Normal bone marrow in the sacrum of young adults: differences between the sexes seen on chemical-shift MR imaging.

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Abstract
Age-related changes in the distribution of fatty and nonfatty bone marrow in the pelvis and femur are well recognized. However, mapping not only of age-related MR appearance of normal bone marrow but also of potential variations in females versus males is necessary to differentiate normal findings from marrow disease. Accordingly, we studied possible sex-related differences in the MR imaging appearance of sacral bone marrow in young adults. A population of 21 healthy men and 21 healthy women (17-42 years old) was prospectively studied with chemical-shift pulse sequences. MR imaging (1.5 T) of the sacrum was done with frequency-selective fat images (SENEX [selective nonexcitation] 300/27) and water images (SENEX 1000/27) by using an elliptical surface coil. Two independent observers compared the signal intensity of sacral marrow with the signal of fatty tissue planes adjacent to the sacrum (fat images) and the signal of gluteus muscle (water images). T2 relaxation times were estimated by using a two-parametric monoexponential fit on SENEX 1000/27 and SENEX 1000/54 images. In both sexes, fat MR images showed a higher fat content and greater heterogeneity in the bone marrow of the lateral masses than in the vertebral bodies of the sacrum (p < .05). Yellow marrow in the lateral masses of the sacrum appeared brighter in men than in women (p < .05). The heterogeneity of fatty marrow did not differ significantly between the sexes. On water MR images, the signal intensity of the sacrum was higher in women (p < .05). T2 relaxation times were longer in nonfatty marrow with a high water signal intensity (41.8 +/- 5.5 msec versus 33.4 +/- 2.6 msec p < .01). Our findings show that the normal appearance of the sacral bone marrow depicted on chemical-shift MR imaging differs between young men and women. Awareness of these differences is important to avoid misinterpretation of normal MR findings seen in patients in this age group.