Sequence functions

A sequence function is a function sequence(system, **protocol) that returns a Sequence, or a list of them with the prescans first and the main sequence last. The objects below read its protocol, label its blocks, write what it returns, time it, and build the excitation a volumetric prescription chooses. The sequences themselves are listed in Sequence catalogue; the structure of a sequence function, its protocol and its prescan chain are described in Sequence modules and functions.

Sequence functions

The protocol of a sequence function is its keyword parameters after system. parameters reads each one’s type and default from the signature, and its unit, choices and description from the NumPy-style Parameters section. pypulseqpp.cli.run() derives the command-line options from the same signature.

Object	Input	Returns	Purpose
parameters	Sequence function	dict of ProtocolParameter, in signature order	Protocol of a sequence function.
ProtocolParameter	Returned by parameters	Frozen record	One protocol parameter: its type, default, unit, choices and description.
Labels	The new value of each label, by Pulseq name	Label events to add to the block	Writer of the label events a block changes.
write	Path, a sequence or a list of sequences	Written paths, in play order	Write a sequence, or a chain of linked sequences, as Pulseq files.
duration	A sequence or a list of sequences	Duration (s)	Return the time a sequence, or a chain, plays.

Excitation choice

A volumetric sequence function offers its excitation as one protocol parameter, whose choices are EXCITATIONS, and builds the module with make_excitation from one slab prescription.

Object	Input	Returns	Purpose
EXCITATIONS	—	("nonselective", "slab", "spsp")	The excitation kinds make_excitation builds.
make_excitation	System, kind, flip angle (deg), slab thickness (m)	Excitation module	Non-selective, slab-selective or water-selective spectral-spatial excitation of one slab.

Spatial saturation bands

A sequence function offers each spatial saturation band as sat<k>_normal_x, _y, _z, sat<k>_position and sat<k>_thickness, in the physical frame, and builds the bands it plays with spatial_saturations; a band of zero thickness is off.

Object	Input	Returns	Purpose
spatial_saturations	System, (normal, position, thickness) per band	List of SpatialSaturation	The bands a sequence function plays, those with a thickness.

Dummy repetitions

A sequence function whose n_dummy is None plays as many dummy repetitions as bring its spoiled steady state within 1 % for a T1 of STEADY_STATE_T1_S, from its repetition time and flip angle.

Object	Input	Returns	Purpose
steady_state_dummies	Repetition time (s), flip angle (deg), T1 (s), tolerance	Repetitions	The repetitions to play before acquiring.
STEADY_STATE_T1_S	—	1.5 s	T1 the dummies are counted for: grey matter at 3 T.

Fat saturation timing

Object	Input	Returns	Purpose
fat_null_flip_deg	Delay to the excitation (s), period (s), fat T1 (s)	Flip angle (deg)	The fat-saturation flip that leaves fat at zero at the excitation, in the steady state.
FAT_T1_S	—	0.37 s	T1 of fat at 3 T.