How to crop bitmap image in WPF?

hey sergey, no, i dont mean rectanguler cropping. what i'm doing is cliping ellipsegeomtry inside a path and then do this:
BitmapImage bmi2 = new BitmapImage();
RenderTargetBitmap rtb = new RenderTargetBitmap(Convert.ToInt32(pathBack.ActualWidth), Convert.ToInt32(pathBack.ActualHeight), 0, 0, PixelFormats.Pbgra32);
pathBack.Clip = circle;
rtb.Render(pathBack);
PngBitmapEncoder pngImage = new PngBitmapEncoder();
pngImage.Frames.Add(BitmapFrame.Create(rtb));
using (Stream fileStream = File.Create(@"C:\Users\Desktop\EW\test\2.png"))
{
pngImage.Save(fileStream);
}
the problem is that when i choose the righth side of the image, the cropped png comes when the circle is on the left side of the image and the right size is blanked. same for each direction...what i mean is that the cropped picture actually does give me what i have chosen but instead of cropping the unselected parts it just made them transparent. i need to remove that. thanks...