Carr-Purcell-Meiboom-Gill experiments

The trick introduced by Meiboom and Gill (14) is to dephase all n pulses in the Carr Purcell train by an angle of 90° with respect to the initial ti/2 pulse. It is easily shown that, without this phase change, imperfections of the 71 pulses are cumulative, whereas with the 90° phase change, a self-compensation occurs for all echoes of even number. The CPMG (Carr-Purcell-Meiboom-Gill) experiment can be handled in two ways ... 

As a biologically relevant example of solid-state NMR on quadrupolar nuclei. Fig, 11 shows experimental and simulated Zn solid-state NMR spectra of zinc diimidazole diacetate which may be considered a model compound for Zn in metalloproteins. The experimental spectrum was obtained by sampling the FID in between the refocusing pulses in a quadrupolar version of the Carr-Purcell-Meiboom-Gill experiment (QCPMG), 5 In this manner, the hundred kHz wide second-order quadrupolar powder pattern is split into a manifold of spin-echo sidebands with the consequence of highly... 

In electron spin echo relaxation studies, the two-pulse echo amplitude, as a fiinction of tire pulse separation time T, gives a measure of the phase memory relaxation time from which can be extracted if Jj-effects are taken into consideration. Problems may arise from spectral diflfrision due to incomplete excitation of the EPR spectrum. In this case some of the transverse magnetization may leak into adjacent parts of the spectrum that have not been excited by the MW pulses. Spectral diflfrision effects can be suppressed by using the Carr-Purcell-Meiboom-Gill pulse sequence, which is also well known in NMR. The experiment involves using a sequence of n-pulses separated by 2r and can be denoted as [7i/2-(x-7i-T-echo) J. A series of echoes separated by lx is generated and the decay in their amplitudes is characterized by Ty. ... 

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... 

T2 measurements usually employ either Carr-Purcell-Meiboom-Gill (CPMG) [7, 8] spin-echo pulse sequences or experiments that measure spin relaxation (Tlp) in the rotating frame. The time delay between successive 180° pulses in the CPMG pulse sequence is typically set to 1 ms or shorter to minimize the effects of evolution under the heteronuc-lear scalar coupling between 1H and 15N spins [3]. 

Ti reports on fast dynamics on a timescale of ps-ns, whereas T2 relaxation depends on both fast and slower dynamics (ps-ns and xs-ms). The experimentally measured T2 relaxation times include an exchange contribution that can be measured by a Carr-Purcell-Meiboom-Gill (CPMG) pulse train (25, 26) or an effective spin-lock field (27-29). The combination of T2 and Tip measurements allows determination of the contribution of chemical exchange to the relaxation time. Eurthermore, relaxation dispersion experiments have been developed to measure slow time-scale xs-ms dynamic processes (30-35). 

Static imaging experiments conducted on fluid-saturated samples are used to determine porosity distributions. Carr-Purcell-Meiboom-Gill (CPMG) imaging is used to evaluate the spin density. The local relaxation is modeled in order to estimate the intrinsic magnetization intensity, which is proportional to the amount of saturating fluid. 

While many NMR active nuclei such as JH, 13C, 31P, and 15N have been used to analyze metabolites (19-21), JH NMR analysis is the most widely used in the field because of the ubiquitous nature of JH and its high NMR sensitivity. Furthermore, ease of analysis and high-throughput capabilities make onedimensional (ID) NMR experiments, including the ID NOESY (nuclear Over-hauser enhancement spectroscopy) and CPMG (Carr-Purcell-Meiboom-Gill)... 

Since the present NMR-SIM version also consider transverse relaxation as a proper process, it is possible to simulate the common T2 measurement experiment using the Carr-Purcell-Meiboom-Gill sequence. 

D. G. Hughes and G. Lindblom, "Baseline drift in the Carr-Purcell-Meiboom-Gill pulsed NMR experiment," J. Magn. Resonance 26, 469-479 (1977). 

As two phases in quadrature will allow Carr-Purcell Meiboom-Gill, T, and or other spin-locking experiments, we recommend this as a minimum. However, providing different rf phases for the transmitter pulses is not difficult and this capability should not increase the cost of the spectrometer very much so you might as well go for four orthogonal phases. As discussed elsewhere, the phase shifts can be performed in the IF stage rather than at the carrier frequency so that one phase shifting network can suffice for all carrier frequencies. 

Phase considerations intrude even in the simplest experiments of observing an FID or an echo. Accurately adjusting the phases of rf pulses can be very important, particularly in experiments involving trains of pulses such as the Carr-Purcell Meiboom-Gill train or the multiple pulse line narrowing sequences. In other sections we have considered how phase shifts originate and how to cope with them. 

Carr-Purcell-Meiboom-Gill (CPMG) experiment. An experiment wherein the net magnetization is allowed tipped into the xy plane, and subjected to a series (or train) of RF pulses and delays to refocus the net magnetization. Maintaining the net magnetization in the xy plane allows the measurement of the T2 relaxation time. 

Since R2 is the only observable that provides information about the /(O) spectral density contribution, accurate measurement of this observable is of particular importance. Transverse relaxation rates are typically measured by either a spin-lock (f ip) or a Carr-Purcell-Meiboom-Gill (CPMG) experiment. In the following, advantages and disadvantages of the two experiments are described with particular consideration of... 

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