The legacy CT object was standardised back in 1993 by the DICOM committee. In 2003 the DICOM WG 21 has decided that it is necessary to create a new CT object to meet the needs of modern CT technology that had evolved substantially.

Furthermore, new applications such as cardiac CT, gated studies, perfusion CT, CT fluoroscopy, contrast tracking and post-processing are not supported by the current standard at that time. Not only are technique attributes insufficient, but also the organisation of increasingly large datasets as single frame objects is awkward, and limitations of the existing definitions of spatial and temporal attributes are apparent.

As a result, supplement 58 (Enhanced CT Image Storage SOP Class) was proposed and later adopted into the final text of DICOM standard.

Although CT was the first enhanced SOP class to be defined, others such as MR have since been added, and the logic and construction are identical, though MR can be even more complicated as “dimensions” can include imaging parameters such as T1, T2 etc.

Due to practical considerations the Standard (Legacy) Image IODs will not be retired, however the use of the new enhanced IODs is encouraged.

Enhanced vs Standard (Legacy) Images

Here is a small list that shows some of the new organisational features/structures for enhanced images:

  • Multi-frame Pixel Data
  • Use of Shared and per-frame functional groups
    • Each functional group contains attributes that likely vary as a group, e.g. Pixel Measures, Plane Orientation, Velocity Encoding, etc.
    • Compact & makes explicit what doesn’t change
  • Dimensions
    • a priori hints as to how the frames are organised
    • Specify intended order of traversal, such as space, then time (e.g., for cardiac cine loops)
  • Stacks
    • Groups of spatially-related slices, repeatable

Other features include:

  • Detailed descriptions of advanced acquisition protocols
  • Accurate and well-defined timing information
  • Quantitative values and colour support (Quantitation mixed with greyscale pipeline in legacy CT)

Conversion between Enhanced and Standard (Legacy) images

With the help of DicomObjects, it is not very difficult to convert enhanced CT (in fact any enhanced imaging object to its legacy IOD) to legacy CT. However, it is impossible to convert legacy CT to enhanced CT objects as so much information simply doesn’t exist in standard CT objects.

4 major steps to convert Enhanced image object to standard images

  1. Split multi-frame to single frame objects
  2. Copy attributes from both shared and per-frame functional group to each newly created single frame object
  3. Copy other related attributes from enhanced to standard objects
  4. Generate new UIDs for all standard objects