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MRI with zero echo time: hard versus sweep pulse excitation


Weiger, M; Pruessmann, K P; Hennel, F (2011). MRI with zero echo time: hard versus sweep pulse excitation. Magnetic Resonance in Medicine, 66(2):379-389.

Abstract

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 T2 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 T2 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-T2 limit.

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 T2 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 T2 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-T2 limit.

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Additional indexing

Item Type:Journal Article, refereed, original work
Communities & Collections:04 Faculty of Medicine > Institute of Biomedical Engineering
Dewey Decimal Classification:170 Ethics
610 Medicine & health
Language:English
Date:2011
Deposited On:22 Jan 2012 20:35
Last Modified:05 Apr 2016 15:27
Publisher:Wiley-Blackwell
ISSN:0740-3194
Publisher DOI:10.1002/mrm.22799
PubMed ID:21381099
Permanent URL: http://doi.org/10.5167/uzh-56458

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