Because of the direct association between post processing algorithms and the histogram analysis the image will be flawed if the histogram is flawed. This can happen in a variety of ways and is the primary focus of this section.In the digital environment today, it is easy to ignore the signs of a poor image due the powerful software designed to "fix" many of these issues. In addition to a poor image that can potentially hide anatomy and pathology, there is an increasing concern with increased dose in digital radiography. This section will help to describe some common errors that occur so that the technologist can recognize and fix them.
Because of the direct association between post processing algorithms and the histogram analysis the image will be flawed if the histogram is flawed. This can happen in a variety of ways and is the primary focus of this section.
PSP storage plates are more sensitive to x-rays, ultraviolet, gamma, and particulate radiation than film emulsion. This is because of the wide dynamic range of a digital detector. In DR this is not a problem because the DR detector is automatically refreshed prior to exposure. However, in CR even common building materials such as, concrete, constantly emit natural radiation and contributes to plate exposure. If PSP storage plates are stored for extended periods of time, artifacts will most likely occur. These artifacts typically appear as small black spots or additional latent images on the plate. If an IP and its PSP storage plate have been stored, unused, for 48 hours it should be erased prior to use.
The Low Exposure Response and Dynamic Range of digital makes scatter control much more important with a digital system (CR) when compared with screen/ film. Like screen/ film, grids are the primary instrument to combat scatter while exposing a patient. Furthermore, protection of CR plates when not in use is important. CR plates should be stored on end outside of the exam room.
A variety of precautions should be taken when using a grid. As in screen/ film, alignment and geometric errors can still occur in digital radiography. Some CR & DR systems have built in grids. This can result in a change of technique when exposing traditionally non-grid anatomy. CR systems are the most prone to grid alignment errors. The more sensitive the system is to low exposure, the more a grid will be necessary. This is especially apparent in a DR system. The appropriate alignment of a grid to the anatomy is critical to a good image. Grid cut-off can cause mottle and low exposure levels to the image. If you are considering a repeat to increase the technique, check the grid alignment first.
The Moiré Effect is a pattern that occurs when the grid lines and the scanning laser are in parallel (the same scanning frequency). This can be avoided by proper calibration from the vendor and is rarely something the technologist can control during an exam. However, the technologist can use a moving grid and avoid extremely short exposure times (under 10msec).
The algorithms in digital radiography use logical "if - then" statements to detect collimator likes within the exposed image (signal). The reason this is done is because when the image is presented for post processing, information outside the collimation lines will distort the histogram. Therefore this information is removed.
Many errors can occur with improper collimation because the detection of a collimation line can be missed or confused with anatomy. All too often, technologists are "opening wide" to expose the entire plate because they believe this will improve their image. This is a poor standard of practice. Aside from an ALARA violation, not collimating will decrease the overall quality of an image.
The attempt to detect collimation lines in an effort to remove the erroneous information within a digital image is termed Field Recognition or Field Detection. Aside from poor collimation, the software designed to detect the field of exposure can also have specific errors associated with it. These errors are called Field Detection Errors. It is important to identify and understand the causes of a field detection error in order to make the proper adjustments to prevent or fix this type of error.
Example of a Field Detection Error:
Reasons for Field Detection Errors:
As the software in digital imaging becomes more advanced, many of these errors are less likely to occur. However, this does not remove the need for diligence from the technologist.
Using split fields (multiple images on a single plate) can be achieved when done correctly. The following guidelines should be used in split fields:
Although computed radiography systems are able to correct for gross technical errors, there is a limit to the minimum dosage required for appropriate penetration and a diagnostic image. The imaging plates require a minimum amount of photon energy and this is supplied by the amount of kilovoltage used for the examination. The various vendors have minimum kVp requirements to help with the necessary photon energy and to help with the dosage to the patient. Each institution operating a computed radiography system must establish radiation dose reduction limits. Inadequate radiation doses result in images that demonstrate quantum mottle or areas devoid of data.
Positioning Artifacts are the most common and most preventable mistake made by the technologist. Errors include:
In the age of digital radiography, it is easy to annotate L or R markers onto the film in the post processing phase of digital acquisition. This is currently not an acceptable practice. Only lead markers placed at the time of exposure will hold in court. This is because there is currently no way to accurately verify what side is correct on a digital image. As technology advances, this may be overruled.