Gradient, PNS and SAR checks

Checks of a complete sequence against the gradient hardware limits, the forbidden gradient bands of a gradient coil, a peripheral-nerve-stimulation model and a VOP SAR model. They are estimates, not a complete scanner or patient-safety assessment. Gradient, PNS and SAR constraints covers what each one computes and the criterion it applies.

Gradient limits

Gradient-derived: the checks evaluate the three physical gradient axes after each block’s rotation, against the limits of system or of the sequence’s own pypulseqpp.Opts. Only the largest per-axis peak is compared with the limit; the simultaneous vector magnitude is reported but not compared.

Object	Input	Returns	Purpose
check_max_grad	Sequence, system limits (max_grad)	(is_ok, report); peaks in Hz/m	Peak gradient amplitude.
check_max_slew	Sequence, system limits (max_slew, gradient raster)	(is_ok, report); peaks in Hz/m/s	Within-block slew rate.
check_grad_continuity	Sequence, system limits (max_slew, gradient raster)	(is_ok, report); steps in Hz/m and Hz/m/s	Amplitude continuity across block boundaries and at the end.

Mechanical resonance

Gradient-derived: the check evaluates the physical gradient axes after the block rotations and the rotation argument, sampled on the sequence’s gradient raster. The forbidden bands are an argument; only gamma is read from system.

Object	Input	Returns	Purpose
check_mech_resonance	Sequence, forbidden bands, window width (s), rotation	(is_ok, report); amplitudes in mT/m	Windowed gradient spectrum against forbidden bands.
mech_resonance_spectrum	Sequence, window index, window width (s), rotation	frequency (Hz) and (3, bins) amplitude (mT/m)	Gradient amplitude spectrum of one window.
read_forbidden_bands	Siemens .asc or GE epiesp.dat path	List of ForbiddenBand	Forbidden bands from a vendor table.
ForbiddenBand	Axis, f_min, f_max (Hz), tolerance (mT/m)	Band record	Forbidden band on one or every physical axis.

Nerve stimulation

Gradient-derived: the check evaluates the slew of each physical gradient axis after the block rotations and the rotation argument, sampled on the sequence’s gradient raster. The nerve model is an argument; only gamma is read from system.

Object	Input	Returns	Purpose
check_pns	Sequence, SAFE or chronaxie model, rotation, trace	(is_ok, report); responses as fractions of threshold	Peripheral nerve stimulation estimate.
read_safe_model	Siemens .asc path	SAFE model, per-axis coefficients	SAFE model from a hardware description.
ChronaxieModel	Chronaxie (s), rheobase (T/m/s), alpha	Model for check_pns	Rheobase–chronaxie nerve model, all axes alike.

SAR

RF-derived: the check evaluates the RF waveforms and RF shims, and reads no gradient, rotation or system. It takes the VOPs as a VopModel, the channel drive calibration drive_per_hz, and local_limit and global_limit in W/kg.

Object	Input	Returns	Purpose
check_sar	Sequence, VopModel, drive_per_hz, limits (W/kg)	(is_ok, report); window-averaged SAR in W/kg	Local and global SAR per window.
read_vops	.mat or .npz path	VopModel	VOPs and global SAR matrix from a file.
example_vops	Channel count	model, drive_per_hz (V/Hz), cp_shim	Synthetic VOP model, for demonstrations and tests only.
VopModel	VOPs (N, Nc, Nc), optional global matrix (Nc, Nc)	Model for check_sar, W/kg per unit drive squared	Virtual observation points.