2D spiral spin echo

A slice-selective excitation and 180-degree refocusing pulse form one spin echo, followed by a spiral interleaf. Spoilers suppress unwanted coherence before the next TR. TE controls T2 weighting; off-resonance affects the spiral readout. Spiral spin echo supports rapid T2-weighted structural imaging.

Constant-density spiral

Sixteen interleaves at a constant pitch.

import pypulseqpp as pp
from pypulseqpp.sequences import se_spiral2D_sequence

baseline = se_spiral2D_sequence(
    n=192, n_shots=16, n_slices=1, te=None, tr=None, n_dummy=0
)
print(f"{baseline.num_blocks} blocks, {baseline.duration()[0]:.2f} s")

112 blocks, 0.40 s

Sequence diagram

baseline.paper_plot()

Sampling order

Each interleaf is the same spiral gradient waveform, rotated to its own angle by a rotation extension.

pp.plot.plot_kspace(baseline, color_by="shot", plane="xy")

Variable density

A dual-density interleaf retains the Nyquist spacing near the origin and increases the pitch at larger radii, reducing the readout duration.

alternative = se_spiral2D_sequence(
    n=192,
    n_shots=16,
    n_slices=1,
    density="dual",
    periphery_undersampling=2.0,
    te=None,
    tr=None,
    n_dummy=0,
)

                   blocks  duration (s)  acquisitions
constant              112          0.40            16
dual density          112          0.35            16

pp.plot.plot_kspace(alternative, color_by="shot", plane="xy")