Abstract: Background and purpose: The apparent diffusion coefficient (ADC) values are the quantitative parameters of diffusion weighted imaging (DWI) which are influenced by many factors, such as b values and region of interest (ROI), and have become the concerns of research. Our study aimed to determine the measurement consistency of ADC in rectal cancer based on different ROIs. Methods: Eighty patients with histologically proven rectal cancer were examined using echo-planar DW MRI values (b value=800 s/mm2). ADC values were measured on ADC map. Mean tumor ADCs were measured according to three distinct slice protocols: ‘whole-slices’,‘single-slice of largest tumor’ and ‘tumor parts of largest independent tumor-containing slices and two adjacent slices’. The freehand ROI was drawn along the border of the high signal of the tumor on the b=800 images to cover the entire tumor area of each slice. Another ROI was drawn using circle tool to cover the entire tumor solid parts as large as possible. After 2 weeks of repeating the measurement between the two physicians. Differences in ADC values were measured using one-way ANOVA. The two protocols were compared for differences in ADC values using paired t-test. We calculated intra- and inter-observer variability using intraclass correlation coefficient (ICC) and Bland-Altman plot. Results: No significant difference was observed by ROIs for three distinct slice protocols (P>0.05). The ADC values obtained by ROIs for the outlined were higher than the ADC values obtained by ROIs for the center analysis (P<0.05). Intraand inter-observer ICC were 0.931, 0.803; 0.913, 0.822 by whole-slices-center and outline ROI respectively. The intraobserver 95% limits of consistency of ADC values were (-0.089 3 to 0.083 1)×10^-3mm²/ s (P: Mean=0, P>0.05), (-0.066 8 to 0.096 3)×10^-3mm²/s (P<0.05) with center and outline ROI, respectively. The inter-observer 95% limits of consistency of perfusion parameters were (-0.127 5 to 0.141 6)×10^-3mm²/s (P>0.05), (-0.112 0 to 0.150 2)×10^-3mm²/s (P<0.05) with center and outline ROI, respectively. Bland-Altman plot showed that whole-slices-center ROI could provide more reproducible and stable results. Conclusion: There was no statistically significant difference among three distinct slice protocols analysis of ADC values in rectal cancer. ADC values by whole-slices-center ROI protocol appropriately improved intra- and inter-observer consistency and could provide more reproducible and stable results with less time. ADC values by whole-slices-outlined ROI protocol appropriately indicated the overall situation, but needed more time.

Key words: Rectal cancer, Diffusion weighted imaging, Apparent diffusion coefficient, Region of interest