B0 Inhomogeneity PPM
Summarize a measured B0 offset map in ppm with peak-to-peak and RMS about the frequency reference, not a fitted shim solution.
Description
A field map expresses local frequency offsets relative to a reference.
A field map expresses local frequency offsets relative to a reference. Ppm rescales those offsets, while peak-to-peak spread and RMS summarize different aspects of inhomogeneity.
The tool returns offsetsPpm (offsets ppm); peakToPeakPpm (peak to peak ppm); rmsPpm (rms ppm). Input measurements and model parameters are supplied by the caller; the tool computes the stated summary rather than acquiring those measurements.
When to use B0 Inhomogeneity PPM
Use B0 Inhomogeneity PPM when you already have offResonanceHz, referenceFrequencyMHz and want offsetsPpm, peakToPeakPpm, rmsPpm. This is useful for checking a supplied calculation, documenting a measurement convention, or comparing observations prepared on the same basis.
How to use B0 Inhomogeneity PPM and understand its results
- Enter offResonanceHz, referenceFrequencyMHz using the field table below. Match the units, ordering and data boundaries described in the calculation.
- Run the tool after checking the stated bounds and matching paired observations or categories. A malformed request gives a tool error rather than a usable measurement.
- Read offsetsPpm, peakToPeakPpm, rmsPpm. Use the calculation and interpretation below to distinguish amounts, ratios, signs and decision flags where applicable.
- Review the supplied parameters and assumptions before using the result in a report or comparing it with another observation.
| Input field | Meaning | Value type and bounds | Requirement |
|---|---|---|---|
| offResonanceHz | Off resonance hz | list of number | Required |
| referenceFrequencyMHz | Reference frequency m hz | number | Required |
Use nonempty signed measured offsets and positive reference MHz. Return each ppm offset, max−min spread and RMS relative to zero.
offsetPpm = offsetHz/referenceMHz; RMS = sqrt(mean(offsetPpm²)) The symbols refer to the supplied quantities or generated values described above; unit conversions and comparison boundaries follow the stated implementation.
{
"input": {
"offResonanceHz": [
-100,
0,
100
],
"referenceFrequencyMHz": 100
},
"output": {
"offsetsPpm": [
-1,
0,
1
],
"peakToPeakPpm": 2,
"rmsPpm": 0.816496580927726
}
}Limitations and assumptions
RMS is not a demeaned standard deviation. No field mapping, unwrapping, masking or shimming is performed.
Numerical inputs must be finite. The tool checks its declared input shape and calculation constraints; those checks do not verify the quality, calibration or representativeness of the supplied observations.
Alternative or Complementary methods
Chemical Shift PPM to Hz Converter: Invert a signed chemical shift into a frequency offset using reference frequency in MHz; this is distinct from existing sample/reference frequency-to-ppm tools. Susceptibility Artifact Shift: Estimate EPI phase-direction displacement from a measured off-resonance map and effective reconstructed phase bandwidth; susceptibility source fields and nonlinear pile-up are not solved.
Methodological background: RAD229: MRI signal and sequence lecture notes.1
References
-
RAD229: MRI signal and sequence lecture notes — Brian Hargreaves and colleagues, Stanford University
Similar or alternative tools
- Chemical Shift PPM to Hz Converter
Invert a signed chemical shift into a frequency offset using reference frequency in MHz; this is distinct from existing sample/reference frequency-to-ppm tools.
- Susceptibility Artifact Shift
Estimate EPI phase-direction displacement from a measured off-resonance map and effective reconstructed phase bandwidth; susceptibility source fields and nonlinear pile-up are not solved.