How do I marshall large structures in VB.NET

   <DllImport("ConvertRaw.dll", CallingConvention:=CallingConvention.Cdecl)>
   Public Function OpenRawFile(<MarshalAs(UnmanagedType.LPStr)> x As String, ByRef CurrentStamp As RawDataHeader) As Integer
   End Function


<StructLayout(LayoutKind.Sequential)>
   Public Structure RawDataHeader
      Public numChannels As Integer
      Public syncpattern As Integer
      Public runnumber As Integer
      Public muxpitch As Integer
      <MarshalAs(UnmanagedType.ByValArray, SizeConst:=4096)>
      Public channels() As Integer
      <MarshalAs(UnmanagedType.ByValArray, SizeConst:=508)>
      Public description() As Byte
      Public scanlistblocks As Integer

End Structure
      Dim stamp1 As New RawDataHeader

      Dim result As Integer
      Dim sizeOfHeader As Integer = Marshal.SizeOf(GetType(RawDataHeader))
      Dim pHeader As IntPtr = Marshal.AllocHGlobal(sizeOfHeader)
      Dim i As Integer
      Dim x As String

      Try
         result = OpenRawFile("C:\Raw_Data\H12091.raw", pHeader)
         stamp1 = DirectCast(Marshal.PtrToStructure(pHeader, GetType(RawDataHeader)), RawDataHeader)
      Finally
         Marshal.FreeCoTaskMem(pHeader)
      End Try

typedef struct {
   int  numChannels;
   int	syncpattern;						
   int	run;							
   int	datastream;
   int  channels[4096];	
   char description[508]		
   int	scanlistblocks;						

}RawDataHeader;