Differences between DICOM Annotation and Overlay

Question

What is the difference between DICOM Overlay and DICOM Annotation?

Answer 1

The hint is in the name: Annotation is one of the embedded objects and Overlay is only a chunk of pixel data.

Answer 2

The only term that DICOM explicitly defines is "Overlay". which refers to a binary raster image which is either stored in unused bits of the pixel data (this is retired but you still find overlays encoded this way in practice) or in separate attributes.

Annotation refer to anything else, including:

• header attributes used to label the images (like patient's name, study date)
• softcopy presentation state objects referring to the annotated image and defining simple vector graphics to draw with the image

I'd say that the term "annotation" refers to everything but overlays.

Answer 3

Overlay is the common term used for text display of header data on the images, this would be configurable by the modaity and usually site or user specific. Annotations are the mark-up of the image by the user, Arrows, ROIs, Rulers etc.

Overlay can be a DICOM object and often called as such for CAD , as the original history was to use DICOM group 6000 for these objects.

Answer 4

Mostly Annotations covers Comment Text, ROI Markings while overlay covers patient details, dicom details like image type, compressions, etc.,

Answer 5

An overlay is a bitmap which is "overlaid" on top of the raw pixel data. Overlays are described by the overlay module.

There are two ways of storing overlays in DICOM. The first is to squeeze it into the unused bits in the raw pixel data. This is possible if, e.g. your bits allocated is more than your bits stored. This method of storing overlays is deprecated. The second approach is using the overlay attribute.

You will notice, in Section C.9.2.1.1 of the DICOM standard, it says:

There are two specific types of overlays. The type is specified in this Attribute [referring to the Overlay Type attribute].

A Region of Interest (ROI) is a specific use of an Overlay. The overlay bits corresponding to all the pixels included in the ROI shall be set to 1. All other bits are set to 0. This is used to specify an area of the image of particular interest.

A Graphics overlay may express reference marks, graphic annotation, or bit mapped text, etc. A Graphics overlay may be used to mark the boundary of a ROI. If this is the case and the ROI statistical parameters are used, they will only refer to the pixels under the boundaries, not those in the included regions.

The overlay bits corresponding to all the pixels included in the Graphics shall be set to 1. All other bits are set to 0.

So as you can see, the concepts of an overlay and annotation are not so cleanly separated. An overlay is a way of storing an image that should be overlaid on top of the raw data. An annotation is some sort of text or comments about the image, which may be stored in an overlay, or it may be stored elsewhere.

For example, an annotation may be burned into the raw pixel data directly (see the Burned in Annotation Attribute).