BitmapSource MemoryStream leak with large images

Question

I'm working with image files over 1 GB, creating a Bitmap from a large BitmapSource and attempting to dispose of the original BitmapSource. The BitmapSource stays in memory. Normally this is an inconvenience as it is eventually collected, but with these large files, clearing the memory immediately is a necessity:

System.Drawing.Bitmap bitmap;
using (MemoryStream outStream = new MemoryStream())
{
    BitmapEncoder enc = new BmpBitmapEncoder();
    enc.Frames.Add(BitmapFrame.Create(bitmapsource)); //large image
    enc.Save(outStream);
    bitmapsource = null;

    Bitmap temp = new System.Drawing.Bitmap(outStream);
    bitmap = temp.Clone(new Rectangle(0, 0, bm.Width, bm.Height), bm.PixelFormat);
    temp.Dispose();
}

GC.Collect();
GC.Collect();

//both bitmap and bitmapsource remain in memory

I've come across a work-around here, but the BitmapSource still persists until much later in the pipeline.