By Dominik Weishaupt, Victor D. Koechli, Borut Marincek
This completely revised moment version succinctly introduces the physics and serve as of magnetic resonance imaging. All very important and clinically suitable facets are awarded in a essentially established demeanour. The emphasis is on useful info together with the most recent tendencies and advancements which are correct for MRI within the medical atmosphere. the hole chapters describe the underlying actual rules of the MR scan and the elemental pulse sequences typical in medical MRI. different chapters are devoted to extra complicated concepts akin to parallel imaging and cardiovascular MR imaging. The ebook is rounded out by way of chapters on MR distinction media, artifacts, high-field imaging, and protection issues. an in depth word list bargains fast entry to crucial MRI terminology. The booklet is meant for readers searching for a simple to appreciate and concise creation to this interesting but a bit of complicated imaging modality at the start in their MRI education.
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Extra info for How does MRI work An Introduction to the Physics and Function of Magnetic Resonance Imaging
Sample text
This is done by first applying a frequency-encoding gradient with negative 7 Basic Pulse Sequences 53 polarity to destroy the phase coherence of the precessing spins (dephasing). 31). Fig. 31. Gradient echo sequence. For the sake of simplicity, a flip angle α of 90° is assumed here as well Since no 180° refocusing pulse is needed to generate gradient echoes, very short repetition times (TR) can be achieved. As TR is a major determinant of the overall scan time of a GRE sequence – and of most other sequences – much faster imaging is possible compared with SE and IR sequences, which is the most important advantage of GRE imaging.
In MR angiography, for instance, this technique is used to fill the center of k-space with the data important for evaluating contrast enhancement patterns. 4 Number of Excitations The number of excitations (NEX) or number of signal averages (NSA) denotes how many times a signal from a given slice is measured. The SNR, which is proportional to the square root of the NEX, improves as the NEX increases, but scan time also increases linearly with the NEX. 5 Imaging Parameters Other parameters affecting the SNR are the sequence used, echo time (TE), repetition time (TR), and the flip angle.
3) but also reduces SNR as more noise is included. Halving the bandwidth improves SNR by about 30%. With a narrow bandwidth, on the other hand, there will be more chemical shift and motion artifacts and the number of slices that can be acquired for a given TR is limited. An interslice gap is a small space between two adjacent slices. It would be desirable to acquire contiguous slices but interslice gaps are necessary in SE imaging due to imperfections of RF pulses. 20), two adjacent slices overlap at their edges when closely spaced.