Midline Shift

Measure signed landmark distances from the explicit reference plane n·x=offset; this does not identify anatomical landmarks or severity.

Description

A plane normal and offset define a geometric midline reference.

A plane normal and offset define a geometric midline reference. Signed perpendicular distances preserve which side of the plane each landmark lies on.

The tool returns signedDisplacementsMm (signed displacements mm); maximumAbsoluteShiftMm (maximum absolute shift 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 Midline Shift

Use Midline Shift when you already have pointsMm, midlineNormal, midlineOffsetMm and want signedDisplacementsMm, maximumAbsoluteShiftMm. This is useful for checking a supplied calculation, documenting a measurement convention, or comparing observations prepared on the same basis.

How to use Midline Shift and understand its results

Enter pointsMm, midlineNormal, midlineOffsetMm 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 signedDisplacementsMm, maximumAbsoluteShiftMm. 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
pointsMm	Points mm	list of list of number	Required
midlineNormal	Midline normal	list of number	Required
midlineOffsetMm	Midline offset mm	number	Required

Use nonempty three-dimensional points and a nonzero three-component normal. Normalize the plane equation and return distances and maximum absolute shift.

distance = (normal·point−offset)/||normal||

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

json

{
  "input": {
    "pointsMm": [
      [
        2,
        0,
        0
      ],
      [
        -1,
        0,
        0
      ]
    ],
    "midlineNormal": [
      1,
      0,
      0
    ],
    "midlineOffsetMm": 0
  },
  "output": {
    "signedDisplacementsMm": [
      2,
      -1
    ],
    "maximumAbsoluteShiftMm": 2
  }
}

Limitations and assumptions

The normal and offset must describe the same plane equation; the offset is normalized with the normal. Landmark detection and clinical interpretation 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

Affine Spatial Normalization: Apply a supplied nonsingular voxel-to-world image affine including translations; voxel indexing and world-axis convention must match the image header, and registration is not fitted. Lesion Volume: Integrate lesion voxel fractions using anisotropic voxel dimensions; segmentation and partial-volume probabilities must be supplied.

Methodological background: NIfTI coordinates and affine transformations.^[1]

References

NIfTI coordinates and affine transformations — Robert W. Cox; National Institute of Mental Health

Similar or alternative tools

Affine Spatial Normalization
Apply a supplied nonsingular voxel-to-world image affine including translations; voxel indexing and world-axis convention must match the image header, and registration is not fitted.
Lesion Volume
Integrate lesion voxel fractions using anisotropic voxel dimensions; segmentation and partial-volume probabilities must be supplied.