Estimating and eliminating the excitation errors in bipolar gradient composite excitations caused by radiofrequency-gradient delay: Example of bipolar spokes pulses in parallel transmission.
Magn Reson Med
; 78(5): 1883-1890, 2017 11.
Article
em En
| MEDLINE
| ID: mdl-28019035
ABSTRACT
PURPOSE:
To eliminate a slice-position-dependent excitation error commonly observed in bipolar-gradient composite excitations such as spokes pulses in parallel transmission. THEORY ANDMETHODS:
An undesired timing delay between subpulses in the composite pulse and their bipolar slice-selective gradient is hypothesized to cause the error. A mathematical model is presented here to relate this mismatch to an induced slice-position-dependent phase difference between the subpulses. A new navigator method is proposed to measure the timing mismatch and eliminate the error. This is demonstrated at 7 Tesla with flip-angle maps measured by a presaturation turbo-flash sequence and in vivo images acquired by a simultaneous multislice/echo-planar imaging (SMS-EPI) sequence.RESULTS:
Error-free flip-angle maps were obtained in two ways 1) by correcting the time delay directly and 2) by applying the corresponding slice-position-dependent phase differences to the subpulses. This confirms the validity of the mathematical description. The radiofrequency (RF)-gradient delay measured by the navigator method was of 6.3 µs, which agreed well with the estimate from flip-angle maps at different delay times. By applying the timing correction, accurately excited EPI images were acquired with bipolar dual-spokes SMS-2 excitations.CONCLUSION:
An effective correction is proposed to mitigate slice-position-dependent errors in bipolar composite excitations caused by undesired RF-gradient timing delays. Magn Reson Med 781883-1890, 2017. © 2016 International Society for Magnetic Resonance in Medicine.Palavras-chave
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Base de dados:
MEDLINE
Assunto principal:
Processamento de Imagem Assistida por Computador
/
Imageamento por Ressonância Magnética
Limite:
Humans
Idioma:
En
Ano de publicação:
2017
Tipo de documento:
Article