ARRT - American Registry of Radiologic Technologists Image Acquisition and Evaluation Questions and Answers — Questions and Answers
Question 1: In computed radiography (CR), what is the primary function of the photostimulable phosphor (PSP) within the imaging plate?
- To absorb the remnant beam energy and store it as a latent image. (Correct answer)
- To convert the latent image directly into a digital signal.
- To emit light instantaneously upon x-ray exposure.
- To focus the electron beam during the image reading process.
Correct answer: To absorb the remnant beam energy and store it as a latent image.
The photostimulable phosphor (PSP) layer, typically made of barium fluorohalide, absorbs the energy from the x-ray photons that pass through the patient. This energy excites electrons within the phosphor, which then become trapped in 'F-centers'. This pattern of trapped electrons forms the latent, or invisible, image.
Question 2: A radiograph of an AP abdomen appears grainy and mottled. The exposure indicator value is significantly lower than the target range. Which of the following technical adjustments would be most effective in correcting this quantum mottle artifact on a repeat exposure?
- Increase the grid ratio.
- Decrease the kVp.
- Increase the mAs. (Correct answer)
- Increase the Source-to-Image Distance (SID).
Correct answer: Increase the mAs.
Quantum mottle, or quantum noise, is a grainy appearance on a radiograph that results from an insufficient number of x-ray photons reaching the image receptor. Increasing the mAs directly increases the number of photons produced, which will improve the signal-to-noise ratio and eliminate the quantum mottle, leading to a smoother, higher-quality image.
Question 3: When changing from an 8:1 ratio grid to a 12:1 ratio grid for a particular examination, which of the following consequences should the technologist anticipate?
- A decrease in patient radiation dose.
- An increase in image contrast. (Correct answer)
- A decrease in the required mAs.
- An increase in the amount of scatter reaching the receptor.
Correct answer: An increase in image contrast.
A higher grid ratio, such as 12:1, has taller and/or more closely spaced lead strips compared to a lower ratio grid like 8:1. This makes the grid more efficient at absorbing scatter radiation before it reaches the image receptor. By removing more scatter, which degrades the image, the resulting radiograph will exhibit higher (or improved) image contrast. However, this comes at the cost of increased patient dose, as mAs must be increased to compensate for the primary radiation that is also absorbed.
Question 4: A technologist is performing a lateral lumbar spine using an Automatic Exposure Control (AEC) system. The patient is positioned correctly, but the resulting image is significantly underexposed. Which of the following is the most likely cause of this error?
- The density control was set to +2.
- The central ray was centered to the AEC detectors.
- The backup timer was set too low. (Correct answer)
- The kVp selected was too high for the body part.
Correct answer: The backup timer was set too low.
The backup timer is a safety feature that terminates the exposure after a certain time to prevent extreme overexposure to the patient and tube overload. If the backup time is set too short, it may terminate the exposure before the AEC system has received enough radiation to create an optimal image, resulting in underexposure. This is especially common in large or dense anatomy where longer exposure times are needed.
Question 5: While reviewing a portable chest radiograph acquired with a DR flat-panel detector, the technologist notices a faint, reversed image of the detector's electronic wiring superimposed over the patient's anatomy. This artifact is known as:
- Moiré effect
- Image lag
- Backscatter (Correct answer)
- Dead pixel artifact
Correct answer: Backscatter
Backscatter is an artifact that occurs when radiation is scattered back towards the detector from objects behind it, such as the bed or cassette holder. This scattered radiation can be strong enough to create an image of the internal components of the detector's backing, which then appears superimposed on the diagnostic image. This is more common with higher exposure factors needed for larger patients or when collimation is insufficient.
Question 6: A digital radiograph of a knee is produced with optimal brightness and contrast. However, the Deviation Index (DI) value is +2.5. How should the technologist interpret this value?
- The image is slightly underexposed, and the technique should be increased.
- The exposure to the image receptor was perfect.
- The image is significantly overexposed, and the mAs should be reduced for future similar exams. (Correct answer)
- The image has a poor signal-to-noise ratio.
Correct answer: The image is significantly overexposed, and the mAs should be reduced for future similar exams.
The Deviation Index (DI) indicates how much the actual exposure to the receptor deviates from a pre-set target exposure. A DI of 0 is ideal. A positive DI indicates overexposure, while a negative DI indicates underexposure. A DI of +1 represents about 25% overexposure, and a DI of +3 represents a 100% (doubling) of the intended exposure. Therefore, a DI of +2.5 signifies significant overexposure, and the technologist should reduce the exposure technique (typically mAs) to adhere to the ALARA principle for subsequent examinations.
ARRT - American Registry of Radiologic Technologists Image Acquisition and Evaluation Questions and Answers
In computed radiography (CR), what is the primary function of the photostimulable phosphor (PSP) within the imaging plate?