Echo train

Fig. 1.22 RARE sequence. Here the formation of the first spin echo is conventional. The CPMG form of spin echo is used to avoid the accumulation of flip angle errors over the echo train. However, before the second echo can be acquired, the phase-encoding has to be rewound to undo the dephasing of the spins. Therefore, a phase encoding step of equal...

The slice selection procedure can be combined with a number of pulse sequences to spatially resolve NMR parameters or to contrast the profiles with a variety of filters. The most commonly used acquisition schemes implemented to sample echo train decays are the CPMG [(jt/2)0—(Jt)90] or a multi-solid echo sequence [(jt/ 2)0-(jt/2)9o]. In these instances, the complete echo train can be fitted to determine... 

Fig. 2.4.7 Profile of a PE gasoline tank wall. T-, and T2 were measured across the sample, and uniform values of about 90 ms and 300 ps, respectively, were obtained, except for EVOH where T2 drops to 30 [is. The profile amplitude is the coefficient at zero frequency of the FT of the signal obtained as the direct addition of the first 8 echoes generated with a solid-echo train.

The skin layers from the palm of the hand were scanned in vivo. A CPMG sequence was applied to sample the echo train decays as a function of depth. The decay was determined by both the relaxation time and the diffusion coefficient. To improve the contrast between the layers, a set of profiles was measured as a function of the echo... 

Figure 2.4.11 Profiles of paintings where different layers can clearly be resolved. A solid-echo train was used with tE = 40 (is, and the first 4 echoes were used to calculate the amplitude. The profiles were reconstructed by moving the sensor in steps of 50 pm in the paint and canvas regions, and 100 pm in the gypsum and wood layers. Using 128 scans per point and a repetition time of 100 ms the total acquisition time per point was 16 s. Profiles of paint based on tempera ( ) and oil ( ) binders show appreciable difference.

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. 232. Spin-echo trains for the determination of the 3C spin-spin relaxation time T2 of carbon tetrachloride neat liquid sample, 90% 13C enriched 25 °C ...

Figure 7.22 The NMR-MOUSE (a) Schematic. The NMR sensor consists of an u-shaped permanent magnet with a solenoidal rf coil placed in the gap. (b) Photo of the NMR-MOUSE testing a tyre, (c) Example of a train of successive Hahn echoes generated according to Carr, Purcell, Meiboom and Gill (CPMG echo train) for carbon-black filled SBR measured by the NMR-MOUSE. The time constant of the echo-envelope defines T...

Sequence TR/TE (ms) Flip angle (°) Echo train length Field of view (cm) Matrix size Slice thickness (mm) Bandwidth... 

With a test sample on the optically flat top surface of the bar, the pulse echo train is reduced in amplitude. This attenuation is owing to the refraction of part of the ultrasonic wave into the test sample at the frequency used. The ratio of successive peak amplitudes may be measured on the oscilloscope and expressed in decibels loss per echo. From this, the loss per echo with no sample on the bar can be substracted to give a value Adb which is related to the mechanical shear impedance of the sample. Rapid changes can be conveniently monitored by a recorder which follows the peak signal of a selected echo. 

Fortunately, the variable t can be adjusted, independent of the desired total length of the echo train, so that during the period r little diffusion occurs and the second exponential factor can be made arbitrarily small, so that valid measurements of T2 can be made. 

Fig. 3.4.1 Homogeneously and inhomogeneously broadened lines, (a) Echo train generated by repeated refocussing of the FID (CPMG method, cf. Fig 2.2.10(b)). (b) The Fourier transform of the slowly decaying echo envelope is the homogeneously broadened line, (c) The Fourier transform of the fast decaying echo is the inhomogeneously broadened line.

Fig. 5.4.11 [Cal 11 ] Modulated gradient NMR for probing spectral densities of diffusive translational motion. The pulse sequence (left) consists of a CPMG echo train with interdispersed gradient pulses G(t) which produces the time-dependent wave vector k(t). The spectrum K(co) of k(t) probes the spectral density of diffusive motion at a single frequency (right).

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