[Mristudio-users] Image proportions mismatch

[susumu mori]

DTI data acquisition uses "echo-planner imaging", with which you can get
one 2D slice image with one shot. For example, if you have 128 x 128
matrix, it reads all 128x128 data points at once.

This is a rapid and high-SNR method, but the drawback is, the image
distorts in the phase encoding direction, which is usually the front-back
orientation

Depending on how you acquire the image, the images are either elongated or
shrunk.
If the entire object get elongated or shrunk linearly (like a rubber band),
the correction would be easy. However, such image distortion is localized
along sinus and ear canals. That means, the distortion is like you pinch or
push a cray object; they are localized. Anatomically, the frontal tip of
the frontal lobe, the bottom of the temporal lobe, and the pons are the
locations typically deformed.

For the frontal lobe, it gets pinched (exploded) or pushed in (imploded).
Usually, if you use a default setting, Philips scanners give you the
pinched appearance of the frontal lobe and the pons get pushed in. Siemens
and GE are opposite.

Your second image seems to have imploded frontal lobe, while the first one
gets exploded. That's my impression. I maybe wrong. If I'm right, you have
to figure out why your second image has atypical imploded appearance.
Somebody might have explicitly changed the direction of the phase encoding
gradient.

On Fri, Jan 4, 2013 at 11:44 AM, Dorian P. <alb.net at gmail.com> wrote:

> They should be but can't say it for sure. They have both par/rec
> files, hence should be a Philips for both.
>
> What does the frontal implosion mean? I also read another comment from
> you, Dr. Mori, regarding different templates for different scanners:
> http://cmrm.med.jhmi.edu/cmrm/forumMRI/topic.php?id=63
>
> I am simply using the single subject template, which I hope will bring
> any frontal implosion/explosion to the same space.
>
> Thank you.
>
> Dorian
>
> 2013/1/4 susumu mori <susumu at mri.jhu.edu>:
> > Are they from the same scanner?
> > First one has exploded frontal lobe, usually what you see with default
> > Philips scans.
> > Second one has imploded frontal lobe, usually with Siemens.
> >
> > This is caused by the orientation of the phase-encoding steps; from
> positive
> > to negative gradients or from negative to positive gradients.
> >
> >
> >
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