Abstract: Objective: To optimize measurement of CT quantitative perfusion parameters for local recurrence and postoperative scar of rectal carcinoma. Method: Ten consecutive rectal carcinoma patients who had undergone Mile's operation were recruited. They were suspected recurrence of cancer because of symptom or review examination. A cine-mode CT perfusion acquisition was performed covering each lesion. The acquisition started 10s after intravenous injection of contrast material (70mL) and lasted 60s. On a perfusion dedicated workstation, regions of interest (ROI) were placed in different ways which contained fixed ROI (24mm²), hot spot (single voxel), hot zone (24mm²) and whole mass. Blood flows (BF), blood volume (BV), mean transit time (MTT) and permeability-surface product (PS) were calculated. Perfusion functional maps were obtained. With the cross-section histogram tool of workstation, the mean value of the highest 5%, 10%, 20%, 40% and 60% of BF, BV, MTT and PS were acquired respectively. All the parameters were measured twice separated by two weeks. All patients with 11 masses received CT guided needle biopsy within one week. Long-term follow-up confirmed the character further. All the perfusion parameters by each method were compared by One-Way ANOVA and two times measurements were compared by Paired-Samples t Test. Results: There were ten patients and 11 lesions(recurrence group: n=6; scar group: n=5). Perfusion parameters varied obviously of different ways of measurement. The lowest value was acquired by whole mass measurement, while the highest value was acquired by the highest 5% measurement. BF, BV, MTT and PS of recurrence group were higher compared with scar group (P<0.05) except the MTT using fixed ROI measurement,MTT and PS using hot spot measurement (P>0.05). There was no statistical difference of perfusion parameters between the first and second measurements (P>0.05) except PS using hot zone (P<0.05). And the whole mass and highest ratio perfusion parameters were more stable (r ≥ 0.963, P<0.05). Conclusion: Compared with conventional measurement, the highest ratio perfusion value obtained by the cross-section histogram tool is more useful, objective and stable for differentiation of local recurrence and postoperative scar of rectal carcinoma. The recommendatory ratio was the highest 5%.