[Mristudio-users] Applying field map to DTI ?

[Susumu Mori]

Unfortunately, MriStudio software doesn't offer the solution #1 (phase map
based) and #3 (inverted distortion).


On Thu, Dec 5, 2013 at 10:11 AM, Dorian P. <alb.net at gmail.com> wrote:

> Thank you Dr. Mori.
>
> I am also looking at time concerns, both in acquisition and
> postprocessing. LDDMM is a bit long to run for clinical purposes. I used to
> do registration in Diffeomap alone (AIR linear -> AIR non-linear using
> white matter map segmented from T1 as template), but this did not resolve a
> recent problem I had. The subject had a tiny tumor (1cm) near the insula. I
> could see the arcuate passing near the tumor in FA maps, but there was no
> way to register this onto MP-RAGE (the arcuate seemed passing through the
> tumor instead). Thus I need some better solution to have millimetric
> precision.
>
> How would I go for applying solutions 1 and 3 Dr Mori, that is applying
> the field map or correcting with inverted gradients (posterior-anterior
> rather than anterior-posterior).
>
> Thank you for your quick response.
>
>
> 2013/12/5 Susumu Mori <smoriw at gmail.com>
>
>> Dorian,
>>
>> DTI, which is based on single-shot echo planar imaging, is known to
>> distort due to B0 field inhomogeneity.
>> We can map this field inhomogeneity using a separately measured field map
>> (thus, a measurement of field is needed in addition to DTI). With this
>> field map, we can theoretically calculate the amount of distortion and undo
>> it.
>>
>> However, this field map technique has some its own difficulties. From
>> what I have heard, this method is not perfect.
>> When we judge if the distortion is well corrected, we usually compare the
>> distortion-corrected DTI image, such as b0 image, with conventional
>> anatomical scans, such as T2-weighted images. If the corrected b0 image
>> matches very well with a T2-weighted image, we say, "it is well corrected".
>>
>> So, an alternative approach is, to nonlinearly (such as LDDMM) register
>> DTI images to a T2-weighted anatomical scan. This approach doesn't require
>> a separate field map data, but do requires a T2-weighted image. This
>> approach is explained in the #8 of Getting Started here (
>> https://www.mristudio.org/wiki/user_manual/diffeomap).
>>
>> Another method is to acquire two sets of b0 image with different
>> phase-encoding gradients, which generate two b0 images with a completely
>> opposite distortion directions. From these two images, a middle image can
>> be calculated. This approach, however, requires such two b0 data to be
>> acquired.
>>
>> So, all three approaches require some extra data. Which of the three is
>> the best? Well, all three are equally good and equally imperfect.
>>
>>
>>  On Thu, Dec 5, 2013 at 9:30 AM, Dorian Pustina <albnet at gmail.com> wrote:
>>
>>>  Hello everybody at MRIstudio,
>>>
>>> I remember reading some paper that said field maps should be applied to
>>> DWI data to correct for deformations and bring everything closer to true
>>> anatomical space. Anybody knows how to apply this field map and whether it
>>> will really correct for the spatial distortions of DTI (thus making it
>>> easier to register DTI on MP-RAGE)?
>>>
>>> Thank you
>>>
>>> Dorian
>>> TJU
>>>
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>>
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>
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