Strategies for non-invasive molecular imaging of acute allograft rejection by gamma scintigraphy and positron emission tomography

The number of patients necessitating treatment for end-stage organ failure, and therefore, the number of allograft recipients, increases. Despite the introduction of new and effective immunosuppressive drugs, acute cellular graft rejection (AR) is still a major risk for graft survival. Hence, early detection and treatment of AR is crucial to limit the inflammatory process and preserve the function of the transplant. Nowadays, AR can only be definitively diagnosed by biopsy. As an invasive procedure, biopsy carries the risk of significant graft injury and is not feasible in patients taking anticoagulant medication. Moreover, limited sampling site (randomly taken exceedingly small portions of tissue) may lead to false negative results, i.e., when rejection is focal or patchy. Thus, in diagnostics, entirely image-based methods would be superior. As AR is characterized by recruitment of activated leukocytes into the transplant several diagnostic strategies exist. We herein review the current approaches (experimental and clinical scenarios with a special focus on renal AR) in noninvasive molecular imaging-based diagnostics of acute AR using either single photon (gamma) imaging or positron emission tomography.