Power Doppler Imaging of Acute Renal Vein Occlusion and Recanalization in Canine Model Fig. 4. Sequential change in mean power Doppler signal during graded renal vein occlusion and recanalization in nine dogs. Bars indicate largest and smallest value in each step. X-axis indicates occlusion state of renal vein, which includes baseline, partial occlusion, complete occlusion, partial recanalization, complete recanalization, and removal of catheter, respectively. Change was statistically significant (p < 0.01). In the SD study, the RI increased as the renal vein was occluded, and decreased as occlusion was relieved (Fig. 5). The sequential change, of the mean RI value in nine dogs, showed a gradual increase during balloon inflation (0.54 0.07, 0.71 0.20, 0.93 0.14 at baseline, partial, and complete occlusion, respectively), and a decrease during deflation (0.74 0.20, 0.59 0.16, 0.55 0.11 at partial, complete recanalization, and catheter removal, respec- tively). The sequential change in the mean RI value was also statistically significant (p < 0.01). DISCUSSION We performed this study to determine whether PD can distinguish the hemodynamic change in acute renal vein obstruction and recanalization. According to our results, PD has the ability to detect the decreased renal perfusion immediately after a partial or complete renal vein occlusion. In addition, PD also demonstrated increased renal perfusion as the occluded renal vein was relieved. Based on these results, we suggest that PD may allow for the early diagnosis of acute RVT prior to the appearance of morphological changes. Power Doppler is based on the integrated Doppler power spectrum, and the color map displays the integrated power of the Doppler signal, which is primarily related to the number of moving red blood cells. It is a technique suitable for assessing slow flow in small vessels because of its greater sensitivity to flow and reduced angle dependence. It is superior to the CD in the imaging of renal perfusion abnormalities and allows improved evalua- tion of the renal parenchymal microvasculature (11 14). Korean J Radiol 9(2), April 2008 Fig. 5. Line graphs showing sequential changes in resistive index during graded renal vein occlusion and recanalization in nine dogs. Recent studies using an experimental animal model have shown that PD shows the hemodynamic change of the kidney with renal artery occlusion by quantification of the PD signal (8, 9). Helenon et al. (15). previously reported the potential of PD imaging for the diagnosis of RVT; however, this study used PD to visualize the vascular pedicle itself. To our knowledge, the use of PD for the evaluation of renal perfusion in renal vein obstruction has not yet been reported. Ultrasonography is the primary diagnostic modality for the evaluation of patients with suspected RVT. The sonographic findings of RVT depend on the stage and severity of the venous occlusion. Immediately after obstruction of the renal vein, the kidney becomes enlarged and echogenic due to edema and hemorrhage. As time progresses, the echogenicity of the kidney decreases with poor corticomedullary differentiation and as cellular infiltration and fibrosis develop, the kidneys again become echogenic and vary in size. At any stage, the echogenic filling defects, representing thrombus formation, may be identified in the renal vein or inferior vena cava (4, 5, 16 19). Most of the previously mentioned gray scale US findings are secondary changes and may be absent in some patients with RVT. These changes are neither highly sensitive nor specific for this diagnosis (5 7). Furthermore, the demonstration of echogenic thrombi of the renal vein, which establishes the diagnosis, is difficult to achieve (6, 7, 20). A spectral and CD image can support the diagnosis by demonstrating either absence of venous signal or partial flow around the thrombus in the main renal vein or inferior vena cava. The venous outflow obstruction also diminishes the arterial flow, which leads to a narrow systolic arterial peak and an absent, reduced or reversed 131