ORIGINAL ARTICLE 18 F-FDG PET/MRI evaluation of retroperitoneal fibrosis: a simultaneous multiparametric approach for diagnosing active disease Verena Ruhlmann 1 & Thorsten Dirk Poeppel 1 & Alexander Sascha Brandt 2 & Johannes Grüneisen 3 & Marcus Ruhlmann 1 & Jens Matthias Theysohn 3 & Michael Forsting 3 & Andreas Bockisch 1 & Lale Umutlu 3 Received: 14 January 2016 /Accepted: 18 February 2016 # Springer-Verlag Berlin Heidelberg 2016 Abstract Purpose The aim of this study was to evaluate integrated 18 F- FDG PET/MRI as a one-stop diagnostic procedure in the as- sessment of (active) idiopathic retroperitoneal fibrosis (RPF) Methods A total of 22 examinations comprising a PET/CT scan followed by a PET/MRI scan in 17 patients (13 men, 4 women, age 58 ± 11 years) with histopathologically confirmed RPF at diagnosis or during follow-up under steroid therapy were analysed in correlation with laboratory inflammation markers (ESR, CRP). The patient cohort was subdivided into two groups: 6 examinations in untreated and 16 in treated patients. Tissue formations in typically periaortic localization suggestive of RPF were visually and quantitatively evaluated. The PET analysis included the assessment of SUVmax and a qualitative score for FDG uptake in RPF tissue in relation to the uptake in the liver. MRI analysis included evaluation of the T2-weighted image signal intensity, contrast enhancement and diffusion restriction (ADC values). Mean values were com- pared using the Mann-Whitney U test. ADC, SUVmax and ESR values were correlated using Pearson’ s correlation. Results MRI analysis revealed restricted diffusion in 100 % and 56 %, hyperintense T2 signal in 100 % and 31 %, and contrast enhancement in the periaortic tissue formation sug- gestive of RPF in 100 % and 62.5 % in the untreated and treated patients, respectively. In the qualitative and quantita- tive PET analysis, statistically significant differences were found for mean FDG uptake scores (2.5 ± 0.8 in untreated patients and 1.1±0.9 in treated patients) and mean SUVmax (7.8 ± 3.5 and 4.1 ± 2.2, respectively). A strong correlation was found between the ADC values and SUVmax (Pearson r -0.65, P =0.0019), and between ESR and CRP values and SUVmax (both r = 0.45, P = 0.061). Conclusion Integrated 18 F-FDG PET/MRI shows high diag- nostic potential as a one-stop diagnostic procedure for the assessment of (active) RPF providing multiparametric sup- portive information. Keywords Retroperitoneal fibrosis . PET/MRI . Multiparametric . One-stop diagnostic procedure Introduction Retroperitoneal fibrosis (RPF) is an uncommon disease that presents as retroperitoneal proliferation of fibrous tissue sur- rounding the retroperitoneal vascular structures and potential- ly leading to vessel or ureteral obstruction, the most frequent complication of RPF [1–3]. Early clinical findings of RPF are nonspecific and include symptoms of chronic back pain, low- er extremity oedema, and deep vein thrombosis [3]. Medical treatment is classically based on steroids to suppress the in- flammatory activity [3, 4]. Diagnosis and management of RPF remains challenging due to a lack of standardized diagnostic criteria for idiopathic * Verena Ruhlmann verena.ruhlmann@uk-essen.de 1 Department of Nuclear Medicine, Medical Faculty, University Duisburg-Essen, University Hospital Essen, Hufelandstraße 55, 45147 Essen, Germany 2 Department of Urology and Paediatric Urology, HELIOS Medical Center Wuppertal, University Hospital Witten/Herdecke, Heusnerstr. 40, 42283 Wuppertal, Germany 3 Department of Diagnostic and Interventional Radiology and Neuroradiology, University Hospital Essen, University Duisburg-Essen, Hufelandstraße 55, 45122 Essen, Germany Eur J Nucl Med Mol Imaging DOI 10.1007/s00259-016-3351-3