fn unwrap_near(ang: f64, prev: f64) -> f64Expand description
Put ang on the 2π-branch nearest prev.
Every phase here reaches us through atan2, which folds onto (−π, π]. That
fold is not harmless: a compartment accumulates (2/3)·χ·γ·B0·TE radians, so
at |χ_total| > π / ((2/3)·γ·B0·TE_max) — 0.09 ppm at 7 T with a 28 ms last
echo, i.e. inside the globus pallidus — the late echoes fold back and the
alternating fit chases the fold into a degenerate fixed point (χ− pinned at
its bound, R2*₀ and the amplitudes driven to zero, χ+ stuck at 0). Walking the
echoes in order and keeping each phase on the branch nearest its predecessor
restores the monotone ramp the model assumes.
The first echo is left on its principal branch: it anchors the series, and at
any sane TE₁ its phase is well inside (−π, π] for |χ| ≤ chi_bound.