[Mristudio-users] Applying field map to DTI ?

[Dorian P.]

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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