[vtkusers] how to crop imagedata after a registration?

[yehe_008]

 Hello,

I'm making a registration between 3D MRI and 3D US data. The size of US data is much bigger than MRI.
I take some landmarks on both images.
Then I use "vtkLandmarkTransform" to get a transform between these landmark points.
At last I apply the transform to MRI data by using "vtkImageReslice".
So I get a huge size MRI data. How can I crop the data to make the new MRI data have same size and almost same content as US data?
Please give me some tips. Thank you for your attention :)

Here's my code
    vtkSmartPointer<vtkPoints> sourcePoints = vtkSmartPointer<vtkPoints>::New();//Input source points
    for ( int t = 0; t < 6; t++ )
    {
        float sourcePoint[3] = {inital_points_mri[t][0], inital_points_mri[t][1], inital_points_mri[t][2]};
        sourcePoints->InsertNextPoint(sourcePoint);
    }

    vtkSmartPointer<vtkPoints> targetPoints = vtkSmartPointer<vtkPoints>::New();//Input target points
    for ( int t = 0; t < 6; t++ )
    {
        float targetPoint[3] = {inital_points_us[t][0], inital_points_us[t][1], inital_points_us[t][2]};
        targetPoints->InsertNextPoint(targetPoint);
    }

    vtkSmartPointer<vtkLandmarkTransform> landmarkTransform = vtkSmartPointer<vtkLandmarkTransform>::New();//get landmark transform
    landmarkTransform->SetSourceLandmarks(sourcePoints);
    landmarkTransform->SetTargetLandmarks(targetPoints);
    landmarkTransform->SetModeToSimilarity ();
    landmarkTransform->Update();

    vtkSmartPointer<vtkImageReslice> transform2 = vtkSmartPointer<vtkImageReslice>::New(); // Apply transform
    transform2->SetInput(MRIimagedata); // Input MRI image
    transform2->AutoCropOutputOn();  // I'm not sure I need it or not?
    // transform2->SetOutputExtent(0,499,0,389,0,358); // set the extent as US data, but not the same content
    landmarkTransform->Inverse();
    transform2->SetResliceTransform(landmarkTransform);
    transform2->Update();
    vtkImageData* transformImage = transform2->GetOutput();
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