Signal formation in echo-shifted sequences

Yiu Cho Chung, Jeffrey L. Duerk

Research output: Contribution to journalArticlepeer-review

16 Scopus citations

Abstract

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 T2 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.

Original languageEnglish (US)
Pages (from-to)864-875
Number of pages12
JournalMagnetic Resonance in Medicine
Volume42
Issue number5
DOIs
StatePublished - 1999
Externally publishedYes

Keywords

  • Fast imaging
  • Gradient-recalled echo
  • Spoiling
  • SSFP

ASJC Scopus subject areas

  • Radiology Nuclear Medicine and imaging

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