Purpose or Case Report: MRI interpretation of pediatric bone marrow is a challenging task due to the highly variable appearance. The bone marrow composition, especially the cellularity, changes with age, benign and malignant hematologic conditions, medications, among other etiologies. Detection of a marrow replacement process on MRI can be missed even by experienced radiologists. Normal hematopoietic bone marrow from birth to 9 years ranges from 20-40% fat (inversely related to cellularity). In malignant marrow replacement processes such as leukemia, bone marrow is highly cellular and fat percentage is low. Currently, no technique is routinely used in clinical practice to quantify bone marrow fat percentage on MRI.
Methods & Materials: A pilot study of patients with suspected malignant marrow replacement recruited by convenience sampling. MRI and bone marrow biopsy were performed within 8 days of one another. Bone marrow biopsy fat % was averaged between two blinded pathologists. MRI obtained from the suspected area of symptoms and core bone marrow biopsy obtained (as standard of clinical care) from the iliac crest. The MRI technique used was the Dixon technique with fat and water separation. Fat fraction is calculated based on the ratio of fat and water proton density. MRI was considered predictive of malignancy if fat fraction % was less than 20.

Results:
Six cases of patients (mean age 7.5y (range 3-14y)) were recruited. Pathologic diagnoses included: leukemia (n=5) or rhabdomyosarcoma (n=1). MRI was performed and analyzed from the: pelvis (n=4), spine (n=1), and elbow (n=1). MRI fat% correlated with pathology, but due to the small sample size was not significant (r=0.6, p=.20). Inter-pathologist correlation was high (r= 0.9, p<0.05). Elbow MRI fat% correlated with pathology (76 v. 67.5%), but was falsely negative in predicting leukemia. Of all other locations, mean fat percentage calculated by MRI versus pathology was similar (average± standard deviation, 3.7± 1.3 v 3.0± 3.0).


Conclusions: MRI can help radiologists detect a malignant marrow replacement process with more confidence using the fat fraction technique. Studies are ongoing to compare these cases to control patients.