Yoo et al. diastolic flow in the segmental renal arteries (3, 21). Despite this, previous studies have demonstrated that the classic SD signs of RVT (absent venous flow and reversed diastolic arterial flow within the kidney) were present only in the acute phase of transplanted kidneys, and these Doppler signs may not be present in the RVT of native kidneys, due to the prompt development of venous collat- erals (3, 16, 22). Data on native kidneys have shown that venous collaterals could be formed in as little as 24 hours and often within the first two weeks after the onset of RVT (23). We also observed the prompt formation of venous collaterals during the occlusion of the renal vein by venography. Our results showed that the mean PD signal, after removal of the catheter, increased compared to the baseline. The mechanism associated with this phenomenon is not fully understood; however, we speculate that it is likely due to compensatory arterial hyperperfusion follow- ing the release of temporary venous occlusion. Advances in technology allow the objective quantifica- tion of the PD signal by counting the number of color pixels in a specific region of interest (8, 9). We quantified the renal perfusion based on the analysis of PD images with the aid of application software developed to quantify the PD signal automatically. This image analysis technique, in which the local statistical PD features such as mean power value (PV), peak PV, standard deviation of PV, percentage PD area, and integrated PV per unit area are extracted within the region of interest as defined by the operator, allow for an objective and reproducible assess- ment of renal perfusion (10). Despite our results, this study had several limitations. Firstly this experimental study was conducted immediately after the balloon occlusion or recanalization of the renal vein. Most patients with RVT in the clinical practice are presented with variable chronicity of occlusion, with the formation of collaterals. Furthermore, thrombosis usually begins in small intrarenal veins. The result of our study may not be applicable to subacute or chronic RVT. Further studies are needed to assess the feasibility of PD imaging in a subacute or chronic model with peripheral thrombosis simulating the clinical situation. Another limitation relates to the technical aspect of PD, which is susceptible to an attenuation effect (i.e. loss of signal with increasing depth). This may not be a problem in superficial small organs such as the prostate. However, for the abdominal viscera includ- ing the kidneys, the attenuation effect is a significant issue. To take into account the US attenuation at the different depths of the investigated tissue, a normalization technique was introduced by Rubin et al. (24), which states that PD signals within a region of interest can be normalized for 132 depth by assigning 100% of the power in regions composed entirely of blood, at similar depths. In our experimental study, the attenuation effect was not so problematic since the mongrel dogs used were about the same size, and their kidneys were located superficially. In addition to the attenuation effect, PD is also susceptible to a motion artifact of the probe or of the patient (breathing and gas), which can simulate blood flow. Therefore, PD has drawbacks when used in patients unable or unwilling to lie still or those who do not hold their breath during scanning (8). On a follow-up examination of patients with RVT, different Doppler parameters by different examiners may significantly affect the results. Therefore, scanning procedures should be performed uniformly to minimize inter- and intra-observer variation. Although our study was performed primarily for the investigation of PD, we additionally examined the sequen- tial change of the RI value during acute venous occlusion and recanalization. Overall, as expected, the RI value increased as the renal vein was occluded and subsequently decreased as the renal vein was recanalized. However, in two of the subject dogs, the RI values decreased during partial occlusion, in comparison with those at baseline values. We speculate that this phenomenon may be caused by the prompt development of collaterals, which allow for venous drainage, which also may contribute to relatively lower RI values during complete occlusion compared to those in the other dogs (Fig. 5). The overall results of the RI change in our study are somewhat in conflict with previous reports which demonstrated the limitations of RI in RVT of the native kidneys. This apparent inconsistency may be attributable to the aforementioned limitations of our study which do not allow for direct extrapolation to RVT with variable chronicity in the native kidneys. However, we think that our observations could be applica- ble to thrombosis of the renal allograft, which usually occurs at the anastomotic site of the renal vein acutely in the early post-transplant period. In conclusion, PD reflects a hemodynamic change in acute renal vein occlusion and recanalization in a canine model. Although there are concerns that should be addressed prior to clinical application, PD may be used as a helpful tool for the early detection of acute RVT and the monitoring of renal perfusion. Further studies will be required to investigate PD in subacute or chronic renal vein obstruction. Acknowledgments This study was supported by a grant from the Korea Health Industry Development Institute. In addition, we are grateful to Hyuk Jae Choi for his animal care expertise. Korean J Radiol 9(2), April 2008