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Chapter-09 Ultrafast and Parallel Imaging

BOOK TITLE: Biomedical Magnetic Resonance: Proceedings of the International Workshop

Author
1. Hennig J.
ISBN
9788180614989
DOI
10.5005/jp/books/10100_9
Edition
1/e
Publishing Year
2005
Pages
21
Author Affiliations
1. University Hospital Freiburg, Freiburg, Germany
Chapter keywords
cardiac imaging, breathing motion, functional brain imaging, cortical signal changes, fast imaging, brain, hemodynamic responses, data acquisition, MR imaging, Fourier NMR, frequency spectrum, spatial information, Fourier transformation, time signal, Fourier imaging data acquisition, spin system, sequence of pulses and gradients, conventional spin-echo imaging, physiological limits, gradient performance, physiological mechanism, nerve stimulation, electrical currents, sinusoidal gradient waveforms, readout gradient GR, constant gradient GP, power requirements, rectilinear sampling, blipped phase encoding gradient, gradient-echo sequences, periodic sequence, multiple pulses, pulse spacing, tissue, parallel imaging, echotrain, hyperpolarization, imaging speed, MR applications

Abstract

In cardiac imaging where the motion of the beating heart needs to be overcome in addition to the overall breathing motion, the borderline between fast and ultrafast lies in a range of one heartbeat for the acquisition of several slices. In functional brain imaging where one wants to observe cortical signal changes anywhere in the brain, one needs fast imaging in the order of one second to cover the whole brain in observations based on hemodynamic responses. Data acquisition in MR imaging is performed using the principles of Fourier NMR. The frequency spectrum encoding the spatial information is derived by Fourier transformation from the time signal. In the conventional approach of Fourier imaging data acquisition is performed line by line after preparing a spin system to an appropriate starting position by a sequence of pulses and gradients preceding the actual data acquisition. This is the approach used in the conventional spin-echo imaging as well as in various gradient-echo imaging approaches. With respect to physiological limits of gradient performance, the relevant physiological mechanism is the involuntary nerve stimulation caused by electrical currents created by the gradients. Originally sinusoidal gradient waveforms have been used for the readout gradient GR and a constant gradient GP was used in order to minimize the power requirements on the gradient systems. With current state-of-the-art technology, a rectilinear sampling with blipped phase encoding gradient is, however, more commonly used. Gradient-echo sequences apply a periodic sequence of multiple pulses at a pulse spacing, which typically is much sorter than the T1 and T2 of the tissue. Parallel imaging will reduce the length of the echotrain. Hyperpolarization with the possibility to increase SNR by several orders of magnitude may offer totally new opportunities to redefine imaging speed in future MR applications.

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