In vivo PET/MR imaging of lymph nodes with [89Zr]Mn3O4@PEG NPs. (a) In vivo PET imaging acquired after subcutaneous injection of [89Zr]Mn3O4@PEG NPs into the left footpad of the mouse (n = 3). Lymph nodes are indicated by arrows. (b) Quantification of the [89Zr]Mn3O4@PEG NPs uptake by the lymph node, paw, heart, liver, kidney, blood, lung, muscle and spleen (n = 3). (c) In vivo MR imaging of the lymph nodes before and after injection of Mn3O4@PEG NPs into the left footpad of the mouse (n = 3). Lymph nodes are indicated by circles.