Pulsed field gradients Subject

Fig. 8.33 Pulse sequence schematic for the gradient ID NOESY experiment. The double pulsed field gradient spin echo (DPFGSE) refocuses only for that resonance subject to the selective pulse. All other magnetization is left defocused in the xy-plane. Ultimately, a NOESY spectrum is recorded only for the selected re-...

The basic element of an NMR diffusion measurement consists of a spin-echo sequence, m combination with the application of static or pulsed field gradients. Several common sequences are shown inFigure 8, and we discuss these only briefly as the subject is covered in... 

A much simpler and more flexible approach, however, extends the scheme of pulsed magnetic field gradients discussed above to include more complex time dependences. To see how this can be done, we write down the general dependence of the phase shift accumulated at time t subject to a space-dependent Larmor frequency ... 

The rf portion of Fig. 14.2 shows a 90°, r, 180° spin echo pulse sequence, rather than a simple 90° pulse. All imaging studies employ either a spin echo sequence or a gradient echo to avoid acquisition of data during the FID, which decays rapidly in the presence of a magnetic field gradient. Instead, data acquisition occurs during the echo, when the rf circuitry is not subject to aberrations... 

Fig. 9. (A) Selective excitation and destruction of magnetization using a magnetic field gradient pulse. PGSE sequences used for diffusional attenuation of the solvent signal, based on the Hahn spin-echo sequence (B) and the stimulated-echo sequence (C). In the Hahn spin-echo sequence the magnetization is always subject to spin-spin relaxation. However, in the stimulated-echo sequence the delays can be set such that A is mainly contained in t2 where the relaxation is longitudinal and thus this sequence is preferable for large solute molecules since the condition T2 < usually holds.

Both of these problems are alleviated to a large extent by moving to an alternative method of selection, the use of field gradient pulses, which is the subject of the next section. However, as we shall see, this alternative method is not without its own difficulties. 

Point resolved spectroscopy (PRESS) and stimulated-echo acquisition mode (STEAM) use three selective RE pulses in combination with linear field gradients to yield localization of the VOI in a single shot. Thus, the localization provided by these techniques is not degraded by subject motion or system instability. Generally, the use of three RF pulses leads to the formation of five coherent pathways for echo formation. Both PRESS and STEAM are designed to yield a particular echo that is localized to the VOI the other echoes are suppressed by the use of spoiler gradients and phase cycling of the RF pulses. The... 

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