Separation of diffusion and slow flow effects by use of flow rephasing and dephasing.

Spin-echo signal intensity alterations due to diffusion and slow flow are investigated in connection with modified Stejskal-Tanner pulse sequences for 2D Fourier MR imaging, one for flow rephasing and the other for flow dephasing gradient waveforms. The theoretical considerations and experimental results concerning the diffusion coefficient measurements of slowly flowing material by these sequences are summarized as follows: (a) By using the flow dephasing sequence with different diffusion and flow-sensitive gradients, slow flow effects can be distinguished from diffusion effects based on the quantitative difference between the diffusion- and flow-sensitive gradient amplitude dependences of these processes. (b) By using the combination of the flow rephasing and dephasing sequences with the same diffusion sensitivity, slow flow effects can be distinguished from diffusion effects directly based on their qualitative difference. By use of the latter approach, several in vivo images are also presented, which mainly represent macroscopic motions of spins, including slow flow such as tissue perfusion.