Relaxation contrast

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 latter case is of interest, for example, when the sample contains metal particles (such as in some types of MRI relaxation contrast agents). 

An example of the application of RARE for rapid image acquisition is shown in Fig. 19, in which a single frame is shown from a series of 2-D images acquired of an oscillatory chemical reaction occurring within a fixed bed. Relaxation contrast is used to discriminate between the reaction products Mn " and Mn (49). In this example, MR offered the opportunity to map the detailed structure of the fixed bed and the product distribution within it. This pulse sequence has also recently been applied to obtain quantitative images of the evolution of a lysozyme urea separation within a chromatography column (50). 

Chemical shift contrast image and relaxation contrast image... 

For monomer Gd(III) complexes the inner and outer sphere mechanisms contribute more or less to the same extent to the overall paramagnetic relaxation enhancement. The development of high relaxivity contrast agents mainly involves increasing the inner sphere term, since the outer sphere contribution can hardly be modified. For the new generation macromolecular agents, therefore, the inner sphere relaxivity becomes much more significant (over 90% of total relaxivity). 

Figure 7.16 T2 weighted Hahn-echo image (a) and quotient image (b) computed as the quotient of two Hahn echo images acquired with different echo times of an unfilled SBR/NR covulcanisate with dimensions 9 mm x 30 mm. The acquired signal has been integrated over the sample thickness of 1 mm. Contrast in the quotient image is determined mainly by transverse relaxation. Contrast in the Hahn-echo image is formed by a mixture of spin density and relaxation...

T2- and Ti -relaxation contrast in the RARE acquisition is exploited such that the images are made selective to lysozyme and urea, respectively. 

Aime, S., Botta, M., Fedeli, R, et al. (2001) High-relaxivity contrast agents for magnetic resonance imaging based on multisite interactions between a f -cyclodextrin oligomer and suitably functionalized Gdlll chelates. Chemistry - A European Journal, 7, 5261-5269. 

Another powerful contrast parameter is spin-lattice, or Tj, relaxation. Spin-lattice relaxation contrast can again be used to differentiate different states of mobility within a sample. It can be encoded in several ways. The simplest is via the repetition time, between the different measurements used to collect the image data. If the repetition time is sufficiently long such that Tj )) Tj for all nuclei in the sample, then all nuclei will recover to thermal equilibrium between measurements and will contribute equally to the image intensity. However, if the repetition time is reduced, then those nuclei for which Tr < Tj will not recover between measurements and so will not contribute to the subsequent measurement. A steady state rapidly builds up in which only those nuclei with Tj contribute in any significant manner. As with -contrast, single images recorded with a carefully selected may be used to select cmdely a short component of a sample. 

---