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The authors regret that a statement on author contributions was not correctly formulated. With this corrigendum we provide the corrected version. Author contributions FW and SK prepared the tissue-engineered vascular grafts and were responsible of the study's coordination. FW, SK, AR and PM performed the tissue analysis and interpretation. SJ, TL, FM and FK were responsible for the study design. VF, MM and NG-W performed the in vitro and in vivo MR monitoring of grafts and the MRI data analysis together with TL, FK, PM and FW. OW and AM performed the PET- CT monitoring of grafts and the data analysis together with FM, PM and FW. HS and KC were responsible for the surgical implantation of the grafts. FW drafted the original manuscript; PM, TL and FK critically revised it. All authors discussed the results and approved the final version of the manuscript. The authors would like to apologise for any inconvenience caused.

Microbubbles (MB) are used as ultrasound (US) contrast agents and can be efficiently targeted against markers of angiogenesis and inflammation. Due to their gas core, MB locally alter susceptibilities in magnetic resonance imaging (MRI), but unfortunately, the resulting contrast is low and not sufficient to generate powerful molecular MRI probes. Therefore, we investigated whether a potent molecular MR agent can be generated by encapsulating superparamagnetic iron oxide nanoparticles (SPION) in the polymeric shell of poly (n-butylcyanoacrylate) (PBCA) MB and targeted them against α vβ3 integrins on the angiogenic vasculature of 4T1 murine breast carcinomas. SPION-MB consist of an air core and a multi-layered polymeric shell enabling efficient entrapment of SPION. The mean size of SPION-MB was 1.61 ± 0.32 μm. Biotin-streptavidin coupling was employed to functionalize the SPION-MB with cyclic RGDfK (Arg-Gly-Asp) and RADfK (Arg-Ala-Asp) peptides. Cells incubated with RGD-SPION-MB showed enhanced transverse relaxation rates compared with SPION-MB and blocking α vβ3 integrin receptors with excess free cRGDfK significantly reduced RGD-SPION-MB binding. Due to the fast binding of RGD-SPION-MB in vivo , dynamic susceptibility contrast MRI was employed to track their retention in tumors in real-time. Higher retention of RGD-SPION-MB was observed compared with SPION-MB and RAD-SPION-MB. To corroborate our MRI results, molecular US was performed the following day using the destruction-replenishment method. Both imaging modalities consistently indicated higher retention of RGD-SPION-MB in angiogenic vessels compared with SPION-MB and RAD-SPION-MB. Competitive blocking experiments in mice further confirmed that the binding of RGD-SPION-MB to α vβ3 integrin receptors is specific. Overall, this study demonstrates that RGD-SPION-MB can be employed as molecular MR/US contrast agents and are capable of assessing the αvβ3 integrin expression in the neovasculature of malignant tumors.