c#: how to read parts of a file? (DICOM)

Question

I would like to read a DICOM file in C#. I don't want to do anything fancy, I just for now would like to know how to read in the elements, but first I would actually like to know how to read the header to see if is a valid DICOM file.

It consists of Binary Data Elements. The first 128 bytes are unused (set to zero), followed by the string 'DICM'. This is followed by header information, which is organized into groups.

A sample DICOM header

First 128 bytes: unused DICOM format.
Followed by the characters 'D','I','C','M'
Followed by extra header information such as:

0002,0000, File Meta Elements Groups Len: 132
0002,0001, File Meta Info Version: 256
0002,0010, Transfer Syntax UID: 1.2.840.10008.1.2.1.
0008,0000, Identifying Group Length: 152
0008,0060, Modality: MR
0008,0070, Manufacturer: MRIcro

In the above example, the header is organized into groups. The group 0002 hex is the file meta information group which contains 3 elements: one defines the group length, one stores the file version and the their stores the transfer syntax.

Questions

• How to I read the header file and verify if it is a DICOM file by checking for the 'D','I','C','M' characters after the 128 byte preamble?
• How do I continue to parse the file reading the other parts of the data?

Answer 1

Something like this should read the file, its basic and doesn't handle all cases, but it would be a starting point:

public void ReadFile(string filename)
{
    using (FileStream fs = File.OpenRead(filename))
    {
        fs.Seek(128, SeekOrigin.Begin);
        if ((fs.ReadByte() != (byte)'D' ||
             fs.ReadByte() != (byte)'I' ||
             fs.ReadByte() != (byte)'C' ||
             fs.ReadByte() != (byte)'M'))
        {
            Console.WriteLine("Not a DCM");
            return;
        }
        BinaryReader reader = new BinaryReader(fs);

        ushort g;
        ushort e;
        do
        {
            g = reader.ReadUInt16();
            e = reader.ReadUInt16();

            string vr = new string(reader.ReadChars(2));
            long length;
            if (vr.Equals("AE") || vr.Equals("AS") || vr.Equals("AT")
                || vr.Equals("CS") || vr.Equals("DA") || vr.Equals("DS")
                || vr.Equals("DT") || vr.Equals("FL") || vr.Equals("FD")
                || vr.Equals("IS") || vr.Equals("LO") || vr.Equals("PN")
                || vr.Equals("SH") || vr.Equals("SL") || vr.Equals("SS")
                || vr.Equals("ST") || vr.Equals("TM") || vr.Equals("UI")
                || vr.Equals("UL") || vr.Equals("US"))
               length = reader.ReadUInt16();
            else
            {
                // Read the reserved byte
                reader.ReadUInt16();
                length = reader.ReadUInt32();
            }

            byte[] val = reader.ReadBytes((int) length);

        } while (g == 2);

        fs.Close();
    }

    return ;
}

The code does not actually try and take into account that the transfer syntax of the encoded data can change after the group 2 elements, it also doesn't try and do anything with the actual values read in.