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With the development of new and improved imaging modalities and less invasive percutaneous techniques, diagnostic and interventional radiology have an increasing and significant role in the pre- and posttransplant evaluation and management of transplant donors and recipients.

22.1. PRETRANSPLANT EVALUATION

The main role of radiological evaluation of potential donors and recipients is to determine the presence of coexistent conditions that will increase the risk of, preclude, or alter transplantation. The multimodality approach is required due to the inherent strengths and weaknesses of each technique. The precise protocol used by different centers is variable depending on local equipment and expertise.

The pretransplant evaluation of the liver recipient involves the following modalities:

22.1.1.1. Duplex Ultrasound

Duplex ultrasound is the least invasive and probably most used imaging modality in the pretransplant evaluation. Its safety, availability, and proven ability to evaluate the hepatic parenchyma; the patency of the portal veins, hepatic artery, hepatic veins, and inferior vena cava; and biliary ductal dilatation are valuable in patient selection; it is the initial study of choice.

22.1.1.2. Computed Tomography

Preoperative, routine computed tomography (CT) scan evaluation is useful in determining the presence of ascites, varices, and other signs of portal hypertension, liver volume, and focal or diffuse lesions. CT arterial portography has aided in detecting liver neoplasms, especially metastases. However, its usefulness in liver transplantation has been limited due to altered hepatic hemodynamics secondary to cirrhosis, and it requires placement of a selective catheter in the splenic or superior mesenteric arteries. Three-dimensional, helical CT angiography (CTA) is a noninvasive technique that is useful to evaluate the normal and variant arterial supply of the liver. The need for intravenous (IV) conventional radiographic contrast may limit the use of CT scan techniques in patients with renal insufficiency or contrast allergies.

22.1.1.3. Magnetic Resonance

Magnetic resonance imaging (MRI) and angiography (MRA) techniques have evolved rapidly and complement traditional modalities. With its tissue-specific contrast agents, multiplanar imaging, and special techniques such as chemical shift and fat suppression, MRI provides improved detection and characterization of focal liver lesions, and is excellent in diagnosing fatty infiltration of the liver and hemochromatosis. Contrast-enhanced MRI can also evaluate hepatic arterial anatomy, portal vein thrombosis, and portal hypertension. However, because MRI is contraindicated in patients with magnetically, mechanically, or electrically activated implants, its application is limited.

22.1.1.4. Conventional Angiography

Visceral angiography is usually reserved as a problem-solving tool in pre-transplant liver evaluation. It is useful in delineating the arterial anatomy, and identifying and characterizing preexisting vascular disease, especially in the selection process for living related transplant donors, when noninvasive modalities such as MRA or CTA have been equivocal. As an invasive technique, it is associated with an approximately 1.7% complication rate that includes contrast reactions, puncture site hematomas, distal embolization, and catheter/guide wire-related arterial injury, among others, and it may be contraindicated in patients with renal insufficiency, coagulopathy, or conventional contrast allergy.

22.1.1.5. Scintigraphy

Hepatobiliary imaging with technetium 99m IDA (iminodiacetic acid) derivatives is valuable in diagnosing and characterizing acute and chronic biliary disease, biliary obstruction, and biliary atresia. Multiple-gated acquisition scans (MUGAs) are used to evaluate the left ventricular ejection fraction. Bone scans are useful in the workup of metastases in patients with history of hepatocellular carcinoma.

22.1.2. Kidney

The diagnostic imaging evaluation of potential renal donors has been traditionally performed with duplex ultrasound of the kidneys, intravenous urograms, and conventional arteriograms, preferably, selective renal arteriograms. However, MRA and CTA, also effective in evaluating the vascular supply of potential donor kidneys, are quickly replacing other modalities.

22.1.2.1. Duplex Ultrasound

Duplex ultrasound is a noninvasive modality that is readily available, cost-effective, and has minimal risk. It is useful in evaluating potential donor kidneys for size, hydronephrosis, radiopaque or radiolucent stones, solid or cystic masses, perirenal collections, and renal parenchymal disease.

22.1.2.2. Computed Tomography

Knowledge of the renal vascular supply is crucial for successful transplantation. Variant renal arterial anatomy has an incidence of about 40%, and about 30% of individuals have multiple renal arteries. Variant venous anatomy is also common. CTA has a sensitivity and specificity of 99.6% for main renal arteries, decreasing to 76.9% and 89.9%, respectively, for accessory arteries. It is minimally invasive and as an intrinsic advantage may identify renal calculi, but since it requires IV radiographic conventional contrast, it has limited application with renal insufficiency or radiographic contrast allergy.

22.1.2.3. Magnetic Resonance

Gadolinium-enhanced MRA, like CTA, is also minimally invasive, but it does not utilize conventional radiographic contrast and can be used in renal insufficiency. In addition, like CTA, it can detect renal lesions and venous anomalies but is less likely to identify calculi. It has a sensitivity and specificity of 71% and 95%, respectively, in detecting accessory renal arteries, and 97% and 92%, respectively, in identifying main renal arteries with 50-99% stenosis. Unfortunately, MRA is contraindicated in some patients.

22.1.2.4. Conventional Angiography

As in liver transplantation, due to alternative modalities, conventional angiography is becoming a problem-solving technique. Nevertheless, it can be performed safely and effectively on an outpatient basis with small catheters and little risk. Alternative intra-arterial contrast agents such as carbon dioxide or gadolinium can be used in patients with contraindications to conventional iodin-ated contrast.

22.2. POSTTRANSPLANT EVALUATION AND MANAGEMENT

Precise communication and coordination among the vascular/interventional radiologists, transplant surgeons, transplant hepatologists, and a nephrologist are absolutely necessary to ensure the best possible outcome for the patients.

22.2.1.1. Vascular

22.2.1.1a. Hepatic Artery Thrombosis (HAT). HAT can occur in 1042% of pediatric and 3-10% of adult liver transplants. Duplex ultrasound is the initial study of choice, with a sensitivity of 90%. Extensive arterial collaterals can result in a false-negative result. The diagnosis is suggested on the basis of absence of flow signal in the intrahepatic and porta hepatis arteries, and a tardus parvus waveform. However, due to the spectrum of abnormalities in the vasculature, visceral arteriography should be performed to confirm the diagnosis. Surgical correction is necessary. CT scan can be used to evaluate consequences of arterial thrombosis such as hepatic infarction, biloma, and abscess formation.

22.2.1.1b. Hepatic Artery Stenosis. This condition is present in 11% of transplants, and ultrasonography is the primary modality for diagnosis. The diagnosis is made when the blood velocity is > 2-3 m/sec, with turbulent flow, or if an intrahepatic tardus parvus pattern is present. As in HAT, visceral arteriography is necessary to confirm the diagnosis. Stenoses are more commonly anastomotic but can occur at adjacent sites. If detected in the immediate postoperative period, these will require surgical repair. Percutaneous transluminal angioplasty (PTA) is useful in treating stenoses 4-6 weeks posttransplant.

22.2.1.1c. Hepatic Artery Pseudoaneurysms. These are rare after liver transplants and potentially fatal if rupture occurs. They usually occur at the arterial anastomosis. They may complicate percutaneous biopsy or biliary drainages and are then intrahepatic. Patients may present with hemobilia, hemoperitoneum or gastrointestinal bleeding, or be asymptomatic, with symptoms found incidentally during routine follow-up. Both contrast-enhanced CT and ultrasound may detect pseudoaneurysms. With ultrasound, they appear as a cystic collection with swirling and disorganized flow. Arteriography shows a well-defined saccular contrast collection with persistent opacification beyond the arterial phase. If intrahepatic, they can be embolized percutaneously with coils from the intravascular approach. Those at the arterial anastomosis require surgical revision.

22.2.1.1d. Inferior Vena Cava (IVC) Thrombosis or Stenosis. This is a rare complication occurring at the anastomosis and may result in Budd-Chiari syndrome or extremity edema. Ultrasound shows focal narrowing, with three- to fourfold increase in velocity if narrowed, or intraluminal echogenic thrombus if thrombosed. The diagnosis is confirmed by venography. Percutaneous translumi-nal angioplasty is the treatment of choice. The initial technical and clinical success is 92%, but the stenosis is often recurrent, requiring reintervention.

22.2.1.1e. Portal Vein Complications. These are also rare, with an incidence rate of 1-13%. Symptoms are usually those of portal hypertension or hepatic failure. Stenosis occurs at the porta hepatis anastomosis. Ultrasound reveals an increased velocity similar to that of an IVC stenosis. If thrombosed, it will show echogenic material. If completely thrombosed, there will be no flow, although some flow can be detected if collaterals have developed. Angiography is best performed via the superior mesenteric artery with intra-arterial vasodilators and imaging into the venous phase. Venous thrombosis and arterial stenoses and occlusions can also be shown by MRA. The treatment of choice for portal vein stenosis is PTA via transhepatic approach. Thrombosis is best managed surgically. Percutaneous thrombolysis and stent placement may be possible.

22.2.1.2. Biliary

Biliary complications can be seen in 12-38% of liver transplant patients.

22.2.1.2a. Bile Leakage. This complication is the most common originating from the T-tube site, choledochocholedochostomy, or choledochojejunos-

tomy. Leaks at sites other than the T-tube insertion or biliary anastomosis may be associated with arterial thrombosis. Patients may present with signs of infection or abnormally elevated bilirubin or liver enzymes. If placed operatively, a T-tube cholangiogram is the best way to evaluate the biliary tree for leaks. Preprocedure antibiotics are given, and contrast is injected by gravity drip. Ultrasound and CT scan may identify indirect evidence of a leak, such as fluid collection that could represent a biloma, and may be used as guidance for percutaneous diagnostic needle aspiration or therapeutic drainage. If a T tube is not present, hepatobiliary imaging with technetium 99m IDA derivatives is useful in detecting bile leaks by showing extraluminal radionuclide. If the cause of the biliary leak is arterial thrombosis, diversion and external drainage are a temporary solution and retransplantation is required. However, many bile leaks may also be managed nonsurgically by draining the biloma percutaneously and diverting bile flow by opening the T tube to external drainage, repositioning the T tube, placing a percutaneous biliary drainage catheter, or endoscopically by placing a nasobiliary tube or stent. Prolonged drainage is required, since leaks may take 1 to 2 months to heal.

22.2.1.2b. Anastomotic and Nonanastomotic Biliary Strictures. These are relatively common, occurring in 4-17% of grafts. Anastomotic strictures, which result from scarring at the suture line, are probably the result of a technical factor, although ischemia could be a contributing factor, and are more commonly seen in patients with sclerosing cholangitis. Nonanastomotic strictures result from bile duct ischemia due to arterial insufficiency caused by hepatic artery stenosis or thrombosis, and may be hilar or intrahepatic. However, other causes include prolonged cold ischemia, chronic ductopenic rejection, ABO blood type incompatibility, infection, and sclerosing cholangitis. CT scan and ultrasound can reveal biliary ductal dilatation associated with strictures. Cholangiography, whether retrograde, endoscopic, or antegrade via direct transhepatic or T-tube route, is most accurate in depicting the extent and location of strictures. However, magnetic resonance cholangiography (MRC) is increasingly able to depict and detect stenoses and occlusions, although it may overestimate the degree of narrowing. Simple anastomotic strictures, whether endoscopic or percutaneous, can be treated by cholangioplasty and stenting. The stent is usually required for extended periods of 6 months or more.

22.2.1.2c. Biliary Obstruction. Several causes of biliary obstruction may complicate liver transplantation. Intrahepatic and extrahepatic stones can be readily diagnosed with ultrasound, cholangiography, or MRC. Small stones can be removed endoscopically. Larger stones can be removed percutaneously in staged procedures or surgically. Strictures can be treated with cholangioplasty, as described earlier. Rarely, obstruction can result from a cystic duct remnant mucocele that can be suggested by CT scan and ultrasound but may require needle aspiration for diagnosis. Therapy of choice is surgery.

22.2.1.3. Rejection

Currently, there are no reliable, noninvasive tests to diagnose organ rejection. Graft biopsy and histological examination remain the standard. Biopsies are performed percutaneously via the transabdominal approach using ultrasound guidance. In patients with severe ascites, coagulopathy, or thrombocytopenia as contraindications for transabdominal biopsy, a transjugular liver biopsy approach is a safe and effective alternative even in the early postoperative period.

22.2.2. Kidney

22.2.2.1. Vascular

22.2.2.1a. Renal Artery Stenosis. This can be present in up to 10% of transplant patients. Early stenosis at the anastomosis is probably a technical problem. Later, distal stenoses could result from rejection or turbulence and may present as hypertension. Ultrasound is a noninvasive modality useful in screening for arterial stenosis. Specific Doppler criteria, such as increased velocity, velocity gradients, and spectral broadening, have been developed. Scintigraphy can also be used to diagnose arterial stenosis. Findings are similar to chronic rejection except when performed with angiotensin-converting enzyme inhibitors, when the condition resembles renal artery stenosis in native kidneys. MRA can also accurately depict stenoses. A conventional arteriogram can then be performed. Carbon dioxide angiography is an alternative for patients with renal insufficiency or contrast allergy. Stenoses should be treated with percutaneous angioplasty, with surgery for recurrent stenoses. Percutaneous intravascular stent placement is another option.

22.2.2.1b. Renal Artery Thrombosis. This is rare and present in < 1% of patients. Ultrasound shows no intrarenal arterial and venous flow. Magnetic resonance can identify parenchymal areas of necrosis and absence of flow in the vessel. Findings can be confirmed with angiography. Surgical exploration is indicated.

22.2.2.1 c. Renal Vein Stenosis. This condition is rare. It may result from perivascular fibrosis or compression from adjacent collection. Sonography shows increased velocities similar to arterial stenosis. Venography can be performed, if necessary, to confirm the diagnosis.

22.2.2.1d. Renal Vein Thrombosis. More common in diabetics, renal vein thrombosis may be related to cyclosporine use, presenting within a week after transplant as oliguria and an enlarged, painful kidney. Ultrasound shows an enlarged kidney with a reversed diastolic arterial waveform and absent venous flow. Thrombus may be identified in an enlarged vein. MRI shows an enlarged kidney with areas of infarction, lack of enhancement on MRA, and nonvisualiza-tion of the renal vein. A surgical thrombectomy may sometimes be possible.

22.2.2.1e. Pseudoaneurysms and Arteriovenous (AV) Fistulas. These can result from surgical technique, percutaneous biopsy, or infection. Small intrarenal pseudoaneurysms may resolve spontaneously. Larger lesions can cause ischemia, hematuria, or hemoperitoneum. Pseudoaneurysms mimic a cyst on ultrasound and have disorganized flow. AV fistulas in ultrasonography have a feeding artery with a high-velocity, low resistance waveform and a pulsatile draining vein. Intraparenchymal pseudoaneurysms may be embolized percutane-ously.

22.2.2.2. Nonvascular

22.2.2.2a. Ureteric Leak and Obstruction. These are the two principal urologic complications of renal transplants. Obstruction early posttransplant is usually secondary to twisting of the ureter or a technical problem at the ureter implantation site. It may also result from extrinsic ureteric compression from adjacent fluid collections. Obstruction late after transplantation is probably secondary to a ureteric stricture resulting from an ischemic damage. Patients may present with oliguria and worsening renal function. Ultrasonography suggests the diagnosis by demonstrating dilated calyces and sometimes a dilated ureter. Echogenie material in the collecting system could represent infection, clots, or stones. An antegrade pyelogram may be required to confirm the diagnosis. Preprocedure antibiotics must always be given, and ultrasound guidance facilitates access. Strictures can be treated by balloon dilatation and stent placement either from the percutaneous antegrade or endoscopic retrograde approach. With the percutaneous approach, results are best when treating obstruction within 3 months of transplantation. Surgical revision may be required. Urine leaks are more common at the distal ureter and may present as fever, pain, oliguria, or wound discharge. Ultrasound and antegrade or retrograde pyelogram will establish the diagnosis. Anechoic or hypoechoic collections will be present per sonography. Pyelogram will show diffuse or contained extraluminal contrast. Scintigraphy can also show radiotracer in unusual areas. Treatment is the same as that for ureteral obstruction with placement of stents. Percutaneous drainage of focal urine collections must also be performed simultaneously.

22.2.2.2b. Perinephric Fluid Collections. Lymphoceles, urinomas, seromas, abscesses, and hematomas may complicate renal transplants. Although their appearance is nonspecific, some features may suggest their diagnosis. Lymphoceles have an incidence rate of around 5%. Their appearance in ultrasound and MRI is that of a simple fluid collection that may contain septations similar to urinomas. Lymphoceles' creatinine concentration may be similar to that of serum, which can be used to differentiate them from urinomas. Most are small, but large lymphoceles can cause mass effects on adjacent structures, such as the ureter, and result in obstruction. Percutaneous transcatheter sclerotherapy with povidone-iodine, alcohol, or bleomycin has been successful. Surgery may be required. Hematomas are easily diagnosed with MRI on the basis of their characteristic appearance on different pulse sequences.

22.2.2.2c. Diminished Renal Function. There are multiple causes of diminished renal function. Transplants affected by hyperacute rejection fail within minutes, are usually removed immediately, and are rarely imaged. Acute rejection (AR) typically occurs 1 to 3 weeks after transplantation and is present in 40-50% of transplants with chronic rejection (CR) occurring 3 months after transplantation. Acute tubular necrosis (ATN) may result from prolonged ischemia and typically appears within the first week, gradually resolving over 2 weeks. Clinical evolution and serial imaging studies are important in differentiating among these entities. Radionuclide scan findings for ATN and AR are similar: marked parenchymal retention and, in severe cases, no excretion into the bladder. They can be separated by their time course. ATN is present in the baseline study and improves over 2 weeks; AR rarely develops in the first week and function deteriorates on serial studies. In chronic rejection (CR), the renal cortex is thinned and there is mild hydronephrosis. There is decreased uptake, normal parenchymal transit, and absent/minimal cortical retention. Cyclosporine toxicity resembles rejection on radionuclide imaging studies. Differentiation is best with cyclosporine level and clinical correlation and serial studies. Ultrasound and MRI have not been reliable in differentiating among types of diminished renal function.

22.2.3. Pancreas

The radiologist's main role in pancreas transplant is in the diagnosis, and to a lesser extent, management of postoperative complications.

22.2.3.1. Vascular Thrombosis

The incidence rate of pancreas graft thrombosis (arterial and venous) is 514%, with venous thrombosis rates of 5%. Differences in the clinical presentation of patients with arterial versus venous thrombosis may suggest the diagnosis. Gadolinium-enhanced MRA is sensitive for detecting vascular compromise. Ultrasound is sensitive and specific in diagnosing venous thrombosis, using as criteria the absence of venous flow and a resistive index 5= 1. CT angiography or selective conventional arteriography can also be used and may be necessary for diagnosis.

22.2.3.2. Rejection

Ultrasound has not been reliable in the diagnosis of graft rejection. MR can identify grafts with infarction and is sensitive for detecting acute rejection but lacks specificity. Percutaneous biopsy is safe and may be needed to establish the diagnosis.

22.2.3.3. Bladder Leak

Bladder and duodenal segment leaks can be seen in 10-20% of bladder-drained pancreas transplants. Ultrasound, CT scan, and MRI can identify abdominal fluid collections that could result from leaks. Ultrasound is limited by overlying abdominal gas. Conventional or scintigraphic cystograms can confirm the diagnosis.

22.2.3.4. Perigraft Collections

Pancreatic pseudocysts, abscesses, hematomas, and urinomas can develop posttransplant. Ultrasound, CT scan, and MRI can identify the collections. Percutaneous drainage may be necessary. Guidance for drainage is most easily performed with ultrasound or CT scan.

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