[Mrtrix-discussion] FA and ADC values for a tract

[Thijs Dhollander]

Hi Helen,

What you're seeing is (probably) correct: FAs larger than 1 and ADCs below
0 can appear due to the tensor estimation in MRtrix being a simple
(unconstrained) linear least squares one.  The tensors are not explicitely
constrained to be positive definite, and thus negative eigenvalues might
appear.  When one or more eigenvalues of a given tensor are negative, the
FA can go beyond 1, while the ADC can go below 0.

Cheers,
Thijs



*Thijs Dhollanderthijs.dhollander at gmail.com
<thijs.dhollander at gmail.com>Tel. +32 475 36 44 27*
Medical Image Computing (MIC), ESAT-PSI, Department of Electrical
Engineering, KU Leuven


On Fri, Mar 21, 2014 at 10:10 PM, Helen Carlson <
Helen.Carlson at albertahealthservices.ca> wrote:

>  Hello all,
>
> I would like to determine the average FA and ADC values for a given track
> (in this case, the arcuate). So I followed previous directions posted here
> and have used the following commands on my arcuate which successfully
> creates a probability mask for FA and ADC:
>
>
>
> tracks2prob -template fa.mif Arcuate.tck FA_mask.mif
>
> tracks2prob -template adc.mif Arcuate.tck ADC_mask.mif
>
>
>
> I then used mrstats to get the average, max, min etc of the tracks by
> using these commands:
>
>
>
> mrstats fa.mif -mask FA_mask.mif
>
> mrstats adc.mif -mask ADC_mask.mif
>
>
>
> Which also seemed to work except that I was expecting my FA to range from
> 0.1 to 1.0 (approximately). Instead, the FA values have a min of 0 and a
> max of 1.13. In addition, my ADC values have a negative minimum (-0.0024).
> It seems these values are out of range and do not make sense but I do not
> know what I am doing wrong.
>
>
>
> Thanks in advance for any help
>
> Helen
>
>
>
>
>
> *Helen Carlson, Ph.D*
>
> Neuroimaging Research Associate
>
> Calgary Pediatric Stroke Program & Neuropsychology
>
> Alberta Children's Hospital
>
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