Basic Principles of MRI

If an atom has an uneven number of neutrons or protons, then the atom has a certain angular momentum. The angular momentum is 

In order to obtain the spatial distribution of the spins, i.e. NMR imaging (MRI), in a sample of interest, weak magnetic field gradients, G, are additionally superimposed to localize the spins. As a result, the NMR signal contains spatial information. The resonance frequency along the sample corresponding to 

Temporal variation of magnetic field gradients along the X, Y, and Z axes a so-called pulse sequence for image acquisition is applied for MRI measurements. Several procedures have been developed for an effective determination of the spatial distribution of the spins. A detailed description of MRI pulse sequences is available in explanations by Callaghan,37 Blumich,38 and Kimmich.39 

Because the induced NMR signal after switching off the 90° RF pulse decays so rapidly as mentioned above, spin echoes are often detected in many practical pulse sequences for MRI visualization. Spin echo is formed by an additional RF pulse applied to the sam-ple.The additional RF pulse is applied after an evolution period, r. This RF pulse that gives rise to inversion of the magnetization components causes the spins to rephrase and is thus referred to the 180° RF pulse as shown in Fig. 2. This rephrasing process contributes to recover the transverse magnetization which had lost in dephasing process and results in the formation of an echo . We detected the echo by the receiver coil. The time from when the 90° RF pulse is applied to when the echo forms is referred to as the echo time , TE and is equal to twice the time between the 90° and 180° pulses, i.e. 2r. 

We need to apply temporal variation of magnetic gradient for localization of the spins between the 90° RF pulse and the echo detection, which is typically in a range of millisecond. In MRI experiments, the sample is divided into either a two-dimensional 

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Although the power of research MRI/MRS machines for human use can be as high as 9.4 T, the FDA has imposed a limit of 3 T for routine clinical human use. Industrial manufacturers of MRI equipment are very careful to conform to all the FDA guidelines regarding magnetic field strength, gradient speed and RF power (see Appendix Basic principles of MRI). 

The structure of this review is as follows. Section II focuses on the basic principles of MRI techniques, and then more advanced techniques which have been used to study catalytic reactors will be introduced in Section III. To illustrate the use of these techniques examples will be given from the field of catalysis, although not necessarily at the scale of the reactor and, in some cases, data for model systems will be shown. Section IV describes methods used to achieve chemical mapping. Section V focuses exclusively on previous research which has used MRI techniques to spatially resolve chemical composition in fixed-bed reactors. 

This section introduces the reader to the basic principles of MRI and the concept of the k-space raster. The basic MRI pulse sequence, the spin-echo imaging sequence, is described at this point. For more detailed discussion of the background theory of MRI the interested reader should refer to texts by Callaghan5 and Kimmich.6... 

Basic principles of MRS. The overall physical principles and characteristics of magnetic resonance spectroscopy (MRS) are identical to those described previously in the MRI section. In fact, magnetic resonance spectroscopy can simply be thought of as just another way of expressing the NMR signals that are recorded during an NMR experiment. Whether it is an MRI or and MRS experiment, virtually all of the same equipment is used and all of the basic NMR principles still apply. The prime difference that separates basic MRS from modern-day MRI is that in MRS, the... 

This article provides a description of the basic principles of NMR spectroscopy. The many applications of this technique, which make it indispensable as a tool for the analytical chemist, are described in subsequent articles, which also include discussion of advanced techniques such as the NMR of solids, multidimensional NMR, and MRI. 

Recently, Britton reviewed the development of MRI to map concentration gradients and visuaHze electrochemical processes in electrochemical cells containing bulk metals, and discussed the achievement, challenges, and solutions [168]. Tsushima and Hirai reviewed the in situ MRI visuahzation of water in operating PEMFCs, described the basic principles and hardware, as well as the design, construction, and material selection of a PEMFC for MRI experiment [169]. 

Delso, Caspar, and Sibylle Ziegler. PET/MRI Systems. European Journal of Nudear Medicine and Molecular Ima ng 36, no. SI (March, 2009) S86-92. A basic review of the principles and challenges associated with combining PET and MRI in one system. 

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