MRI technique

MRA describes any of the several MRI techniques that are used to depict arteries. These can be divided into contrast-based techniques and noncontrast-based techniques. 

Fig. 2.6.10 Specialized experimental set-up for microfluidic flow dispersion measurements. Fluid is supplied from the top, flows via a capillary through the microfluidic device to be profiled and exits at the bottom. The whole apparatus is inserted into the bore of a superconducting magnet. Spatial information is encoded by MRI techniques, using rf and imaging gradient coils that surround the microfluidic device. They are symbolized by the hollow cylinder in the figure. After the fluid has exited the device, it is led through a capillary to a microcoil, which is used to read the encoded information in a time-resolved manner. The flow rate is controlled by a laboratory-built flow controller at the outlet [59, 60].

The discussion above that led to Eqs. (4.2.6 and 4.2.7) assumes that the no-slip condition at the wall of the pipe holds. There is no such assumption in the theory for the spatially resolved measurements. We have recently used a different technique for spatially resolved measurements, ultrasonic pulsed Doppler velocimetry, to determine both the viscosity and wall slip velocity in a food suspension [2]. From a rheological standpoint, the theoretical underpinnings of the ultrasonic technique are the same as for the MRI technique. Flence, there is no reason in principle why MRI can not be used for similar measurements. 

The MRI technique has also been used with systems of more practical importance, such as a polymer melt [19]. Here a low density polyethylene melt was... 

Up to this point, water mobility values obtained are average values for an entire sample. However, if magnetic field gradients in the x, y, and z directions are incorporated into a pulsed NMR experimental setup, the spatial distribution aspects of water mobility (7), T2, and D) can also be measured via the use of magnetic resonance imaging (MRI) techniques. 

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. 

Gd(III) chelates have played an important role in the development of clinical applications of MRI technique by adding relevant physiological information to the superb anatomical resolution attainable with this imaging modality. 

A more phenomenological approach 25) allows to overcome this limitation. The space around each particle is divided between two regions the boundary is echo-time dependent. In the first one, near the particle, the gradients are too large for the refocusing pulses to be effective, so that the moments situated in this region will be rapidly dephased. These protons contribute a fast signal decay that is unobservable with MRI techniques. 

Since this early work, interest has been focused mainly on oscillations and traveling waves during chemical reactions 49, 126-129). Butler et al. 130) employed MRI techniques to investigate the extent of reaction in a single crystal of... 

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