Selective excitation gradient echoes

OPPOSE Double pulsed field gradient spin-echo (selective excitation) 9.3.3... 

Figure 3 presents an example of such a situation. The 2Q-HoMQC spectrum of apo-cytochrome c was acquired in 93% H2O at 480 ixM concentration on a Varian Unity/INOVA 600 MHz NMR instrument overnight, using a pulse sequence with gradient coherence selection and weak gradient spin-echo during excitation delays and the evolution period [29], respectively (I.P., not published). The spectral windows were 8 kHz both in F2 and F. ... 

Figure 4 Methods of selectivity enhancement using pulsed field gradients employing (a) selective excitation pulses, (b) a selective refocusing pulse and (c) a double piUsed field gradient spin echo (DPFGSE) sequence with inversion pulses...

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 3 Excitation profiles of the Watergate sequence obtained using either a selective 180° pulse sandwiched between two hard 90° pulses (left panel) or a 3-9-19 binomial sequence in place of the S element of the gradient echo (right panel) (see also Figure 2).

For chemical-shift imaging of plants, standard spin-echo imaging sequences are used. Chemical-shift resolution is introduced by selective excitation in the absence of gradients, chemical-shift difference methods (cf. Section 6.2.4), or by saturation of unwanted resonances. Good chemical-shift resolution requires high-field strengths, and microscopic... 

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.

Martin et al.3H measured the diffusion of methylene chloride (CH2C12) in liquid crystalline solvents by following the decay of the double quantum coherence. Zax and Pines39 measured diffusion of benzene in a nematic liquid crystalline solution using a pulse sequence (Fig. 22) which non-selectively excites MQC, permits this to evolve in the presence of a magnetic field gradient, then uses a series of A identical gradient-delay periods to selectively provide an echo which arises solely from the A-quanlum coherence in the evolution period t. ... 

Figure 9.18. Selective excitation sequences based on (a) a single and (b) a double pulsed field gradient spin-echo. The element S represents any selective 180° inversion pulse or pulse sequence.

Figure 9.19. Clean selective excitation with the double pulsed field gradient spin-echo sequence using a 40 ms Gaussian 180° pulse and gradients of 0.07 0.07 0.03 0.03 Tm- .

When implementing this sequence it may be necessary to add attenuation to the transmitter to increase the duration of each pulse so that the shorter elements do not demand very short (< 1 xs) pulses (note the similarity with the requirements for the DANTE hard-pulse selective excitation described above). The binomial sequences can be adjusted to provide an arbitrary overall tip angle by suitable adjustment of the tip angles for each element. For example, inversion of all off-resonance signals can be achieved by doubling all elements relative to the net 90 condition. Exactly this approach has been exploited in the gradient-echo methods described below. 

In Check it 5.4.1.11 the improved selective ID COSY experiment using a selective refocusing n pulse are calculated. These category of selective COSY experiments based on a gradient flanked selective spin echo generate less artefacts and are superior to the ID COSY pulse sequences with a selective excitation pulse. Essentially the flanking gradients cancel the artefacts in a similar manner as a "perfect EXORCYCLE" scheme [5.140]. 

Sequences must be optimized to achieve a high spatial resolution and a fast acquisition time. As sequences must allow analysis of the cartilage with high contrast to the surrounding tissue, thin sliced 3D spoiled gradient echo sequences with spectral fat suppression (Eckstein et al. 1998) or with selective water excitation (Hyhlik-Durr et al. 2000) are usually performed. The water excitation protocol allows the water-bound protons in the cartilage to be excited selectively and directly. Therefore, no prepulse is required as in conventional fat-suppressed imaging protocols. 

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