This chapter describes the concepts of the display pipeline.
The X-Ray intensity (I) at the image receptor is inversely proportional to the exponential function of the product of the object’s thickness (x) traversed by the X-Ray beam and its effective absorption coefficient (μ): I ~ e - μ x .
The X-Ray intensity that comes into contact with the image receptor is converted to the stored pixel data by applying specific signal processing. As a first step in this conversion, the amplitude of the digital signal out of the receptor is linearly proportional to the X-Ray intensity. In further steps, this digital signal is processed in order to optimize the rendering of the objects of interest present on the image.
The Enhanced XA IOD includes attributes that describe the characteristics of the stored pixel data, allowing to relate the stored pixel data to the original X-Ray intensity independently from the fact that the image is “original” or “derived”.
When the attribute Pixel Intensity Relationship (0028,1040) equals LIN:
P ~ I : The pixel values (P) are approximately proportional to X-Ray beam intensity (I).
When the attribute Pixel Intensity Relationship (0028,1040) equals LOG:
P ~ x : The pixel values (P) are approximately proportional to the object thickness (x).
In order to ensure consistency of the displayed stored pixel data, the standard display pipeline is defined.
On the other side, the stored pixel data is also used by applications for further analysis like segmentation, structure detection and measurement, or for display optimization like mask subtraction. For this purpose, the Pixel Intensity Relationship LUT described in PS 3.3 C.18.104.22.168.13.1 defines a transformation LUT enabling the conversion from the stored pixel data values to linear, logarithmic or other relationship.
For instance, if the image processing applied to the X-Ray intensity before storing the Pixel Data allows returning to LIN, then a Pixel Intensity Relationship LUT with the function “TO_LINEAR” is provided. The following figure shows some examples of image processing, and the corresponding description of the relationship between the stored pixel data and the X-Ray intensity.
Figure FFF.1.5-1 Examples of Image Processing prior to the Pixel Data Storage
No solution is proposed in the Enhanced XA SOP Class to standardize the subtractive display pipeline. As the Enhanced XA image is not required to be stored in a LOG relationship , the Pixel Intensity Relationship LUT may be provided to convert the images to the logarithmic space before subtraction. The creation of subtracted data to be displayed is a manufacturer-dependent function.
As an example of subtractive display, the pixel values are first transformed to a LOG relationship, and then subtracted to bring the background level to zero and finally expanded to displayable levels by using a non-linear function EXP similar to an exponential.
Figure FFF.1.5-2 Example of Manufacturer- Dependent Subtractive Pipeline with Enhanced XA