Encoding, MRI

Single point (or constant time) imaging (SPI) is a phase encoding MRI method which uses reasonably large, pulsed magnetic field gradients (up to 0.1 T m ). 2-44 Band-selective radiofrequency pulses are not used and... 

A problematic artifact associated with MRI arises when the imaged subject moves duriag acquisition of the / -space data. Such motion may result ia a discontiauity ia the frequency-encoded or phase-encoding direction data of / -space. When Fourier transformed, such a discontiauity causes a blurred band across the image corresponding to the object that moved. Such an artifact ia an image is referred to as a motion artifact. 

Fig. 2.6.1 Schematic of an experiment with remote detection. The basic steps are (a) the polarization of the sensor medium, (b) NMR or MRI encoding using rf pulses and magnetic field gradients and (c) signal detection. The NMR or MRI information travels between the locations (b) and (c).

The situation is different in an experiment where the acquisition dimension of the conventional, direct detection experiment itself measures only the magnitude of a signal, as opposed to its time evolution. This is the case for example in an MRI experiment with phase encoding in all three dimensions. Here, it is the ID sensitivity that has to be compared between remote and direct detection, because the direct FID is no longer sampled point-by-point. Also, by following the treatment of ID sensitivity in Ref. [20], this yields ... 

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].

Fig. 4.2.5 Unaliased (A) and aliased (B) velocity images for a 0.6% aqueous carboxy-methylcellulose solution obtained by MRI. The vertical axes represent the data obtained in the velocity encoded direction and the horizontal axis represents data obtained in the spatially encoded direction. Aliasing is achieved by setting the velocity encode gradients so large that the maximum phase evolution of a given fluid element for a pulsed gradient step exceeds Ji or 2jt...

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