dicom学习笔记之二：数据元素编码

        VM：value multiply =如果数据类型是字符串，就会用反斜杠把相关的值区分开来；如果是字节流，就需要用长度和当前的类型进行除法运算来获取。

        

        在显式语法中，如果VR值是OB, OW, OF, SQ, UT, UN类型

Tag		VR		Value Length	Value
Group Number (16-bit unsigned integer)	Element Number (16-bit unsigned integer)	VR(2 byte character string) of "OB", "OW", “OF”, “SQ”, “UT” or "UN"	Reserved (2 bytes) set to a value of 0000H	32-bit unsigned integer	Even number of bytes containing the Data Element Value(s) encoded according to the VR and negotiated Transfer Syntax. Delimited with Sequence Delimitation Item if of Undefined Length.
2 bytes	2 bytes	2 bytes	2 bytes	4 bytes	'Value Length' bytes if of Explicit Length

        否则的话，     

Tag		VR	Value Length	Value
Group Number (16-bit unsigned integer)	Element Number (16-bit unsigned integer)	VR (2 byte character string)	(16-bit unsigned integer)	Even number of bytes containing the Data Element Value(s) encoded according to the VR and negotiated Transfer Syntax.
2 bytes	2 bytes	2 bytes	2 bytes	'Value Length' bytes

        在隐式的情况下：

Tag		Value Length	Value
Group Number (16-bit unsigned integer)	Element Number (16-bit unsigned integer)	32-bit unsigned integer	Even number of bytes containing the Data Elements Value encoded according to the VR specified in PS 3.6 and the negotiated Transfer Syntax. Delimited with Sequence Delimitation Item if of Undefined Length.
2 bytes	2 bytes	4 bytes	'Value Length' bytes or Undefined Length

 在dcmtk代码如下：

 /* read the value in the length field. In some cases, it is 4 bytes wide, in other */
    /* cases only 2 bytes (see DICOM standard (year 2000) part 5, section 7.1.1) (or the */
    /* corresponding section in a later version of the standard) */
    if (xferSyn.isImplicitVR() || nxtobj == EVR_na)   //note that delimitation items don't have a VR
    {
        inStream.read(&valueLength, 4);            //length field is 4 bytes wide
        swapIfNecessary(gLocalByteOrder, byteOrder, &valueLength, 4, 4);
        bytesRead += 4;
    } else {                                       //the transfer syntax is explicit VR
        DcmVR vr(newTag.getEVR());
        if (vr.usesExtendedLengthEncoding())
        {
            Uint16 reserved;
            inStream.read(&reserved, 2);           // 2 reserved bytes
            inStream.read(&valueLength, 4);        // length field is 4 bytes wide
            swapIfNecessary(gLocalByteOrder, byteOrder, &valueLength, 4, 4);
            bytesRead += 6;
        } else {
            Uint16 tmpValueLength;
            inStream.read(&tmpValueLength, 2);     // length field is 2 bytes wide
            swapIfNecessary(gLocalByteOrder, byteOrder, &tmpValueLength, 2, 2);
            bytesRead += 2;
            valueLength = tmpValueLength;
        }
    }