Request for: pixel (px)

[thomas-feilkas-bl]

Wikipedia provides the following description (https://en.wikipedia.org/wiki/Pixel):

In digital imaging, a pixel (abbreviated px), pel,[1] or picture element[2] is the smallest addressable element in a raster image, or the smallest addressable element in a dot matrix display device.

The definition for pixel would fit into chapter 4.6 Prefixes and Units Used in Information Technology.

The use case triggering this request is medical imaging data, in particular x-ray imaging data when using the DICOM standard. When acquiring 3d data like CTs or MRIs, the coordinates for single slices of images are defined to be in millimeters relative to the frame of reference (coordinate system root) of the data. If an object is defined within a CT scan, for instance a liver is outlined, the coordinates of the object are defined in millimeters in the frame of reference of the CT.

For x-ray images there is no frame of reference in DICOM as the nature of the imaging does not necessarily allow for it. The reason is best described at https://en.wikipedia.org/wiki/Projectional_radiography but the basic principle is the size of an object in the image depends on where it was located during image acquisition. So an object of diameter 25mm may end up occupying more or less pixels in the image depending on its location relative to detector and source. A conversion function ("calibration") from imaging pixels to mm can be established but it may not exist for the x-ray data. The conversion function is also depending on the location where the calibration happened.

So if we want to measure the distance between the two points of interest in a spinal x-ray and store the location of the measurement, the coordinate system to use is pixels in the original image data as this coordinate system can not change. To be able to express the distance in mm, the conversion function is applied to the data in pixels. When storing this kind of data for a patient it is important to be very precise about the units having been used. Since the default units are mm in DICOM, a measurement of a tumor with a length of 100 pixels may be interpreted as 100 mm if no unit of measurement is defined. Since DICOM adopted the UCUM standard for expressing units of measurements, it would be helpful to be able to express dimensions in imaging pixels as well.

[incansvl]

Another example of a name for a countable entity, as per #224 ?