Flat panel detector definitions (Fill fraction, dels, pixels, matrix size, pitch)

Flat panel digital detectors are separated into small individual components termed detector elements (DELs), and the size of individual DELs is referred to as the pixel pitch. Whereas the matrix size is the number of DELs in each direction on the detector.

See our illustrated post on detector definitions: https://howradiologyworks.com/digital... .

Chapters:
00:00 Digital X-Ray Imaging Sampling Terminology
02:52 Detector Elements
04:00 Fill fraction
04:30 Matrix Size
04:45 Sampling Frequency
05:20 Example Calculation of Sampling Frequency
06:35 Example Calculation of Fill Fraction

Digital X-Ray Imaging Sampling Terminology
Here we discuss the terminology relating to the size of each detector element so that when definitions like detector pitch or fill fraction get thrown around you will have a good understanding of the meaning.

Detector Elements
Like digital photography, X-ray images are formed with digital elements (DELs). When the image is being stored after it is acquired, or when it is being displayed on a monitor the individual elements are referred to as picture elements (pixels).

Just to keep things clear we use a different terminology to describe the physical detector elements (DELs).

The detector pitch is then defined as the distance from end to end within a DEL. Therefore, smaller DEL size will yield a smaller pitch.


Additionally, within each DEL there is a region that can detect x-rays and an inactive region (such as the electronics of each DEL). The region where x-rays can interact is referred to as the active area. The region that cannot detect x-rays is referred to as the non-active area.

The ratio of active area of DEL to the whole size of each DEL is called “fill fraction”.


The detector fill fraction will therefore be a number that is between 0 and 1. The greater the fill fraction the more x-rays will be captured in the measurements. Thus, a higher fill fraction will be more dose efficient. In general, as the size of each DEL gets smaller the challenge is to ensure that the Fill Fraction stays high since there is associated electronics for each DEL.

Rad Take-Home Point: A digital x-ray detector can be separated into detector elements (DELs), and each element has a fill fraction which indicates the geometric efficiency of the detector to collect the x-ray signal.

The detector matrix is composed of many individual DELs. The matrix size is a two-dimensional number. If matrix size is 1024 x 1024, this means that the matrix has more than one million DELs.

There are also medical flat panel detectors that have matrices of 4288×4288.

We can put these numbers into context we can compare to digital cameras where the matrix size is typically specified in megapixels. A 1024×1024 detector is equal to 1 MegaPixel. A MegaPixel is defined as 220 which is slightly greater than 1 million. The detector that is 4288×4288 is equivalent a 17.5 MegaPixel sensor in terms of the number of Dels.

Rad Take-home Point: The matrix size of a digital x-ray detector indicates how many elements in the whole detector.

Another important characteristic of a digital flat panel detector is the sampling frequency in the detector. This is another way of expressing the size of each Del.

The sampling frequency is inversely proportional to the pixel pitch.

If DELs are larger (i.e. have greater pitch) then the sampling, sampling frequency will be smaller. If DELs are smaller then the sampling frequency will be higher.

Rad Take-home Point: The sampling frequency is inversely related to the Pitch of each Del.