¿Cómo convertir PDF a imágenes usando C # e ImageMagick?

Me gustaría convertir un archivo PDF a .GIF usando C # y magicknet.dll. He agregado la referencia a MagickNet Dll a mi proyecto.

MagickNet.Magick.Init();
MagickNet.Image img = new MagickNet.Image("d:/aa.pdf");
img.Write("d:/bb.gif");
MagickNet.Magick.Term();
img.Dispose();
System.Runtime.InteropServices.SEHException was unhandled by user code
  Message="External component has thrown an exception."
  Source="ImageMagickNET"
  ErrorCode=-2147467259
  StackTrace:
       at Magick.Image.{ctor}(Image* , basic_string\,std::allocator >* )
       at ImageMagickNET.Image..ctor(String imageSpec)
       at Test1._Default.Button1_Click(Object sender, EventArgs e) in C:\Users\PANKAJ\Documents\Visual Studio 2008\Projects\Test1\Test1\Default.aspx.cs:line 31
       at System.Web.UI.WebControls.Button.OnClick(EventArgs e)
       at System.Web.UI.WebControls.Button.RaisePostBackEvent(String eventArgument)
       at System.Web.UI.WebControls.Button.System.Web.UI.IPostBackEventHandler.RaisePostBackEvent(String eventArgument)
       at System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String eventArgument)
       at System.Web.UI.Page.RaisePostBackEvent(NameValueCollection postData)
       at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
  InnerException: 

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