After renal transplantation 5 to 10% of patients experience allograft rejection. Rapid and accurate diagnosis is vital for implementation of additional immunosuppressive therapy. Currently, a renal biopsy is essential for the diagnosis of renal allograft rejection. However, this is an intervention associated with complications like bleeding, patient discomfort and hospital admission. Additionally, limited biopsy sample size may lead to false negative results. So, the introduction of a new non-invasive diagnostic tool for allograft rejection could have major implications for the care of renal transplant recipients. For the purpose of visualizing infiltrating T lymphocytes with positron emission tomography (PET), the tracer 18-Fluor-Interleukin-2 ([18F]FB-IL2) has been developed. The investigators hypothesized that a high correlation exists between [18F]FB-IL2 uptake and the extend of T cell infiltration into renal transplants with signs of rejection

Inclusion Criteria:

• Female or male aged between 18 and 80 years.
• Renal Transplant recipients
• The patient understands the purpose and risks of the study and has given written informed consent to participate in the study.
• All patients will have a clinical indication for renal biopsy.

Exclusion Criteria:

• Patients with multiple-organ transplants.
• Female patients who are pregnant or unwilling to use adequate contraception during the study.
• Claustrophobia
• Altered mental status, or any psychiatric condition that would prohibit the understanding or rendering of informed consent.
• A clinical reason for an immediate start of a therapeutic intervention with immunosuppressive medication.