Myelination Index

Compute reference-normalized T1w/T2w ROI ratios as an explicit myelin-sensitive imaging index; intensity bias correction is external and this is not a myelin fraction.

Description

A T1-weighted to T2-weighted signal ratio is an imaging index whose scale depends on acquisition.

A T1-weighted to T2-weighted signal ratio is an imaging index whose scale depends on acquisition. Normalizing by a reference region reduces one supplied scale difference.

The tool returns roiRatios (roi ratios); referenceMeanRatio (reference mean ratio); normalizedIndex (normalized index). Input measurements and model parameters are supplied by the caller; the tool computes the stated summary rather than acquiring those measurements.

When to use Myelination Index

Use Myelination Index when you already have roiT1Weighted, roiT2Weighted, referenceT1Weighted, referenceT2Weighted and want roiRatios, referenceMeanRatio, normalizedIndex. This is useful for checking a supplied calculation, documenting a measurement convention, or comparing observations prepared on the same basis.

How to use Myelination Index and understand its results

Enter roiT1Weighted, roiT2Weighted, referenceT1Weighted, referenceT2Weighted 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 roiRatios, referenceMeanRatio, normalizedIndex. 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
roiT1Weighted	Roi t1 weighted	list of number	Required
roiT2Weighted	Roi t2 weighted	list of number	Required
referenceT1Weighted	Reference t1 weighted	list of number	Required
referenceT2Weighted	Reference t2 weighted	list of number	Required

Calculate paired ratios separately in the ROI and reference; all signals must be positive and each pair of lists aligned. Region sizes may differ.

normalized index = mean(ROI T1/T2)/mean(reference T1/T2)

The symbols refer to the supplied quantities or generated values described above; unit conversions and comparison boundaries follow the stated implementation.

json

{
  "input": {
    "roiT1Weighted": [
      20,
      40
    ],
    "roiT2Weighted": [
      10,
      10
    ],
    "referenceT1Weighted": [
      10,
      10
    ],
    "referenceT2Weighted": [
      10,
      10
    ]
  },
  "output": {
    "roiRatios": [
      2,
      4
    ],
    "referenceMeanRatio": 1,
    "normalizedIndex": 3
  }
}

Limitations and assumptions

Ratios are not quantitative myelin measurements by themselves. Registration, bias correction and reference-region validity are external.

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. Neuromelanin Contrast: Compute neuromelanin-sensitive ROI/reference contrast and CNR using reference sample SD; scanner sequence and ROI selection are external, and contrast is not neuron density.

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.
Neuromelanin Contrast
Compute neuromelanin-sensitive ROI/reference contrast and CNR using reference sample SD; scanner sequence and ROI selection are external, and contrast is not neuron density.