Laura LeBlanc, 1204502
In Computed Radiography (CR), a photostimulable storage phosphor imaging plate is used to acquire an x-ray image. These imaging plates are reusable cassettes and eliminate costs of non-reusable material such as film screen. A CR reader is then needed to extract the latent image from the phosphor. Once the material has been extracted, the CR reader system erases the phosphor plate and the cassette is once again ready to be exposed.
Erasure
Any residual metastable electrons are moved to ground state essentially by flooding the plate with an intense white light. A diagram of this can be seen in Figure 1. Erase can be seen as the last step in retrieval of an image, before the cassette can be exposed again.
Erasure
Any residual metastable electrons are moved to ground state essentially by flooding the plate with an intense white light. A diagram of this can be seen in Figure 1. Erase can be seen as the last step in retrieval of an image, before the cassette can be exposed again.
This QC lab was performed to test whether or not the CR reader was accurately and sufficiently erasing the cassettes, allowing them to be reused immediately. When a cassette has been completely erased the resulting image should be uniform and free from residual artifacts. Any remnants of the previous image results in an artifact known as “image ghosting”. Image ghosting occurs when the plate is not completely erased prior to the next exposure. Ghosting can be improved by subsequent signal erasure techniques (erasing the cassette using the CR Reader). Clearly, this provides undesirable additional steps while performing a radiographic study and when time is crucial for a particular study, one would not want to have to erase the cassette more than once. Another problem image ghosting can cause is a gradual increase in noise and non-uniformity which could potentially cause misdiagnosis and will shorten the life of the photostimulable phosphor plate. | In Figure 2, an image taken from Bushong, 2008, you can see there is a pelvis at the top of the image, which includes bowel pattern at the bottom. This is an example of image ghosting and is the result of an incomplete erasure of the imaging plate before the cassette was reused for a chest radiograph was performed. |
The following video gives a run through demonstration of how to perform the lab with step-by-step directions:
The erasure function was measured quantitatively and qualitatively. As seen in the video by selecting a narrow window width and adjusting the level, this would have revealed any image ghosting artifacts on the screen. With the level slowly being raised, there is no evidence of remnant ghosting from the previous exposure of the pennies - this is a qualitative measurement. There is, however, artifacts seen on the peripheral image likely due to the age and possibly overuse of the cassettes.
Other factors that may have contributed to artifacts seen on the imaging plate include:
Other factors that may have contributed to artifacts seen on the imaging plate include:
- Foreign objects such as dirt and debris can fall onto the imaging plate leaving light coloured specks.
- CR scanner malfunction can cause skipped scan lines, missing pixels and distorted images. Memory issues, digitization problems and communication errors also may contribute to the aforementioned.
- CR image receptors are more sensitive to scatter radiation because of the lower k-edge values. This means that the effects of scatter and background noise can cause an irreversible decrease in contrast.
- Defects in the imaging plate such as scratches and scuffmarks can lead to false-negatives in fractures or signs of pneumothorax.
- Rough handling may have also caused the peripheral artifacts seen on the cassette; it is important to ensure safe and proper handling of cassettes.
The CR reader system used was Kodak, which uses exposure index (EI) values that are generated with each exposure. The theory is that if the plate was completely erased, the second exposure should yield an EI of 0, given the acceptable EI range of 0-80. The EI value acquired was 122 meaning that the CR reader only passed the qualitative measurement. It is important to take corrective measures, which may include: ensuring that the CR reader was properly calibrated, the cassette is of working standard and the x-ray equipment itself is working to par. One way of reducing the probability of obtaining ghost artifacts in a clinical setting is to erase all cassettes at the beginning of the day so that you can ensure it was properly erased. It is also important to erase cassettes before use, after they have set for 24 or more consecutive hours. |
References:
Al Khalifah, K., & Brindhaban, A. (2011, January 1). Effect of delay between erasure and image
acquisition in CR. Retrieved January 28, 2015, from http://www.ncbi.nlm.nih.gov.
Bushong, S. (2008). Radiologic science for technologists: Physics, biology, and protection (9th ed.). St.
Louis, Mo.: Mosby/Elsevier.
Papp, J. (2011). Quality management in the imaging sciences (4th ed.). St. Louis, Mo.: Mosby Elsevier.
Al Khalifah, K., & Brindhaban, A. (2011, January 1). Effect of delay between erasure and image
acquisition in CR. Retrieved January 28, 2015, from http://www.ncbi.nlm.nih.gov.
Bushong, S. (2008). Radiologic science for technologists: Physics, biology, and protection (9th ed.). St.
Louis, Mo.: Mosby/Elsevier.
Papp, J. (2011). Quality management in the imaging sciences (4th ed.). St. Louis, Mo.: Mosby Elsevier.