Which isotope is known to localize by capillary blockade in lung scans?

The correct choice, MAA (macroaggregated albumin), is a radiopharmaceutical specifically designed for lung scanning, particularly in assessing pulmonary perfusion. MAA consists of small particles that can effectively block capillaries when injected into the bloodstream, forming a useful tool for highlighting areas of the lung that may have reduced blood flow or perfusion.

When MAA particles are administered intravenously, they preferentially localize in the pulmonary capillary bed due to their size. As they travel through the circulatory system, these particles become trapped in the small blood vessels of the lungs, enabling the effective visualization of lung perfusion through imaging techniques. This process is known as capillary blockade, as the particles create a blockage that allows for the assessment of circulation in different areas of the lungs.

Other isotopes listed, such as Technetium-99m, Indium-111, and Lutetium-177, have different applications and properties in imaging and therapy. Technetium-99m is widely used in a variety of nuclear medicine scans but does not localize via capillary blockade in the same manner as MAA. Indium-111 is mainly utilized in certain diagnostic applications, and Lutetium-177 is primarily used in targeted