Jeffrey J. Peterson
Thomas H. Berquist
▪ NORMAL MARROW: BASIC CONCEPTS
Bone marrow consists of trabeculae (15%) and cellular constituents (85%) (erythrocytic, leukocytic, fat cells, and reticulum cells).
Hemopoietic cells (blood cells) comprise red marrow. The remainder, or inactive marrow, is yellow marrow. Red marrow is 40% water, 40% fat, and 20% protein. Yellow marrow is 80% fat, 15% water, and 5% protein.
At birth, nearly the entire skeleton is composed of red marrow. Red marrow converts to yellow marrow in a predictable pattern from peripheral to central. The adult marrow pattern is usually attained by 25 years of age.
Adult marrow pattern is primarily red in the axial skeleton (skull, spine, pelvis, ribs, and sternum) and proximal humeri and femora.
Magnetic resonance imaging (MRI) features of normal marrow are predictable on T1- and T2-weighted sequences. Yellow marrow has signal intensity similar to subcutaneous fat on T1-weighted sequences and low signal intensity on T2-weighted sequences. Red marrow has lower intensity than fat on T1-weighted and signal intensity near that of fat (high) on T2-weighted sequences. Short-T1 inversion recovery (STIR) sequences show red marrow as even higher intensity compared with T2-weighted sequences.
Chan BY, Gill KG, Rebsamen SL, et al. MR imaging of pediatric bone marrow. Radiographics. 2016;36:1911-1930.
Vogler JB, Murphy WA. Bone marrow imaging. Radiology. 1988;168:679-693.
▪ MARROW RECONVERSION
Hematopoietic demand may result in conversion of yellow to red marrow.
The sequence of red marrow conversion occurs in reverse of the red to yellow marrow conversion, occurring as one develops adult marrow pattern.
Reconversion begins in the axial skeleton and moves peripherally.
Reconversion is symmetric, but not necessarily uniform.
Patients without bone marrow disorders may develop expanded red marrow termed “hyperplasia.”
Marrow reconversion, or hyperplasia, may be the result of multiple conditions:
Marrow replacement diseases
Cyanotic heart disease
Imaging of red marrow reconversion is best accomplished with MRI. Signal intensity changes may be difficult to differentiate from infiltrative diseases. T1-weighted images show areas of decreased signal intensity. T2-weighted and STIR sequences show signal intensity higher than fat.
Shellock FG, Morris E, Deutsch AL, et al. Hematopoietic marrow hyperplasia: high prevalence on MR images of the knee in asymptomatic marathon runners. Am J Roentgenol. 1992;158:335-338.
Vande Berg BC, Levouvet FE, Moyson P, et al. MR assessment of red marrow distribution and composition in the proximal femur: correlation with clinical and laboratory parameters. Skeletal Radiol. 1997;26:589-596.
▪ MYELOID DEPLETION
In patients with myeloid depletion, the hemopoietic (red) marrow is replaced by fatty (yellow) marrow.
Fat replacement may be diffuse or focal, depending on the extent and duration of the process.
Conditions leading to myeloid depletion include
Radiation changes are focal, so fat replacement occurs in the treated region. Changes occur as early as 3 to 7 weeks. With low doses, recovery may occur. Patients receiving more than 50 Gy are usually irreversible.
Fatty marrow has fat signal intensity on T1- and T2-weighted sequences.
FIGURE 11-3. Radiation therapy. Axial T1-weighted image shows a malignant soft tissue mass with fatty marrow in the femur. Compare with normal lower signal intensity red marrow in the opposite femur.
Cavenagh EC, Weinberger E, Shal DW, et al. Hematopoietic marrow reconversion in pediatric patients undergoing spinal irradiation: MR depiction. Am J Neuroradiol. 1995;16:461-467.
▪ MARROW ISCHEMIA
Causes of bone marrow ischemia and osteonecrosis are numerous.
Sickle cell anemia
Focal changes are most common in the hip, knee, and shoulder.
Avascular necrosis occurs in the subchondral epiphysis, and bone infarction occurs in the metaphysis and diaphysis.
Image changes may be evident on radiographs in advanced cases. MRI is the technique of choice for imaging early ischemic changes.
Magnetic resonance (MR) imaging often depicts the “double line sign” at the periphery of the osteonecrosis. This consists of an inner high signal intensity line representing granulation tissue and an outer low signal intensity line representing sclerosis. This is reported in up to 80% of cases of osteonecrosis.
FIGURE 11-5. Avascular necrosis of the hip. Coronal T1-weighted (A) and coronal T2-weighted (B) magnetic resonance (MR) images show avascular necrosis involving both femoral heads. There is acute collapse of the necrotic femoral articular surface on the right associated with diffuse marrow edema through the right proximal femur. Finding on the left is more chronic with serpiginous sclerosis seen at the margin of the marrow ischemia.
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