Relaxation parameter imaging

The different information obtainable from a chemical-shift weight and a relaxation-parameter image is illusiraled in Fig. 8.3,2 Pra3] In covulcanization of different rubber sheets, for example, sheets from SBR and NR, an interface may arise depending on the materials and the conditions of vulcanization (Fig. 8.3.2(a)). A sufficiently long acquisition delay t without chemical-shift refocusing introduces the chemical-shift... 

The combination of microscopic and macroscopic information is made possible by what can be called parameter imaging . In the general sense, it consists of the encoding of properties such as spectral line shifts, relaxation times, diffusion coefficients, etc., in the image by suitable combination of corresponding modules into one pulse sequence. Parameter images are to be distinguished from mere... 

R. L. Kleinberg 1996, (Utility of NMR T2 distributions, connections with capillary pressure, day effect, and determination of the surface relaxivity parameter n >). Magn. Reson. Imaging 14 (7/8), 761—767. 

This method however assumes that a static calibration is sufficient for a dynamic sample. It corresponds that a PEM sample in a fuel cell has well defined and fixed relaxation times. If the PEM however is truly dynamic and undergoes microstructural variation or other evolution then the relaxation times will change and the image will change even though the water content is not changing. For more reliable approach, it is feasible to use mapping of relaxation parameters in the PEM, because relaxation parameters such as T1 and T2 also depends on water contents as performed by Balcom et al.15... 

From a fit of Equation (10) to spatially resolved relaxation curves, images of the parameters A, B, T2, q M2 have been obtained [3- - 32]. Here A/(A + B) can be interpreted as the concentration of cross-links and B/(A + B) as the concentration of dangling chains. In addition to A/(A + B) also q M2 is related to the cross-link density in this model. In practice also T2 has been found to depend on cross-link density and subsequently strain, an effect which has been exploited in calibration of the image in Figure 7.6. Interestingly, carbon-black as an active filler has little effect on the relaxation times, but silicate filler has. Consequently the chemical cross-link density of carbon-black filled elastomers can be determined by NMR. The apparent insensitivity of NMR to the interaction of the network chains with carbon black filler particles is explained with paramagnetic impurities of carbon black, which lead to rapid relaxation of the NMR signal in the vicinity of the filler particles. 

Often the information on NMR relaxation parameters carried by image contrast is insufficient to address a particular problem. We can then look to the rich information content of the spectrum itself. Generally, spectroscopy of the entire body is not of much value, and in vivo spectroscopy is usually carried out as localized spectroscopy, that is, over a part of the body. There are various ways of restricting the operation of the spectrometer to a particular region, and they fall into two broad classes those that depend on the physical dimensions of the rf coil and those that use field gradients in the pulse sequences. Often these approaches are combined. At this time, the use of spectroscopic examinations has not become part of the repertoire of clinical practice, despite a history of in vivo spectroscopy almost as old as MRI itself. In vivo spectroscopy has had a number of landmark successes in solving problems in metabolism research in both animals and humans, but there have been no spectroscopic applications that have been demonstrated to be more effective than other methods for the routine diagnosis of disease. 

From a fit of (7.1.6) to spatially resolved relaxation curves, images of the four parameters A, B, T2, and qM2 have been obtained [Hafl, Knol, Kuhl]. Here A/(A -F B) is interpreted as the concentration of cross-linked chains, and B/(A -F B) as that of the dangling chains [Fiill]. 

Relaxation and spin-diffiision parameter imaging of strained polymers... 

Fig. 10.3.8 Tje-parameter imaging of shear bands in poly (carbonate), (a) Relaxation of transverse magnetization under OW4 irradiation. The decay is decomposed into two exponentials with relaxation times Tje.shon and r2e,iong- (b) Spin-density image, (c) Parameter image of Tj shorc (d) Parameter image of 7 2e,iong- Adapted from [Wei5] with permission from Wiley-VCH.

NMR parameter images can be translated to material property images by calibration or relationships known from theory. For example, cross-link density can be linked to the transverse relaxation decay [101-103] and the longitudinal relaxation decay in the rotating frame [104, 105]. Relaxation of transverse magnetization in cross-linked elastomers is nonexponential (Fig. 

To obtain information about the polymer, a material parameter imaging is used. Different properties can be utilized to provide image contrast without adding other substances. Such properties include the relaxation times Ti (relaxation time of longitudinal magnetization decay, spin-lattice relaxation) and T2 (relaxation time of transversal magnetization decay, spin-spin relaxation), as well as the chemical shift. Frequently, the decay of magnetization is measured. For liquids, a simple... 

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