Influence of Magnetic Field Strength on T2* Decay and Phase Effects in Gradient Echo MRI of Vertebral Bone Marrow

Abstract

Objective: The goal of this study was twofold: first, to quantify the influence of magnetic field strength (B-0) on T2* and T2(susceptibility) relaxation rates in vertebral bone marrow; and second, to determine the significance of in-phase/out-of-phase phenomena in GE MRI as a function of TEs at different B-0 values. Materials and Methods: Magnetic resonance imaging of the spine was performed at 1.5, 1.0, and 0.2 T using a GE sequence (2D-FLASH) with variable TE. Vertebral body signal intensities were measured and plotted against TE values. These curves were used to calculate T2* decay of bone marrow at different B-0 values. Results: At all field strengths, we observed exponential signal decay, modulated by a cosine function, the periodicity of which was proportional to B-0. The T2* values of vertebral bodies were 9.7 +/- 1.8 ms at 1.5 T, 17.2 +/- 2.4 ms at 1.0 T, and 53 +/- 5 ms at 0.2 T. True T2 relaxation time (independent of field strength) was 62 +/- 4 ms. The T2(susceptibility) was 11.7 ms at 1.5 T, 24.8 ms at 1.0 T, and 585 ms at 0.2 T. Conclusion: We conclude that at low B-0, susceptibility effects become less important and T2* approaches true T2 relaxation