Getting a handle on an element of the opening parent dialo

I have a dialog that opens another window with

{
type: 'button',
id: 'browseButton',
label: 'Browse...',
onClick: function() {
window.open(CKEDITOR.config.imageBrowserUploadUrl, '', 'top=200,left=100,width=640,height=465,scrollbars,resizable');
}
}

In this other window I would like to retrieve a handle on an element of the opening parent dialog.

But this does not work:

alert(window.opener.document.getElementById("imageFile"));

How to get a handle on an element sitting in the opening parent dialog ?

Thanks !