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Signal formation in echo‐shifted sequences
Author(s) -
Chung YiuCho,
Duerk Jeffrey L.
Publication year - 1999
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/(sici)1522-2594(199911)42:5<864::aid-mrm5>3.0.co;2-8
Subject(s) - nuclear magnetic resonance , echo (communications protocol) , echo time , physics , signal (programming language) , steady state free precession imaging , flash (photography) , offset (computer science) , magnetic resonance imaging , computer science , optics , medicine , radiology , computer network , programming language
Echo shifted (ES) gradient‐recalled echo sequences (whose TE > TR) have found important applications in fMRI and MR thermometry since their introduction. The technique increases T∗ 2 weighting in BOLD imaging and temperature change sensitivity in phase‐based temperature imaging compared to FLASH sequences. Yet, inconsistent observations have previously been reported when variants of this technique were used in various MRI experiments. Previous understanding of the ES sequences (the “FLASH‐like” postulation) was not sufficient to explain these observations. This work provides an in‐depth study on the various ES sequences. It was found that there are two types of ES sequences: ES‐FLASH that spoils coherent transverse magnetization and ES‐GRE, which is based on SSFP signals. A signal expression was derived for the clinically popular TR‐periodic ES‐GRE sequence with one echo shift. This signal expression reduced to the “FLASH‐like” postulation under certain conditions. The new knowledge about the echo formation mechanism in ES sequences helps explain the experimental observations previously reported. Moreover, the resonance offset angle based analysis demonstrates an elegant methodology to analyze short TR imaging sequences. Magn Reson Med 42:864–875, 1999. © 1999 Wiley‐Liss, Inc.