Unit 3: Troubleshooting in Digital Radiography
At the conclusion of this unit, learners should be able to:
- Identify the primary considerations of quality assurance in digital radiography.
- Identify characteristics that affect image quality.
- Identify common artifacts associated with digital radiography.
Quality Assurance in Digital Radiography
Although the world of radiology has been markedly advanced by the evolvement of digital imaging, the need for quality assurance is still very important in the day-to-day operations of a medical imaging department. Testing the performance of the digital imaging equipment is important not only to meet state and national standards, but to ensure a high quality image. Policy and procedures are implemented to guarantee criteria such as:
- Ensure that patients are exposed to minimum radiation using the ALARA (as low as reasonable achievable) philosophy.
- Produce optimum image quality for diagnosis.
- Reduce the cost of radiology operations.
Reading assignment - Chapter 10 of the Seeram text
Characteristics of Image Quality
Although the technological advances in digital radiography have lead to improved efficiency and patient care, it is important to be aware of the factors that affect image quality. Optimization strategies have been used in digital modalities to reduce signal noise and improve image quality while keeping radiation doses within acceptable limits. Radiologic Technologists in facilities utilizing CR or DR systems should be able to recognize opportunities with digital systems to reduce radiation exposure while reducing image noise and improving overall image quality. The key factors in evaluating for image quality are spatial resolution, noise, and detective quantum efficiency (DQE).
Spatial resolution is controlled by the matrix size and how many pixels can be displayed by the monitor. There is a direct correlation between matrix and pixel size and spatial resolution. As the matrix size increases, the pixel size decreases, and the spatial resolution increase creating a sharper image.
There are two different classifications of noise to be aware of; electronic system noise or quantum mottle noise. Quantum mottle noise is the only image noise that affects image quality and can be controlled by the radiographer. Quantum mottle noise is a result of an inefficient number of photons reaching the imaging plate due to an error in the preset exposure factors (mAs and kVp). This can produce a grainy image that can be easily corrected by adjusting the mAs or kVp, whichever is appropriate for the clinical situation.
The final factor affecting image quality is detective quantum efficiency or DQE. DQE is the measure of the sensitivity and accuracy by which the image receptor converts the incoming data to the output viewing device. The relative value of DQE for various image receptors means that fewer x-rays are required by the higher DQE receptors to produce an image; this translates into lower patient radiation dose. DQE shows that both characteristics of digital image receptors suggest that patient radiation dose should be less with digital imaging than with screen-film imaging.
Just as there were artifacts with film/screen radiography, there are unique artifacts associated with digital radiography systems. These following list touches on key artifacts and can be associated to both equipment and technologist error.
- Phantom or ghost images - These artifacts are due to an incomplete erasure of the imaging plate. Extreme overexposure of an imaging plate may cause ghost images and may require two primary erasures.
- Scratches or tears - Scratches or tears are caused by damage to the imaging screen. These artifacts are permanent and cannot be repaired.
- Light spots- These are usually caused by dust or other foreign material on the imaging plate. Cleaning the imaging plates on a regular basis is a QC process that can be utilized to avoid these types of artifacts. Please see the video below on how to properly clean an imaging plate.
- White line on image - a white line on the image is due to dust on the light guide in the CR reader. Service should be contacted to clean the reader as soon as possible.
- Dropout artifacts -Dropout is a reduction in image resolution that may be in a specific area or an all over reduction in image quality. Dropout artifacts are usually caused by dust accumulation in the CR reader and can be avoided with regularly scheduled QA and cleaning of the equipment.
- Fogging - Imaging plates are much more sensitive than regular film/screen cassettes and therefore are easily fogged by background radiation. Imaging plates that have not been used in a 12 hour period should be manually erased using the primary erasure function on the CR reader.
- Quantum mottle - technical error due to inefficient exposure factors that can be corrected by adjusting mAs or kVp.
The final exam: Troubleshooting in Digital Radiography Quiz
Answer key: DR quiz answer key
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