Computed Radiography (CR) is a method to generate digital images in x-ray radiography that uses photo-stimulated luminescence to store the x-ray exposure into a latent image, that is then digitized. Computed Radiography is rapidly replacing film screen radiography, and at the same time a newer technology termed Digital Radiography (DR) is beginning to displace CR systems.
What is the workflow for Computed Radiography ?
The basic workflow of computed radiography is to acquire a latent image (i.e. energy stored in the panel), a laser based reader is used to readout the panel such that a digital image is created that can be visualized on a monitor or transferred to the PACS.
Computed Radiography follows a similar clinical workflow to screen film radiography, except you get to skip all of the chemical processing. The technical parameters are set in the same manner of SID, Bucky, kVp and mAs. The exposure is taken and this creates a latent image, i.e. stored energy in the CR panel.
The CR panel is then taken to a digitizer where the latent image is read out from the panel and a digital image is produced. A laser is scanned over the panel which releases stored energy in the panel as visible light. The visible light is measured and converted to a digital image.
Another strong light is used to clear the panel by releasing any additional stored energy after the first read step. This step is required before re-using the panel. Otherwise there could be a contribution from the previous exposure to the current acquisition.
The CR reader (a.k.a. the digitizer) also saves a digital image onto the console. The console is also typically connected to the PACS system so that the digital images.
The computed radiography workflow is very similar to the traditional film workflow in that the CR panel is housed within a Bucky tray and removed after each x-ray acquisition.
One major advantage of computed radiography is that the dark room and film processing chemicals are no longer required.
A second advantage of the computed radiography workflow is that the images are directly digitized and sent to the PACS. Using a film based workflow there would need to be an additional step to scan the x-ray film in order to produce a digital image.
Therefore, the transition from film to Computed Radiography has been easier than some transitions as in most cases the existing x-ray equipment can be used with the CR system and a complete overall of the table, and x-ray tube are not required in order to transition from film to a CR system.
What is the physics behind Computed Radiography?
Photostimulated Luminescence is the fancy terminology that describes the process where energy is stored within a material, and when a little additional energy is added to the system by shining a light on the material the material itself will emit light that can be measured. An example material that produces Photostimulated Luminescence is (BaFBr).
The physical steps in Computed Radiography are:
1. The panel is irradiated by the remnant beam (i.e. the x-rays that have passed through the patient).
2. X-rays store energy as PSLCs, i.e. PhotoStimulated Luminescence Complexes. The number of complexes stored is proportional to the incident x-ray exposure.
3. A laser is used to convert the stored energy to visible light.
4. A photomultiplier tube is used to convert the visible light to an electrical signal
5. An analog to digital converter (ADC) is used to convert the measured electrical signal into a number (i.e. a digital signal).
These are the physical steps involved in converting an x-ray signal to stored energy that can then be read out and digitized. In this figure only a small section of the panel is shown. In reality the complete panel needs to be read out such that the latent image is converted into a digital image.
How is the latent image read out in Computed Radiography?
In computed radiography the latent image is read out by scanning a laser over the panel and for each small region a signal is measured. These small regions on the detector become the image pixels in the digital image.
The steps to read out the latent image in Computed Radiography are:
- Scan the laser along a given row in the detector using a mirror that can tilt via a motorized drive.
- Light will be emitted by the photo stimulated phosphor.
- Collect the light via a light guide, that is directed as input to a photo-multiplier tube.
- The light that is input to the photomultiplier tube will be converted to an electrical signal, and that signal.
- The signal is then amplified (i.e. increased in magnitude)
- Finally an analog to digital conversion (ADC) takes place to convert the measurement to a number for each pixel in the image (I.e. a digital signal).
Why Computed Radiography needs a new name?
The name Computed Radiography is a misnomer. Computed Radiography does involve Radiography but it doesn’t actually involve significant computation.
This is in contrast to Computed Tomography (CT) which really does perform computations in order to generate an image. In CT the data acquired on modern systems is 2D data and the images reconstructed are 3D images and significant computing is required to generate the images.
The name Computed Radiography was developed in the early days of Computed Tomography and is purely a marketing name. Now it only serves to confuse all new students entering the field as they are trying to figure out why it is called Computed Radiography.
If I was on the naming committee for this technology I think that something like Stored Radiography (SR) or Stored and Digitized Radiography (SDR) would be much more accurate.
Since the name CR has been around for so long it is unlikely to change anytime soon, but at least we are here for you to feel justified with your distaste for the name computed radiography.
Another misnomer is that DR (Digital Radiography) is the first general purpose digital radiography. However, CR generates digital images as well and was in significant clinical practice before DR.
Now that you are familiar with the basic concepts behind computed radiography make sure that you checkout the physics behind digital radiography (DR) as well since DR systems are replacing many CR systems.