Organ transplantation is the preferred medical treatment for patients with end-state organ failure.Two major challenges facing organ transplantation are graft rejection and shortage of donor organs.If we can detect early signs of acute rejection, we may be able to reduce chronic rejection and organ loss, thus reducing the need for repeating transplantation and making the donors organs available to those on transplantation waiting list.The current gold standard for diagnosing and staging cardiac rejection after heart transplantation is endomyocardial biopsy, which is not only invasive but also prone to sampling errors.Since repeated tests are necessary, it would be desirable to have a non-invasive method for detecting acute rejection.