Final answer:
The moire effect in computed radiography with a grid is likely caused by grid frequency less than 60 lines/cm and grid lines running parallel to the laser scanning direction. The Bucky being turned off is not a relevant cause. The correct answer is A. 1 & 2 only.
Step-by-step explanation:
The question you've asked pertains to the moire effect in computed radiography when a grid is used. The moire effect can occur due to the interference patterns created by the grid lines with the scanning laser in the imaging plate (IP) system. To answer your question, the moire effect will likely be caused by both a grid frequency less than 60 lines per centimeter (which would be 1) and grid lines running parallel to the direction the laser scans the IP (which would be 2).
The Bucky must be turned on to prevent the moire effect, so option 3 is not a cause of the moire effect when the grid is used properly. Therefore, the correct answer to your question is A. 1 & 2 only.
Seeing that a grid with low frequency, like one with less than 60 lines/cm, will not match well with the high resolution of computed radiography readers, this creates room for the overlapping of frequencies and the moire effect. Similarly, when grid lines run parallel to the laser scanning direction, there's a risk of the grid pattern being registered by the laser, which can again result in the moire effect. However, when the Bucky is turned off, the grid is not being used, so moire caused by grid use is not a concern in that condition.