timing.tex

\chapter{Controlling sequence timing}
\label{ch:timing}

The default module duration is set to the value specified in \texttt{modules.txt}, however the duration can be extended in real-time by setting the value of the 'textra' column in {\tt scanloop.txt} to a non-zero value.
This allows the sequence timing to be controlled dynamically, e.g., for the purpose of varying TE or TR during a scan.

If the minimum module duration exceeds the prescribed duration in {\tt modules.txt}, the minimum module duration is used (without warning).
It is therefore perfectly fine to set the module duration in {\tt modules.txt} to '0', since this guarantees that the minimum duration will be used which is often the desired behavior.

Figure~\ref{fig:timing} shows detailed timing information for the three module types: gradients-only, RF excitation, and data acquisition.
For gradients-only modules, the minimum module duration is equal to the waveform duration plus the controls variables (CVs) 'start\_core', 'timetrwait', and 'timessi'.
These have been determined empirically, and are currently set to 224us, 64us, and 100us, respectively.
For RF and acquisition modules, the module duration must be extended by 'myrfdel' and 'daqdel', respectively, to account for gradient delays with respect to RF transmission and data acquisition, respectively.
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\myfigure{timing}{0.75}{
Detailed timing diagram for the three module types:
(a) gradients-only, (b) RF excitation, and (c) data acquisition.
The labels correspond to Control Variables (CVs) in{\toppe}.
Note that \texttt{playseq.m} uses timing values in \texttt{timing.txt} (see sequence examples in the Matlab repository) to reproduce the precise sequence timing one should expect to observe on the scanner.
}
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