If you are asking me for a number I'm not going to be able to give you the number. What I can tell you is that the rule of thumb when you deal with radioactive material is what Dr. Gulenchyn mentioned earlier: as low as reasonably achievable. We know that thallium increases the radiation by a factor of 1.12. Now, if you have to decide whether to let a patient die and not do a test and the patient was going to die, you are going to use thallium. But as I said, it's not the ideal isotope in the 21st century.
On June 9th, 2009. See this statement in context.