Dark t1 bone marrow.

Bone marrow contains fat and water. Using magnetic resonance (MR) T1-weighted images, fat appears light gray and water appears dark gray. Black or low signal structures include normal cortical bone, bone islands, hemosiderin deposits, calcification, ossification, metal artifact, gas, and foreign material. Reactivation of bone marrow reduces the signal of fatty marrow. Additional sequences and supplementary imaging with conventional radiographs and computed tomography are often helpful in determining the nature of the low signal lesions. Genetic bone disorders such as melorheostosis and other syndromes may give low signal lesion in a typical and often diagnostic pattern. MRI is an important method of showing the fracture line as a low signal lesion on T1-weighted images and is the preferred imaging method when conventional radiographs are unhelpful and fracture is suspected. Infection will cause edema and later sclerosis and necrosis with a mixed low and high signal pattern. Infarction will cause a mixed pattern depending on the stage of the process, but this includes low signal on T1-weighted images. Ossification and calcification in primary and secondary tumors appear as low signal areas on T1-weighted images. Recognition and understanding of lesions that cause low signal on T1-weighted images is an important tool when interpreting MR images.