerebral infarction. When, more specifically, the changes on DWI and ADC indicate impairment in the random movement of water molecules in a given tissue. With slower movement, as in energy-depleted tissues with failing ATP-dependent sodium/potassium channels, high cellularity, or a highly viscous extra-cellular space, there is restricted diffusion (of water) and a corresponding bright signal on DWI. These and other considerations are the focus of this week's episode of BrainWaves. Produced by James E. Siegler. Music courtesy of Uncanny and Kevin McLeod. Sound effects by Mike Koenig. BrainWaves' podcasts and online content are intended for medical education only and should not be used for clinical decision making. Be sure to follow us on Twitter @brainwavesaudio for the latest updates to the podcast. REFERENCES Filippi M, Iannucci G, Cercignani M, Assunta Rocca M, Pratesi A and Comi G. A quantitative study of water diffusion in multiple sclerosis lesions and normal-appearing white matter using echo-planar imaging. Archives of neurology. 2000;57:1017-21. Meyer JR, Gutierrez A, Mock B, Hebron D, Prager JM, Gorey MT and Homer D. High-b-value diffusion-weighted MR imaging of suspected brain infarction. AJNR American journal of neuroradiology. 2000;21:1821-9. Roberts TP and Rowley HA. Diffusion weighted magnetic resonance imaging in stroke. Eur J Radiol. 2003;45:185-94. McKinney AM, Short J, Truwit CL, McKinney ZJ, Kozak OS, SantaCruz KS and Teksam M. Posterior reversible encephalopathy syndrome: incidence of atypical regions of involvement and imaging findings. AJR Am J Roentgenol. 2007;189:904-12. Maier SE, Sun Y and Mulkern RV. Diffusion imaging of brain tumors. NMR Biomed. 2010;23:849-64. Steiner I, Budka H, Chaudhuri A, Koskiniemi M, Sainio K, Salonen O and Kennedy PG. Viral meningoencephalitis: a review of diagnostic methods and guidelines for management. European journal of neurology : the official journal of the European Federation of Neurological Societies. 2010;17:999-e57. Hegde AN, Mohan S, Lath N and Lim CC. Differential diagnosis for bilateral abnormalities of the basal ganglia and thalamus. Radiographics. 2011;31:5-30. Aliaga ES and Barkhof F. MRI mimics of multiple sclerosis. Handbook of clinical neurology. 2014;122:291-316. Starkey J, Kobayashi N, Numaguchi Y and Moritani T. Cytotoxic Lesions of the Corpus Callosum That Show Restricted Diffusion: Mechanisms, Causes, and Manifestations. Radiographics. 2017;37:562-576.