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Frequency selective rf pulse

Slice selection is accomplished by applying a frequency-selective rf pulse in the presence of a gradient. [Pg.333]

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 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. 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... [Pg.70]

Figure 2 RF and magnetic field gradient pulse sequence used in NMR microscopy along with the associated trajectory through Ir-space. The frequency-selective 180° pulse is used to select a layer of spins (the slice plane) to participate in the spin echo and hence to contribute to the image. NT=acquisition time TE=/echo time. Figure 2 RF and magnetic field gradient pulse sequence used in NMR microscopy along with the associated trajectory through Ir-space. The frequency-selective 180° pulse is used to select a layer of spins (the slice plane) to participate in the spin echo and hence to contribute to the image. NT=acquisition time TE=/echo time.
A 2D NMR experiment involves a selection of pulses, delays, frequencies, RF phases and amplitudes,... [Pg.337]

Siegel et al. showed that enhancement of the CT can also be obtained using hyperbolic secant (HS) pulses to invert selectively the STs [74], Unlike the DFS waveform, whose frequency sweep is generated by a constant rf-pulse phase while modulating the amplitude, the HS pulse utilizes both amplitude and phase modulation, yielding an enhancement exceeding that obtained by DFS or RAPT [61, 74, 75]. Most recently, the pulse sequence called wideband uniform-rate smooth truncation (WURST) [76] was introduced to achieve selective adiabatic inversion using a lower power of the rf-field than that required for the HS pulses [77,78]. One of its applications involved more efficient detection of insensitive nuclei, such as 33S [79]. [Pg.136]

Fig. 5. Pulse sequence for MR detection of vibration using a radiofrequency field gradient. A binomial 1331 radiofrequency pulse (pulse length D, interpulse delay r) is applied in-phase with the mechanical wave. Thus the vibration period 7V is equal to 4(D + r). The number of cycles can be increased to ensure a better frequency selectivity. The constant RF field gradient generated by a dedicated RF coil allows space encoding without using conventional static field gradients (from Ref. 16 with permission from Elsevier). Fig. 5. Pulse sequence for MR detection of vibration using a radiofrequency field gradient. A binomial 1331 radiofrequency pulse (pulse length D, interpulse delay r) is applied in-phase with the mechanical wave. Thus the vibration period 7V is equal to 4(D + r). The number of cycles can be increased to ensure a better frequency selectivity. The constant RF field gradient generated by a dedicated RF coil allows space encoding without using conventional static field gradients (from Ref. 16 with permission from Elsevier).

See other pages where Frequency selective rf pulse is mentioned: [Pg.43]    [Pg.407]    [Pg.315]    [Pg.746]    [Pg.315]    [Pg.746]    [Pg.214]    [Pg.317]    [Pg.315]    [Pg.3422]    [Pg.310]    [Pg.920]    [Pg.369]    [Pg.866]    [Pg.43]    [Pg.407]    [Pg.315]    [Pg.746]    [Pg.315]    [Pg.746]    [Pg.214]    [Pg.317]    [Pg.315]    [Pg.3422]    [Pg.310]    [Pg.920]    [Pg.369]    [Pg.866]    [Pg.474]    [Pg.160]    [Pg.16]    [Pg.241]    [Pg.156]    [Pg.309]    [Pg.547]    [Pg.66]    [Pg.305]    [Pg.722]    [Pg.1522]    [Pg.387]    [Pg.17]    [Pg.168]    [Pg.438]    [Pg.396]    [Pg.340]    [Pg.179]    [Pg.952]    [Pg.359]    [Pg.2]    [Pg.3]    [Pg.46]    [Pg.14]    [Pg.14]    [Pg.15]   
See also in sourсe #XX -- [ Pg.43 ]

See also in sourсe #XX -- [ Pg.310 ]




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