Longitudinal/transverse relaxation times

In polymers one will often particularly be interested in very slow dynamic processes. The solid echo technique just described is still limited by the transverse relaxation time T being of the order of a few ps at most. The ultimate limitation in every NMR experiment however, is not T but the longitudinal relaxation time T, which for 2H in solid polymers typically is much longer, being in the range 10 ms to 10 s. The spin alignment technique (20) circumvents transverse relaxation and is limited by Tx only, thus ultraslow motions become accessible of experiment. 

To extract information about xj from NMR data, the transverse relaxation time Tj may be used as well as the longitudinal time T. For gaseous nitrogen it was done first with Ti in [81] and confirmed later [82] when T was measured and used for the same goal. The NMR linewidth of 15N2 is the inverse of T2, and the theory, relating to Ti to x.1, is well known [39, 83]. For the case of diatomic and linear molecules the formula is... 

Dynamic parameters for heterogeneous systems have been explored in the liquid, liquid like, solid like, and solid states, based on analyses of the longitudinal or transverse relaxation times, chemical exchange based on line-shape analysis and separated local field (SLF), time domain 1H NMR, etc., as summarized in Figure 3. It is therefore possible to utilize these most appropriate dynamic parameters, to explore the dynamic features of our concern, depending upon the systems we study. 

The real power of MRI is the ability to exploit the inherent NMR properties of different tissues and tissue pathologies. The main MR parameters are known as longitudinal-relaxation time-constant (Tl), describing the time for the magnetization to recover to its equilibrium along the main-field axis after RF perturbation transverse-relaxation time-constant (T2), describing the... 

In conclusion, compared with transverse relaxation, no impressive results regarding longitudinal relaxation measurements in meat science have been demonstrated at present, as it has mainly been shown that Ti expresses differences in water content. Taking into consideration the faster acquisition of transverse relaxation times compared with longitudinal relaxation, and the more hidden features in the longitudinal relaxation, it can be speculated if longitudinal relaxation will ever gain same attraction in meat science as transverse relaxation. 

Fig. 8. Evolution of the longitudinal and transverse relaxation times (Ti and T2, respectively) as a function of (for a fixed measurement frequency Vo = 400 MHz) assuming that the considered spin is subjected to random fields whose correlation function is proportional to being the correlation time. Notice the continuous...

Recapitulating, the SBM theory is based on two fundamental assumptions. The first one is that the electron relaxation (which is a motion in the electron spin space) is uncorrelated with molecular reorientation (which is a spatial motion infiuencing the dipole coupling). The second assumption is that the electron spin system is dominated hy the electronic Zeeman interaction. Other interactions lead to relaxation, which can be described in terms of the longitudinal and transverse relaxation times Tie and T g. This point will be elaborated on later. In this sense, one can call the modified Solomon Bloembergen equations a Zeeman-limit theory. The validity of both the above assumptions is questionable in many cases of practical importance. 

A NMR study of water adsorbed on silica gel has been made by Zimmerman el al. 18). Transverse (Ta) and longitudinal (Ti) relaxation times of various amounts of water adsorbed at 25° have been obtained with the use of the spin-echo technique and a two-phase behavior of both Ta and T relaxation times has been observed as illustrated in Figs. 10a and b. Generally only one T value is obtained, as for a single phase, except for x/m g HaO/g solid) values in the vicinity ol x/m = 0.126. Two values of Ta... 

Nuclear Magnetic Resonance (NMR) Spectroscopy. Longitudinal and transverse relaxation times (Ti and T2) of 1H and 23Na in the water-polyelectrolytes systems were measured using a Nicolet FT-NMR, model NT-200WB. T2 was measured by the Meiboom-Gill variant of the Carr-Purcell method (5). However, in the case of very rapid relaxation, the free induction decay (FID) method was applied. The sample temperature was changed from 30 to —70°C with the assistance of the 1180 system. The accuracy of the temperature control was 0.5°C. 

---