About DICOM

DICOM is a communication protocol used for transfer of medical images. DICOM protocol is an object oriented protocol and has two parts as follows:
1. IODs
2. Services

IODs (Information Object definition):
IOD can be explained as the data members of the object oriented system. For example CT image is an IOD for is generated by say CT machine. IODs can be of two types
A. Normalized: Normalized IODs talk about only 1 entity. For example Patient. This IOD will have information only about the patient.
B. Composite: Composite IOD will contain information about more that one logical entities. For example CT image IOD will contain information about Patient, Study, and Series etc.

Services:
Services can are analogous to methods in object oriented system. For example, Print can be a service.

Whenever you talk about DICOM transfer you must specify following two things
a. What is being done
b. On what it is being done

Answer to first question is the service that Store or Print and answer to the second question is CT image. Hence Print CT image as a pair is required and sufficient to express DICOM communication. This pair is called as SOP (Service Object Pair).

Now when you talk about Print CT image, there must be an application which is capable of serving this SOP request. This application that serves this request is called as SCP (Service Class Provider). Also when there must be an application which will be able to make this request of SOP - Print CT image. The application that makes a request is called as SCU (Service Class User).

Now let’s look at what is there exactly inside an IOD. IOD contains DICOM Data Set. Like any other data structures, DICOM data set contains data elements. Structure of DICOM data element is as follows:

Tag VR VL Value
Group Element
Tag is made up of two elements as group and element. All the related information is kept in a group e.g. Patient Name, Age, Sex etc will be present in the same group but with different element number. This specifies the data type of the data that is present in the data element. This is the length of the data that can come. This is the actual value.

Transfer syntax:
1. Byte ordering: can be either little endian or big endian. This specifies whether MSB will appear first or last while reviving bytes. In Big endian style MSB is received later and LSB is received first. In case of little endian style LSB is received later after MSB.
2. VR type: It can be either Implicit VR or Explicit VR. If it is implicit VR then data type (VR) for each data element is not specified the data element. But one has to maintain a look up for each tag and its corresponding VR. In case of Explicit VR data type of the data element is mentioned in the data element itself.
3. Compression Type: If the pixel data is compressed the compression algorithm applied has to be mentioned in the transfer syntax itself.

Note: If there a compression applied then byte ordering is presumed as little endian and VR type is implicit VR.

Communication model in DIOM: