CPMG

These experiments yield T2 which, in the case of fast exchange, gives the ratio (Aoi) /k. However, since the experiments themselves have an implicit timescale, absolute rates can be obtained in favourable circumstances. For the CPMG experiment, the timescale is the repetition time of the refocusing pulse for the Tjp experiment, it is the rate of precession around the effective RF field. If this timescale is fast witli respect to the exchange rate, then the experiment effectively measures T2 in the absence of exchange. If the timescale is slow, the apparent T2 contains the effects of exchange. Therefore, the apparent T2 shows a dispersion as the... 

Dissolution of the reaction product with R = mesityl in THF, followed by workup, afforded CpMg[But(NC6H2Me3-2,4,6)2](THF) as a white crystalline solid that was stable to loss of tetrahydrofuran at room temperature (Scheme 20). ... 

Volatility studies revealed that the unsolvated complex CpMg[But (NC6H3Pr2-2,6)2] sublimed unchanged at 180°C/0.05Torr, and was recovered... 

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. 2.4.4 Profile of a sample made from two latex layers 70 pm thick separated by a 150 pm thick glass spacer. The full profile is the combination of 10 images with an FOV of 50 pm, covering a total depth of 500 pm. Each of these images is the FT of the echo signal obtained as the addition of the first 16 echoes acquired during a CPMG sequence and 512 scans with...

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.

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... 

Fig. 2.4.13 CPMG decays of the pigments terra rossa ( ), azurro oltremare ( ), terra uerde (A), ocra giana ( ) and terra siena bruciata (f ). Based on the different decay times the color of the paint can be determined from the NMR signal.

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.

However, some aspects of the spin dynamics are better described using functions other than Fourier series. For example, the magnetization decay in a CPMG [28] experiment follows an exponential form,... 

The idea of exploration of relaxation correlation was first reported in 1981 by Peemoeller et al. [23] and later by English et al. [24] using an inversion-recovery experiment detected by a CPMG pulse train. This pulse sequence is shown in Figure 2.7.1. 

Diffusion-relaxation correlation has been utilized to study biological tissues, e.g., compartmentalization in tissues [32-35]. In many reports, a sequence that combines a stimulated echo-type sequence with a pulsed field gradient and a CPMG as a detection has been described [35]. Other pulses sequences have also been used to study the diffusion-relaxation correlation, e.g., Ref. [36]. 

Fig. 2.7.2 Diffusion-relaxation correlation se- The detection (2nd) segment for both is a quences using pulsed field gradients, (a) The CPMG pulse train that is similar to that in first segment is a spin-echo with the echo Figure 2.7.1. The amplitude or the duration of appearing at a time 2tcpi after the first pulse, the gradient pairs in both sequences is (b) The first segment is a stimulated echo incremented to vary the diffusion effects, appearing at a time tcpi after the third pulse.

Hiirlimann and Venkataramanan [43] have derived the kernel for the inversion-recovery CPMG experiment in an inhomogeneous field ... 

When Tcp is longer in a CPMG sequence under a constant gradient, the diffusion effect can be observed. For the Flahn echo coherence pathway where every jt pulse refocuses the dephasing, the diffusion contribution is,... 

Hence, a series of measurements with several Tcp values will provide a data set with variable decays due to both diffusion and relaxation. Numerical inversion can be applied to such data set to obtain the diffusion-relaxation correlation spectrum [44— 46]. However, this type of experiment is different from the 2D experiments, such as T,-T2. For example, the diffusion and relaxation effects are mixed and not separated as in the PFG-CPMG experiment Eq. (2.7.6). Furthermore, as the diffusion decay of CPMG is not a single exponential in a constant field gradient [41, 42], the above kernel is only an approximation. It is possible that the diffusion resolution may be compromised. 

When the measurement of a distribution of diffusivity is desired, improved signal-to-noise is needed. This can be achieved by using a diffusion-editing pulse sequence [7] rather than the CPMG sequence with varying echo spacing. This... 

We can perform spatially resolved Carr-Purcell-Meiboom-Gill (CPMG) experiments, and then, for each voxel, use magnetization intensities at the echo times to estimate the corresponding number density function, P(t), which represents the amount of fluid associated with the characteristic relaxation time t. The corresponding intrinsic magnetization for the voxel, M0, is calculated by... 

We represent the NMR relaxation distribution by the continuous number density function, P( t), of characteristic relaxation time t. Our measurements correspond to a series of CPMG echoes, represented by... 

The CPMG pulse sequence is composed of the initial 90° radiofrequency (rf) excitation pulse followed by a series of 180° rf pulses spaced to allow a train of... 

Figure 4.1.1 CPMG pulse sequence designed for three-dimensional imaging. TE is echo time, and Gi, G2 and G3 represent the gradient magnetic fields along the directions of zlt z2 and z3, respectively.

Our method is demonstrated with experiments on a Bentheimer sandstone sample. The sample was prepared to be cylindrically shaped with a diameter of 2.5 cm and a length of 2.0 cm. The sample was fully saturated with de-ionized water under vacuum. We performed the CPMG imaging experiment described in the previous section to measure the magnetization intensity at 50 echoes spaced by 4.6 ms for each of 32 x 16 x 16 voxels within the field of view of 3.0 cm x 3.0 cm x 3.0 cm. The corresponding voxel size is 0.938 mm x 1.88 mm x 1.88 mm. We used 1 s of repetition time (TR) and the total imaging time was 4.3 min. 

CPMG pulse sequence Carr-Purcell-Meiboom-Gill pulse sequence. A pulse sequence used for removing broad signals from a spectrum by multiple defocusing and refocusing pulses. 

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