In my opinion, thallium is not an ideal isotope these days for nuclear medicine. It is however an acceptable alternative. It exposes patients to too much radiation. Exams done using that isotope emit more radiation than any other in nuclear medicine. To give you an example, a regular bone scan exam results in dosimetry of 8 millisieverts, meaning the dose absorbed by the patient. Eight millisieverts is acceptable. A cardiac exam using thallium will result in an approximately 30 millisieverts dosimetry, which is quite high.
Furthermore, that exam produces a lot of radiation and the image quality is not what we can expect from a nuclear medicine test today. The resolution is not as good and when patients are even slightly overweight, it becomes extremely difficult to see the heart. In such cases, diagnostic errors may occur, when we use thallium. That said, for patients who are thin and underweight, it remains an acceptable radioactive tracer that provides good results in those conditions. However, if a patient is even slightly overweight, I would rather use positron emission tomography, using myocardial perfusion agents. In such cases, these agents have half-lives of approximately 10 minutes. So this must be done locally, on site, for patients. This must be used on site. This makes the technology extremely inaccessible.