[Mristudio-users] Is DTIStudio based on DICOM patient coordinate system or image coordinate system?

[susumu mori]

Hi Qiqi,

I want to follow up Hangyi's comment.

First of all, we have so many ways to define coordinates and
the nomenclature could be also confusing. There are two very clear
definition;

1) Image Coordinates: At the very end of data acquisition process, we get an
image. The brain could seem upright or tilted. It doesn't matter. We have
image X, Y, and Z. We do color-coded visualization (x: red, y: green, z:
blue) and fiber tracking in this Image Coordinates. In the end, this is the
most important coordinates. Likewise, the gradient table should be defined
in this Image Coordinates. If a line of a table says, [1, 0, 0], this should
be a gradient along the first axis of the image matrix (= image X).

2) Physical (gradient hardware) Coordinates: The very beginning of the
coordinate system in the data acquisition process is the Physical
Coordinates. This is there even before we put a subject. It's a physical
entity. In this coordinate, gradient [1, 0, 0] means, "apply X gradient
hardware".

Usually, difference between Image and Physical Coordinates is only
permutation. For example, X hardware gradient (Physical [1, 0, 0]) could be
Image Y (Image [0, 1, 0]) depending on image prescription. However, things
get complicated once you use oblique imaging as will be discussed later.

In addition, there are two more ways;
3) Patient Coordinates: I believe that this is defined by the parameters we
prescribe in the console. FH is foot-head, RL is right-left, and IS is
inferior-superior. This tells overall orientation of the human body. By
specifying these, we can link the Physical Coordinates and the patient
position. For example, if we put a patient in head-first, face-up, and axial
imaging, RL aligns to X gradient, FH to Z, and IS to Y gradient systems.
Note that Patient Coordinates are not really anatomical coordinates. For
example, even if we specify which is the RL orientation, the actual patient
RL anatomy could be tilted. The exact anatomical RL axis is not considered
here. Patient Coordinates just define the overall patient position.

4) Phase/Frequency/Slice orientation: This defines axial/coronal/sagittal
image orientations.

You don't have to worry all these unless you use oblique imaging. As soon as
oblique angles are introduced, then these coordinate systems start to
dissociate and things get very complicated. Please correct me if I'm wrong,
but I believe Physical and Patient coordinates are always aligned within the
relationship of permutation. Image Coordinates and Phase/Frequency/Slice are
aligned within the relationship of permutation. Between these two groups, we
need coordinate conversion calculation as soon as imaging plane is tilted
(oblique imaging).

Important questions are;

1) Which coordinate system DtiStudio uses?
A: Always Image Coordinates. Gradient tables have to be in Image
Coordinates.
2) Which coordinate system the vendor-provided gradient tables are?
A: It depends. GE and Philips (gradient overplus = off) dynamically adjust
the gradient table such that the gradient tables always follow the Image
Coordinates. Namely, you can use the table as is no matter what the oblique
angles are.
A: For Siemens and Philips (gradient overplus = on), the gradient tables
remain in Physical Coordinates. That means, you have to recalculate the
gradient table based on oblique angles you used before fed into DtiStudio.
For Siemens you can click "consider rotation if applicable" checkbox (if you
are using Mosaic). For Philips, you can use a gradient table calculate at
http://godzilla.kennedykrieger.org/~jfarrell/OTHERphilips/GUI.html.
A: I believe Toshiba uses the GE way so that you don't need table
recalculation based on oblique angles, but not 100% sure.
3) Which coordinate systems DICOM/Mosaic headers use?
A: I am not 100% sure but so far our experience says, "Physical
Coordinates". DtiStudio has a function to extract a gradient table from
certain DICOM/Mosaic files. When they are extracted, they seem in Physical
Coordinates. That means, you have to rotate them based on oblique angles
("consider rotation" needs to be checked).
4) How can I be sure if I'm doing it correctly?
A: Please do the following test;
a> Scan a subject in a regular way. Acquire one DTI dataset without any
oblique angles.
b> Tile the image plane severely, maybe 30-40 degree. Acquire one DTI
dataset.
c> Ask the subject to rotate the brain severely, maybe 30-40 degree. Acquire
one DTI without oblique angles.
d> Tile the image plane so that the brain looks upright. Acquire one DTI
dataset.

If everything is working correctly, in all these dataset images should look
right;
Image a and d: The brain looks upright. the corpus callosum is red at the
mid-sagittal. All color should look symmetric.
Image b and c: The brain looks tilted. The color of the corpus callosum do
not look symmetric.
All images: the fiber tracking should appear the same in 3D view. Simply put
an ROI in the genu of the corpus callosum, and you can immediately tell if
they look the same or something is not working

Hope this helps.

Susumu

2010/10/24 chichesterchiche <seigeweb at hotmail.com>

>  Dear DTIStudio Experts,
>
>    We know we need to input the gradient table in DTIStudio. However,
> when doing tensor map calculation, which coordinate system is DTIStudio
> based on? Is DTIStudio based on DICOM patient coordinate system or image
> coordinate system? I acquired the DWI images by TOSHIBA MRI and the gradient
> vectors are based on patient coordiante. Can I input the
> patient-coordinate-based gradient vector to DTI Studio directly, or do I
> need to transform the patient-coordinate-based gradient vector to image
> coordinate system at first?
>
> Thank you.
>
> Qiqi
>
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