On the fluid-tissue contrast behavior of high-resolution steady-state sequences

In general, MR image contrast is expected to be resolution independent, but a pronounced loss of contrast is observed between fluids and tissues with contemporary musculoskeletal protocols (typical inplane resolution << 1 mm) using nonbalanced steady-state free precession, such as double echo steady state. For nonbalanced steady-state free precession, diffusion sensitivity increases with increasing spoiler moments which increase with decreasing voxel size, suggesting diffusion damping as the major cause for the observed contrast variation. This is confirmed by simulations and measurements indicating that for fluids, diffusion effects become apparent already for resolutions Δx < 1 mm, whereas tissues typically require Δx < 200 μm. Gradient spoiling, however, is generically not minimized but frequently applied along the readout direction. For anisotropic steady-state free precession scans, the loss of contrast between fluids and tissues from diffusion can thus be minimized by simply moving the spoiler gradients to the lowest resolution direction.