This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Radiology
[{'content': 'In endodontics, a long cone open-ended cylinder is used as an adjunct with paralleling technique in order to: \nSelect and explain. \nA. Increase the apparent size of the focal spot, B. To prevent distortion of image, C. To prevent elongation of the image, D. To prevent shortening of image', 'role': 'user'}, {'content': "Answer: B \nExplanation: Image size distortion (magnification) is the increase in size of the image on the radiograph compared with the actual size of the object. The divergent paths of photons in an X-ray beam cause enlargement of the image on a radiograph. Image size distortion results from the relative distances of the focal spot-to-image receptor and object-to-image receptor. Increasing the focal spot-to-image receptor distance and decreasing the object-to-image receptor distance minimizes image magnification.\xa0\xa0\nKey Point:\nThe use of a long, open-ended cylinder as an aiming device on an X-ray machine thus reduces the magnification of image\xa0on a periapical view. As previously mentioned, this technique also improves image sharpness by increasing the distance between the focal spot and the object.\nReference: White and Pharoah's Oral Radiology Principles and Interpretation 7th edition page no 84", 'role': 'assistant'}]