QMedical Command Reference

DICOM Data

The basic unit of information in DICOM files is the data element, which consists of:

• a data element tag;
• a value length;
• a value field (with possibly multiple values); and
• an optional value representation field.

The data element tag is a unique identifier for a data element composed of an ordered pair of numbers: a group number followed by an element number. A DICOM file consists of a collection of data elements in ascending order of group number (primary key) and element number (secondary key).

A data element can be standard or private.

Standard data elements are listed in the DICOM data element dictionary, which is defined in Part 6 of the DICOM Standard. Standard data elements always have data element tags with even group numbers. The DICOM Dictionary is indexed by data element tag and specifies for each data element its description (semantics), value multiplicity range, value representation and whether a data element has been retired.

Private data elements are defined by an implementer, to communicate information that is not contained in standard data elements. Private data elements have odd group numbers.

Data elements are organized into data sets. Each data set consists of a series of data elements in ascending order of data element tag. A DICOM file is a data set.

Data sets can be recursively nested. The mechanism employed by DICOM for nesting data sets is by use of the special data elements of value representation sequence of items (SQ). Data elements of value representation SQ may contain zero or more items each of which may contain one data set.

QMedical provides full access to the data elements present in a DICOM file at any level of nesting. QMedical uses reference numbers to address individual data elements within the data hierarchy. To retrieve the reference numbers for all data elements at a given level, use the QMed_GetElements command. Command QMed_FindElement returns the reference number for a particular element at a given level, identified by its group and element number.

Given a particular element reference number, you can get full information about the data element:

• QMed_GetDictElementInfo provides DICOM dictionary information, such as group number, element number and description.
• QMed_GetElementDataInfo provides information about the particular element instance as it is encoded in the DICOM file: element data size, number of items, type of the element.

To retrieve the content of a data element, more than one commands can be used depending on the type of the data element and the desired 4D type of the returned value(s).

• QMed_GetElementData returns the content of a data eleent as a BLOB. To further manipulate such data it is useful to know its endianness and/or pixel representation, since a BLOB may consist of a series of multi-byte values which may be in the big or little endian format and correspond to signed or unsigned values. This information is provided by the QMed_GetMetaInfo method.
• The QMed_GetElementDataAsLong, QMed_GetElementDataAsReal and QMed_GetElementDataAsText methods are useful to retrieve the data element content in a particular format.