[mcencini]

Requested by Matteo

What changed

• Waveform form of the description. read_mrd_description reads the description as ISMRMRD waveforms 1024 (header), 1025 (event list) and 1026 (one per RF definition), the form pulserver sends from Sequence description: one repetition at the k-space centre, as ISMRMRD waveforms pulserver#339. It passes physiological waveforms over. The TEXT message form is no longer read: pulserver no longer sends it, and no stored series holds one. A new description_from_waveforms reads the description off a stored series' waveforms (ismrmrd.Waveform objects or (id, data) pairs).
• RF raster default. EpgEngine.simulate and simulate_subspace now take rf_raster_time_s from the description by default, instead of 1 µs.
• 20 mm rule. A pulse whose bandwidth over its slice-select gradient exceeds NONSELECTIVE_THICKNESS_M (20 mm) is tabulated without bandwidth, so a slice profile takes it at its centre rotation: a slab the encoding resolves. A description stating no gradient or no bandwidth keeps the old behaviour.

Why

• On 7 T data (3D FSE, 3 Nov 2026), the description was not stored with the series, because ISMRMRD HDF5 has no table for TEXT.
• With the 1 µs default, a description on the scanner's 2 µs raster had every flip halved. Shaped echo trains came out 2–30× low.

Validation

• pytest tests/sequence: 1006 passed after removing the TEXT path (the full suite passed at 1490 before that). New tests:
  • the physiological waveforms ahead of the description are passed over;
  • a kept series reads its description from its waveforms;
  • a pulse is integrated on the raster its description states;
  • a slab thicker than the threshold is profiled as its centre.
• ruff check and ruff format --check: clean.
• End to end on the 7 T series: pulserver's waveforms, decoded here with default arguments, reproduce fse_description for the same train to 0.07 %, with or without exact_slice_profile. The slabs read as 180 and 255 mm.

Documentation impact

The developer guide's MRD stream section now gives the waveform layout. explanations/description.md and explanations/epg.md list the new claims with their tests.

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