Signal losses in diffusion preparation: comparison between spin-echo, stimulated echo and SEASON.

Diffusion-weighted magnetic resonance imaging and spectroscopy commonly apply a spin-echo or stimulated echo preparation including sensitizing field gradients. The article reports on a systematic numerical approach to an optimum diffusion preparation considering undesired signal losses caused by relaxation. A large range of possible applications on whole-body units and animal scanners is covered. Instructions for an optimized type and timing of the diffusion preparation are provided for the readership, based on the desired diffusion weighting (b-value), the available maximum field gradient amplitudes, the RF pulse durations and gradient ramp times, and the relaxation characteristics of the specimen (or tissue) of interest. In addition, a new type of diffusion preparation named SEASON (simultaneous Spin-Echo And Stimulated echO preparatioN) is introduced and compared with spin-echo and stimulated echo diffusion preparation. It is demonstrated that spin-echo preparation is superior to stimulated echo preparation in all cases with T2 approximately T1 and in all cases with relatively low diffusion weighting resulting in short duration of diffusion sensitizing gradients delta << T2. For tissues with T2 << T1 (as musculature or red bone marrow) stimulated echo preparation becomes superior to spin-echo preparation for high ratios b/A2 (h-value indicates diffusion weighting, A is the maximum gradient amplitude). The new SEASON technique allows a higher yield in signal intensity compared to spin-echo or stimulated echo preparations in clinically relevant cases.