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 gradient axes after each block’s rotation, against the limits of system or of the sequence’s own pypulseqpp.Opts. They take no prescription rotation, so these are the physical axes only once TransformFOV has composed one into the rotation extensions. 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.