Gradient echo based sequences

Gradient echo-based sequences, such as constructive interference in steady state sequences, also are used for the imaging of the inner ear region (Casselman et al. 1996 Held et al. 1997). However, the specific absorption rate of these sequences may be higher than that of 3D fast spin echo-based sequences, and susceptibility artifacts may be more pronounced, especially at 3 T scanners. 

Fig. 1.20 Gradient-echo based pulse sequences based on low flip angles. When low flip angles and short image repetition times are employed at the expense of transverse magnetization during the course of the complete image acquisition, this represents a FLASH sequence (without ). The combination of flip angle and repetition time can be adjusted in...

Fig. 1.21 Echo Planar Imaging (EPI) pulse sequence. Gradient-echo based multiple echoes are used for fast single-shot 2D imaging. Slice selection along Gs and frequency encoding along C, are utilized. Phase encoding is realized using short blipped gradient pulses along Gp.

Figure 2 Gradient-echo-based water suppression pulse sequences, (a) WATERGATE (b) water-flip-back (c) excitation sculpting (d and e) examples of the S pulse train that is sandwiched between the gradient echo (d) water-selective inversion...

Figure 9.24. Solvent signal suppiession by a diffusion filter. The sample is 1 mM lysozyme in 50 50 H20 D20 and suppression in (a) was achieved using a H stimulated-echo-based sequence employing a diffusion delay A of 50 ms, bipolar gradient pairs of 4 ms total duration ( ) and gradient strengths of 0.43 Tm. The standard spectrum is shown in (b).

There are a number of NMR methods available for evaluation of self-diffusion coefficients, all of which use the same basic measurement principle [60]. Namely, they are all based on the application of the spin-echo technique under conditions of either a static or a pulsed magnetic field gradient. Essentially, a spin-echo pulse sequence is applied to a nucleus in the ion of interest while at the same time a constant or pulsed field gradient is applied to the nucleus. The spin echo of this nucleus is then measured and its attenuation due to the diffusion of the nucleus in the field gradient is used to determine its self-diffusion coefficient. The self-diffusion coefficient data for a variety of ionic liquids are given in Table 3.6-6. 

Fig. 1.19 Spin-echo based pulse sequence to each gradient pulse, A the separation between encode velocity change. The gradients are each pair of bipolar gradient pulses and tm the stepped pair-wise independently (2D VEXSY) mixing time between the bipolar gradient pairs, or simultaneously (1 D VEXSY). For a VEXSY The opposite polarity of the bipolar gradient experiment, 7q to k4 are usually applied along pair is realized by an inversion 180° pulse, the same spatial direction. 8 is the duration of...

The MRA techniques discussed in the following base on gradient echo sequences and can generally be divided into flow-dependent and flow-independent ones. The main flow dependent techniques are time-of-flight MRA (TOF-MRA) and phase contrast MRA. CE-MRA, in contrast, is blood flow-independent, although flow phenomena can still influence... 

Time-of-flight MRA is based on gradient echo sequences with very short repetition times (Laub and Kaiser 1988). The repeated HF excitations cause a relative spin saturation - i. e. signal reduction - in stationary tissue, while inflowing unsaturated blood is depicted with high signal. A saturation... 

FySy), where = E", (Schleucher et al., 1994). This Hamiltonian can be implemented using a heteronuclear Hartmann-Hahn mixing sequence that creates an effective Hamiltonian = 2TrJ f (FySy + F S ) that is embedded between two 9(f I,S)y pulses. A heteronuclear isotropic Hartmann-Hahn (HIHAHA) transfer step (Quant et al., 1995a Quant, 1996) can be used for in-phase COS-CT, for example, from S to F (Sattler et al., 1995a). CQS-CT mixing steps yield sensitivity-improved experiments and are especially useful for experiments that are based on gradient echoes. 

As a consequence of having three time periods for the stimulated echo instead of two for the Hahn echo, a considerable variety of imaging schemes can be designed by assigning different functions to the individual pulses. Either one, two, or all three pulses can be made selective, and the selective pulses can be combined with suitable gradients. Thus, imaging sequences which provide more information than those based on Hahn echoes can be designed (cf. Section 7.2.4). 

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.

The most robust experiment to date [34] (Fig. 8.39b) is based upon the double pulsed field gradient spin-echo (DPFGSE) sequence as a means of selecting a target resonance from which the NOEs ultimately develop. This... 

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