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

Wed Mar 26 12:58:31 PDT 2014

Hi Thijs.

It is great to know that the new release of mrtrix will have tools for AFD.
In the meantime, I have played around with some scripts to obtain such a
measurement in a tract-specific way and I would appreciate your input and
that of others in this list. The scripts do this:

1. Perform tractography and select a set of streamlines for analysis.
2. For each streamline segment (i.e., the straight line between two
adjacent steps), find the FOD lobe that is most parallel (dot product
between streamline segment and FOD peaks).
3. Once the FOD lobe has been identified, extract its amplitude
(tract-specific AFD).

I am assuming that if DWI signal is normalized (as you mentioned), then FOD
amplitudes should be comparable between subjects. Also, since there is no
spatial normalization of images, there is no need to modulate the AFD to
compensate for the spatial transformation.

If you find any flaws with this, please let me know. Oh, and if you could
give us a teaser of what new features we will see in the new release of
mrtrix, that would be fantastic!

Dr. Luis Concha
Instituto de Neurobiología
Laboratorio C-13
UNAM, Campus Juriquilla
Boulervard Juriquilla 3001
Juriquilla, Querétaro.
C.P. 76230
México
Tel (442) 2 38 10 54
Fax (442) 2 38 10 46
http://personal.inb.unam.mx/lconcha/

On Wed, Mar 26, 2014 at 1:47 PM, Thijs Dhollander <
thijs.dhollander at gmail.com> wrote:

> Hi Helen,
>
> Opinions on FA/ADC quantification can be wildly varying... :-)  There's no
> problem calculating and reporting their values, but interpretation is a big
> issue: they can be quite sensitive in many cases, but they are always far
> from specific.  To avoid starting a long rant on how we should abandon
> these measures as quickly as possible, I'd rather refer to this nice
> article:
> http://www.sciencedirect.com/science/article/pii/S1053811912007306 ; it
> contains some nice and clear passages explaining some of the relevant
> issues to be wary of, as well as further references that are worth a good
> read.
> So the question remains: what's better then?  There's quite some
> alternatives, but I personally particularly like apparent fibre density
> (AFD).  A nice and in-depth article on it can be found here:
> http://www.sciencedirect.com/science/article/pii/S1053811911012092 .
> Last year's ISMRM had an abstract on some further improvements on analysing
> AFD: http://cds.ismrm.org/protected/13MPresentations/abstracts/0841.pdf(if you or anyone in your lab attended, they can access it;  the
> accompanying oral presentation can be found at
> http://cds.ismrm.org/protected/13MPresentations/0841/ ).  The next big
> version of MRtrix (expected to be released somewhere around this year's
> ISMRM in May... ;-)) will contain much more tools that provide you with the
> means to perform such analyses.
> In the mean time, you can already easily obtain an (orientationally
> averaged) AFD map by simply calculating the average of your (normalized)
> DWI images from a single shell acquisition.  The same contrast (up to a
> fixed constant factor) can also be obtained by opening up the result from
> an SH fit of the signal (or even a CSD outcome) in mrview, and looking at
> the first coefficient (i.e. the 0th order).
>
> Hope this already helps somewhat!
>
> Cheers,
> Thijs
>
> PS: now that I've mentioned the upcoming ISMRM; I'd like to encourage
> everyone on this list that still has many questions (or not) to surely
> attend and look for each other -- discussions on these new and better ways
> of quantifying are always much more lively in person :-)
>
>
>
> *Thijs Dhollanderthijs.dhollander at gmail.com <thijs.dhollander at gmail.com>
> Tel. +32 475 36 44 27 <%2B32%20475%2036%2044%2027>*
> Medical Image Computing (MIC), ESAT-PSI, Department of Electrical
> Engineering, KU Leuven
>
>
> On Wed, Mar 26, 2014 at 4:52 PM, Helen Carlson <
> Helen.Carlson at albertahealthservices.ca> wrote:
>
>> Hello Thijs
>>
>> Thank-you for your reply to my question. I would like to quantify changes
>> in white matter over time, specifically pre/post treatment. Is there a
>> better way to do this than using the mean FA and ADC? Or is this
>> measurement appropriate?
>>
>> Thanks again
>>
>> Helen.
>>
>>
>>
>> *From:* mrtrix-discussion-bounces at www.nitrc.org [mailto:
>> mrtrix-discussion-bounces at www.nitrc.org] *On Behalf Of *Thijs Dhollander
>> *Sent:* March 23, 2014 9:47
>> *To:* mrtrix-discussion at www.nitrc.org
>> *Subject:* Re: [Mrtrix-discussion] FA and ADC values for a tract
>>
>>
>>
>> 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 Dhollander thijs.dhollander at gmail.com
>> <thijs.dhollander at gmail.com>Tel. +32 475 36 44 27
>> <%2B32%20475%2036%2044%2027>*
>> 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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