SSFP

In imaging, faster accumulation by means of the steady-state free procession (SSFP) has been used successfully for 15N, but the method has so far not been reported for spectroscopy, although it should prove useful there also, provided proper attention is paid to phasing problems. 

The steady-state free-procession (SSFP) technique was used with no NOE, and in the case of aniline the T of the 15N was reduced by addition of chromium acetylacetate as a relaxation reagent to the sample. The 15N spectrum of aniline showed a single broad line (Avi/2 = 42 Hz) in which, because of proton exchange125,126, there is no evidence of N H coupling (J = 80 Hz). 

Fig. 2.2.11 The SSFP technique. A steady state of transverse magnetization is maintained by rapid application of rf pulses. A typical flip angle a is 90°. The equilibrium value of the SSFP magnetization depends on the resonance offset i2o and the pulse spacing r. Adapted from [Call] with permission from Oxford University Press.

Instead of the SSFP method, pulsed excitation is used [Bot2, Scol]. However, the lines are broadened for slow modulation frequencies, and sideband artefacts result if the modulation frequency is increased beyond 1 / Tf [Fei 1, Man 1 ]. The sidebands appear at multiples of the modulation frequency. If the modulation frequency is increased beyond the spectral width, they can be filtered out. To achieve such high-gradient modulation frequencies, the gradient coils need to be part of a resonant circuit. Modulation frequencies of up to 60 kHz can be achieved on small-gradient systems. 

Under repetitive pulsing the transverse magnetization in the steady state is given by the SSFP formula (2.2.34),... 

The SRTS sequence consists of a preparatory pulse and an arbitrary long train of the phase-coherent RF pulses of the same flip angle applied with a constant short-repetition time. As was noted above, the "short time" in this case should be interpreted as the pulse spacing T within the sequence that meets the condition T T2 Hd. The state that is established in the spin system after the time, T2, is traditionally defined as the "steady-state free precession" (SSFP), ° and includes two other states (or sub-states) quasi-stationary, that exists at times T2effective relaxation time) and stationary, that is established after the time " 3Tie after the start of the sequence.The SSFP is a very particular state which requires a specific mechanism for its description. This mechanism was devised in articles on the basis of the effective field concept and canonical transformations. Later approaches on the basis of the average-Hamiltonian theory were developed. ... 

The term "short SSFP sequence" (SSS) will be also used to designate sequences with duration not exceeding the spin-spin relaxation time T2. Sequences with duration less than T2 do not result in any appreciable changes of the initial spin temperature, and therefore its constituent pulses can in this case be regarded as a group of individual pulses with its effect determined by the effect of each pulse in accordance wifh its sequence order in the group, as one can see in paper. When condition 2t < T is also complied with, than all transients created by each pulse are added into one continuous signal. 

The fact that the duration of transient signals after SSFP type sequences can exceed T (Tj is a time constant of the free induction decay) can be used to eliminate spurious signals. The SSS can replace conventional pulses in the multiple-pulse sequences with intervals between pulses longer than T2 while detecting explosives at frequencies higher than 3 MHz (RDX, HMX, tetryl), where the duration of... 

Suppose both blocks consist of the SSFP type sequences. At the time > T2 after the beginning of the first pulse sequence the process in the spin system can be considered as a quasi-stationary one. Such state of the spin system may be described by the density matrix... 

T2-weighted gradient-echo sequences with the steady-state free precession (SSFP) technique were initially usedby Wallner et al. (1991), Morimoto et al. (1992),HALL-CRAGGsetal. (1993), and IsmzAKiet al. (1993). Subsequently, other investigators have utilized two-dimensional (2D) and three-dimensional (3D) heavily T2-weighted fast spin-echo sequences... 

PVR perspective volume rendering SSFP steady-state free precession... 

Figure 4 The response from nitrogen-14 in a sample of HMX to the SSFP pulse sequence with offset -5 kHz. The resonance frequency is 5.301 MHz at 295 K.

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