[Mristudio-users] Applying field map to DTI ?

[Susumu Mori]

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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