Backbuffer texture

Is it possible to get the backbuffer texture? as a texture2d not texture.

or even turn a texture into a texture2d?

Imagine i have drawn stuff to the backbuffer, and afterwords do this:

        RenderTarget2D rt = m_objGaussianBlur.DrawToRenderTarget(gd);
        sp.Begin(SpriteSortMode.Immediate, BlendState.NonPremultiplied); 
        sp.Draw(rt, gd.Viewport.Bounds, Color.Blue * 0.8f);
        //paint unblured on top

since setting a render target actually clears it (with lovely purple), it’s contents are lost. And I failed to change the presentation parameters rendertarget usage property.Had no effect when i changed the value to preserve contents.

Or is there any other technique for this?

Thanks, Roger.