Shuffled echo-resolved 3D FSE

Shuffled 3D FSE uses the same refocusing-train design as conventional FSE and differs in two separate choices. The support is a variable-density Poisson-disc draw, which determines which views are acquired. The ordering is that of T2 Shuffling [TAM17]: each train is a group of contiguous views in raster order, and the echo position of each view within its train is random, which determines when each view is acquired. Together, the variable-density support and the random echo positions give each echo time a subset of views spread over the sampled extent without a regular pattern, the sampling condition of echo-resolved subspace reconstruction [TAM17]; no reconstruction is performed here.

from pypulseqpp import sequences

Fse3DApp = sequences.fse3D_sequence.Fse3DApp

P = {
    "n_x": 128,
    "n_y": 96,
    "n_z": 48,
    "fov_x": 0.20,
    "fov_y": 0.20,
    "fov_z": 0.12,
    "etl": 64,
    "te": 100e-3,
    "tr": 1.4,
    "ry": 2,
    "rz": 2,
    "n_acs_y": 8,
    "n_acs_z": 6,
    "n_dummy": 0,
    "ordering": "shuffling",
    "flip_modulation": "optimized",
    "wave_amplitude": 0.0,
}
app = Fse3DApp(**P)
seq = app.design()

Variable-density sampling

A fully sampled calibration region is embedded in a variable-density Poisson-disc support, selected with make_cartesian_plane_sampling(..., sampling="poisson"). The echo index of each view is then assigned by make_shuffling_order() rather than by its distance from the k-space centre.

Echo-time distribution

The refocusing train is designed as in Conventional 3D fast spin echo, whose figures show the schedule; shuffling changes the assignment of views to echoes, not the refocusing schedule. With a 64-echo train, each echo index occurs throughout the sampled extent, so the contrast evolution along the train is not confined to a radial k-space band. The figure gives the distance of every acquired view from the k-space centre against its echo time: every echo time samples views from the centre to the edge of the support.

References

[TAM17] (1,2)

Tamir JI, Uecker M, Chen W, Lai P, Alley MT, Vasanawala SS, Lustig M. T2 shuffling: sharp, multicontrast, volumetric fast spin-echo imaging. Magnetic Resonance in Medicine. 2017;77(1):180-195. https://doi.org/10.1002/mrm.26102