How to get a Rect from Image or RectTranform?

I need to take screen shoot of specific area of my UI, so I need a Rect to use in Texture2D.ReadPixel().

I tried to put an Image on the RectTranform and a script like this:

  Rect data=Image.GetPixelAdjustedRect();    
 
 //then draw it with        
 
      void OnGUI()
             {
                 GUI.Box(data,"test");
             }

The result :(the white box is what I'm trying to capture, you can see the darker GUI draw on the upper left)

Im also tried to use RectTranform.GetWorldCorners() and Camera.WorldToScreenPoint() to create a Rect but no luck even it seem closer :((

Please help!