<div dir="ltr">Dorian,<div><br></div><div>DTI, which is based on single-shot echo planar imaging, is known to distort due to B0 field inhomogeneity.</div><div>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.</div>
<div><br></div><div>However, this field map technique has some its own difficulties. From what I have heard, this method is not perfect.</div><div>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".</div>
<div><br></div><div>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 (<a href="https://www.mristudio.org/wiki/user_manual/diffeomap">https://www.mristudio.org/wiki/user_manual/diffeomap</a>).</div>
<div><br></div><div>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.</div>
<div><br></div><div>So, all three approaches require some extra data. Which of the three is the best? Well, all three are equally good and equally imperfect. </div></div><div class="gmail_extra"><br><br><div class="gmail_quote">
On Thu, Dec 5, 2013 at 9:30 AM, Dorian Pustina <span dir="ltr"><<a href="mailto:albnet@gmail.com" target="_blank">albnet@gmail.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div dir="ltr">Hello everybody at MRIstudio,<div><br></div><div>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)?</div>
<div><br></div><div>Thank you</div><div><br></div><div>Dorian</div><div>TJU</div></div>
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