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MRI with zero echo time: Hard versus sweep pulse excitation
Author(s) -
Weiger Markus,
Pruessmann Klaas P.,
Hennel Franciszek
Publication year - 2011
Publication title -
magnetic resonance in medicine
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.696
H-Index - 225
eISSN - 1522-2594
pISSN - 0740-3194
DOI - 10.1002/mrm.22799
Subject(s) - excitation , amplitude , sweep frequency response analysis , duty cycle , flip angle , physics , bandwidth (computing) , nuclear magnetic resonance , radio frequency , sensitivity (control systems) , optics , acoustics , computational physics , computer science , magnetic resonance imaging , telecommunications , electronic engineering , voltage , radiology , medicine , quantum mechanics , engineering
Zero echo time can be obtained in MRI by performing radiofrequency (RF) excitation as well as acquisition in the presence of a constant gradient applied for purely frequency‐encoded, radial centre‐out k ‐space encoding. In this approach, the spatially nonselective excitation must uniformly cover the full frequency bandwidth spanned by the readout gradient. This can be accomplished either by short, hard RF pulses or by pulses with a frequency sweep as used in the SWIFT (Sweep imaging with Fourier transform) method for improved performance at limited RF amplitudes. In this work, the two options are compared with respect to T 2 sensitivity, signal‐to‐noise ratio (SNR), and SNR efficiency. In particular, the SNR implications of sweep excitation and of initial or periodical acquisition gaps required for transmit‐receive switching are investigated. It was found by simulations and experiments that, whereas equivalent in terms of T 2 sensitivity, the two techniques differ in SNR performance. With ideal, ungapped simultaneous excitation and acquisition, the sweep approach would yield higher SNR throughout due to larger feasible flip angles. However, acquisition gapping is found to take a significant SNR toll related to a reduced acquisition duty cycle, rendering hard pulse excitation superior for sufficient RF amplitude and also in the short‐ T 2 limit. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.