Function heidi_params 

fn heidi_params(
    config: &InversionConfig,
    metadata: &ScanMetadata,
) -> Result<HeidiParams, PipelineError>

HEIDI’s parameters with the scan’s: NESTA’s result depends on χ’s scale in upstream’s phase units (see the HEIDI module docs), so the field strength and echo times come from metadata, and config.heidi.te picks the echo time of the field (by default the last, as qsm_heidi’s phase2susceptibilityfactor uses).