Sequence repetition time

Because NMR signals are weak, it may be desirable to use a pulse-sequence repetition time (Tr) much less than five times the longitudinal relaxation time (Tj) in order to maximize the SNR per unit acquisition time. The strong spatial dependence of the flip angle also causes the relaxation state of the nuclei to vary strongly with position. Figure 4 shows the spatial sensitivity of a surface coil for various TrS, demonstrating the imprecise boundary of the sensitive volume. 

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. 2.4.5 Profile of a phantom made of three 2-mm thick rubber layers separated by glass slides of 2- and 1-mm thick. The CPMG sequence was executed with the following parameters repetition time = 50 ms, tE = 0.12 ms, number of echoes = 48 and 64 accumulations. The profile was scanned with a spatial resolution of 100 pm in 5 min.

Fig. 2.4.14 Profile of a multi-layer polymer coating used to protect concrete surfaces from environmental corrosion. The profile is the signal amplitude resulting from the addition of the first 32 echoes acquired with a CPMG sequence with tE = 50 ps. It has an FOV of 8 mm and was measured with a spatial resolution of 100 pm. Using 256 scans per point and a repetition time of 100 ms, the total acquisition time per point was 25 s.

Most clinical examinations apply robust spin-echo or fast spin-echo sequences. These types of sequences provide tissue contrast changes by variation of the chosen repetition time TR (time interval between succeeding RF excitations) and echo time TE (time delay between RF excitation and signal acquisition). 

The manner in which the RF pulse is applied is critical to NMR analysis. A very simplified pulse sequence is a combination of RF pulses, signals, and intervening periods of recovery, as illustrated in Figure 6.80. The main components of the pnlse sequence are the repetition time, TR, which is the time from the application of one RF pulse to the application of the next RF pulse (measured in milliseconds) and the echo time, TE. The repetition time determines the amount of relaxation that is allowed to occur between the end of one RF pulse and the application of the next. Therefore, the repetition time determines the amount of Ti relaxation that has occnrred. The echo time is the time from the application of the RF pnlse to the peak of the signal induced in the coil (also measured in milliseconds). The TE determines how much decay of transverse magnetization is allowed to occur before the signal is read. Therefore, TE controls the amount of T2 relaxation that has occnrred. 

The imaging of conversion within the fixed bed was achieved by using a distortionless enhancement by polarization transfer (DEPT) spectroscopy pulse sequence integrated into an imaging sequence, as shown in Fig. 44. In theory, a signal enhancement of up to a factor of 4 (/hZ/c 7i is the gyromagnetic ratio of nucleus i) can be achieved with DEPT. In this dual resonance experiment, initial excitation is on the H channel. Consequently, the repetition time for the DEPT experiment is constrained by Tih (< T lc) where Tn is the Ty relaxation time of... 

The third method involves a three pulse sequence, 90 — r — 180° — x — 90°, with a repetition time of tr s. This pulse sequence refocuses the magnetization vector M0 into its equilibrium position within the repetition time, thus representing a pulse driven relaxation acceleration. This technique, known as DEFT NMR [23, 24] (driven equilibrium Fourier transform NMR) can be understood by following the behavior of the magnetization vector Mq under the influence of the pulse sequence in the rotating frame of reference (Fig. 2.17(a-e)). 

The repetition time tr of the pulse sequence is independent of 7j, which may be different for nonequivalent nuclei. The optimum repetition time has been found to be t, = 4 r [22]. DEFT NMR requires careful adjustment of pulse widths for 90° and 180° pulses and (computer-controlled) pulse programming for accurate timing between pulses and pulse sequences. Other methods for improving signal noise using other pulse sequences and spin echo trains have been described [22, 25]. DEFT NMR, however, appears to be the most efficient method so far, as long as Tj and T2 are of the same order of magnitude. 

Fig. 4. 50 MHz DD/MAS 13C NMR spectrum of bulk polyethylene with a viscosity-average molecular weight of 3.0 x 106 at room temperature. The spectrum was obtained by pulse sequence I with a repetition time, X( = 17,000 s. The chemicalshift is based on that of tetramethylsilane (TMS)...

Fig. 14. DD/MAS 13C NMR spectra of different polyethylene samples crystallized under high pressure, obtained by a n/4 single pulse sequence [ti/4-FID-t ] (a) Fraction 1, (b) Rigidex 9, (c) HO20-54P, (d) Hifax. The repetition time x( is indicated in each spectrum...

Fig. 20. Equilibrium spectrum and line shape analysis in the IB2 carbon range for the sample PI6. The spectrum was obtained by a single pulse sequence with a repetition time of 50 s...

Figure 22-(a) shows the DD/MAS spectrum in the resonance range of a-methyl-ene carbon at 0 °C. This spectrum represents the thermal equilibrium state of this sample, because it was obtained by a single pulse sequence with the repetition time of 600 s longer than 5 times the longest Tic in the system. The spectrum (b) is that of the crystalline component, which was obtained with use of Torchia s pulse sequence [53]. In the equilibrium spectrum, the noncrystalline contribution (amorphous plus interfacial) can be seen upfield to the crystalline component. Figure 23 shows the elementary line shapes of the amorphous and crystalline-amorphous interphases that comprise the noncrystalline resonance. 

Spectrum (a) shows the DD/MAS 13C NMR spectrum of the a-methylene carbon that was obtained by a single pulse sequence with a repetition time of 0.8 s. This is 0.8 s is longer than 5 times the Tic of the noncrystalline component and much shorter than Tic of the crystalline component (cf. Table 10). Hence, this spectrum represents the contribution from the noncrystalline component that consists of amorphous and crystalline-amorphous interphases. Spectrum (b) is a partially relaxed spectrum transversely for 600 ps. Since 600 ps is much longer... 

Molecular Conformation of sPP gel. Figure 27 shows the DD/MAS 13C NMR spectrum of sPP gel. This spectrum was obtained by a single-pulse sequence (tt /2—FIDdd-tt)ii with the repetition time ty more than 5 times the longitudinal relaxation time Tic. Hence, this spectrum reflects the thermal equilibrium state of the gel. For comparison, the spectrum of the bulk ttgg crystal of this sample... 

Fig. 27. Equilibrium DD/MAS 13C NMR spectrum of sPP/o-dichlorobenzene gel (13.6 wt%), obtained by a n/2 single pulse sequence with a repetition time of 300 s. In the lower part the equilibrium spectrum of the bulk sPP crystal in ttgg form is shown for reference. The arrows indicate the resonance of the amorphous component of each carbon...

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... 

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