Full Title: Diffusion MRI for rectal cancer staging: ADC measurements before and after ultrasonographic gel lumen distension

Abstract 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 1.5 Tesla 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 1.122 mm2 /sec, with 95% Confidence Interval (CI) = 1.02-1.22; using gel lumen distension, higher border ADC mean value of 1.269 mm2 /sec (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 1.038 mm2 /sec (95% CI = 0.91-1.16) for diffusion sequences acquired without endorectal distension and 1.127 mm2 /sec (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.