Individually optimized 3D fast spin echo

Individually parameterized 3D FSE assigns different echo-train lengths and repetition times to the shots that acquire central and peripheral k-space [BUO25]. From the central to the peripheral shots, TR and the minimum and maximum angles of the refocusing schedule [BUS08b] follow a cubic smooth-step transition between their two limits; the echo-train length follows the same transition, rounded to integer values. The views are assigned by an adaptive radial order. Longer trains and a different TR at the periphery can reduce scan time, while the contrast at the centre of k-space is set by the parameters of the central shots.

from pypulseqpp import sequences

Fse3DApp = sequences.fse3D_sequence.Fse3DApp

P = {
    "n_x": 96,
    "n_y": 64,
    "n_z": 20,
    "fov_x": 0.20,
    "fov_y": 0.20,
    "fov_z": 0.12,
    "etl": 40,
    "etl_periphery": 72,
    "te": 200e-3,
    "tr": 1.2,
    "tr_periphery": 1.6,
    "refocusing_angle_deg": 120.0,
    "n_dummy": 0,
    "ordering": "radial",
    "flip_modulation": "optimized",
    "wave_amplitude": 0.0,
}
app = Fse3DApp(**P)
seq = app.design()

Train parameters

TR and the schedule’s minimum and maximum angles follow a cubic smooth-step transition, \(3u^2 - 2u^3\) with \(u\) from 0 at the first (central) shot to 1 at the last (peripheral) shot [BUO25]; the echo-train length follows the same transition rounded to integers. Every schedule passes through the prescribed 120 degrees at the effective-TE echo. The schedules of representative shots are plotted up to each shot’s own train length: the central shots reach lower minimum angles, and the peripheral shots have shallower minima and longer trains.

Adaptive radial ordering

The ordering ranks (shot, echo) slots jointly by distance from the effective-TE echo and by position in the central-to-peripheral transition [BUO25]. Views are ranked by k-space radius; the innermost views fill the slots nearest the effective-TE echo of the central shots, and within each group of one view per shot the views are assigned to shots in order of angle. Colour gives the train length and TR of the shot that acquired each view.

References

[BUO25] (1,2,3)

Buonincontri G, et al. Proceedings of the International Society for Magnetic Resonance in Medicine. 2025; abstract 566-05-007.

[BUS08b]

Busse RF, Brau ACS, Vu A, Michelich CR, Bayram E, Kijowski R, Reeder SB, Rowley HA. Effects of refocusing flip angle modulation and view ordering in 3D fast spin echo. Magnetic Resonance in Medicine. 2008;60(3):640-649. https://doi.org/10.1002/mrm.21680