Diclofenac is a nonsteroidal anti-inflammatory drug that is frequently prescribed in order to reduce inflammation and pain. It is a non-selective inhibitor of the enzyme cyclooxygenase (COX), resulting in a reduction of prostaglandin-synthesis and may consequently reduce renal perfusion. Therefore, it is potentially harmful in patients at risk for renal function impairment. Diclofenac is available for oral and topical application and has been reported to reach measurable plasma concentration after repeated topical application(1). With contrast-enhanced ultrasound (CEUS) a discreet decrease of renal perfusion within one hour after oral diclofenac application could be detected in healthy subjects(2), while no changes in para-aminohippuric acid (PAH)-clearance, the clinical standard to measure renal blood flow over a period of 24 hours, was found(3). Arterial spin labeling (ASL) is a robust method for accurate quantification of renal perfusion and results are non-dependent on the investigator. The purpose of this study was to evaluate whether systemic or topical application of diclofenac is associated with renal perfusion impairment in healthy subjects measured by ASL.Ten healthy subjects were examined on three different days within a period of two weeks: 1) baseline examination without any medication, 2) 60-100 minutes after oral application of 50 mg diclofenac-sodium (Voltaren)(4), 3) after topical application of diclofenac (Voltaren Emulgel Gel, 1,16%) over 4 days (three applications per day) on an area of 21 x 27cm. Blood pressure, serum creatinine and diclofenac blood levels were measured at each time point. MRI was performed on a 1.5 Tesla scanner (Avanto, Siemens, Germany) and renal perfusion was measured by using a flow alternating inversion recovery (FAIR) trueFISP ASL sequence as described previously(5). Alternating, after slice-selective and global inversion, images were acquired in free breathing in the coronal plane with the following parameters: TR/TE 4.0/2.0 ms, inversion time = 1200 ms, FOV 420x420 mm, slice thickness 5 mm, repetitions 30. Following motion correction of ASL images with elastic registration, maps of renal perfusion were calculated. ROIs were placed in the upper, middle and lower third of the kidneys and mean perfusion values were calculated. Paired t-tests were used for statistical analysis. Values are given as mean±SEM.Out of 10 healthy subject, 3 were male, 7 were female. Mean age was 30±2 years (22-49 years), mean body weight 65±4 kg (53-80 kg), mean height 73±3 cm (160-188 cm). Mean blood pressure was 111±4/78±3 mmHg. Representative perfusion maps at baseline and after oral and topical application of diclofenac are shown in figure 1. After oral application of diclofenac, renal perfusion significantly decreased compared to baseline conditions (321±13 vs. 345±16 ml/(min*100g), p<0.01, figure 2). No significant changes were found after topical application compared to baseline (346±20 ml/(min*100g), p=0.4, ns, figure 2). ASL can detect discreet changes in renal perfusion that occur after single oral application of 50 mg diclofenac in healthy subjects. No significant changes in renal perfusion were detected after topical application of diclofenac. The fact that diclofenac induced measurable changes in renal perfusion even in healthy subjects should be considered in therapy plans of patients with pre-existing renal function impairment. ASL being able to detect discreet changes in renal perfusion could be useful for further studies on nephrotoxic side effects of novel medication or nephroprotective effects of novel treatment strategies in clinical studies since it is a non-invasive, robust and quantitative technique.