G-Ratio
Compute the aggregate g-ratio sqrt(AVF/(AVF+MVF)) from calibrated volume fractions; not an arithmetic average of individual axon diameter ratios.
Description
An aggregate imaging g-ratio uses supplied axonal and myelin volume fractions to estimate relative axonal size within total fiber volume.
An aggregate imaging g-ratio uses supplied axonal and myelin volume fractions to estimate relative axonal size within total fiber volume. It is a regional model-based proxy.
The tool returns fiberVolumeFraction (fiber volume fraction); aggregateGRatio (aggregate g ratio). Input measurements and model parameters are supplied by the caller; the tool computes the stated summary rather than acquiring those measurements.
When to use G-Ratio
Use G-Ratio when you already have myelinVolumeFraction, axonVolumeFraction and want fiberVolumeFraction, aggregateGRatio. This is useful for checking a supplied calculation, documenting a measurement convention, or comparing observations prepared on the same basis.
How to use G-Ratio and understand its results
- Enter myelinVolumeFraction, axonVolumeFraction 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 fiberVolumeFraction, aggregateGRatio. 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 |
|---|---|---|---|
| myelinVolumeFraction | Myelin volume fraction | number | Required |
| axonVolumeFraction | Axon volume fraction | number | Required |
Require fractions in [0,1], positive combined fraction and sum no greater than one.
fiber fraction = axonFraction+myelinFraction; g = sqrt(axonFraction/fiberFraction) The symbols refer to the supplied quantities or generated values described above; unit conversions and comparison boundaries follow the stated implementation.
{
"input": {
"myelinVolumeFraction": 0.2,
"axonVolumeFraction": 0.6
},
"output": {
"fiberVolumeFraction": 0.8,
"aggregateGRatio": 0.8660254037844386
}
}Limitations and assumptions
The fractions must represent disjoint compartments. Calibration and biological assumptions are external; this is not a direct microscopic diameter measurement.
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
Myelin Water Fraction: Sum short-T2 component amplitudes relative to a supplied fitted relaxometry spectrum; the short-pool cutoff is caller supplied and amplitudes are not fitted here. Average Fractional Anisotropy: Compute voxel FA from nonnegative tensor eigenvalues and an ROI-weighted average, rather than averaging tensor eigenvalues before deriving FA.
Methodological background: Whole-Brain In-vivo Measurements of the Axonal G-Ratio in a Group of 37 Healthy Volunteers.1
References
-
Whole-Brain In-vivo Measurements of the Axonal G-Ratio in a Group of 37 Healthy Volunteers — Mohammadi et al.; Frontiers in Neuroscience
Similar or alternative tools
- Myelin Water Fraction
Sum short-T2 component amplitudes relative to a supplied fitted relaxometry spectrum; the short-pool cutoff is caller supplied and amplitudes are not fitted here.
- Average Fractional Anisotropy
Compute voxel FA from nonnegative tensor eigenvalues and an ROI-weighted average, rather than averaging tensor eigenvalues before deriving FA.