Voxel-based morphometry

Voxel based morphometry (VBM) is a neuroimaging analysis technique that allows investigation of focal differences in brain anatomy, using the statistical approach of statistical parametric mapping. In traditional morphometry, volume of the whole brain or its subparts is measured by drawing regions of interest (ROIs) on images from brain scanning and calculating the volume enclosed. However, this is time consuming and can only provide measures of rather large areas. Smaller differences in volume may be overlooked. VBM registers every brain to a template, which gets rid of most of the large differences in brain anatomy among people. Then the brain images are smoothed so that each voxel represents the average of itself and its neighbors. Finally, the image volume is compared across brains at every voxel.

One of the first VBM studies and one that came to attention in main stream media was a study on the hippocampus brain structure of London taxicab drivers. The VBM analysis showed the back part of the posterior hippocampus was on average larger in the taxi drivers compared to control subjects while the anterior hippocampus was smaller. London taxi drivers need good spatial navigational skills and scientists have usually associated hippocampus with this particular skill.

Another famous VBM paper was a study on the effect of age on gray and white matter and CSF of 465 normal adults. The VBM analysis showed global gray matter was decreased linearly with age, especially for men, whereas global white matter did not decline with age.

A key description of the methodology of voxel-based morphometry is Voxel-Based Morphometry—The Methods &mdash; one of the most cited articles in the journal NeuroImage. The usual approach for statistical analysis is mass-univariate (analysis of each voxel separately), but pattern recognition may also be used, e.g., for classifying patients from healthy.

VBM for brain asymmetry
Usually VBM is performed for examining differences across subjects, but it may also be used to examine neuroanatomical differences between hemispheres detecting brain asymmetry. A technical procedure for such an investigation may use the following steps:
 * 1) Construction of a study-specific brain image template with a balanced set of left and right handed and males and females.
 * 2) Construction of white and grey matter templates from segmentation.
 * 3) Construction of symmetric grey and white matter templates by averaging right and left cerebral hemispheres.
 * 4) Segmentation and extraction of brain image, e.g., removal of scalp tissue in the image.
 * 5) Spatial normalization to the symmetric templates
 * 6) Correction for volume change (applying a Jacobian determinant)
 * 7) Spatial smoothing.
 * 8) Actual statistical analysis by the general linear model, i.e., statistical parametric mapping.

General technical references

 * Tutorial: A Critical Analysis of Voxel Based Morphometry (VBM)
 * Voxel-Based Morphometry Should Not Be Used with Imperfectly Registered Images
 * Why Voxel-Based Morphometry Should Be Used
 * Voxel Based Morphometry at the BIC
 * Voxel-Based Morphometry at Johns Hopkins University
 * Voxel-Based Morphometry at Wikibooks
 * VBM tutorial by John Ashburner

Voxel-basierte Morphometrie Morfometria basata sui voxel Voxel alapú morfometria