Signal Acquisition

Figure 2. The spin-echo imaging pulse sequence may be modified to phase encode position or velocity, to observe chemical shift vs. position, or to observe signal attenuation due to diffusion in the presence of a gradient. Solid lines signify features present in standard 2-dimensional imaging experiments, dashed lines represent optional gradients foi velocity and diffusion experiments. The multiple lobes of the Phase Encode Gradient refer to the variation of gradient amplitude with each scan, starting at negative, through zero to positive. The Bipolar Velocity Encoding gradient varies in the same way. Gradients connected by brackets (F. E. Gradient, F. E. Dephase) illustrate that use of any gradient results in phase accumulation unless balanced by another gradient opposite in sense. The Refocusing Pulse reverses the sense of any phase previously accumulated. Interchanging components oi gradient time lines gx, gy and gz allows acquisition of spatial, diffusion and velocity information from 3 orthogonal planes.

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