How I Do It: MRI of the Bone with Marrow-specific Sequences.

Normal bone marrow is composed of red marrow, which is hematopoietically active (producing red blood cells), and yellow marrow, which is hematopoietically inactive (composed mainly of fat cells). In infancy, bone marrow is mostly red marrow and converts to fatty yellow marrow in a systematic and predictable manner over time. Therefore, depending on the patient's age, the MRI appearance of normal bone marrow will change. In addition, red marrow reconversion from fatty yellow marrow wherein red replaces yellow bone marrow is a physiologic process that can occur due to stressors such as anemia, obesity, or chronic illness. Bone marrow conversion and reconversion are commonly encountered in routine practice, occasionally making the differentiation of normal from abnormal marrow challenging. True disorders of the bone marrow must be distinguished from normal marrow and include bone marrow replacement, infiltration, hyperemia-mediated reactive processes, ischemia with resultant necrosis, and bone marrow depletion. This review outlines a systematic approach to evaluating bone marrow at routine MRI, highlighting the role of clinically available noncontrast "marrow-specific" MRI sequences, which include T1-weighted spin-echo imaging, chemical shift imaging, and diffusion-weighted imaging with apparent diffusion coefficient mapping, for definitively characterizing the marrow signal for a focal or diffuse abnormality.