orthotopic liver transplantation is an important treatment option for patients with end - stage liver disease . in the united states , 109,126 liver transplants were performed between january 1 , 1988 and april 30 , 2011 with 6291 transplants performed in 2010 . indications for liver transplant include cirrhosis secondary to hepatitis c , hepatitis b , cryptogenic hepatitis , and autoimmune disease . cholestatic liver diseases for which liver transplantation may be required include biliary cirrhosis , caroli 's disease , and primary sclerosing cholangitis . other reasons for transplant include biliary atresia , acute hepatic necrosis , metabolic disorders , malignant neoplasms in certain cases , and budd chiari syndrome . advances in surgical techniques over the past several decades , along with improvements in organ preservation and immunosuppressive therapy have improved patient outcomes after liver transplantation . post - operative complications , however , can limit this success , especially if not recognized early enough to allow for graft salvage . among the most common and clinically significant complications to occur in the post - operative period are those of vascular origin . ultrasound with doppler is the primary imaging modality for the evaluation of hepatic transplants , providing detailed assessment of vascular flow in the graft . however , a multimodality approach including ct angiography , mri , or conventional angiography may be necessary to diagnose the complications . this article describes the vascular anatomy of liver transplants and imaging appearances of the major vascular complications that may occur within the hepatic artery , portal vein , and venous outflow tract . all our images have been obtained from a single major urban medical center where all 52 transplants have been performed since our program began three years ago . all liver transplant patients undergo a liver sonogram with comprehensive doppler evaluation on post - operative day one . the sonographic examination algorithm includes gray - scale evaluation of the graft and biliary tree , color , and spectral doppler of the main hepatic artery , portal vein , hepatic veins and ivc , their anastomoses and intrahepatic branches . further imaging is based on clinical follow - up including daily monitoring of laboratory values . if clinical parameters are abnormal , repeat sonography is performed and depending on the results , ct , mri , or angiography may be requested . typically , the donor hepatic artery is anastomosed end - to - end to the recipient hepatic or celiac artery . in recipients with a diseased or small hepatic artery , a donor iliac artery or ptfe ( polytetrafluoroethylene ) interposition graft hepatic arterial complications may lead to graft ischemia and are associated with high patient morbidity and mortality . common complications of graft ischemia include biliary stenosis or necrosis as the biliary tree relies solely on the hepatic arteries for its blood supply [ figures 1 and 2 ] . ( a ) axial ct angiography image ( cta ) performed 1-month post - transplant shows a patent interposition graft ( white arrow ) between the abdominal aorta ( black arrow ) and donor hepatic artery . ( c ) coronal reformat cta demonstrates patent intrahepatic arterial collaterals ( black arrows ) . ( d ) mrcp coronal thick slab demonstrates two biliary strictures in the common duct secondary to arterial ischemia ( white arrows ) . 60-year - old woman status post - liver transplant with an interposition graft between the aorta and the donor hepatic artery with questionable findings for graft stenosis on sonography . ( a ) coronal mip reformatted image of contrast - enhanced mri reveals areas of interruption of the contrast column in the graft ( white arrows ) between the aorta ( block arrow ) and donor hepatic artery ( black arrow ) consistent with significant stenoses . top image : long segment of turbulent flow in the graft with color aliasing ( white arrow ) . bottom right : spectral doppler reveals persistent graft stenosis post - angioplasty with elevated peak systolic velocity in the distal graft . ( f ) stenoses were treated with balloon angioplasty with excellent results ( black arrows ) and a stent was placed in the proximal graft ( white arrow ) . the incidence of hepatic artery thrombosis is 4 - 12% in adults and 42% in children . risk factors include donor and recipient arterial caliber discrepancy , use of an interposition conduit , acute rejection , and excessive cold ischemia time . early thrombosis is a dreaded complication associated with liver failure , biliary leak , sepsis , and a high mortality rate of 20 - 60% . prompt diagnosis is crucial , generally necessitating immediate graft revascularization . even with early recognition of thrombosis , 60% of patients will still require retransplantation . patients with hepatic artery thrombosis can occasionally present , many years post - transplant , with sepsis and chronic rejection . hepatic artery stenosis occurs in 5 - 11% of liver transplant recipients usually in the first 100 days post - transplant . it is often at the anastomotic site , due to clamp injury and intimal trauma . the normal hepatic arterial spectral flow waveform demonstrates a low resistance pattern with a brisk systolic upstroke . the systolic upstroke acceleration time should be < 0.08 s. the normal hepatic arterial resistivity index ( ri ) ranges from 0.50 to 0.80 . in the first 72 hours post - transplant , the ri may be greater than 0.80 due to vascular spasm and post - surgical swelling , although it generally returns to normal in a few days . 
  in main hepatic artery thrombosis , the artery may not be seen on gray - scale . abnormal doppler findings of hepatic artery thrombosis are absent color power doppler and spectral flow in the main or intra - hepatic arteries . hepatic arterial collaterals may develop in chronic thrombosis and demonstrate low intrahepatic arterial ris , mimicking stenosis . therefore , the sensitivity of ultrasound for the detection of hepatic artery thrombosis decreases as the interval following transplant increases . with hemodynamically significant hepatic artery stenosis , a segmental increase in flow velocity is found at the post - stenotic jet . a focal peak systolic velocity ( psv ) in the main hepatic artery of greater than 2.0 m / s indicates significant stenosis . color doppler imaging can be used to rapidly identify the line of maximal velocity associated with the stenotic site , by carefully reducing the color doppler scale to produce the slightest degree of aliasing , which can be recognized as a change in doppler color at the brightest portion of the color spectrum . intrahepatic arteries may have a tardus - parvus waveform with a prolonged systolic upstroke acceleration time and low intrahepatic ri(s ) of less than 0.50[figure 2 ] . in a recent study by park et al , the use of post - stenotic hepatic arterial flow velocity in conjunction with the finding of a tardus - parvus wave pattern can significantly increase the positive predictive value of doppler for diagnosing hepatic artery stenosis . pseudoaneurysms are a rare but potentially catastrophic complication of hepatic transplantation , often occurring at the anastomotic site . they may develop secondary to infection , graft biopsy , or transhepatic catheter / drain placement . they are often clinically occult or may present with nonspecific graft dysfunction or abnormally elevated serum liver enzyme levels . contrast - enhanced ct is superior to sonography in identifying small pseudoaneurysms , as bowel gas may obscure a tortuous extrahepatic hepatic artery . ct angiography or conventional angiography is performed if there is suspicion of graft - related bleeding or unexplained graft malfunction . the donor portal vein is usually anastomosed end - to - end to the recipient portal vein . however , if there is pre - existing thrombosis of the recipient portal vein , thrombectomy or placement of a jump graft between the donor portal vein and recipient superior mesenteric or splenic vein can be performed . mild focal narrowing at the anastomosis is a common finding which does not necessarily imply stenosis . on doppler examination turbulent flow with elevated portal venous flow velocities may be present in the early post - operative period , decreasing over time in the absence of significant stenosis . portal vein stenosis and thrombosis are rare complications with an incidence of 1 - 2% . predisposing factors include previous portal vein surgery or thrombosis , high downstream resistance due to an inferior vena cava ( ivc ) stricture , hypercoagulable state , and technical problems during surgery such as mismatched vessel size or vascular kinking . treatments include angioplasty , stent placement , thrombolysis , surgical repair with thrombectomy , venous jump graft or portosystemic shunt creation . gray - scale findings of portal vein thrombosis may include a hyperechogenic clot ; however , because acute thrombus can be isoechoic or hypoechoic , color doppler imaging should be performed . with acute thrombosis , the portal vein distends ; in chronic thrombosis , the vessel caliber decreases . very slow portal venous flow may simulate thrombosis , even with power doppler , and so further evaluation with other imaging modalities such as ct , mri or angiography may be appropriate . if arterial flow is detected within the clot , sonographic findings of portal vein stenosis include vascular narrowing with segmental color doppler aliasing and post - stenotic dilatation . duplex imaging reveals at least a 3- to 4-fold velocity gradient across the stenosis[figure 3 ] . in a study by mullan et al , a peak portal vein flow velocity of 80 cm / s diagnosed portal vein stenosis with a sensitivity of 100% and specificity of 84% . in a study by chong et al , a peak portal vein velocity of greater than 125 cm / s had only 73% sensitivity but a specificity of 95% . with chronic portal thrombosis and cavernous transformation , an attempt should be made to differentiate large venous collaterals in the porta hepatis from the main portal vein and intrahepatic branches . ( a ) gray scale shows narrowing at the portal vein anastomosis ( arrow ) . ( d ) spectral doppler with high velocity at the post - stenotic jet , which is 3.5 times higher than peak velocity in the pre - stenotic portal vein . decreased portal venous flow velocity may be seen downstream from portal vein stenosis , but it can also be associated with increased hepatic resistance secondary to rejection or recurrent parenchymal disease . vascular steal phenomenon directing portal flow away from the graft may occur when large pre - transplant portosystemic shunts , commonly splenorenal , are not ligated at the time of transplant . in vascular steal , the portal venous flow may have decreased velocity or variable direction . a pulsatile in orthotopic liver transplantation , there are two main surgical techniques for reestablishing the venous outflow tract . in one , the intrahepatic segment of the recipient inferior vena cava is explanted with the diseased liver and replaced en bloc with the donor intrahepatic cava and graft . anastomosis usually is performed end - to - end , at both the suprahepatic and infrahepatic segments of the recipient 's residual ivc [ figure 4a ] . ( b ) piggyback technique : the donor ivc or hepatic vein is anastomosed to the recipient common hepatic vein stump ( arrow ) . an alternative transplant procedure focuses on maintaining an intact recipient inferior vena cava avoiding venovenous bypass intraoperatively . in the piggyback technique , a widely used form of caval - preserving surgery , a short segment of the donor ivc is anastomosed to the common trunk of the recipient 's main hepatic veins [ figure 4b ] . in partial hepatic transplants , one of several techniques can be used to anastomose the donor hepatic vein(s ) to the recipient ivc . complications involving the venous outflow tract are uncommon , reportedly occurring in < 1 - 10% depending on whether the procedure is orthotopic or partial . patients can develop stenosis or thrombosis of the inferior vena cava or main hepatic veins , most commonly at an anastomotic site . patients with inferior vena caval or hepatic venous complications may present with pain , ascites , hepatomegaly , pleural effusion , peripheral edema , or abnormal serum liver enzyme levels . caval obstruction can result in renal failure . in the acute post - surgical period , caval stenosis is usually due to anastomotic stricture or caval compression from graft swelling , hematoma or other post - operative fluid collection . with pediatric or living - related donor grafts , vascular redundancy and graft rotation in the latter group may result in venous kinking or frank torsion . delayed stenosis can result from patient growth or graft enlargement , fibrosis , chronic thrombus , or neointimal hyperplasia . , findings of venous outflow compromise may include caval or anastomotic narrowing with pre - stenotic dilatation of the ivc and/or hepatic veins . color doppler imaging frequently reveals vascular narrowing and aliasing at the stenotic site where flow velocities are abnormally elevated . isoechoic or hypoechoic thrombus , not visible on gray - scale imaging , may be identified as an intravascular filling defect with color doppler [ figure 5 ] . 68-year - old man three months status post liver transplant with ivc clot demonstrated with color doppler ( arrow ) . duplex / spectral imaging typically demonstrates dampening with loss of pulsatility of the normally triphasic flow wave patterns in the pre - stenotic cava and/or hepatic veins . a monophasic wave pattern in the cava and hepatic veins is nonspecific , often seen in the acute post - surgical period [ figure 6 ] . even a persistent monophasic flow wave pattern in the hepatic veins is non - specific for venous complications , as it is also seen with graft rejection or recurrent parenchymal disease . post - transplant caval or hepatic venous triphasic wave pattern has a high negative predictive value for downstream stenosis and graft rejection . with hemodynamically significant stenosis , there is an at least 3- to 4-fold flow velocity gradient across the stenosis [ figure 7 ] . ( b ) same patient one month after surgery with spectral doppler showing normal triphasic hepatic venous flow . the spectral waveform is dampened with maximum flow velocity measured at 20 cm / s . ( c ) spectral doppler shows dampened waveform and increased flow velocity at the post - stenotic jet 4.8 times higher than the maximum flow velocity in the pre - stenotic ivc . no intravascular spectral flow is demonstrated in regions of venous thrombosis . in severe cases of venous outflow stenosis or in thrombosis , there can be reversed flow in one or more main hepatic veins or in the portal vein . post contrast computed tomography / angiography ( ct / cta ) or magnetic resonance imaging / angiography / cholangiopancreatography ( mri / mra / mrcp ) are ancillary modalities used to evaluate vascular and nonvascular complications of hepatic transplants [ 
       figure 1 ] . these imaging techniques are especially valuable in patients with complex , atypical , or obscured post - surgical anatomy on sonography , or in patients with inconclusive or conflicting ultrasound findings . these include demonstrating  nutmeg  enhancement pattern of the congested liver in the setting of venous outflow obstruction , hepatic infarction 
       [ figure 8 ] , and biliary abnormalities . these modalities also demonstrate the extent of post - operative fluid collections and other abnormalities , which may be partially obscured by gaseous interference on sonography 
       [ figure 9 ] . ( b ) axial pre - contrast fat saturated t1-weighted mri image demonstrates hemorrhagic infarction in the right hepatic dome evidenced by increased signal ( arrows ) . ( c ) axial post - contrast t1-weighted mri image with subtraction showing no enhancement in the infarcted area ( arrows ) . 52-year - old man s / p liver transplant with extrinsic portal vein compression by an inflammatory mass . mri axial t2-weighted images shows ( a ) signal void of the pre - stenotic main portal vein ( white arrow ) at the confluence of splenic vein ( black arrow ) and superior mesenteric vein . ( b ) an inflammatory mass which is hyperintense on t2-weighted images ( small black arrows ) causes extrinsic compression of the main portal vein evidenced by decreased caliber of signal void ( long white arrow ) . ( c ) signal void of the post - stenotic portal vein at the hilum ( arrow ) . conventional angiography is generally used for problem solving and in preparation for intended vascular interventions . during catheterization , subsequent balloon angioplasty and stent placement are frequently successful in salvaging the graft , especially if early diagnosis is made [ figure 2 ] . all our images have been obtained from a single major urban medical center where all 52 transplants have been performed since our program began three years ago . all liver transplant patients undergo a liver sonogram with comprehensive doppler evaluation on post - operative day one . the sonographic examination algorithm includes gray - scale evaluation of the graft and biliary tree , color , and spectral doppler of the main hepatic artery , portal vein , hepatic veins and ivc , their anastomoses and intrahepatic branches . further imaging is based on clinical follow - up including daily monitoring of laboratory values . if clinical parameters are abnormal , repeat sonography is performed and depending on the results , ct , mri , or angiography may be requested . typically , the donor hepatic artery is anastomosed end - to - end to the recipient hepatic or celiac artery . in recipients with a diseased or small hepatic artery , a donor iliac artery or ptfe ( polytetrafluoroethylene ) interposition graft hepatic arterial complications may lead to graft ischemia and are associated with high patient morbidity and mortality . common complications of graft ischemia include biliary stenosis or necrosis as the biliary tree relies solely on the hepatic arteries for its blood supply [ figures 1 and 2 ] . ( a ) axial ct angiography image ( cta ) performed 1-month post - transplant shows a patent interposition graft ( white arrow ) between the abdominal aorta ( black arrow ) and donor hepatic artery . the superior mesenteric artery ( sma ) ( c ) coronal reformat cta demonstrates patent intrahepatic arterial collaterals ( black arrows ) . ( d ) mrcp coronal thick slab demonstrates two biliary strictures in the common duct secondary to arterial ischemia ( white arrows ) . 60-year - old woman status post - liver transplant with an interposition graft between the aorta and the donor hepatic artery with questionable findings for graft stenosis on sonography . ( a ) coronal mip reformatted image of contrast - enhanced mri reveals areas of interruption of the contrast column in the graft ( white arrows ) between the aorta ( block arrow ) and donor hepatic artery ( black arrow ) consistent with significant stenoses . top image : long segment of turbulent flow in the graft with color aliasing ( white arrow ) . bottom right : spectral doppler reveals persistent graft stenosis post - angioplasty with elevated peak systolic velocity in the distal graft . ( f ) stenoses were treated with balloon angioplasty with excellent results ( black arrows ) and a stent was placed in the proximal graft ( white arrow ) . the incidence of hepatic artery thrombosis is 4 - 12% in adults and 42% in children . risk factors include donor and recipient arterial caliber discrepancy , use of an interposition conduit , acute rejection , and excessive cold ischemia time . early thrombosis is a dreaded complication associated with liver failure , biliary leak , sepsis , and a high mortality rate of 20 - 60% . prompt diagnosis is crucial , generally necessitating immediate graft revascularization . even with early recognition of thrombosis , 60% of patients will still require retransplantation . patients with hepatic artery thrombosis can occasionally present , many years post - transplant , with sepsis and chronic rejection . hepatic artery stenosis occurs in 5 - 11% of liver transplant recipients usually in the first 100 days post - transplant . it is often at the anastomotic site , due to clamp injury and intimal trauma . the normal hepatic arterial spectral flow waveform demonstrates a low resistance pattern with a brisk systolic upstroke . the systolic upstroke acceleration time should be < 0.08 s. the normal hepatic arterial resistivity index ( ri ) ranges from 0.50 to 0.80 . in the first 72 hours post - transplant , the ri may be greater than 0.80 due to vascular spasm and post - surgical swelling , although it generally returns to normal in a few days . 
  in main hepatic artery thrombosis abnormal doppler findings of hepatic artery thrombosis are absent color power doppler and spectral flow in the main or intra - hepatic arteries . hepatic arterial collaterals may develop in chronic thrombosis and demonstrate low intrahepatic arterial ris , mimicking stenosis . therefore , the sensitivity of ultrasound for the detection of hepatic artery thrombosis decreases as the interval following transplant increases . with hemodynamically significant hepatic artery stenosis , a segmental increase in flow velocity is found at the post - stenotic jet . a focal peak systolic velocity ( psv ) in the main hepatic artery of greater than 2.0 m / s indicates significant stenosis . color doppler imaging can be used to rapidly identify the line of maximal velocity associated with the stenotic site , by carefully reducing the color doppler scale to produce the slightest degree of aliasing , which can be recognized as a change in doppler color at the brightest portion of the color spectrum . intrahepatic arteries may have a tardus - parvus waveform with a prolonged systolic upstroke acceleration time and low intrahepatic ri(s ) of less than 0.50[figure 2 ] . in a recent study by park et al , the use of post - stenotic hepatic arterial flow velocity in conjunction with the finding of a tardus - parvus wave pattern can significantly increase the positive predictive value of doppler for diagnosing hepatic artery stenosis . pseudoaneurysms are a rare but potentially catastrophic complication of hepatic transplantation , often occurring at the anastomotic site . they may develop secondary to infection , graft biopsy , or transhepatic catheter / drain placement . they are often clinically occult or may present with nonspecific graft dysfunction or abnormally elevated serum liver enzyme levels . contrast - enhanced ct is superior to sonography in identifying small pseudoaneurysms , as bowel gas may obscure a tortuous extrahepatic hepatic artery . ct angiography or conventional angiography is performed if there is suspicion of graft - related bleeding or unexplained graft malfunction . the donor portal vein is usually anastomosed end - to - end to the recipient portal vein . however , if there is pre - existing thrombosis of the recipient portal vein , thrombectomy or placement of a jump graft between the donor portal vein and recipient superior mesenteric or splenic vein can be performed . mild focal narrowing at the anastomosis is a common finding which does not necessarily imply stenosis . on doppler examination turbulent flow with elevated portal venous flow velocities may be present in the early post - operative period , decreasing over time in the absence of significant stenosis . portal vein stenosis and thrombosis are rare complications with an incidence of 1 - 2% . predisposing factors include previous portal vein surgery or thrombosis , high downstream resistance due to an inferior vena cava ( ivc ) stricture , hypercoagulable state , and technical problems during surgery such as mismatched vessel size or vascular kinking . treatments include angioplasty , stent placement , thrombolysis , surgical repair with thrombectomy , venous jump graft or portosystemic shunt creation . gray - scale findings of portal vein thrombosis may include a hyperechogenic clot ; however , because acute thrombus can be isoechoic or hypoechoic , color doppler imaging should be performed . with acute thrombosis , the portal vein distends ; in chronic thrombosis , the vessel caliber decreases . very slow portal venous flow may simulate thrombosis , even with power doppler , and so further evaluation with other imaging modalities such as ct , mri or angiography may be appropriate . if arterial flow is detected within the clot , tumor recurrence must be suspected . sonographic findings of portal vein stenosis include vascular narrowing with segmental color doppler aliasing and post - stenotic dilatation . duplex imaging reveals at least a 3- to 4-fold velocity gradient across the stenosis[figure 3 ] . in a study by mullan et al , a peak portal vein flow velocity of 80 cm / s diagnosed portal vein stenosis with a sensitivity of 100% and specificity of 84% . in a study by chong et al , a peak portal vein velocity of greater than 125 cm / s had only 73% sensitivity but a specificity of 95% . with chronic portal thrombosis and cavernous transformation , an attempt should be made to differentiate large venous collaterals in the porta hepatis from the main portal vein and intrahepatic branches . ( a ) gray scale shows narrowing at the portal vein anastomosis ( arrow ) . ( d ) spectral doppler with high velocity at the post - stenotic jet , which is 3.5 times higher than peak velocity in the pre - stenotic portal vein . decreased portal venous flow velocity may be seen downstream from portal vein stenosis , but it can also be associated with increased hepatic resistance secondary to rejection or recurrent parenchymal disease . vascular steal phenomenon directing portal flow away from the graft may occur when large pre - transplant portosystemic shunts , commonly splenorenal , are not ligated at the time of transplant . in vascular steal , the portal venous flow may have decreased velocity or variable direction . a pulsatile portal venous waveform may be normal in the early post - operative period . if there is cardiac disease or arterioportal shunting , the pulsatile waveform will persist . in orthotopic liver transplantation , there are two main surgical techniques for reestablishing the venous outflow tract . in one , the intrahepatic segment of the recipient inferior vena cava is explanted with the diseased liver and replaced en bloc with the donor intrahepatic cava and graft . anastomosis usually is performed end - to - end , at both the suprahepatic and infrahepatic segments of the recipient 's residual ivc [ figure 4a ] . ( b ) piggyback technique : the donor ivc or hepatic vein is anastomosed to the recipient common hepatic vein stump ( arrow ) . an alternative transplant procedure focuses on maintaining an intact recipient inferior vena cava avoiding venovenous bypass intraoperatively . in the piggyback technique , a widely used form of caval - preserving surgery , a short segment of the donor ivc is anastomosed to the common trunk of the recipient 's main hepatic veins [ figure 4b ] . in partial hepatic transplants , one of several techniques can be used to anastomose the donor hepatic vein(s ) to the recipient ivc . complications involving the venous outflow tract are uncommon , reportedly occurring in < 1 - 10% depending on whether the procedure is orthotopic or partial . patients can develop stenosis or thrombosis of the inferior vena cava or main hepatic veins , most commonly at an anastomotic site . patients with inferior vena caval or hepatic venous complications may present with pain , ascites , hepatomegaly , pleural effusion , peripheral edema , or abnormal serum liver enzyme levels . caval obstruction can result in renal failure . in the acute post - surgical period , caval stenosis is usually due to anastomotic stricture or caval compression from graft swelling , hematoma or other post - operative fluid collection . with pediatric or living - related donor grafts , vascular redundancy and graft rotation in the latter group may result in venous kinking or frank torsion . delayed stenosis can result from patient growth or graft enlargement , fibrosis , chronic thrombus , or neointimal hyperplasia . , findings of venous outflow compromise may include caval or anastomotic narrowing with pre - stenotic dilatation of the ivc and/or hepatic veins . color doppler imaging frequently reveals vascular narrowing and aliasing at the stenotic site where flow velocities are abnormally elevated . isoechoic or hypoechoic thrombus , not visible on gray - scale imaging , may be identified as an intravascular filling defect with color doppler [ figure 5 ] . 68-year - old man three months status post liver transplant with ivc clot demonstrated with color doppler ( arrow ) . duplex / spectral imaging typically demonstrates dampening with loss of pulsatility of the normally triphasic flow wave patterns in the pre - stenotic cava and/or hepatic veins . a monophasic wave pattern in the cava and hepatic veins is nonspecific , often seen in the acute post - surgical period [ figure 6 ] . even a persistent monophasic flow wave pattern in the hepatic veins is non - specific for venous complications , as it is also seen with graft rejection or recurrent parenchymal disease . post - transplant caval or hepatic venous triphasic wave pattern has a high negative predictive value for downstream stenosis and graft rejection . with hemodynamically significant stenosis , there is an at least 3- to 4-fold flow velocity gradient across the stenosis [ figure 7 ] . ( b ) same patient one month after surgery with spectral doppler showing normal triphasic hepatic venous flow . the spectral waveform is dampened with maximum flow velocity measured at 20 cm / s . ( c ) spectral doppler shows dampened waveform and increased flow velocity at the post - stenotic jet 4.8 times higher than the maximum flow velocity in the pre - stenotic ivc . no intravascular spectral flow is demonstrated in regions of venous thrombosis . in severe cases of venous outflow stenosis or in thrombosis , there can be reversed flow in one or more main hepatic veins or in the portal vein . post contrast computed tomography / angiography ( ct / cta ) or magnetic resonance imaging / angiography / cholangiopancreatography ( mri / mra / mrcp ) are ancillary modalities used to evaluate vascular and nonvascular complications of hepatic transplants [ 
       figure 1 ] . these imaging techniques are especially valuable in patients with complex , atypical , or obscured post - surgical anatomy on sonography , or in patients with inconclusive or conflicting ultrasound findings . these include demonstrating  nutmeg  enhancement pattern of the congested liver in the setting of venous outflow obstruction , hepatic infarction 
       [ figure 8 ] , and biliary abnormalities . these modalities also demonstrate the extent of post - operative fluid collections and other abnormalities , which may be partially obscured by gaseous interference on sonography 
       [ figure 9 ] . ( b ) axial pre - contrast fat saturated t1-weighted mri image demonstrates hemorrhagic infarction in the right hepatic dome evidenced by increased signal ( arrows ) . ( c ) axial post - contrast t1-weighted mri image with subtraction showing no enhancement in the infarcted area ( arrows ) . 52-year - old man s / p liver transplant with extrinsic portal vein compression by an inflammatory mass . mri axial t2-weighted images shows ( a ) signal void of the pre - stenotic main portal vein ( white arrow ) at the confluence of splenic vein ( black arrow ) and superior mesenteric vein . ( b ) an inflammatory mass which is hyperintense on t2-weighted images ( small black arrows ) causes extrinsic compression of the main portal vein evidenced by decreased caliber of signal void ( long white arrow ) . ( c ) signal void of the post - stenotic portal vein at the hilum ( arrow ) . conventional angiography is generally used for problem solving and in preparation for intended vascular interventions . during catheterization , subsequent balloon angioplasty and stent placement are frequently successful in salvaging the graft , especially if early diagnosis is made [ figure 2 ] . sonography is the primary imaging tool for diagnosis of early and delayed vascular complications in liver transplant patients . after rejection , this review of the normal sonographic post - transplant vascular anatomy and the imaging appearances of the major hepatic artery , portal vein , and venous outflow tract complications that may occur will assist the radiologist in identifying post - transplant vascular pathology . recognition of the doppler and cross - sectional imaging manifestations of normal and abnormal hepatic graft vasculature is essential for appropriate and timely management to ensure graft salvage . in cases of inconclusive ultrasound findings , a multimodality approach ,
orthotopic liver transplantation is an important treatment option for patients with end - stage liver disease . 
 advances in surgical technique , along with improvements in organ preservation and immunosuppression have improved patient outcomes . 
 post - operative complications , however , can limit this success . 
 ultrasound is the primary imaging modality for evaluation of hepatic transplants , providing real - time information about vascular flow in the graft . 
 graft vascular complications are not uncommon , and their prompt recognition is crucial to allow for timely graft salvage . 
 a multimodality approach including ct angiography , mri , or conventional angiography may be necessary in cases of complex transplant vascular anatomy or when sonography and doppler are inconclusive to diagnose the etiologies of these complications . 
 the purpose of this article is to familiarize radiologists with the normal post - transplant vascular anatomy and the imaging appearances of the major vascular complications that may occur within the hepatic artery , portal vein , and venous outflow tract , with an emphasis on ultrasound .