Which isotope should not be used for a first pass study?

In nuclear medicine, a first pass study is designed to assess the initial passage of a radiopharmaceutical through an organ or vascular bed, typically focusing on the heart. For this purpose, a radiotracer that provides reliable and rapid imaging during the first circulation through the organ is necessary.

99mTc-MAA (macroaggregated albumin) is commonly used for pulmonary perfusion studies rather than for first pass cardiac studies. It is primarily designed to assess blood flow to the lungs and is trapped in the capillary bed, making it less suitable for tracking the rapid passage through the heart and providing the dynamics required during a first pass evaluation.

In contrast, isotopes like 123I, 68Ga, and 201Tl are better suited for first pass studies due to their ability to provide clear imaging of cardiac function and flow. 123I can be used for thyroid imaging and certain cardiac studies, while 68Ga is often used in PET imaging for various applications, including cardiac and tumor assessments. 201Tl has a long established role in cardiac imaging and can effectively visualize first pass perfusion.

Therefore, 99mTc-MAA is not appropriate for a first pass study due to its intended uses and characteristics, making it the correct choice