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E1969. Atlas of Bone Marrow Maturation
Authors
  1. Desi Schiess; University of Texas Southwestern Medical Center
  2. Lelia Williams; University of Texas Southwestern Medical Center
  3. Cory Pfeifer; University of Texas Southwestern Medical Center
Background
Bone consists of trabecular bone and two types of marrow: red (hematopoietic) and yellow (fatty). Its composition – and therefore MRI appearance – changes with normal maturation (red to yellow conversion) and stress (red marrow reconversion). Bone marrow maturation follows a predictable pattern throughout the body and within individual bones. Maturation is typically complete by age 25, when red marrow is limited to the axial skeleton and proximal appendicular skeleton. It is important for the radiologist to understand the pattern of normal marrow maturation and its imaging appearance in order to be able to distinguish it from pathologic processes such as neoplastic replacement (leukemia, lymphoma, sarcoma, metastases), non-neoplastic edema due to trauma, infection, inflammation (osteomyelitis, juvenile inflammatory arthritis), or expected post-treatment changes (radiation, chemotherapy, colony stimulating factor, bone marrow transplant). Repetitive microtrauma is a unique pattern of injury in children as the physes are weak and more prone to injury.

Educational Goals / Teaching Points
Review the normal stages of bone marrow maturation in the lower extremities of infants, children, and adolescents with MR imaging examples. Review pathologic cases of abnormal bone marrow (i.e. leukemia). Review the MR appearance of expected post-treatment changes (i.e. fatty marrow after radiation).

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
The normal changes that occur with bone marrow maturation are most pronounced in childhood. There is growing use of magnetic resonance imaging for marrow assessment in pediatric patients due to its lack of radiation and exquisite tissue contrast. Trabecular bone demonstrates minimal signal intensity due to lack of mobile protons. Red marrow is mildly hyperintense relative to skeletal muscle and intervertebral disks on T1-weighted images and demonstrates mild enhancement due to its vascularity, which decreases with age. Yellow marrow follows the signal intensity of subcutaneous fat without significant contrast enhancement. T1-weighted images demonstrate optimal contrast between red and yellow marrow. Fluid-sensitive sequences are most useful for evaluating pathology. Contrast-enhanced images are more useful in older children and adults who have less vascular marrow. Red marrow reconversion starts in the proximal metaphysis, followed by the distal metaphysis, and finally within the diaphysis. Severe stress can result in epiphyseal marrow reconversion, marrow cavity expansion, or extramedullary hematopoiesis. Tumors most often occur in the region of red marrow due to its richer vascularity. Infiltrative processes typically demonstrate T1 hypointensity relative to muscle.

Conclusion
It is important to be familiar with the patterns of normal and abnormal bone marrow conversion. The radiologist may be the first provider to detect unsuspected disease and can have a significant impact on patient management.