[Mristudio-users] left-right flipping

[susumu]

Suppose you have a straight tract with 5-pixel diameter and 50-pixel long.
If you do fiber tracking, you get 5 x 50-pixel tract. However, if there is
one location where the diameter is only 3 pixel, it is likely that the
tractography gives you 3 x 50 for the entire length. Does it make sense?

-----Original Message-----
From: mristudio-users-bounces at mristudio.org
[mailto:mristudio-users-bounces at mristudio.org] On Behalf Of Linda Lanyon
Sent: Wednesday, June 25, 2008 1:58 PM
To: 'DTI Studio, ROI Editor, Landmarker Questions/Support'
Subject: Re: [Mristudio-users] left-right flipping

Hi Susumu

Thanks so much for your very helpful reply!

Just to clarify, at #2 do you mean that a narrow section of the path might
be too narrow for the fibers to significantly influence the overall
diffusion in the voxel, so the tensor is not appropriately anisotropic, the
FA value drops and tractography stops at that point?

Best wishes
Linda  

-----Original Message-----
From: mristudio-users-bounces at mristudio.org
[mailto:mristudio-users-bounces at mristudio.org] On Behalf Of susumu
Sent: Monday, June 23, 2008 11:21 PM
To: 'DTI Studio, ROI Editor, Landmarker Questions/Support'
Subject: Re: [Mristudio-users] left-right flipping

Hi Linda,

Please note that DtiStudio follows the Radiological convention like all the
scanners do; the right side of the image in axial and coronal slices are the
patient's left. So far we haven't noticed any situations in which right-left
is flipped in DtiStudio. If you want to confirm, you can put a syringe or a
water bottle to one side of the brain during a scan.

You need to be careful if your data went through other software programs.
Many Neurology-oriented programs follow the Neurology convention. Default
Analyze format also is Neurology convention. If the brain orientation is
flipped, usually you can tell it immediately because the axial slice becomes
inverted.

As for the right-left, we are also one of them who published the right-left
asymmetry based on tractography. However, we have to be careful. We know
that the total white matter volume is almost the same between the right and
the left hemisphere. Most white matter tracts should have the same sizes.
The reason why tractography gives us the asymmetry results could be
complicated;

1) There are regions with lower FA in one of the hemisphere.
2) The tract shape and size is more convoluted in one of the hemisphere (the
size of the tractography is heavily influenced by the narrowest section of
the entire path).
3) The SNR (or signal intensity) is inhomogeneous due to the nature of the
RF coil.

Please note that I've never seen that anybody (including myself) tested by
scanning the same person twice; once with face up and once with face down,
to reject the #3.

Hope this helps.

Susumu

-----Original Message-----
From: mristudio-users-bounces at mristudio.org
[mailto:mristudio-users-bounces at mristudio.org] On Behalf Of llanyon
Sent: Monday, June 23, 2008 2:33 PM
To: mristudio-users at mristudio.org
Subject: [Mristudio-users] left-right flipping

Hi everyone

We are looking at the arcuate fasciculus (AF) with DTI Studio (version
2.40). Just about all the published literature says that the AF is larger in
the left hemisphere than the right. However, we are seeing the reverse
pattern, and are concerned about a possible flip of the data in the x
direction. We have acquired our data on a 3T Philips Achieva Scanner. I have
check L-R laterality using lesion patient data and there does not seem to be
a flip, but I wondered whether anyone else has had any issues like this?

Thanks
Linda Lanyon

----------------------------------------------------------------------------
-------------------------
Linda Lanyon
Post-doctoral Research Fellow, Michael Smith Foundation for Health Research
University of British Columbia
Human Vision & Eye Movement Laboratory
Vancouver General Hospital Eye Care Centre
3rd Floor Research Labs - Rm 365
2550 Willow St, Vancouver, B.C., Canada
Phone: +1-604-875-4111 x 69003
Fax: +1 604-875-4663
email: llanyon at eyecarecentre.org
web: http://www.neuroophthalmology.ca/UBCNeuroOp/JBarton/labstaff/Linda.htm
Director, Society for Canadian Women in Science and Technology:
www.scwist.ca
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