2D Cartesian gradient echo

A spoiled gradient-echo (SPGR) acquisition applies one low-flip-angle slice-selective excitation before an unbalanced Cartesian readout in each TR. Gradient spoiling and quadratic RF/receiver phase cycling suppress coherent residual transverse magnetisation. TR and flip angle primarily determine T1 weighting, with T2* decay during TE. SPGR is widely used for T1-weighted structural imaging.

Fully sampled acquisition

Every phase-encode line of one slice is acquired.

import pypulseqpp as pp
from pypulseqpp.sequences import gre2D_sequence

baseline = gre2D_sequence(n_x=192, n_y=192, n_slices=1, te=None, tr=None, n_dummy=0)
print(f"{baseline.num_blocks} blocks, {baseline.duration()[0]:.2f} s")
print(
    f"TE {baseline.get_definition('TE')[0] * 1e3:.2f} ms, "
    f"TR {baseline.get_definition('TR')[0] * 1e3:.2f} ms"
)

960 blocks, 1.83 s
TE 4.02 ms, TR 9.54 ms

Sequence diagram

baseline.paper_plot()

Sampling order

The acquisition order places central k-space near the temporal midpoint of the scan. Colour encodes repetition index.

pp.plot.plot_kspace(baseline, color_by="order", plane="xy", show_trajectory=False)

In-plane acceleration

ry=2 acquires one phase-encode line in two and adds a fully sampled calibration region at the centre of k-space, from which a parallel-imaging reconstruction estimates the coil sensitivities.

alternative = gre2D_sequence(
    n_x=192, n_y=192, n_slices=1, ry=2, n_acs_y=24, te=None, tr=None, n_dummy=0
)

                   blocks  duration (s)  acquisitions
full                  960          1.83           192
ry = 2                540          1.03           108

pp.plot.plot_kspace(alternative, color_by="order", plane="xy", show_trajectory=False)