The isotope Zr has been applied to the tracking and quantification of molecular antibodies with PET cameras (a method called "immuno-PET").
The biological half-life of antibodies is typically on the order of days, see daclizumab and erenumab by way of example. To visualize and quantify the distribution of such antibodies in the body, the PET isotope Zr is well suited because its physical half-life matches the typical biological half-life of antibodies, see table above.