Representative coronal and sagittal PET imaging slices of 4T1-tumor bearing mice (tumors indicated by arrowheads) after intravenous injection of a) 89Zr-POM clusters and b) free89Zr as control at different time points. c) Quantification of 89Zr-POM uptake in the blood, liver, spleen, kidney, 4T1 tumor, and muscle at various time points post-injection (n = 3, mean ± s.d.). d) Biodistribution of 89Zr- POM clusters at 24 h after intravenous injection into 4T1-tumor bearing mice as determined by 89Zr radioactivity measurement in various tissues and organs (n = 3, mean ± s.d.).