Selective rf pulses

Figure 5.5.8 shows the volume selective spectroscopy pulse sequence used. This pulse sequence combines elements of NMR spectroscopy and MRI pulse sequences three slice selective rf pulses are applied in three orthogonal directions to obtain 1H spectra from pre-determined local volumes within the sample [30]. In this particular application, spectra were recorded from local volumes of dimension 1.5 mm x 1.5 mm x 0.5 mm within the fixed bed the data acquisition time for each spectrum was 3 min. Figure 5.5.9(a) shows the local volumes selected within slice 3 of the bed, as identified in Figure 5.5.7. In Figure 5.5.9(b) the 1H spectra recorded from within these volumes are shown data are presented only for the range of... 

In one spatial dimension (usually denoted as the slice-select direction), a selective RF pulse with a certain... 

Fig. 12. Sequences for volume selective single voxel spectroscopy. Both techniques work with three slice-selective RF-pulses. (a) The Point RESolved Spectroscopy (PRESS) sequence generates a volume selective double spin-echo. The entire time delay between the initial 90° excitation and the echo is sensitive to transverse relaxation, (b) The Stimulated Echo Acquisition Mode (STEAM) sequence generates a stimulated echo. Maximal signal intensity (without relaxation effects) is only half the signal intensity of PRESS under comparable conditions, but slice profiles are often better (only 90° pulses instead of 180° pulses) and the TM interval is not susceptible to transverse relaxation, (c) The recorded echo signal is only generated in a volume corresponding to the intersection of all three slices.

Slice selection is accomplished by applying a frequency-selective rf pulse in the presence of a gradient. 

The transformation of the density matrix by a 90° pulse and its subsequent evolution as the magnetization precesses freely depend on whether the pulse is applied only to one nucleus or to both I and S. We treat first the situation that would normally occur in a heteronuclear system, where a 90x pulse is applied only to the I spins—a 90pulse. This treatment is, of course, also applicable to a homonu-clear system subjected to a selective rf pulse. 

Fig. 6.2.5 [FRAl] Timing diagrams of basic STEAM imaging sequences. Schemes (a)-

Selective pulses are widely used in many pulse sequences [5.88, 5.89], not just for solvent signal suppression. The transformation of an n dimensional experiment into a (n-x) dimensional experiment by the application of x selective pulses not only reduces experiment time but it also keeps the acquired data matrix to a minimum. The implementation of selective pulses can be easily incorporated into pulse sequence design but the choice of selective pulse and associated parameters depends upon the current problem under investigation. When implementing pulse sequences that use selective rf pulses the following aspects must be considered ... 

Figure 1 ISIS. In eight separate acquisitions combinations of up to three frequency-selective RF pulses invert the longitudinal magnetization in three orthogonal slices prior to a nonselective excitation pulse and collection of the free induction decay. The inversion pulses modify the resultant phase of the transverse magnetization existing after excitation. For three-dimensional localization, the inversion pulses are applied and the data added or subtracted according to the protocol in Table 1.

Figure 4 The chemical shift displacement artifact. (A) An applied linear magnetic gradient encodes spatial position in the resonant frequencies of two particular spectrum peaks (represented by the sloping lines). Peak 1 has a different chemical shift to peak 2. A selective RF pulse, centered on frequency and with bandwidth A/, will excite a slice at a different position for each peak as shown. (B) Increasing the strength of the linear magnetic gradient reduces the difference in slice position - however, to achieve slices with the same spatial width as in (A), a larger bandwidth RF pulse must be used.

The opposite extreme regime, when co coq, involves the so-called soft or selective RF pulses. In this case, pulses of long duration and low power can be used to excite just one of the transitions, which is achieved by a suitable choice of the resonance offset (72) and also of the pulse length. The main aspect to be considered here is that each transition has associated with it a specific frequency and a different effective nutation fi equency, which depends on the values of I and m. For selective excitation of a transition between the levels m + and m, it is found that the ideal soft jr/2 pulse must be shorter than the corresponding hard itjl pulse (which excites all transitions) by a factor of -3-1) —m m 1). For excitation of just the central transition (1 /2 -1 /2) for half-integer spin nuclei, the... 

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