<P> Medical imaging may provide a limited amount of information regarding bone marrow . Plain film x-rays pass through soft tissues such as marrow and do not provide visualization, although any changes in the structure of the associated bone may be detected . CT imaging has somewhat better capacity for assessing the marrow cavity of bones, although with low sensitivity and specificity . For example, normal fatty "yellow" marrow in adult long bones is of low density (- 30 to - 100 Hounsfield units), between subcutaneous fat and soft tissue . Tissue with increased cellular composition, such as normal "red" marrow or cancer cells within the medullary cavity will measure variably higher in density . </P> <P> MRI is more sensitive and specific for assessing bone bone composition . MRI enables assessment of the average molecular composition of soft tissues, and thus provides information regarding the relative fat content of marrow . In adult humans, "yellow" fatty marrow is the dominant tissue in bones, particularly in the (peripheral) appendicular skeleton . Because fat molecules have a high T1 - relaxivity, T1 - weighted imaging sequences show "yellow" fatty marrow as bright (hyperintense). Furthermore, normal fatty marrow loses signal on fat - saturation sequences, in a similar pattern to subcutaneous fat . </P> <P> When "yellow" fatty marrow becomes replaced by tissue with more cellular composition, this change is apparent as decreased brightness on T1 - weighted sequences . Both normal "red" marrow and pathologic marrow lesions (such as cancer) are darker than "yellow" marrow on T1 - weight sequences, although can often be distinguished by comparison with the MR signal intensity of adjacent soft tissues . Normal "red" marrow is typically equivalent or brighter than skeletal muscle or intervertebral disc on T1 - weighted sequences . </P> <P> Fatty marrow change, the inverse of red marrow hyperplasia, can occur with normal aging, though it can also be seen with certain treatments such as radiation therapy . Diffuse marrow T1 hypointensity without contrast enhancement or cortical discontinuity suggests red marrow conversion or myelofibrosis . Falsely normal marrow on T1 can be seen with diffuse multiple myeloma or leukemic infiltration when the water to fat ratio is not sufficiently altered, as may be seen with lower grade tumors or earlier in the disease process . </P>

Where are yellow and red bone marrow located