(Radiographics. 2000;20:607-622.)
© RSNA, 2000
Complications of Renal Transplantation: Evaluation with US and Radionuclide Imaging1
Elizabeth D. Brown, MD,
Michael Y. M. Chen, MD,
Neil T. Wolfman, MD,
David J. Ott, MD and
Nat E. Watson, Jr, MD
1 From the Department of Radiology, Wake Forest University School of Medicine, Medical Center Blvd, Winston-Salem, NC 27157-1088. Presented as a scientific exhibit at the 1998 RSNA scientific assembly. Received March 30, 1999; revision requested May 12 and received June 7; accepted June 9. Address reprint requests to M.Y.M.C. (e-mail: mchen@wfubmc.edu).
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Abstract
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Following renal transplantation, patients are often evaluated with ultrasonography (US) or radionuclide imaging to assess renal function and the presence of possible complications. Both modalities are inexpensive, noninvasive, and nonnephrotoxic. A basic understanding of the surgical techniques commonly used for renal transplantation is useful when imaging these patients in order to recognize complications and to direct further imaging or intervention. The most frequent complications of renal transplantation include perinephric fluid collections; decreased renal function; and abnormalities of the vasculature, collecting system, and renal parenchyma. Perinephric fluid collections are common following transplantation, and their clinical significance depends on the type, location, size, and growth of the fluid collection, features that are well-evaluated with US. Causes of diminished renal function include acute tubular necrosis, rejection, and toxicity from medications. Radionuclide imaging is the most useful modality for assessing renal function. Vascular complications of transplantation include occlusion or stenosis of the arterial or venous supply, arteriovenous fistulas, and pseudoaneurysms. Although the standard for evaluating these vascular complications is angiography, US is an excellent noninvasive method for screening. Other transplant complications such as abnormalities of the collecting system and renal parenchyma are well-evaluated with both radionuclide imaging and US.
Index Terms: Kidney, radionuclide studies 81.12172 • Kidney, transplantation, 81.455 • Kidney, US, 81.12983 • Transplantation, 81.455
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LEARNING OBJECTIVES FOR TEST 1
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After reading this article and taking the test, the reader will be able to:
• Describe the most common complications of renal transplantation.
• Analyze the US and radionuclide imaging findings of these renal transplant complications.
• Discuss which complications need additional evaluation or treatment.
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Introduction
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Renal transplantation has become the treatment of choice for end-stage renal disease. With improved transplantation technology and new immunosuppressive agents such as cyclosporine, OKT3, and FK506, 1-year survival rates for grafts are reported to be 80% for mismatched cadaveric renal grafts; 90%, nonidentical living related grafts; and 95%, human lymphocyte antigen–identical grafts. The half-life of grafts from living related donors varies between 13 and 24 years, depending on the match (1).
Ultrasonography (US) is often the imaging method chosen for transplant evaluation early in the postoperative period, and it can be used for long-term follow-up as well. US also is used to guide diagnostic and therapeutic interventions, such as biopsy or fluid aspiration, when complications of transplantation develop. Radionuclide imaging is an excellent modality for assessing graft function, both qualitatively and quantitatively, while screening for common complications.
In this article, we discuss the general surgical techniques of renal transplantation as a basis for understanding potential complications, which include perinephric fluid collections; decreased renal function; and abnormalities of the vasculature, collecting system, and renal parenchyma. The most common complications are described, and their appearances on US and radionuclide images are illustrated.
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Surgical Techniques
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Surgical techniques for renal transplantation vary by institution. The right or left lower quadrant is chosen for the incision, based on prior surgery or preference of the surgeon. Preservation of the hilar fat and the adventitia surrounding the ureter at the time of organ harvesting helps maintain the blood supply. Division of the epigastric artery, round ligament, or spermatic cord may be necessary to provide adequate space for the graft. Most commonly, the external iliac artery and vein are chosen for anastomosis. The arterial supply is created from an end-to-side anastomosis with the external iliac artery. Multiple renal arteries can be anastomosed as a Carrel patch, joined together, or anastomosed separately. The venous anastomosis with the external iliac vein is also an end-to-side type and is done with nonabsorbable monofilament sutures (2) (Fig 1). Small accessory renal veins can be sacrificed because of internal collateral vessels.
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Figure 1. Drawing shows the typical end-to-side anastomosis used in renal transplantation: renal artery (RA) and vein (RV) to the external iliac artery (EIA) and vein (EIV). Ao = aorta, IVC = inferior vena cava, K = renal transplant.
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The ureteral neocystostomy is then performed in such a way as to prevent reflux to the transplant. In complex cases or in patients undergoing repeat surgery on the collecting system, the recipient's native ureter may be used as a conduit to the bladder.
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Complications
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Perinephric Fluid Collections
Postoperative fluid collections are common following transplantation and include hematomas, seromas, urinomas, lymphoceles, and abscesses. The appearance of peritransplant fluid collections is nonspecific, but differentiation of the fluid type may be attempted based on the radiologic appearance of the collection and the postoperative interval. Ultimately, the diagnosis can be made by using percutaneous aspiration of the fluid as needed. The appearance and complications of a fluid collection depend on its composition as well as its location. For example, blood surrounding an extraperitoneal transplant may be echogenic or may have multiple septations and result in compression of the renal parenchyma (Fig 2), whereas hemorrhage in the setting of an intraperitoneal transplant likely results in free fluid without mass effect (3).
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Figure 2. Hematoma in a patient who presented with pain over the graft after falling against a bed rail. Longitudinal US image shows an echogenic mass (arrowheads) surrounding the renal transplant.
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Hematomas.—Small crescentic peritransplant fluid collections seen immediately after transplantation are most likely hematomas or seromas and should be considered a normal sequela of surgery (4) (Fig 3). Size, location, and growth determine the significance of these collections. Because an increase in size may indicate the need for surgical intervention, the size of any such collections should be documented on the baseline US scan. More complex collections identified later in the postoperative period with clinical evidence of infection may represent abscesses (Fig 4).
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Figure 3. Color Doppler US image obtained immediately after transplantation shows a small fluid collection (arrow). This is an expected finding in the early posttransplantation period.
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Figure 4. Abscess in a patient who presented with fever and elevated white blood cell count. Longitudinal US image shows a complex fluid collection (arrows) superficial to the renal transplant.
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Urinomas.—Urine leaks are relatively rare complications following transplantation and manifest in the early postoperative period with pain, swelling, and discharge from the wound. Leaks at the ureterovesicle anastomosis are related to surgical technique or distal ureteral necrosis. Urine leaks elsewhere in the collecting system usually develop secondary to ischemia, with resulting necrosis of the collecting system that is either limited to a small area or involves the entire ureter. Ischemia may result from harvesting of the graft, anastomotic technique, vascular supply variation, rejection, or medication.
Although the appearance of urinomas is nonspecific on US images, internal septations are seen less often than in hematomas (4). Radionuclide imaging studies will show progressive radiotracer activity in the abnormal collection, definitively demonstrating that the fluid is urine (5) (Fig 5).
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Figure 5a. Urinoma. Anterior (a) and right lateral (b) views from a technetium-99m mercaptoacetyltriglycine (MAG3) study show abnormal radionuclide activity around the transplant (arrows).
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Figure 5b. Urinoma. Anterior (a) and right lateral (b) views from a technetium-99m mercaptoacetyltriglycine (MAG3) study show abnormal radionuclide activity around the transplant (arrows).
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Lymphoceles.—Lymphoceles usually occur 4–8 weeks after surgery and affect up to 15% of patients. The cause of these collections is likely the disruption of the normal lymphatic channels during perivascular dissection or disruption of hilar lymphatics. Most lymphoceles are discovered incidentally, are asymptomatic, and do not require therapy. However, because of their potential to exert mass effect, lymphoceles can impair renal function by producing hydronephrosis or can cause conditions such as edema of the leg, abdominal wall, scrotum, or labia (2).
On US images, a rounded collection may be seen along the mid-ureter, associated with hydronephrosis (Figs 6–8). In the patient with leg swelling, US performed to exclude a deep venous thrombosis may show extrinsic compression of the external iliac vein by a rounded fluid collection (4). On radionuclide images, a large photopenic region can be seen to exert a mass effect on the transplant (Fig 9).
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Figure 7. Hydronephrosis secondary to a lymphocele. Color Doppler US image shows the hydronephrotic kidney adjacent to a large fluid collection, seen inferior to the kidney and superior to the bladder (arrow).
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Figure 8a. Lymphocele. (a) Transverse US image of the midpole region of a renal transplant shows a large anechoic fluid collection adjacent to the renal hilum. (b) Longitudinal color Doppler image shows hydronephrosis resulting from extrinsic compression of the collecting system. (c) Transverse US image obtained after repair of the lymphocele shows that the hydronephrosis has decreased and the fluid collection is absent.
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Figure 8b. Lymphocele. (a) Transverse US image of the midpole region of a renal transplant shows a large anechoic fluid collection adjacent to the renal hilum. (b) Longitudinal color Doppler image shows hydronephrosis resulting from extrinsic compression of the collecting system. (c) Transverse US image obtained after repair of the lymphocele shows that the hydronephrosis has decreased and the fluid collection is absent.
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Figure 8c. Lymphocele. (a) Transverse US image of the midpole region of a renal transplant shows a large anechoic fluid collection adjacent to the renal hilum. (b) Longitudinal color Doppler image shows hydronephrosis resulting from extrinsic compression of the collecting system. (c) Transverse US image obtained after repair of the lymphocele shows that the hydronephrosis has decreased and the fluid collection is absent.
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Figure 9a. Lymphocele. (a) Views from a Tc-99m MAG3 study demonstrate the appearance of the normal transplant in the right lower quadrant. (b) Views from a repeat study performed 2 months later show that the upper pole of the transplant is compressed by a large photopenic defect (arrows).
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Figure 9b. Lymphocele. (a) Views from a Tc-99m MAG3 study demonstrate the appearance of the normal transplant in the right lower quadrant. (b) Views from a repeat study performed 2 months later show that the upper pole of the transplant is compressed by a large photopenic defect (arrows).
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Lymphoceles can be treated with either percutaneous or surgical techniques. Percutaneous therapy varies from simple aspiration to placement of a drain (Fig 10), with or without sclerotherapy. Recurrence rates are reported to be lower with use of a sclerosing agent such as povidone-iodine (6) or ethanol (7). Surgical techniques include open or laparoscopic marsupialization of the lymphocele into the peritoneal cavity (8).
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Figure 10a. Percutaneous drainage of a lymphocele. (a) Longitudinal US image obtained during needle placement shows the echogenic needle within the fluid collection (arrow). (b) On a US image obtained after placement of the pigtail catheter, the catheter can be seen as a larger echogenic structure coursing into the collection (arrow).
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Figure 10b. Percutaneous drainage of a lymphocele. (a) Longitudinal US image obtained during needle placement shows the echogenic needle within the fluid collection (arrow). (b) On a US image obtained after placement of the pigtail catheter, the catheter can be seen as a larger echogenic structure coursing into the collection (arrow).
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Diminished Renal Function
Causes of diminished renal function include acute tubular necrosis, rejection (hyperacute, acute, and chronic), and drug nephrotoxicity.
Acute Tubular Necrosis.—Acute tubular necrosis is the most common cause of "delayed graft function," which is defined as the need for dialysis in the first week following transplantation (9). Its causes include prolonged ischemia (cold or warm) and reperfusion injury. Theoretically, kidneys can be stored for 48 hours at 4° C following perfusion, but cold ischemia times of more than 24–30 hours result in a higher frequency of acute tubular necrosis (10). Acute tubular necrosis is initially present in most cadaveric grafts and resolves spontaneously over the first 2 weeks, depending on the degree of ischemic insult. It is infrequently seen in patients whose transplants are from living related donors, presumably because of the diminished cold ischemia time of the transplants. With radionuclide imaging (iodine-131 orthoiodohippurate and Tc-99m MAG3), the most conspicuous findings are delayed transit with delayed time to maximal activity (T-max), delayed time from maximum to one-half maximal activity (T-1/2), and a high 20 to 3 minute ratio. On sequential images, marked parenchymal retention is seen (5) (Figs 11, 12).
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Figure 11a. Acute tubular necrosis following transplantation with a cadaveric graft. (a, b) Views from a Tc-99m MAG3 study obtained at 16 minutes (a) and at 65 minutes (b) show the increasing parenchymal activity in the graft. (c) Time/activity curve shows the delayed time to peak activity.
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Figure 11b. Acute tubular necrosis following transplantation with a cadaveric graft. (a, b) Views from a Tc-99m MAG3 study obtained at 16 minutes (a) and at 65 minutes (b) show the increasing parenchymal activity in the graft. (c) Time/activity curve shows the delayed time to peak activity.
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Figure 11c. Acute tubular necrosis following transplantation with a cadaveric graft. (a, b) Views from a Tc-99m MAG3 study obtained at 16 minutes (a) and at 65 minutes (b) show the increasing parenchymal activity in the graft. (c) Time/activity curve shows the delayed time to peak activity.
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Figure 12a. Focal acute tubular necrosis of the upper pole following transplantation with a cadaveric graft. (a) View from a Tc-99m MAG3 study obtained at 10 minutes shows the delay of tracer activity in the upper pole of the graft (arrow). (b) Delayed view from the Tc-99m MAG3 study shows increasing parenchymal activity in the same region (arrow). (c) Time/activity curve of the upper pole region of interest demonstrates the delayed time to peak activity, with progressively increasing activity over 15 minutes.
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Figure 12b. Focal acute tubular necrosis of the upper pole following transplantation with a cadaveric graft. (a) View from a Tc-99m MAG3 study obtained at 10 minutes shows the delay of tracer activity in the upper pole of the graft (arrow). (b) Delayed view from the Tc-99m MAG3 study shows increasing parenchymal activity in the same region (arrow). (c) Time/activity curve of the upper pole region of interest demonstrates the delayed time to peak activity, with progressively increasing activity over 15 minutes.
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Figure 12c. Focal acute tubular necrosis of the upper pole following transplantation with a cadaveric graft. (a) View from a Tc-99m MAG3 study obtained at 10 minutes shows the delay of tracer activity in the upper pole of the graft (arrow). (b) Delayed view from the Tc-99m MAG3 study shows increasing parenchymal activity in the same region (arrow). (c) Time/activity curve of the upper pole region of interest demonstrates the delayed time to peak activity, with progressively increasing activity over 15 minutes.
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Rejection.—In hyperacute rejection, the graft fails immediately at vascular anastomosis and is removed. For this reason, transplants affected by hyperacute rejection are rarely imaged. Acute rejection is relatively common following transplantation, with up to 50% of patients experiencing at least one episode in the first year. Patients may present with malaise, fever, weight gain, or a painful kidney. However, patients receiving cyclosporine may be asymptomatic (10). The radionuclide imaging findings of acute rejection are characterized by diminished flow (5). These findings are similar to those of acute tubular necrosis, and the two entities can be differentiated by the time course of the findings. Acute rejection rarely develops in the first few days after transplantation and instead will manifest with a decrease in function on serial radionuclide imaging studies. Thus, obtaining a baseline scan is extremely important.
Chronic rejection is the most common cause of late graft loss. Renal function progressively declines and eventually fails. The diagnosis is made histologically by demonstrating proliferation of graft arteries and arterioles, interstitial cellular infiltration and fibrosis, tubular atrophy, and glomerular changes (10). A graft with chronic rejection will have a thin cortex and mild hydronephrosis on both gray-scale US and radionuclide images. Chronic rejection is characterized by diminished uptake of radiopharmaceuticals (5) and also by normal parenchymal transit with absent or minimal cortical retention. When chronic rejection is advanced, there may be parenchymal retention of radiotracer (5).
Drug Nephrotoxicity.—Drug nephrotoxicity is another cause of diminished renal function. Many drugs are potentially nephrotoxic, and their effects are accentuated by dehydration and decreased renal perfusion. Cyclosporine has the greatest nephrotoxic potential, with its vasoconstrictive effect on the afferent glomerular arterioles (10). On radionuclide images, acute cyclosporine toxicity resembles mild acute rejection, with depressed effective renal plasma flow and parenchymal retention. Patients with a toxic reaction to cyclosporine have radionuclide imaging findings similar to those of chronic rejection, with decreased effective renal plasma flow and no parenchymal retention (5). Findings should be correlated with cyclosporine levels. In the short term, nephrotoxicity from cyclosporine is dose dependent and responds to a reduction in dosage (10).
Differential Diagnosis with US.—US findings of diminished renal function include renal enlargement, increased cortical thickness, increased or decreased echogenicity of the renal cortex, loss of corticomedullary differentiation (Fig 13), prominent pyramids (Fig 14), collecting system thickening, and effacement of the central sinus echo complex. However, gray-scale US findings are subjective, and the negative predictive values vary between 17% and 50%. Although duplex Doppler US has not proved to be as accurate in the evaluation of transplant rejection as initially thought, most groups now use an elevated resistive index ( > 0.8) as a nonspecific parameter of renal transplant dysfunction; reversal of diastolic flow may also be seen in rare cases (Figs 15 –18). Differentiation among acute tubular necrosis, acute interstitial rejection, acute cellular rejection, chronic rejection, and cyclosporine toxicity is usually made by using percutaneous US-guided biopsy.
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Figure 15. Normal transplant. On a duplex color Doppler US image of a normal transplant, the spectral waveform shows a brisk systolic upstroke and high diastolic flow. Resistive index is normal.
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Figure 16. Acute tubular necrosis following transplantation with a cadaveric graft. Duplex color Doppler US image obtained 2 days after transplantation shows a spectral waveform with a brisk systolic upstroke and low end-diastolic flow, an appearance typical for acute tubular necrosis. Resistive index is elevated.
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Figure 18a. Severe transplant rejection. (a) Duplex color Doppler US image shows a spectral waveform in which the arterial flow in diastole is reversed. Differential diagnosis for this finding includes acute tubular necrosis and renal vein thrombosis. (b) On another duplex image, the spectral waveform shows that the renal vein is patent, thus the diagnosis of renal vein thrombosis is excluded. Findings from biopsy confirmed transplant rejection.
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Figure 18b. Severe transplant rejection. (a) Duplex color Doppler US image shows a spectral waveform in which the arterial flow in diastole is reversed. Differential diagnosis for this finding includes acute tubular necrosis and renal vein thrombosis. (b) On another duplex image, the spectral waveform shows that the renal vein is patent, thus the diagnosis of renal vein thrombosis is excluded. Findings from biopsy confirmed transplant rejection.
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Vascular Complications
Vascular complications are seen in less than 10% of renal transplant recipients, but they are an important cause of graft dysfunction (11). In contrast to other causes of transplant dysfunction, vascular complications have a high associated morbidity and mortality. Once identified, vascular lesions are usually easily repaired.
Although angiography remains the standard for diagnosis of vascular complications, US performed with duplex and color Doppler modes is an excellent noninvasive modality for evaluating the affected vessels (4,12). A variety of radionuclides can be used to image vascular complications in the renal transplant recipient. The most commonly used agent is Tc-99m DTPA (diethylenetriaminepentaacetic acid). Image acquisition should begin immediately after intravenous injection of the agent and should include a flow phase to evaluate the vascular anatomy. Analysis of the images can be quantitative or qualitative, depending on preference of the radiologist (13).
Renal Artery Stenosis.—Renal artery stenosis is the most common vascular complication of transplantation, reported in up to 10% of patients (14,15). Evaluation for renal artery patency should be performed in patients with severe hypertension refractory to medical therapy or with hypertension combined with either an audible bruit or unexplained graft dysfunction. Moderate hypertension alone is not a reliable marker for renal artery stenosis, because up to 65% of transplant recipients have nonrenovascular hypertension.
Stenoses usually occur at the anastomosis or at the proximal donor artery and are directly related to surgical technique (14,15). Distal donor artery stenoses are less common and are thought to be caused by intimal injury from the perfusion cannula, although some researchers suggest that strictures in both proximal and distal arteries are caused by rejection. Recipient arterial stenoses are rare and result from vascular clamp injury or native atherosclerotic disease.
On radionuclide images, the findings of renal artery stenosis are similar to those of chronic rejection. With administration of angiotensin-converting enzyme inhibitors, the findings simulate those of renovascular hypertension in native kidneys (5).
Doppler US has proved to be an excellent noninvasive modality for evaluating the renal artery. Initially, the course of the artery is mapped by using color Doppler techniques. With proper adjustment of the controls, the stenotic segments will appear as regions of focal color aliasing (Fig 19), which can then be evaluated with duplex Doppler techniques to characterize and grade the abnormality (16). Doppler criteria for significant stenosis include: (a) velocities of greater than 2 m/sec or focal frequency shift of greater than 7.5 KHz (when a 3-MHz transducer is used), (b) a velocity gradient between stenotic and prestenotic segments of more than 2:1, and (c) marked distal turbulence (spectral broadening) (17,18). An observation of tardus parvus waveforms may be helpful, but they are variably present. If no significant flow abnormality is identified after a complete Doppler examination is performed, stenosis can be excluded (16).
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Figure 19a. Renal artery stenosis. (a) Color Doppler US image shows the renal artery in the region of the anastomosis. Note aliasing in the region of the stenosis (arrow). (b) Pulsed-wave color Doppler US image shows a peaked systolic waveform with short acceleration time in the stenotic segment (arrow). Velocity at the prestenotic region is 1.0 m/sec, and velocity at the stenotic segment is 2.2 m/sec.
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Figure 19b. Renal artery stenosis. (a) Color Doppler US image shows the renal artery in the region of the anastomosis. Note aliasing in the region of the stenosis (arrow). (b) Pulsed-wave color Doppler US image shows a peaked systolic waveform with short acceleration time in the stenotic segment (arrow). Velocity at the prestenotic region is 1.0 m/sec, and velocity at the stenotic segment is 2.2 m/sec.
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Angiography should be performed in patients with clinical and Doppler US findings of renal artery stenosis or in patients with clinical findings of stenosis despite normal US results. At angiography, the stenosis can be graded and possibly treated (Fig 20). Alternatively, patients may be evaluated with magnetic resonance or computed tomographic (CT) angiography. Patients with abnormal Doppler US findings but with no clinical abnormalities should be followed up clinically (16).
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Figure 20a. Renal artery stenosis. (a) Duplex Doppler US image shows a tardus parvus waveform distal to the stenosis. (b) Angiogram obtained with a right common iliac artery injection before angioplasty shows the stenotic segment at the anastomosis (arrow). (c) Angiogram obtained after angioplasty shows resolution of the stenosis (arrow).
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Figure 20b. Renal artery stenosis. (a) Duplex Doppler US image shows a tardus parvus waveform distal to the stenosis. (b) Angiogram obtained with a right common iliac artery injection before angioplasty shows the stenotic segment at the anastomosis (arrow). (c) Angiogram obtained after angioplasty shows resolution of the stenosis (arrow).
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Figure 20c. Renal artery stenosis. (a) Duplex Doppler US image shows a tardus parvus waveform distal to the stenosis. (b) Angiogram obtained with a right common iliac artery injection before angioplasty shows the stenotic segment at the anastomosis (arrow). (c) Angiogram obtained after angioplasty shows resolution of the stenosis (arrow).
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Transplant recipients experiencing chronic rejection may also have segmental renal artery stenosis, with scarring in the parenchyma and resulting in compromised renal function. Segmental renal artery stenosis manifests at duplex Doppler evaluation as prolonged acceleration time in the intrarenal arteries, with normal profiles in the main renal artery. With careful evaluation of the intervening segmental and interlobar arteries, asymmetric flow can be demonstrated. These findings correlate with the typical angiographic appearance of beaded intrarenal vessels.
Renal Vein Thrombosis.—Renal vein thrombosis is rarely a cause of transplant dysfunction. It occurs within the first week after transplantation (14) and is manifested clinically by sudden oliguria, graft tenderness, and swelling. Causes include surgical technique, compression of the renal vein by fluid collections, and hypovolemia. An increased frequency of renal vein thrombosis in left lower quadrant allografts has been attributed to compression of the left common iliac vein between the sacrum and the right common iliac artery (silent iliac compression syndrome). The usual outcome of renal vein thrombosis is infarction, and a transplant nephrectomy is usually performed to prevent infection (16).
On gray-scale US images, the allograft may appear swollen and hypoechoic. At Doppler US examination, venous flow is absent, and the arterial waveform shows reversed, plateauing diastolic flow (19). Although reversal of diastolic flow is nonspecific (it is also seen in severe rejection and acute tubular necrosis), the combination of this waveform with absent venous flow is virtually diagnostic of renal vein thrombosis (20) (Fig 21).
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Figure 21. Renal vein thrombosis. Duplex Doppler US image shows a spectral waveform in which the diastolic flow in the renal artery is reversed and plateauing. No venous signal could be obtained.
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Renal Artery Thrombosis.—Renal artery thrombosis is a rare complication of transplantation that occurs soon in the early postoperative period and almost invariably leads to graft loss. Associated findings include severe rejection, severe tubular necrosis, and faulty surgical anastomosis (21).
Renal artery thrombosis is diagnosed at US when duplex and color Doppler techniques fail to demonstrate intrarenal venous and arterial flow. However, because these findings may mimic those of severe rejection, angiography may be warranted in problem cases (16).
Renal Vein Stenosis.—Renal vein stenosis usually results from perivascular fibrosis and compression from adjacent, large perinephric fluid collections. The high velocities caused by stenosis are seen on color Doppler US images as an area of color aliasing, similar to the findings in cases of arterial stenosis. Although no published criteria exist for grading the stenosis, Tublin and Dodd (16) used a cutoff of a threefold to fourfold increase in velocity to indicate a clinically significant stenosis (16).
Intrarenal Arteriovenous Fistulas and Pseudoaneurysms.—Intrarenal arteriovenous fistulas and pseudoaneurysms are the result of vascular trauma during percutaneous biopsy. Arteriovenous fistulas may form when an artery and vein are lacerated; pseudoaneurysms result when only the artery is lacerated. The majority of these lesions are small, are clinically insignificant, and usually resolve spontaneously; thus, the frequency of this complication is unknown. When lesions are sizable, marked arteriovenous shunting may result in renal ischemia. Hematuria or perigraft hemorrhage may result when large arteriovenous fistulas or pseudoaneurysms rupture. When symptomatic or large, intrarenal arteriovenous fistulas and pseudoaneurysms may be effectively treated with embolization (4).
Arteriovenous fistulas are easily identified at color and duplex Doppler US. Larger fistulas are seen as a focal flurry of disorganized color flow outside the borders of the normal renal vasculature, an appearance that is attributed to the vibration in the tissue surrounding the fistula. The feeding artery and draining vein may be visualized with appropriate gain and wall filter settings. Smaller fistulas may be manifested only by the color aliasing of focally increased arterial velocities. Regardless of the size of the fistula, its feeding artery will show a high-velocity, low-resistance waveform at Doppler interrogation (Fig 22). The draining vein may show a pulsatile "arterialized" flow (4).
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Figure 22a. Intrarenal arteriovenous fistula following biopsy. (a) Color Doppler image shows an abnormal focus of increased flow in the midpole (arrow). (b) Duplex color Doppler US image of this same area shows marked turbulence in the waveform. (c) Angiogram obtained with an aortic injection shows early filling of the right external iliac vein (arrow). (d) Angiogram obtained with subselective injection of the main renal artery shows early filling of the transplant renal vein (arrow). (e) On an angiogram obtained after repair, early draining veins are not visualized.
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Figure 22b. Intrarenal arteriovenous fistula following biopsy. (a) Color Doppler image shows an abnormal focus of increased flow in the midpole (arrow). (b) Duplex color Doppler US image of this same area shows marked turbulence in the waveform. (c) Angiogram obtained with an aortic injection shows early filling of the right external iliac vein (arrow). (d) Angiogram obtained with subselective injection of the main renal artery shows early filling of the transplant renal vein (arrow). (e) On an angiogram obtained after repair, early draining veins are not visualized.
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Figure 22c. Intrarenal arteriovenous fistula following biopsy. (a) Color Doppler image shows an abnormal focus of increased flow in the midpole (arrow). (b) Duplex color Doppler US image of this same area shows marked turbulence in the waveform. (c) Angiogram obtained with an aortic injection shows early filling of the right external iliac vein (arrow). (d) Angiogram obtained with subselective injection of the main renal artery shows early filling of the transplant renal vein (arrow). (e) On an angiogram obtained after repair, early draining veins are not visualized.
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Figure 22d. Intrarenal arteriovenous fistula following biopsy. (a) Color Doppler image shows an abnormal focus of increased flow in the midpole (arrow). (b) Duplex color Doppler US image of this same area shows marked turbulence in the waveform. (c) Angiogram obtained with an aortic injection shows early filling of the right external iliac vein (arrow). (d) Angiogram obtained with subselective injection of the main renal artery shows early filling of the transplant renal vein (arrow). (e) On an angiogram obtained after repair, early draining veins are not visualized.
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Figure 22e. Intrarenal arteriovenous fistula following biopsy. (a) Color Doppler image shows an abnormal focus of increased flow in the midpole (arrow). (b) Duplex color Doppler US image of this same area shows marked turbulence in the waveform. (c) Angiogram obtained with an aortic injection shows early filling of the right external iliac vein (arrow). (d) Angiogram obtained with subselective injection of the main renal artery shows early filling of the transplant renal vein (arrow). (e) On an angiogram obtained after repair, early draining veins are not visualized.
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Pseudoaneurysms mimic simple or complex renal cysts on gray-scale US images. However, when color Doppler US evaluation is performed, flow can be appreciated in the lesion. With careful adjustment of the color scale, the pseudoaneurysm neck can be seen to contain alternating jets of forward and reverse flow. Duplex Doppler US imaging of the lesion will show a "to-and-fro" waveform. Pseudoaneurysms with broad necks may not have the typical Doppler waveform and may show a more disorganized flow, with a high-velocity, low-resistance waveform identified in the neck.
Extrarenal Arteriovenous Fistulas and Pseudoaneurysms.—Extrarenal arteriovenous fistulas and pseudoaneurysms are extremely uncommon. They typically occur as a result of surgical technique rather than percutaneous biopsy. Extrarenal pseudoaneurysms also may be a complication of perivascular infection. The significance of an extrarenal arteriovenous fistula depends on its size. However, all extrarenal pseudoaneurysms, regardless of size, are considered clinically significant because of the high frequency of perianastomotic infection and the possibility of spontaneous rupture. The color and duplex Doppler US features of these extrarenal lesions are the same as those of their intrarenal counterparts (4) (Fig 23).
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Figure 23a. Extrarenal pseudoaneurysm. (a) Duplex color Doppler US image shows turbulent forward and reverse flow in the main renal artery. (b) Angiogram obtained with renal artery injection shows a pseudoaneurysm of the renal artery at the anastomosis (arrow). The pseudoaneurysm was repaired surgically.
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Figure 23b. Extrarenal pseudoaneurysm. (a) Duplex color Doppler US image shows turbulent forward and reverse flow in the main renal artery. (b) Angiogram obtained with renal artery injection shows a pseudoaneurysm of the renal artery at the anastomosis (arrow). The pseudoaneurysm was repaired surgically.
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Abnormalities of the Collecting System
Urinary tract obstruction after renal transplantation occurs in less than 5% of patients. More complete hilar dissections in recipients of grafts from live donors may explain the greater frequency of urinary tract complications following transplantation. Causes of urinary tract obstruction include edema at the anastomosis and ischemia leading to fibrosis and stricture. Infection, hematoma, lymphocele, mucosal edema, and kinking of the ureter are other causes of obstruction (16) (Figs 7, 8).
Patients with obstruction are typically asymptomatic, and the diagnosis is made during evaluation because of a rising creatinine level. US will demonstrate dilated calices, although this is a nonspecific finding because it is also seen in cases of diminished ureteral tone resulting from denervation of the transplant (4). The utility of Doppler US assessment of resistance and pulsatility in the setting of urinary tract obstruction after transplantation has been evaluated but has not yet been determined (4,22).
Treatment of urinary tract obstruction consists of stent placement, balloon dilatation, or correction of the source of extrinsic compression of the collecting system, such as a lymphocele. However, surgical reconstruction may be required for long or recurrent strictures.
The presence of material within a dilated collecting system is usually clinically significant. Highly echogenic, weakly shadowing masses in the collecting system are consistent with fungus balls, whereas low-level echoes may suggest pyonephrosis or hemonephrosis (16). Other abnormalities of the collecting system include calculi and urothelial tumors.
Abnormalities of the Renal Parenchyma
Focal hypoechoic or hyperechoic areas within the renal parenchyma may be identified on US images. These focal areas may or may not be contour deforming and are nonspecific findings, representing pyelonephritis, infarction, or rejection. A focal infarction may be suggested by absent flow at power Doppler evaluation (Fig 24). Later in the posttransplantation course, similar findings may represent renal cell carcinoma or posttransplantation lymphoproliferative disorder (PTLD).
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Figure 24a. Cortical infarct. (a) On a gray-scale US image, the region of cortical infarct is hypoechoic (arrowhead). (b) Power color Doppler image shows a paucity of blood flow in the region of the infarct (arrowhead).
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Figure 24b. Cortical infarct. (a) On a gray-scale US image, the region of cortical infarct is hypoechoic (arrowhead). (b) Power color Doppler image shows a paucity of blood flow in the region of the infarct (arrowhead).
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PTLD results from a loss of the normal immune mechanism, which allows an unchecked proliferation of B cells to occur. This dysfunction results in a spectrum of disease, ranging from mild diffuse polyclonal lymphadenopathy to malignant monoclonal lymphoma (23). The most common manifestation of this disorder at radiologic examination is lymphadenopathy, but the disorder can also affect any of the solid organs or hollow viscera. PTLD can even affect the graft parenchyma (24) (Fig 25). On radionuclide images, focal photopenic defects in the renal contour may be seen in cases of infarction or recurrent infections (Fig 26).
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Figure 25a. PTLD involving a renal transplant. (a) US image shows a hypoechoic mass (arrowhead) in the midpole of the graft. (b) On a CT image, the mass has low attenuation (arrowhead).
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Figure 25b. PTLD involving a renal transplant. (a) US image shows a hypoechoic mass (arrowhead) in the midpole of the graft. (b) On a CT image, the mass has low attenuation (arrowhead).
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Figure 26. Scarring from recurrent pyelonephritis in a child with vesicostomy and a renal transplant for posterior urethral valves. View from a Tc-99m MAG3 study shows multiple photopenic defects (arrows). The recurrent infections were attributed to the vesicostomy bag and decreased in frequency after discontinuation of its use.
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US-guided Biopsy
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Percutaneous biopsy is an invaluable diagnostic tool in transplant recipients with diminished renal function. Although core needle biopsies are more common, some centers also use fine-needle biopsies (16). Postbiopsy complications include perirenal hemorrhage, arteriovenous fistula, pseudoaneurysm, and collecting system laceration. The potential for these complications is diminished with the use of real-time US to guide the biopsy procedure. By using color Doppler imaging, the intrarenal and extrarenal vessels can be identified and avoided. When US is used to guide the needle toward the renal cortex, samples containing multiple glomeruli can be obtained, thereby improving the diagnostic yield of each needle pass.
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Conclusions
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Given the prolonged survival now possible for renal transplant recipients because of newer medical and surgical technologies, it is increasingly important to use noninvasive methods of screening these patients and evaluating their symptoms or signs of complications. By understanding the appearances of potential complications as depicted with the most commonly used modalities, radionuclide imaging and US, early diagnosis of complications may lead to therapeutic interventions that prolong the life of the graft.
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Abstract
LEARNING OBJECTIVES FOR TEST...
