MRI of bone marrow: opposed-phase gradient-echo sequences with long repetition time.

Signal intensity for opposed-phase gradient-echo (GE) sequences of tissues composed of fat- and water-equivalent cells such as red bone marrow is extremely sensitive to variation of the ratio of both cell populations (fat-to-water ratio QF/W). Because most bone marrow pathology results in variation of QF/W, GE sequences are characterized by high-contrast imaging of pathology. The aim of this study was to evaluate the influence of TR, TE, FA, QF/W and histology on signal intensity. Signal intensity of opposed-phase GE sequences as a function of TR, TE, FA, and QF/W was measured for a fat-water phantom and cadaver specimens of normal bone marrow (red and yellow) and pathological bone marrow (tumors). All specimens were correlated to histology. Opposed-phase GE imaging of red bone marrow pathology results in low-signal-intensity imaging of intact red bone marrow and high-signal-intensity positive contrast imaging of pathology associated with a change in QF/W. In first-order approximation the signal intensity of pathology is linearly correlated to the change in QF/W. Opposed-phase GE imaging is a sensitive imaging technique for red bone marrow pathology. Relative contrast of red bone marrow pathology is similar to fat-suppressed imaging techniques. Acquisition time is identical to T1-weighted SE sequences.