Constraint checks

Constraint checks#

TL;DR

  • A .seq file records the system limits the sequence was designed against, not the system it will be played on. The two differ when the file is used at another site, when a site derates its gradient limits, or when a raster is finer in the design script than on the amplifier.

  • The gradient-derived checks evaluate the gradient axes after each block’s ROTATIONS extension, which are the logical axes of a design; the SAR check evaluates the RF waveforms and RF shims. The physical axes also need the prescription rotation: it is passed as rotation to check_pns() and check_mech_resonance(), and applied with TransformFOV before the other gradient checks.

  • Every check returns a boolean verdict and a report, whether or not it passes. Gradient amplitude, slew-rate and continuity reports are in Hz/m and Hz/m/s, mechanical-resonance amplitudes in mT/m, PNS responses as fractions of the model threshold, and SAR in W/kg.

  • The gradient amplitude, slew-rate and continuity checks read only the sequence and its system limits; the PNS, mechanical-resonance and SAR checks require site or coil data supplied as arguments. Timing is checked separately, by check_timing().

  • Gradient amplitude, slew rate and continuity are evaluated pointwise, PNS over the whole sequence from rest, and mechanical resonance and SAR over windows. Any window exceeding its threshold or limit makes the verdict false.

The checks in pypulseqpp.safety evaluate a finished sequence against stated limits and models. They are design-time estimates. They do not replace the scanner’s own gate before download, nor its hardware monitor during the scan, and they do not establish patient safety. This page states the conventions the checks share: which system a check evaluates against, the frame it reads gradients in, the units of its report, and the interval it evaluates over.

Checks#

Check

Compares

Needs

check_max_grad()

the largest per-axis gradient amplitude

max_grad from the system limits

check_max_slew()

the largest per-axis slew rate within a block

max_slew and the gradient raster from the system limits

check_grad_continuity()

the amplitude step across each block boundary

max_slew and the gradient raster from the system limits

check_pns()

a nerve model’s response to the slew of each axis

a SAFE or chronaxie model

check_mech_resonance()

the windowed gradient amplitude spectrum

a forbidden-band table

check_spl()

peak and A-weighted average sound pressure of the loudest repetition, played without end

an acoustic transfer function per physical axis

check_sar()

window-averaged local and global SAR

virtual observation points, a drive calibration and SAR limits

The first three are explained in Gradient amplitude, slew rate and continuity, and the others in Peripheral nerve stimulation, Mechanical resonance and Specific absorption rate.

Designed and played systems#

A .seq file records the system limits the sequence was designed against, in its [DEFINITIONS] and in the system its writer used. It does not record the system it will be played on. The two are identical when the designer entered the limits of the scanner the sequence will run on, and differ when the file is used at another site, when a site derates its gradient limits, or when a raster is finer in the design script than on the amplifier.

Frames, rotations and reports#

The gradient-derived checks — amplitude, slew rate, continuity, PNS and mechanical resonance — evaluate the gradient axes after each block’s ROTATIONS extension has been applied: the logical axes of a design. The SAR check is RF-derived: it evaluates the RF waveforms and RF shims, and no gradient axis or rotation enters it.

The physical gradient axes are reached by a prescription rotation, composed after each block’s own. check_pns() and check_mech_resonance() take one directly, as their rotation argument. The gradient amplitude, slew-rate and continuity checks read the rotations the sequence holds, so a prescription is applied to them by transforming the sequence first with TransformFOV, which composes it into the rotation extensions, and checking the result.

Every check returns a boolean verdict and a report. The report is returned whether or not the check passes, and states the values found, where they were found and the limit or threshold they were compared with. Gradient amplitude, slew-rate and continuity reports are in Hz/m and Hz/m/s; gamma on the system limits converts them to mT/m and T/m/s. Mechanical-resonance amplitudes are in mT/m, PNS responses are fractions of the model threshold, and SAR is in W/kg.

Required data#

The first three checks read only the sequence and its system limits. The last three require site or coil data that no sequence file contains — a nerve model’s coefficients, a gradient assembly’s forbidden bands, a body model’s VOPs and the transmit chain’s calibration — supplied as arguments.

Timing is checked separately, by check_timing(), which establishes that every event time is addressable on the raster its event is played on and that the transmit and receive dead times are respected. A sequence whose gradients are within every limit can still be unplayable because one delay is off the raster.

Evaluation interval#

Gradient amplitude, slew rate and continuity are pointwise: every sample of the scan is either within the limit or outside it, and the largest value determines the verdict.

The nerve models are dynamic. Their response at one sample depends on the preceding slew history, so the stimulation check is evaluated over the whole sequence in one pass, starting from rest, rather than over a representative repetition.

The mechanical-resonance and SAR checks are evaluated over windows rather than samples. The resonance check slides a window of a stated length along the sequence and transforms each; the SAR check averages RF energy over each repetition detected from the block definitions. Any window exceeding its threshold or limit makes the verdict false.

See also#