15. DTI in MS Flashcards
intro to MS
chronic of CNS
dissemination of WM lesions in time and space
multiple areas of WM inflammation, demyelination and gliosis
what do T2 images show of MS
On conventional T2 images the MS lesions appear as areas of high signal intensity compared with the adjacent normal regions.
These T2 hyperintensities reflect several pathological changes: oedema, inflammation, demyelination, remyelination, axonal loss, gliosis
Therefore, conventional imaging has a lack of histopathological specificity that accounts, in part, for the modest correlation between clinical disability and MRI parameters
what is NAWM
diffuse damage in WM that appears normal on MRI but doesn’t mean person doesn’t have disability
what does DTI tell us
In the brain white matter diffusion is greater along the axons and restricted in the directions perpendicular to the axons. This property is termed diffusion anisotropy.
From the DT, several indices can be derived, such as:
1) fractional anisotropy (FA), which quantifies the preferential direction of diffusion within a voxel,
2) mean diffusivity (MD), which measures the magnitude of water diffusion without regard to its directionality
3) and principal diffusivities:
- lambda 1, which is the largest eigenvalue, and is parallel to the fibers
- lambda 2 and 3, which are respectively the intermediate and smallest eigenvalues, and are perpendicular to the fibers
why DT in ms
1) because changes in diffusion parameters reflect pathological changes in the tissue structure (e.g. membranes, subcellular structures)
and (2) because DT is able to detects pathological changes that are not visible on conventional imaging, such as those in the NAWM
The fact that DTI can detect pathological changes which are not visible on conventional MRI has important clinical relevance because pathology in NAWM and NAGM contributes to disability in patients with MS.
using ROI analysis
ROI analysis consists of drawing regions of interest on specific areas of the brain, using anatomical knowledge, and then quantifying the diffusion parameters, such as FA or MD, within those areas.
when to do ROI analysis
When the study is focused on a particular region that has been chosen a priori
The area is easily defined
we are not short of time
Disadvantages are
correct for multiple comparisons, and Bonferroni correction can be unnecessarily conservative reducing the sensitivity of the measurement
this method is dependent on the operator, and we have to pay attention on the inter-observer reproducibility and on defining guidelines for positioning the ROIs.
poor inter-observer reproducbility
method of ROI and tractography
define a given tract in all subjects
calculate FA along tract
compare FA between subjects
problem - requires manual intervention to specify tract, hence doesn’t investigate whole brain
how histogram analysis MS
The histogram of a diffusion parameter is a frequency distribution showing the number of voxels with a particular range of parameter values.
From the histogram of each DTI parameter, the following variables can be derived:
the peak location (the location of the modal value);
the peak height (the proportion of voxels at modal value);
the mean
MS patients show:
1) a reduction of the MD peak height (which reflects the amount of truly normal tissue)
2) A shift towards the right for MD
3) and towards the left for FA.
This corresponds to an increase of the average MD and a decrease of
the average FA values in MS patients vs healthy subjects.
when histogram analysis
no a priori hypothesis exists about the location of pathology
operator dependent bias is an issue
time is an issue
but… info on the location of abnormalities is lost
partial volume effect due to atrophy has a statistically significant effect on all FA histogram variables
segmentation of DTI images is an issue
how to VBM analysis
align all subjects data to standard space
gm segmentatio
smooth gm
do voxelwise stats
The VBM-style analysis consists of performing an analysis on a voxel-by-voxel basis in order to localise changes related to tissue abnormalities.
when to do vbm analysis
all location across the brain are tested in an unbiased way
specific location of significant group differences or correlations is automatically shown
but registration algorithm is an issue, alignment difficult
why is DTI not routinely used for diagnosis of MS
Because the changes in diffusion parameters of the lesions are not the same.
For example, MS lesions might show increased MD values, such as in this study.
why has DTI not used in treatment trials
lack of standardization of measurements for multi-centre studies.
And also because the exact relationship between long. changes and pathological abnormalities is unclear.
