Sampling and scan-loop schedules

Routines that select the Cartesian views an acquisition encodes, order an already selected set into loop, shot and echo positions, and supply the per-shot orientations and per-repetition RF values of a scan loop. None of them creates events or labels: the sequence application scales the gradients and creates the LIN, PAR, ECO and other labels from their results, as Sampling support and ordering describes.

Cartesian sampling support

Support is returned as zero-based encoded view indices, y or (y, z) on an (n_y, n_z) grid with the k-space centre at (n_y // 2, n_z // 2), in two disjoint lists: calibration views and the other acquired views.

Object	Input	Returns	Purpose
make_cartesian_axis_sampling	Prescription of one axis: matrix size, acceleration, ACS, partial Fourier	(calibration, imaging): encoded view indices y	Lines of a 2D acquisition, or partitions of a stack-of-* acquisition.
make_cartesian_plane_sampling	Prescription of a ky-kz plane, with sampling='lattice' or 'poisson'	(calibration, imaging): encoded view indices (y, z)	Views of a 3D Cartesian acquisition.

Boolean sampling masks

mask[y, z] is True where the view (y, z) is acquired; np.argwhere(mask) lists those views as encoded indices.

Object	Input	Returns	Purpose
make_caipirinha_mask	Grid shape, ry, rz, shift	Boolean mask, True where acquired	CAIPIRINHA lattice anchored at (0, 0).
make_poisson_disc_mask	Grid shape, acceleration, calibration block, seed	Boolean mask, True where acquired	Variable-density Poisson-disc support.
make_random_mask	Grid shape, acceleration, calibration block, seed	Boolean mask, True where acquired	Uniform-random support.

Traversal ordering

Object	Input	Returns	Purpose
make_traversal_order	Number of positions, scheme	Permutation of range(n)	Loop order over slices, partitions or selected lines.

Echo-train ordering

Each routine takes centred coordinates of already selected Cartesian views: an (N, 2) or (N,) array with the k-space centre at the origin, obtained from encoded indices as views - (n_y // 2, n_z // 2). It returns trains, where trains[s][e] is the row index into that array of the view acquired at echo e of shot s. The values are indices, not coordinates. The orderings do not change the support: T2 Shuffling reorders views already selected, for example by the Poisson-disc support of make_cartesian_plane_sampling(..., sampling='poisson').

Object	Input	Returns	Purpose
make_linear_order	Centred coordinates, train length	Indices as [shot][echo]	Raster-order bands, one per echo.
make_centric_order	Centred coordinates, train length	Indices as [shot][echo]	Distance-ranked bands; centre at echo 0 or center_echo.
make_radial_order	Centred coordinates, train length	Indices as [shot][echo]	One angular wedge per shot, centre-out within it.
make_radial_adaptive_order	Centred coordinates, train length	Indices as [shot][echo]	Distance bands folded about center_echo.
make_shuffling_order	Centred coordinates, train length, seed	Indices as [shot][echo]	Random echo positions within raster-contiguous trains (T2 Shuffling).

EPI shot offsets

The offsets are relative to echo 0 of one shot. The scan loop chooses each shot’s origin (y0, z0); echo e then acquires (y0, z0) + offsets[e].

Object	Input	Returns	Purpose
make_epi_shot_offsets	EPI prescription: echo-train length, scheme, acceleration, segments, CAIPI	(etl, 2) offsets (Δky, Δkz) relative to echo 0	Linear, segmented blipped-CAIPI and zigzag traversals of one shot.

Non-Cartesian orientations

Object	Input	Returns	Purpose
calc_uniform_angles	Count, angular span	Angles (rad)	Equal spacing over the span.
calc_golden_angles	Count, half or full circle	Angles (rad), modulo 2π	Golden-angle increment.
calc_tiny_golden_angles	Count, tiny-golden index	Angles (rad), modulo 2π	Smaller golden increment.
calc_raga_angles	Count, approximation order	Angles (rad) from a finite equidistant set	Rational approximation of the golden angle.
calc_projection_shell	Views per shell, shots, scheme	Unit directions (n_views, 3) and rotations (n_shots, 3, 3)	3D radial shell and its per-shot rotations.

RF phase and flip schedules

Object	Input	Returns	Purpose
make_rf_spoiling_schedule	Number of repetitions, increment	Phases (rad), one per repetition	Quadratic RF spoiling.
make_phase_cycling_schedule	Number of repetitions, cycle	Phases (rad), one per repetition	Repeated phase cycle.
make_traps_schedule	Echo-train length, target angle	Refocusing angles (rad), one per echo	Stabilising variable refocusing angles.