Normal age-related conversion of bone marrow in the mandible: MR imaging findings

The mandible is one of the most common sites for osteomyelitis and other marrow-based diseases. Therefore, knowledge of the normal patterns of marrow distribution could help evaluate marrow-based diseases. The purpose of this study was to assess the age-related normal sequence of conversion from hematopoietic to fatty marrow in the mandible as depicted on MR images.

We prospectively reviewed T1-weighted MR images of the mandible for the distribution of hematopoietic and fatty marrow. Forty-five subjects 4 months to 25 years old with no known marrow abnormality were examined with the spin-echo technique. Marrow conversion was assessed in the condyle, ramus, angle, and body of the mandible using visual grading based on homogeneity, signal intensity, and a signal-intensity ratio determined by the intensities of the surrounding subcutaneous fat and air.

Conversion of hematopoietic to fatty marrow occurred first in the mandibular body, followed by the angle, ramus, and finally the condyle. The marrow in the region distal to the ramus had almost fully converted to fatty marrow by the third decade of life, but the remaining regions contained some hematopoietic marrow. Further substantiating these results, the signal-intensity ratio increased up to about 90% in the angle and 70% in the ramus by the age of 10 years and then leveled off. On the other hand, the signal-intensity ratio in the condyle reached 60% by age 15 and remained unchanged for the following 10 years.

The normal age-related conversion from hematopoietic to fatty marrow in the mandible follows a well-defined sequence, first seen in the mental region early in childhood, then in the body, the ramus, and finally the condyle.