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': 'Isotope used in myocardial perfusion scan is: \nSelect and explain. \nA. Technetium, B. Thallium, C. Stannous pyrophosphate, D. Gallium', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Ans. Thallium', 'role': 'assistant'}]