Relaxation-time contrast

In Section II.3 we have seen that a specific chemical species existing in a given physicochemical environment is characterized by specific values of 7) and T2, and that this fact is important both in the implementation of imaging pulse sequences to obtain quantitative information and in the modification of the pulse sequences to image selectively one species and/or phase within the sample. While exploitation of relaxation time contrast is not likely to become a routine approach for chemical mapping in reactors, there will be niche applications in which it will continue to have use—three of these are identified below. The limitations of the approach derive from that fact that the relaxation times characterizing a system will not only be influenced by chemical composition but also by temperature and the proximity of the molecules to a solid surface or interface. The three case studies illustrated below in which relaxation time contrast has been used with considerable success are (i) an... 

In these studies, chemical conversion was determined in situ by measuring the lH resonance associated with OH groups present. In practice two such resonances exist associated with chemical species inside and outside the catalyst particles, respectively. The difference in chemical shift between these intra- and inter-particle species arises because of the different electronic environment of the molecules inside the catalyst particles compared to their environment in the bulk fluid in the inter-particle space. In this work, chemical conversion was determined from the MR signal acquired from species in the inter-particle space of the bed because the signal from inside the catalyst particles is also going to be influenced, to an unknown extent, by relaxation time contrast. In addition to possible relaxation contrast effects, there will also be modifications to the chemical shifts of individual species resulting from adsorption onto the catalyst this may cause peak broadening and reduces the accuracy with which we can determine the chemical shift of the species of interest. As follows from eqn (11) which describes the esterification reaction of methanol and acetic acid to form methyl acetate and water ... 

The majority of the early MRI studies specific to catalysis addressed the heterogeneity in structure and transport within catalyst pellets. In-plane spatial resolution achieved in these investigations was approximately 30 pm, and the pellets themselves were of typical dimension 1-5 mm. In the majority of cases, investigations addressed the pure (usually oxide) support so that the quantitative nature of the data obtained was not lost because of the presence of metal (which introduces an unknown degree of nuclear spin relaxation time contrast into the images). 

Clearly, the basic imaging scheme can be extended to include relaxation-time contrast for discrimination of variations in cross-link density and strain, and the ID MRI-MOUSE (magnetic resonance imaging MOUSE) can be extended with further gradient coils to permit imaging in three dimensions. Numerous applications of the MRI-MOUSE can be envisioned in soft matter analysis, in particular in those areas, where imaging with conventional equipment has proven to be successful, and where smaller, less expensive, and mobile devices are in need. 

The intensity of the signal acquired by the spin-warp technique depends on the echo time tE and on the repetition time t of successive scans. The echo time introduces T2 weights to the pixels of the image, resulting in relaxation-time contrast in cases where T2 is a function of space coordinates x, y, and z- Within the validity of the Bloch equations, the weight of the echo at point r = (x, y, z) is given by... 

A challenge of stray field (and wide-line methods) is that the types of contrast that can be introduced are rather limited, and usually based on rotating frame relaxation times. Contrast arising from the commonly observed spectroscopic parameters, particularly the chemical shift cannot be achieved. 

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