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.
Description
Off-resonance displaces signal in the phase-encoding direction according to effective bandwidth per pixel.
Off-resonance displaces signal in the phase-encoding direction according to effective bandwidth per pixel. A field-map offset and a pixel size convert that shift to millimetres.
The tool returns signedShiftPixels (signed shift pixels); signedDisplacementsMm (signed displacements mm). Input measurements and model parameters are supplied by the caller; the tool computes the stated summary rather than acquiring those measurements.
When to use Susceptibility Artifact Shift
Use Susceptibility Artifact Shift when you already have offResonanceHz, effectivePhaseHzPerPixel, phasePixelMm and want signedShiftPixels, signedDisplacementsMm. This is useful for checking a supplied calculation, documenting a measurement convention, or comparing observations prepared on the same basis.
How to use Susceptibility Artifact Shift and understand its results
- Enter offResonanceHz, effectivePhaseHzPerPixel, phasePixelMm 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 signedShiftPixels, signedDisplacementsMm. 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 |
| effectivePhaseHzPerPixel | Effective phase hz per pixel | number | Required |
| phasePixelMm | Phase pixel mm | number | Required |
Use nonempty signed offsets and positive effective phase bandwidth and pixel size. Preserve input order and signs.
shiftPixels = offResonanceHz/effectivePhaseHzPerPixel; displacementMm = shiftPixels×phasePixelMm The symbols refer to the supplied quantities or generated values described above; unit conversions and comparison boundaries follow the stated implementation.
{
"input": {
"offResonanceHz": [
-50,
0,
50
],
"effectivePhaseHzPerPixel": 10,
"phasePixelMm": 2
},
"output": {
"signedShiftPixels": [
-5,
0,
5
],
"signedDisplacementsMm": [
-10,
0,
10
]
}
}Limitations and assumptions
The supplied bandwidth must already account for acquisition details. No field map, displacement-field orientation or image unwarping is computed.
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 Artifact Pixels: Estimate signed frequency-encoding chemical-shift displacement using supplied shift, reference frequency and pixel bandwidth; direction depends on readout polarity. 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.
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 Artifact Pixels
Estimate signed frequency-encoding chemical-shift displacement using supplied shift, reference frequency and pixel bandwidth; direction depends on readout polarity.
- 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.