CXCR4 and neoplasm: To create an alternative parameter for CXCR4 affinity that more closely correlated to tumor uptake in vivo, we titrated the tumor uptake of [68Ga]Pentixafor, [18F]RPS-534, [18F]RPS-544, and [18F]RPS-547 in μPET/CT images by co-injection of increasing masses of AMD-3100.