NORMAL AGE-RELATED CONVERSION OF BONE-MARROW IN THE MANDIBLE - MR-IMAGING FINDINGS

OBJECTIVE. The mandible is one of the most common sites for osteomyeli tis and other marrow-based diseases. Therefore, knowledge of the norma l patterns of marrow distribution could help evaluate marrow-based dis eases. The purpose of this study was to assess the age-related normal sequence of conversion from hematopoietic to fatty marrow in the mandi ble as depicted on MR images. SUBJECTS AND METHODS. We prospectively r eviewed T1-weighted MR images of the mandible for the distribution of hematopoietic and fatty marrow. Forty-five subjects 4 months to 25 yea rs old with no known marrow abnormality were examined with the spin-ec ho technique. Marrow conversion was assessed in the condyle, ramus, an gle, and body of the mandible using visual grading based on homogeneit y, signal intensity, and a signal-intensity ratio determined by the in tensities of the surrounding subcutaneous fat and air. RESULTS. Conver sion of hematopoietic to fatty marrow occurred first in the mandibular body, followed by the angle, ramus, and finally the condyle. The marr ow in the region distal to the ramus had almost fully converted to fat ty marrow by the third decade of life, but the remaining regions conta ined 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 o ther hand, the signal-intensity ratio in the condyle reached 60% by ag e 15 and remained unchanged for the following 10 years. CONCLUSION. Th e 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 t he condyle.