Spin-echo phase encoded velocity

Fig. 27. MR flow images of unsteady-state flow of water through a fixed bed with a column-to-particle diameter ratio of 2. Velocity maps measured by a standard spin-echo phase encoding velocity measurement and the GERVAIS pulse sequence are compared for values of Re of (a) 200 and (b) 300. In each case (i) is the standard spin-echo phase encoding velocity image, and (ii) and (iii) are the and flow vectors measured using GERVAIS.

In order to avoid flow artifacts it may be advisable to replace the spatial encoding pulses (right-hand box) by velocity compensated pulses such as shown in Figure 2.9.4(e) for phase encoding. The amplitude of the Hahn spin-echo is attenuated by hydrodynamic dispersion. Evaluation of the echo attenuation curve for fixed intervals but varying preparation gradients (left box) permits the allocation of a hydrodynamic dispersion coefficient to each voxel, so that maps of this parameter can be rendered. 

An example of a velocity-encoding method is the bipolar-gradient method.35 Here, two identical pulses are applied during the dephasing and rephasing periods of a spin-echo sequence. Since the two pulses are in opposite polarity with respect to the time domain, the phase shift (p(t) in Eq. (28) is dominated by the velocity term, i.e. the first-order term of t in the expansion. 

Figure 6 (A) Pulsed gradient spin echo sequence used to encode spin magnetization phase for molecular translational motion. (B) Velocity and diffusion maps for a water molecule flowing through a 2 mm diameter capillary. The images are shown as stackplots. The velocity profile is Poiseuille while the diffusion map is uniform. Courtesy of RW Mair, MM Britton and the author.

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