Please try the option ImageType = ImageType.OfficeCompatibleEmf, although the generated Emf image still has some issues in Image Viewer, but it displays good in PPT(which is your case). Also, you can try to set the option HorizontalResolution and VerticalResolution, it will improve the image quality.
We are glad to know that your issue has been resolved using the suggested option. We have closed the topic now. If you have any additional questions or comments, please do not hesitate to reach out to us again.