RF and echo time definition

[mrpaul]

Where during the RF excitation pulse is the start of the echo time defined? For symmetric sinc pulses I see it being defined from the center. But for hard pulses it seems to be defined from the end of the pulse duration.

What is there meaning of this time point? Are there any suggestions for reading material on this subject?

Thanks

[aelster]

Dear MrPaul,

Great question! I've never given that much thought before and am not sure there is any official definition of exactly where TE should start.

I quickly scanned through a number of papers with diagrams showing hard (rectangular) pulses and can't find any where TE began at the end of the pulse. All the papers I found measured TE from the middle of the rectangle. Because hard pulses tend to be relatively short in duration the difference is minimal.

I think that the unwritten rule is that for any symmetric RF pulse (sinc, rectangle, or otherwise), the middle of the pulse in the time domain should be chosen as the start of the TE interval.

Asymmetric pulses (e.g., truncated sinc, SLR, and various adiabatic pulses) are more tricky. Looking again through diagrams in the literature it seems the overwhelming majority of authors use the peak of the pulse in the time domain as the starting point for TE. If anyone else has better information, please chime in!