WPF : Save Image + InkCanvas

Here are some parts of the code :

In the Xaml file :

<window.datacontext>
<local:displayviewmodel>


<inkcanvas grid.row="1" x:name="myInkcanvas0">
<Image x:Name="img0" Source="{Binding Path0}" />

<inkcanvas grid.row="1" x:name="myInkcanvas1">
<Image x:Name="img1" Source="{Binding Path1}"/>


<Button Content="Save" Click="Save"/>


In the main cs file :

private void Save(object sender, RoutedEventArgs e)
{
SaveFile(((DisplayViewModel)this.DataContext).Path0, myInkcanvas0);
SaveFile(((DisplayViewModel)this.DataContext).Path0, myInkcanvas1);
}

private void SaveFile(string sPath, InkCanvas cnv)
{
FileStream stream = new FileStream(sPath, FileMode.Open, FileAccess.ReadWrite);
RenderTargetBitmap rtb = new RenderTargetBitmap(1200, 300, 0, 0, PixelFormats.Default);
rtb.Render(cnv);

BmpBitmapEncoder encoder = new BmpBitmapEncoder();
encoder.Frames.Add(BitmapFrame.Create(rtb));
encoder.Save(stream);
stream.Close();

}

--> Exception : the process can not access to the file myfile.png because it is used by another process.