[IGSTK-Users] CTImageReader voxel origin

[Andinet Enquobahrie]

Hi Chris,

Most likely your IDL application is not taking into account patient
orientation (direction cosines ) information during index-physical
coordinate system conversion.

In IGSTK ( by way of the ITK image readers and image classes) ,
we use spacing, origin and orientation information to convert  between
the two coordinate systems.

This should probably the first thing you might want to check. Check your
input dicom CT images  Orientation (0020,0037) . This tag specifies the
direction cosines of the first row and the first column with respect
to the patient. 

Let us know what you find out...

-Andinet


> Hello,
>
> I am trying to integrate another application (in IDL) with IGSTK.  The 
> purpose of the IDL code is to locate the centers of fiducial markers 
> from a CT image.  The code outputs the x, y, and z coordinates of the 
> markers.  I would like to use these values with 
> igstkLandmark3DRegistration to register tracker space to CT space.  
> However, the coordinates I receive from my IDL code are offset by an 
> obscure amount from their corresponding values as reported by IGSTK.  
> (I find the IGSTK-reported coordinates using the "picker" code from 
> NeedleBiopsy). My CTImageSpatialObject's parent is m_WorldReference 
> with the Identity transform relationship between them.
>
> In this setup, how can I determine where the origin of the CT image 
> is?  Is there anyway to easily transform the CTImage so that all 
> coordinates are positive and the origin (0,0,0) is located in the 
> corner of the first slice?  I cannot simply find the translation 
> between the two and apply it because the values I get from IGSTK are 
> not as precise as the ones I get from IDL.  Also, the relationship 
> changes for different CT Images.
>
> Thanks,
> Chris
>
>
> Christopher Heath
> Vanderbilt University School of Engineering
> christopher.c.heath at vanderbilt.edu
>
>
>
>


-- 
==========================================================
Andinet A. Enquobahrie, PhD
R&D Engineer
Kitware Inc.

28 Corporate Drive
Clifton Park, NY 12065-8662
Phone: 518-371-3971 x124
www.kitware.com