Broadband CPMG sequence with short composite refocusing pulses

[Display omitted] ► Composite refocusing pulse of same duration and amplitude as standard 180° pulse. ► Significant increase in SNR for CPMG measurements in inhomogeneous fields. ► Increased decay time when T1/T2>1. ► Increased sensitivity to diffusion. We demonstrate that CPMG sequences with pha...

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Bibliographic Details
Published in:Journal of magnetic resonance (1997) 2013-05, Vol.230, p.64-75
Main Authors: Koroleva, Van D.M., Mandal, Soumyajit, Song, Yi-Qiao, Hürlimann, Martin D.
Format: Article
Language:English
Subjects:
Algorithms
Amplitudes
Broadband
CPMG
Decay
Diffusion
Diffusion Magnetic Resonance Imaging - methods
Electromagnetic Fields
Excitation
Inhomogeneous fields
Magnetic Resonance Spectroscopy - methods
Power consumption
Signal Processing, Computer-Assisted
Trains
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Summary:[Display omitted] ► Composite refocusing pulse of same duration and amplitude as standard 180° pulse. ► Significant increase in SNR for CPMG measurements in inhomogeneous fields. ► Increased decay time when T1/T2>1. ► Increased sensitivity to diffusion. We demonstrate that CPMG sequences with phase-modulated refocusing pulses of the same duration as the standard 180° pulses can generate echo trains with significantly increased amplitudes compared to the standard CPMG sequence in the case when there is a large range of Larmor frequencies across the sample. The best performance is achieved with symmetric phase-alternating (SPA) composite refocusing pulses of the form α−yβ+yα−y. In comparison to standard 180° pulses, we show that with SPA refocusing pulses with α≈27° and β≈126°, it is possible to double the signal-to-noise ratio without increasing the total pulse duration or power consumption of the refocusing pulses. The increased bandwidth of these pulses more than compensates for the decrease in performance in the vicinity of resonance. To achieve the full benefit of the broadband nature of the SPA pulses in a CPMG sequence, it is necessary to combine these refocusing pulses with a broadband excitation pulse. When it is not possible to use a short, high amplitude excitation pulse, we show that phase-alternating (PA) excitation pulses are suitable for this purpose. We present a detailed analysis of the underlying spin dynamics of these new pulse sequences and confirm the simulations with experiments. We show that for samples with T1/T2>1, the new sequences in grossly inhomogeneous fields do not only generate echoes with an increased amplitude, but also with an increased decay time. Finally, we analyze the diffusion properties and show quantitatively that the broadband sequences have a substantially higher diffusion sensitivity compared with the standard CPMG sequence.
ISSN:1090-7807
1096-0856
DOI:10.1016/j.jmr.2013.01.006