Imaging relaxation contrast

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... 

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. 

The porosity of the particles could be shown by encapsulating an aqueous solution of a gadolinium-based magnetic resonance imaging (MRI) contrast agent [Gd-diethylenetriamine penta acetic acid (DTPA)]. As the Gd complex is still accessible by water from the capsule exterior, changing the water s proton relaxation time, it can be concluded, that the shell is porous and allows water to diffuse into the capsules, but restricts the complex to the interior [78]. 

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