"The sign describes the resemblance to a mushroom cap invading the rectosigmoid colon 1. The overlying mucosa and submucosa are T2 hyperintense, like the top skin of a mushroom 1. The hypertrophic muscularis propria is heterogenously T2 hypointense, like the gills of a mushroom, radiating towards a central stalk due to fibrotic convergence at the serosal surface 1."