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Objectives: To compare Apparent Diffusion Coefficient (ADC) measurements in rectal neoplastic lesions before and after lumen distension obtained with sonography transmission gel., Methods: From January 2014 to July 2016, 25 patients (average age 63.7, range 41-85, 18 males) were studied for pre-treatment rectal cancer staging using a 1.5T MRI. Diffusion MRI was obtained using echo-planar imaging with b=800 value; all patients were studied acquiring diffusion sequences with and without rectal lumen distension obtained using sonography transmission gel. In both diffusion sequences, two blinded readers calculated border ADC values and small ADC values, drawing regions of interest respectively along tumour borders and far from tumour borders. Mean ADC values among readers - for each type of ADC measurement - were compared using Wilcoxon matched pairs signed rank test. Correlation was assessed using Pearson analysis., Results: Border ADC mean value for diffusion MR sequences without endorectal contrast was 1.122mm 2 /sec, with 95% Confidence Interval (CI)=1.02-1.22; using gel lumen distension, higher border ADC mean value of 1.269mm 2 /s (95% CI=1.16-1.38) was obtained. Wilcoxon matched pairs signed rank test revealed statistical difference (p<0.01); a strong Pearson correlation was reported, with r value of 0.69. Small-ADC mean value was 1.038mm 2 /s (95% CI=0.91-1.16) for diffusion sequences acquired without endorectal distension and 1.127mm 2 /s (95% CI=0.98-1.27) for diffusion sequences obtained after endorectal gel lumen distension. Wilcoxon analysis did not show statistical difference (p=0.13). A very strong positive correlation was observed, with r value of 0.81., Conclusions: ADC measurements are slightly higher using endorectal sonographic transmission gel; ROI should be traced far from tumour borders, to minimize gel filled-pixel along the interface between lumen and lesion. Further studies are needed to investigate better reliability of ADC in rectal cancer MRI using sonographic gel intraluminal distension., (Copyright © 2016 Elsevier Ireland Ltd. All rights reserved.)