Presentation States & Graphic Layers
Before PACS, there was film. The danger with soft copies of images as opposed to film is that images can be modified by the different users of the system. This means that an original image that showed a pathology could be modified in such a manner that this pathology could no longer be visible. In order to avoid misdiagnosis, the original image need to be accessible at all times. Furthermore, when a modified image is sent to another location, the original need to be available. The idea of presentation states was introduced into the DICOM standard to satisfy the requirements and to prevent the inefficiencies of storing multiple instances of each image.
A presentation state is a transparent overlay that is placed on top of an existing image. This allows a user to modify the appearance of the overlay, which in turns alters the appearance of the image without changing the original pixel data. The presentation files contain only the information regarding modifications to the image, and hence are very small files.
The size difference is best understood when quantified. X-Ray images can get as large as 30 - 40 MB. Storing three instances of each image will take up between 90 and 120 MB of storage space. A presentation state file is usually smaller than 2 KB. Storing three instances or even 10 instances of each image is basically equivalent to storing one instance. So, thanks to presentation states we can have an unmodified original image, as well as each user can view the image altered in a manner that best suits them.
Information stored in presentation states is mostly composed of image annotations, image enhancements such as window level adjustments and image modifications such as shutters. The image annotation information is grouped in Graphic Layers. Each graphic layer contains a variety of image annotations, as well as information about the fonts and colors used to draw these annotations. A Presentation State can contain multiple Graphic Layers.
The RamSoft PowerReader implements the above described mechanisms for all image manipulations. The original image files are not modified and can always be accessed if necessary. The topics in this section of the manual explain how to work with Presentation States and Graphic Layers.