Yoo et al. PD signal was mapped into a pixel with higher intensity in a linear fashion. The software calculated the integrated PD signal within the region of interest drawn by an operator along the border of the kidney (Fig. 1). For the SD imaging, peak systolic and end diastolic velocity was obtained by means of a manual caliper measurement. The resistive index (RI) ([peak systolic velocity-end diastolic velocity]/peak systolic velocity) was calculated as a mean of three subsequent cardiac cycles. Statistics The sequential change of the PD signal and RI was assessed by plotting the PD signal at each step during occlusion and recanalization of the renal vein in each dog. The statistical analyses were performed using two-way analysis of variances (ANOVAs) and Duncan multiple- range tests to assess the sequential change of the mean value of the PD signal and RI in nine dogs. Statistical significance was set a priori at p < 0.01. RESULTS The PD images revealed a decrease in the signal intensity as the renal vein was occluded, and an increase after recanalization, as measured on the visual scale reading (Fig. 2). The time course of the PD signal during renal vein occlusion and recanalization in a dog is shown in Fig. 3. Prior to a balloon occlusion, the mean standard deviation PD signal was 2.88 106 5.43 105. During the partial and complete occlusion, the PD signal decreased to 2.23 106 9.63 105 and 1.15 106 8.82 105, respectively. After balloon deflation, the PD signal increased to 1.60 106 1.32 105, and 2.92 106 1.18 105 during partial and complete recanalization, respectively. After removing the balloon catheter, the PD signal was 3.52 106 8.73 105, which was greater than at the observed baseline. The sequential change of the mean PD signal value was statisti- cally significant (Fig. 4, p < 0.01). Fig. 3. Time course analysis of power Doppler signal during graded renal vein occlusion and recanalization in a dog. Note decrease of power Doppler signal during occlusion as well as increase in power Doppler signal during recanalization. B C A 130 D E F Fig. 2. Sequential change of the longitudinal power Doppler image of same region of dog kidney, during graded renal vein occlusion and recanalization. Baseline (A), partial occlusion (B), complete occlusion (C), partial recanalization (D), complete recanalization (E), and removal of catheter (F). Korean J Radiol 9(2), April 2008