Capability tiers (C0–C4)
A replay pack declares what it can replay. The tiers are additive: a higher tier stores more per-walker channels, so it can answer more questions — but every tier shares the same walk and the same geometry.
| Tier | Name | What it stores | What you can decode |
|---|---|---|---|
| C0 | Gradient | the walk, as a compact trajectory basis | any gradient waveform — PGSE, OGSE, STE, arbitrary G(t); any b, Δ, δ, direction; real RF / B₁ pulses (vector-Bloch replay, so refocusing follows the actual pulse) |
| C1 | Bulk relaxation | per-compartment T₂/T₁ | any TE/TR weighting; a walk to T_max contains every shorter TE as a prefix |
| C2 | Surface | boundary local-time | surface relaxivity ρ at the walls, and the apparent T₂ it produces |
| C3 | Field | per-walker susceptibility basis (13-channel) | any B₀ and χ (isotropic + anisotropic), any orientation; GRE dephases, spin-echo refocuses |
| C4 | MT | two-pool qMT parameters (f_b, k_f, T₂ᵇ) | magnetization-transfer ratio, Z-spectrum |
The field tier is subtle on purpose
C3 is where a lossy shortcut can quietly break the physics. A spin-echo refocuses the static field to machine precision regardless of substrate shape; only diffusing spins carry a residual (the diffusion×susceptibility cross-term). That cancellation is delicate — a codec that looks fine on a gradient-echo can destroy it. So the fidelity check grades any C3 codec against the substrate's own field-noise floor, on the spin-echo cross-term, not just the easy gradient-echo case.
One walk, all tiers
A single master walk backs C0–C4 at once. The tiers are what got recorded alongside the trajectory, not separate simulations — and each is stored as a compact basis the decode contracts against, not an expanded trace. That is why replay is cheap and a pack is small.