Chemical Shift Artifact Pixels
Estimate signed frequency-encoding chemical-shift displacement using supplied shift, reference frequency and pixel bandwidth; direction depends on readout polarity.
Description
Chemical-shift frequency offsets displace signal along readout according to bandwidth per pixel.
Chemical-shift frequency offsets displace signal along readout according to bandwidth per pixel. The displacement sign follows the supplied shift convention.
The tool returns signedShiftPixels (signed shift pixels); signedDisplacementMm (signed displacement 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 Chemical Shift Artifact Pixels
Use Chemical Shift Artifact Pixels when you already have chemicalShiftPpm, referenceFrequencyMHz, readoutHzPerPixel, pixelSizeMm and want signedShiftPixels, signedDisplacementMm. This is useful for checking a supplied calculation, documenting a measurement convention, or comparing observations prepared on the same basis.
How to use Chemical Shift Artifact Pixels and understand its results
- Enter chemicalShiftPpm, referenceFrequencyMHz, readoutHzPerPixel, pixelSizeMm 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, signedDisplacementMm. 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 |
|---|---|---|---|
| chemicalShiftPpm | Chemical shift ppm | number | Required |
| referenceFrequencyMHz | Reference frequency m hz | number | Required |
| readoutHzPerPixel | Readout hz per pixel | number | Required |
| pixelSizeMm | Pixel size mm | number | Required |
Use positive reference frequency, bandwidth per pixel and pixel size; allow a signed chemical shift.
shiftPixels = chemicalShiftPpm×referenceMHz/readoutHzPerPixel; displacementMm = shiftPixels×pixelMm The symbols refer to the supplied quantities or generated values described above; unit conversions and comparison boundaries follow the stated implementation.
{
"input": {
"chemicalShiftPpm": -3.5,
"referenceFrequencyMHz": 100,
"readoutHzPerPixel": 200,
"pixelSizeMm": 1
},
"output": {
"signedShiftPixels": -1.75,
"signedDisplacementMm": -1.75
}
}Limitations and assumptions
This describes readout displacement, not opposed-phase cancellation or phase-encode distortion. Reconstruction sign conventions need separate matching.
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.