[IGSTK-Developers] Hi, question about ImageSpatialObjectRepresentation code

[Hui Zhang]

Hi, David,

It is a good implementation method if the exact transform is applied 
directly to vtkImageActor. Does it mean that we should always set the 
DisplayExtent of vtkImageActor to
(ext[0], ext[1], ext[2], ext[3], 0, 0)
for the extract slice is a 2D image from the original input?

What I did in my former class is:
use vtkImageReslice to extract 2D image content, and then assign the output 
to vtkTexture. Creat a vtkPlane object according to the oblique and slice 
definition, which has the same transform with vtkReslice. Assign the 
vtkTexture to that plane to display the oblique view.

Except the oblique view, another view / imagerepresentation proposed by the 
doctors here in Georgetown Hospital is slab-MIP view, which is a thick or 
multi-slice-blend view (multi parallel slices around the real displayed 
slice). The blend function can be MIP or alpha blending (ray sum). These 
kinds of view/imagerepresentations are used  to display the spine around the 
tracked needle for vertebroplasy experiment, and to display the tisse plane 
in front of the tracked needle for liver biopsy experiment.

My implentation is to use vtkImageReslice to extract those multi-slices and 
write the code to perform MIP or alpha blending computation before assigning 
it to the vtkTexture.

Just wish these would be helpful to provide a better 
view/imagerepresentation in IGSTK. I can attach some oblique and slab-MIP 
views from our current system for some demostration.

Regards
James


----- Original Message ----- 
From: "David Gobbi" <dgobbi at atamai.com>
To: "Hui Zhang" <zhang at isis.imac.georgetown.edu>
Cc: "'IGSTK-developers'" <igstk-developers at public.kitware.com>
Sent: Monday, November 14, 2005 7:00 PM
Subject: Re: [IGSTK-Developers] Hi, question about 
ImageSpatialObjectRepresentation code


> Hi James,
>
> You're right, vtkImageReslice can be used for orthogonal and oblique
> slices, but even if ImageSpatialObjectRepresentation uses
> vtkImageReslice to extract the slice, it should still use vtkImageActor
> to display it (but vtkImageActor would not be used to select the
> orientation, and probably not the slice number either).
>
> The trick is that whatever transform is used as the ResliceTransform for
> vtkImageReslice, the exact same transform also has to be used as the
> UserTransform for the vtkImageActor. That ensures that all of the pixels
> in the resliced image are displayed at the correct position in 3D space.
>
> It is also necessary to make sure that the OutputExtent of
> vtkImageReslice and the DisplayExtent of vtkImageActor are matched to
> one another.
>
> And here is where I start rambling...
>
> Choosing the pixel spacing for the oblique slice is difficult if the
> original voxel spacing of the image is not isotropic. I have used two
> different ways to choose the pixel spacing:
>
> 1) Use the direction cosines of the oblique slice to calculate an
> "oblique" pixel spacing based on a weighted combination of the original
> x, y, and z spacings
>
> 2) My favorite method: if the View is a parallel projection (like most
> 2D views) then it is possible to calculate exactly how many millimetres,
> in patient coordinates, there are per screen pixel on the monitor. Then
> the OutputExtent, OutputSpacing and OutputOrigin of vtkImageReslice can
> be set so that each pixel in the resliced image maps to a screen pixel
> in the View2D.
>
> I prefer (2) because it uses vtkImageReslice to combine the reslicing
> and the image zoom into one trilinear interpolation step. In method (1),
> vtkImageReslice performs trilinear interpolation for the oblique
> reslicing, and then vtkImageActor applies a bilinear interpolation step
> to achieve the zoom. Because two interpolations steps are done, method
> (1) provides an image that is less true to the original.
>
> - David
>
>
>
>
> Hui Zhang wrote:
>
>>Hi,
>>
>>I check the current ImageSpatialObject / ImageSpatialObjectRepresentation 
>>code from IGSTK and find it is easy to use this class to display the 2D 
>>slice image from Axial / Sagittal / Coronal view. The code use the 
>>vtkImageActor to set the orientation and slice number, as below:
>>
>>switch( m_Orientation )
>>    {
>>    case Axial:
>>      m_ImageActor->SetDisplayExtent( ext[0], ext[1], ext[2], ext[3], 
>> m_SliceNumber, m_SliceNumber );
>>      break;
>>    case Sagittal:
>>      m_ImageActor->SetDisplayExtent( m_SliceNumber, m_SliceNumber, 
>> ext[2], ext[3], ext[4], ext[5] );
>>      break;
>>    case Coronal:
>>      m_ImageActor->SetDisplayExtent( ext[0], ext[1], m_SliceNumber, 
>> m_SliceNumber, ext[4], ext[5] );
>>      break;
>>    }
>>
>>My question is, through this category, we can't display any oblique image 
>>slice and integrate them into the same class. If the surgeons need get 
>>oblique view along the planned path or tracked tool, or even slab-MIP 
>>view, the program can't make any use of the existed 
>>ImageSpatialObjectRepresentation class. That means, we'll have two 
>>different ImageRepresenation classes, one for the fixed-axis, and one for 
>>the oblique axis. What we did before in our current navigation system is, 
>>we use vtkImageReslice instead of vtkImageActor, and made the orthogonal 
>>image representations (Axial / Coronal / Sagittal) are three specific 
>>oblique views. So, in my question, can we implement with vtkImageReslice 
>>in ImageSpatialObjectRepresentation class, instead of vtkImageActor? And 
>>from this implementation, the orthogonal image representation can be 
>>easily a derived class or just three specific settings.
>>
>>Regards,
>>
>>---------------------------------------------------------------------------------------------
>>Hui (James) Zhang, Ph.D.
>>
>>Imaging Science and Information Systems (ISIS) Center
>>Department of Radiology
>>Georgetown University Medical Center
>>2115 Wisconsin Avenue, Suite 603
>>Washington, DC, 20007
>>
>>Work phone: 202-687-2902
>>Work fax: 202-784-3479
>>Cell phone: 240-476-9361
>>MSN: zh_skyfox at hotmail.com
>>----------------------------------------------------------------------------------------------
>>
>>
>>
>>------------------------------------------------------------------------
>>
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>>
>>
>
>
>