**4. Magnetic resonance imaging**

Kalb *et al*. provide a recent overview about MR-based approaches for functional and structural evaluations of renal grafts including a section on diagnostics of AR [20]. Beside exact anatom‐ ical information, MR can assess different aspects of renal function. Typical MR findings oc‐ curring in AR are enlargement of the graft (due to edema) with loss of corticomedullary differentiation and elevated cortical relative signal. There might be edema of and surrounding the kidney and the ureter. The high spatial and temporal resolution of MR allows perfusion imaging which might be useful to distinguish AR from ATN. 3D gradient echo perfusion imaging might show enhancement of the cortex and markedly delayed excretion of contrast [20]. Recent research with blood-oxygen level-dependent (BOLD) MR was promising for dif‐ ferentiating AR from ATN and a normal functioning kidney [21,22]. Furthermore, MR renog‐ raphy has been applied for diagnosis of the cause of acute dysfunction after kidney transplantation [23,24]. These two studies rely on quantitative evaluation of the shape of the renal enhancement curve to diagnose acute dysfunction. One can observe delayed and lower medullary enhancement in ATN whereas cortical and medullary enhancement curves de‐ crease in AR. However, further studies verifying the results are needed and still some issues about gadolinium-containing contrast agents and nephrogenic systemic fibrosis and gadoli‐ nium nephrotoxicity need to be resolved. More recently, Yamamoto *et al*. proposed a new quantitative analysis method of MR renography, including a multicompartmental tracer ki‐ netic renal model for diagnosis of AR and ATN, but state in their paper that findings in patients with normal graft function, AR, and ATN showed a substantial overlap with those of the normal population [25]. Another strategy followed was imaging of macrophage infiltration with ultrasmall superparamagnetic iron oxide particles [26]. Grafts with AR showed signifi‐ cant accumulation of iron particles but only within a time frame of 72 h which is much too late for potential clinical application.
