The vertebral body is composed of three tissues: bone, hematopoietic (red) marrow, and fatty (yellow) marrow. With aging, the bone becomes demineralized and the ratio of hematopoietic to fatty marrow changes. MR studies of patients of varying ages have revealed that the T1 relaxation time of bone marrow progressively decreases with age. The T1 shortening is due to replacement of hematopoietic marrow by fatty marrow. At birth the vertebral marrow consists entirely of hematopoietic tissue. The fatty component increases to about 15 per cent by age 10, 35 per cent by age 25, and 60 per cent by age 80.

      As a result of increased marrow fat, the T2 relaxation time also decreases with age except in women over 50. The greater loss of bone mineral in postmenopausal women is postulated as the cause of the variation. Bone mineral affects the T2 relaxation time of marrow. Magnetic susceptibility effects reduce the apparent T2 of protons at bonetissue interfaces, making those protons invisible to MR. With osteoporosis (loss of bone mineral), the protons are "unmasked," resulting in an increase in spin density and T2 relaxation time. Endnote

      There are many diseases that affect the bone marrow, including metastatic disease, tumors of blood cell origin, Endnote blood dyscrasias and anemias, and radiation therapy. In normal marrow there are two populations of protons (water and fat protons), and the two populations are more or less balanced. Any disease process that upsets that balance affects the MR signal. In the adult, the active red marrow of the body is found mostly in the axial skeleton. Primary and secondary marrow malignancies preferentially infiltrate areas of active hematopoiesis, so the spine and pelvis are commonly affected. Replacement of the normal marrow by abnormal soft-tissues results in increased marrow cellularity, a greater number of water protons, and prolongation of T1 and T2. Endnote  

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