Sequence modules

Sequence modules#

TL;DR

  • A SequenceModule solves a reusable block layout independently of the acquisition loop. It contains block tuples, named mutable event templates and a timing reference, center.

  • center is the module timing reference in seconds from its start, usually an RF pulse centre or an echo. For an inversion module followed by an excitation module, the recovery delay for inversion time \(T_I\) is \(T_I - (t_{\mathrm{inv}} - t_{\mathrm{centre,inv}}) - t_{\mathrm{centre,exc}}\), independent of block boundaries.

  • An acquisition loop may change a named event template before add_block, for example by setting an RF phase offset or scaling a phase-encode gradient. Previously registered blocks and the module’s internal sequence remain unchanged.

  • A readout constructed with te=None uses its shortest realizable echo time. Requested bandwidths and times are rasterized, and the module reports the achieved values.

  • Module construction separates fixed waveform and timing design from per-view encoding, which mirrors the Pulseq distinction between event definitions and playout instances. Sampling order and repetition structure belong to the sequence application.

A SequenceModule solves a reusable block layout independently of the acquisition loop. The module contains block tuples, named mutable event templates, and a timing reference.

SequenceModule
├── blocks: [(event, ...), ...]
├── events: named mutable event templates
└── center: timing reference (s)

Timing centre#

The center attribute gives the module timing reference in seconds from its start, usually an RF pulse centre or an echo. For an inversion module followed by an excitation module, the recovery delay required for inversion time (T_I) is

\[ t_{\mathrm{delay}} = T_I - (t_{\mathrm{inv}} - t_{\mathrm{centre,inv}}) - t_{\mathrm{centre,exc}}. \]

This definition composes pulse-centre timing without depending on block boundaries.

Module contents#

Module family

Blocks and events

Responsibility

Excitation

RF event, selection gradient, rephaser

Excitation or refocusing geometry and pulse timing.

Preparation

RF and gradient preparation blocks

Inversion, T2 preparation, saturation, diffusion, or magnetisation transfer.

Readout

Prephasing, ADC event, readout gradients, rewinding or spoiling

Echo timing, acquisition bandwidth, and sampling trajectory.

Named events are published both as attributes and through events. An acquisition loop may change an event template before add_block, for example by setting an RF phase offset or scaling a phase-encode gradient. Previously registered blocks and the module’s internal sequence remain unchanged.

A readout constructed with te=None uses its shortest realizable echo time. Requested bandwidths and times are rasterized, and the achieved values are reported by the module.

Representation boundary#

Module construction separates fixed waveform and timing design from per-view encoding. This mirrors the Pulseq distinction between event definitions and playout instances; see Storage, deduplication and file revisions. Sampling order and repetition structure belong to Sequence applications.

See also#