eight male new zealand white rabbits , weighing approximately 3 kilograms each , were included in this study .
these rabbits were anesthetized with an intravenous injection of ketamin ( 40 mg / kg ) and xylazine ( 5 mg / kg ) , and placed in the right decubitus position ; thus , allowing a retroperitoneal approach to the left renal hilum . the left flank was incised , the retroperitoneal space dissected , and the left renal hilum exposed through the posteromedial aspect of the kidney .
the segmental left renal artery supplying the lower pole was isolated and ligated with surgical silk .
the incised flank was immediately closed in four rabbits following the surgical procedure in order to cause a permanent segmental infarction . in the remaining four rabbits ,
the segmental arterial obstruction was released 60 minutes later after ligation , to cause transient renal ischemia and reperfusion , and then the incised flank closed .
gray - scale and contrast - enhanced color doppler us imaging were performed using a sequoia 512 system ( siemens medical solution , mountain view , ca , usa ) , equipped with a linear array transducer of 8 - 15 mhz .
the doppler us gain was gradually increased until background noise was observed , which was then reduced just enough to remove the noise from the appearance of the kidney .
intravenous anesthesia was performed with ketamin ( 40 mg / kg ) and xylazine ( 5 mg / kg ) prior to the us scan .
sh u 508a ( levovist ; schering ag , berlin , germany ) was used as the us contrast agent , which consisted of galactose microparticles ( 99.9% ) and palmitic acid ( 0.1% ) .
a total of 300 mg of the us contrast agent ( 100 mg per kilogram ) was intravenously administered as a manual bolus injection .
us images were obtained on preoperative day one , immediate after wound repair , and when scheduled on postoperative days ( pod ) one , three , seven , 14 and 28 . immediately after the final us imaging on pod 28 ,
all rabbits were sacrificed with an overdose of intravenous thiopental sodium , and the left kidneys harvested for histological examination , which were fixed in formalin and stained with hematoxylin and eosin .
the imaging features of the involved renal parenchyma were compared between the renal ischemia and infarction groups , on gray - scale us images , for the presence or absence of parenchymal swelling , echogenicity changes and tissue loss .
parenchymal swelling was graded as follows : mild ( mildly swollen with no obliteration of renal sinus ) , moderate ( moderately swollen enough to compress the renal sinus by less than half of the normal sinus thickness ) and severe ( severely swollen enough to compress the renal sinus by more than half of normal sinus thickness ) .
echogenicity changes or tissue loss were graded as follows : mild ( echogenicity change or tissue loss of less than one third of the lower polar parenchyma ) , moderate ( echogenicity change or tissue loss between one third and two thirds of the lower polar parenchyma ) and severe ( echogenicity change or tissue loss of more than two thirds of the lower polar parenchyma ) .
on contrast - enhanced color doppler us images , the perfusion defects of the involved renal parenchyma were compared between the ischemia and infarction groups , and graded as follows : mild ( perfusion defect of less than one third of the lower polar parenchyma ) , moderate ( perfusion defect between one third and two thirds of the lower polar parenchyma ) and severe ( perfusion defect of more than two thirds of the lower polar parenchyma ) .
eight male new zealand white rabbits , weighing approximately 3 kilograms each , were included in this study .
these rabbits were anesthetized with an intravenous injection of ketamin ( 40 mg / kg ) and xylazine ( 5 mg / kg ) , and placed in the right decubitus position ; thus , allowing a retroperitoneal approach to the left renal hilum . the left flank was incised , the retroperitoneal space dissected , and the left renal hilum exposed through the posteromedial aspect of the kidney .
the segmental left renal artery supplying the lower pole was isolated and ligated with surgical silk .
the incised flank was immediately closed in four rabbits following the surgical procedure in order to cause a permanent segmental infarction . in the remaining four rabbits ,
the segmental arterial obstruction was released 60 minutes later after ligation , to cause transient renal ischemia and reperfusion , and then the incised flank closed .
gray - scale and contrast - enhanced color doppler us imaging were performed using a sequoia 512 system ( siemens medical solution , mountain view , ca , usa ) , equipped with a linear array transducer of 8 - 15 mhz .
the doppler us gain was gradually increased until background noise was observed , which was then reduced just enough to remove the noise from the appearance of the kidney .
intravenous anesthesia was performed with ketamin ( 40 mg / kg ) and xylazine ( 5 mg / kg ) prior to the us scan .
sh u 508a ( levovist ; schering ag , berlin , germany ) was used as the us contrast agent , which consisted of galactose microparticles ( 99.9% ) and palmitic acid ( 0.1% ) .
a total of 300 mg of the us contrast agent ( 100 mg per kilogram ) was intravenously administered as a manual bolus injection .
us images were obtained on preoperative day one , immediate after wound repair , and when scheduled on postoperative days ( pod ) one , three , seven , 14 and 28 . immediately after the final us imaging on pod 28 ,
all rabbits were sacrificed with an overdose of intravenous thiopental sodium , and the left kidneys harvested for histological examination , which were fixed in formalin and stained with hematoxylin and eosin .
the imaging features of the involved renal parenchyma were compared between the renal ischemia and infarction groups , on gray - scale us images , for the presence or absence of parenchymal swelling , echogenicity changes and tissue loss .
parenchymal swelling was graded as follows : mild ( mildly swollen with no obliteration of renal sinus ) , moderate ( moderately swollen enough to compress the renal sinus by less than half of the normal sinus thickness ) and severe ( severely swollen enough to compress the renal sinus by more than half of normal sinus thickness ) .
echogenicity changes or tissue loss were graded as follows : mild ( echogenicity change or tissue loss of less than one third of the lower polar parenchyma ) , moderate ( echogenicity change or tissue loss between one third and two thirds of the lower polar parenchyma ) and severe ( echogenicity change or tissue loss of more than two thirds of the lower polar parenchyma ) .
on contrast - enhanced color doppler us images , the perfusion defects of the involved renal parenchyma were compared between the ischemia and infarction groups , and graded as follows : mild ( perfusion defect of less than one third of the lower polar parenchyma ) , moderate ( perfusion defect between one third and two thirds of the lower polar parenchyma ) and severe ( perfusion defect of more than two thirds of the lower polar parenchyma ) .
all rabbits , with the exception of one , survived until pod 28 , the last day of us imaging .
various abscesses occurred in the perirenal area or in the left flank wall , but these responded to antibiotics .
table 1 summarizes the imaging features of the gray - scale us in the ischemic and infarction groups .
parenchymal swellings in the ischemic and infarction groups peaked on pod three and seven , respectively , by normalized by pod 14 .
renal ischemic and infarction areas showed hypoechoic parenchyma before pod seven , which were then replaced with a hyperechoic area .
the hyperechoic area in the renal ischemic group was greatest at pod seven , and then gradually reduced until pod 28 .
a hyperechoic area in the infarction group was observed at pod seven , which did not alter until pod 28 .
parenchymal tissue loss was observed in both groups at pod 14 , with the infarction group showing more tissue loss than the ischemia group .
table 2 summarizes the perfusion defect changes in the two groups according to the postoperative day . the ischemic group showed the most extensive perfusion defect area on pod one .
this imaging feature improved from pod three , and appeared as a mild perfusion defect on pod 28 .
normal renal perfusion of the lower polar parenchyma did not fully recover in the ischemic group .
however , the hyperechoic areas that showed blood flow recovery after reperfusion regained normal echogenicity and perfusion , but the hyperechoic areas that were not reperfused became infarcted .
the rabbits that underwent renal infarction showed complete perfusion defect of the lower polar parenchyma and no recovery of renal perfusion at pod 28 .
the pathological specimens obtained from the ischemic group showed minimal tissue loss of the lower polar parenchyma , as a result of coagulation necrosis of the renal tubules and glomeruli . however , those obtained from the infarction group showed severe tissue loss and hemorrhage , as a result of extensive necrosis of the lower polar parenchyma .
table 1 summarizes the imaging features of the gray - scale us in the ischemic and infarction groups .
parenchymal swellings in the ischemic and infarction groups peaked on pod three and seven , respectively , by normalized by pod 14 .
renal ischemic and infarction areas showed hypoechoic parenchyma before pod seven , which were then replaced with a hyperechoic area .
the hyperechoic area in the renal ischemic group was greatest at pod seven , and then gradually reduced until pod 28 .
a hyperechoic area in the infarction group was observed at pod seven , which did not alter until pod 28 .
parenchymal tissue loss was observed in both groups at pod 14 , with the infarction group showing more tissue loss than the ischemia group .
table 2 summarizes the perfusion defect changes in the two groups according to the postoperative day . the ischemic group showed the most extensive perfusion defect area on pod one .
this imaging feature improved from pod three , and appeared as a mild perfusion defect on pod 28 .
normal renal perfusion of the lower polar parenchyma did not fully recover in the ischemic group .
however , the hyperechoic areas that showed blood flow recovery after reperfusion regained normal echogenicity and perfusion , but the hyperechoic areas that were not reperfused became infarcted .
the rabbits that underwent renal infarction showed complete perfusion defect of the lower polar parenchyma and no recovery of renal perfusion at pod 28 .
the pathological specimens obtained from the ischemic group showed minimal tissue loss of the lower polar parenchyma , as a result of coagulation necrosis of the renal tubules and glomeruli . however , those obtained from the infarction group showed severe tissue loss and hemorrhage , as a result of extensive necrosis of the lower polar parenchyma .
many clinical and experimental studies have described the findings of renal infarction with the use of us , ct or mri ( 1 - 10 ) .
renal infarction is usually diagnosed based on the ct findings , including a sharply demarcated area of poor contrast enhancement , a hyperdense cortical rim and subcapsular fluid collection ( 3 , 4 , 10 ) .
mri can also be useful for the detection of renal infarction , and in this case usually appears as areas of low signal intensity on both t1- and t2-weighted mr images , and as a region of poor contrast enhancement on contrast - enhanced mr images ( 1 , 2 , 6 , 7 ) .
doppler us has the ability to depict focal perfusion defects by clearly visualizing fine small vessels in the renal parenchyma ( 5 , 9 ) .
moreover , gray - scale us can show echogenicity changes in the infarction areas , including hypoechoic and hyperechoic involved parenchyma in the early and late stages of the infarction , respectively ( 8) .
however , previous studies have primarily focused on the renal infarction , so have not addressed the transient renal ischemic changes . moreover ,
these must be differentiated from renal infarction , as renal ischemia is theoretically reversible ; whereas , renal infarction is not .
therefore , it is of some great importance to obtain imaging features that are able to differentiate renal ischemia from renal infarction during evaluation of the management and prognosis of patients with a renal vascular impairment . in the present study ,
gray - scale us and contrast - enhanced color doppler us were able to depict potentially differentiating features of renal ischemia and renal infarction . during the early pod , parenchymal swelling , hypoechogenicity and reduced renal perfusion were common us imaging features in both the ischemic and renal infarction groups .
however , the reperfused hyperechoic areas , unlike the infarct areas , returned to being normal parenchyma . this imaging feature should be considered a differential us imaging feature of renal ischemia and renal infarction . on the contrary , hyperechoic areas without reperfusion proved irreversible , and became infarcted .
reported that renal parenchymal hyperechogenicity followed hypoechogenicity after arterial occlusion , and these regions corresponded pathologically to infarction ( 8) .
however , our study demonstrated that reperfused hyperechoic renal parenchyma was able to recover normal echogenicity and perfusion .
the hyperechoic area corresponded pathologically to an ischemic penumbra area , which does not determined whether this lesion returned to normal or became infarcted .
this should depend on the presence or absence of arterial reperfusion to the renal ischemic area .
( 11 ) , who reported that renal infarction occurs when renal artery was occluded for more than 90 minutes . in our pilot study , 60 minutes was found to be the optimal ischemic duration for depiction of renal ischemia and infarction on us .
30 minute duration failed to visualize the ischemic or infarction area on us because the involved area was too small ; whereas , 90 minute duration made it impossible to differentiate renal ischemia from infarction as all involved area had become totally infarcted .
our study had several limitations ; first , a relatively small number of cases were included .
second , us imaging features at the early stage of renal ischemia and infarction did not correlate with those of the pathological examination .
third , subcapsular rim enhancement was not observed on the contrast - enhanced color doppler us .
this was why the subcapsular arteries might have been injured when the left kidneys were isolated from the perirenal space . in conclusion
, gray - scale and contrast - enhanced color doppler us were found to have the potential to depict various imaging features as a result of renal ischemia and renal infarction . a hyperechoic region , with normal blood flow , may be a reversible us feature to provide a clue to the differentiation of renal ischemia from renal infarction .
 objectiveto characterize the imaging features on gray - scale and contrast - enhanced color doppler us images which differentiate renal ischemia from renal infarction.materials and methodsthe segmental renal arteries of eight healthy rabbits were surgically ligated . in four of these rabbits , 
 the ligated renal artery was released 60 minutes after arterial occlusion to cause transient ischemia . in the remaining four rabbits , 
 the arterial ligation was retained to cause a permanent infarction . 
 the gray - scale and contrast - enhanced color doppler us imaging features of the involved renal parenchyma of both ischemia and infarction groups were compared with respect to the presence or absence of parenchymal swelling , echogenicity changes , tissue loss and perfusion defects.resultsparenchyma swelling , echogenic changes , tissue loss and perfusion defects were found to be more extensive in the infarction than the ischemia group . 
 the hyperechoic areas reperfused with blood flow recovered normal echogenicity and perfusion , whereas the hyperechoic areas without reperfusion became renal infarcts.conclusiongray-scale and contrast - enhanced color doppler us showed that the hyperechoic areas with reperfusion may reverse to normal parenchyma and allow the differentiation of renal ischemia from renal infarction . 
MATERIALS AND METHODS
Surgery
US imaging
Imaging analysis
RESULTS
Analysis of Gray Scale US
Analysis of Contrast-Enhanced Color Doppler US
Pathologic Correlation
DISCUSSION