eMedicine - Kidney Transplantation, Surgical Complications : Article by Lennard A Nadalo

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Kidney Transplantation, Surgical Complications

Article Last Updated: Feb 1, 2007

Background

The first renal transplantation was performed in 1954. The kidney remains the most frequent organ transplanted. A major breakthrough occurred with the release of cyclosporine in 1983; this drug helps to control transplant rejection. With improved surgical techniques and medical management of rejection, renal transplantation has become the treatment of choice for end-stage renal disease (ESRD).

The use of immunosuppressive agents such as cyclosporine, OKT3, and FK506 has resulted in a 1-year survival rate for mismatched cadaveric renal grafts of 80%. A 90% 1-year graft survival rate has been reported with nonidentical grafts from living related donors and a 95% 1-year success rate for grafts with identical human lymphocyte antigen. The half-life of grafts from living related donors varies from 13-24 years. Other medical managements have further extended the functional life of renal transplants while ensuring a better quality of life for the transplant recipient.

Surgical techniques for transplantation have improved, and the means by which kidneys are obtained from living related donors was recently advanced with the use of laparoscopic surgical techniques. The frequency of left kidney harvesting via a laparoscopic approach has resulted in more frequent transplantation of kidneys with multiple renal arteries. Both the laparoscopic approach and selection of donor kidneys that have multiple renal arteries demand careful preoperative assessment of the potential renal donor. The frequency of renal transplantations with multiple renal arteries may be as high as 30%.

Perioperative complications occur in 15-20% of renal transplants. Most initial complications can be corrected if detected promptly. The radiologist should help to select the most effective imaging methods for evaluating the many problems encountered in renal transplantation. In the author's experience, ultrasonography (US) is a safe, rapid, and portable means to first detect most surgical emergencies.

Nuclear medicine scanning and flow studies remain the primary means for evaluating vascular supply to the transplant after surgery. The main advantage of nuclear medicine scans is that they demonstrate the pathophysiology involved. Recent developments in Doppler US and MRI show promise in improving the quality of renal vascular imaging without the use of potentially nephrotoxic intravenous (IV) contrast materials.

CT scanning and CT-guided interventions remain an important means for examining the preoperative renal donor candidate and for evaluating the complications that develop in patients who undergo renal transplants. The complementary nature of nuclear medicine studies and US in the imaging evaluation of hydronephrosis, renal artery stenosis, flank pain, renal masses, pyelonephritis, and kidney transplant was confirmed in the author's test group.

In more recent years, MRI of the abdomen has evolved into an excellent alternative means for the diagnosis of most renal transplantation complications and for the examination of the living related donor. The contrast agents used for MRI are nontoxic to the transplanted kidney, and MRI often can be used to assess renal function, vascular supply, and postoperative complications. However, MRI remains expensive and may be contraindicated in certain patients. Whenever 2 or more diagnostic techniques are used in the diagnosis of renal transplantation, comparison between the techniques is often very useful. Comparison of recent, current studies with remote, prior examinations is critical. New fluid collects are of greater significance than are slowly resolving fluid collections.

Pathophysiology

The process of organ transplantation begins with the evaluation of patients with end-stage kidney disease. Chronic renal failure makes the potential transplant recipient more susceptible to a wide range of neoplasms, including carcinoma of the kidney. The significance of an atypical cyst or a small renal deformity is much greater in chronic renal insufficiency. The renal transplant recipient is often chronically ill. Pulmonary hypertension greater than 35 mm Hg has been detected in more than a third of patients receiving chronic hemodialysis. Dialysis access-induced venous stenosis is common. Stenosis of the superior vena cava may result in severe upper extremity and cervical venous stasis.

The next stage in the renal transplantation process involves the selection and examination of a suitable donor. In the best circumstance, the renal donor is alive and related to the transplant recipient. The success of transplants involving living related subjects is much higher than those involving cadaveric transplants. Traditionally, the potential donor was examined with intravenous urography (IVU) and angiography. Although a number of pretransplantation urologic evaluations have been recommended in the literature, no standard exists. More preoperative evaluations are performed with MRI and CT and fewer donors are examined with the older methods. CT urography offers ease of performance and a high degree of detection of renal stones, anomalies, and (less commonly) renal masses in the kidneys of potential donors.

Generally, kidneys are harvested from living related donors using a left-flank laparoscopic approach. Hand-assisted laparoscopic techniques have been reported to result in shorter operative and warm ischemia times. The need for excellent preoperative imaging is directly related to the challenge of the laparoscopic procedure as well as the additional time (27-30%) needed to transplant a kidney with two or more renal arteries. Concomitant surgery has been performed for live donors using a laparoscopic approach. Simultaneous surgical interventions at the time of kidney donation benefit the donor and do not compromise the overall transplantation process.

Renal transplantation surgery must address multiple renal arteries, which may be treated with primary or secondary anastomosis. The left renal vein is longer and easier to use; therefore, harvesting of the left kidney is favored. Recently described techniques allow for longer arterial length on the harvested renal transplant. Longer donor kidney vessels potentially make a right renal donor kidney more acceptable. Dual pediatric donor kidneys are left connected to the donor aorta, which is then anastomosed to the external iliac artery. The vessels of the ureter must be preserved. In general, the ureter should be straight but not retracted. The distal ureter is implanted into the bladder through a tunnel without opening the bladder.

Frequency

United States

The frequency of organ transplantation has increased consistently during the past 10 years. In 1995, surgeons performed 20,030 transplants; 11,788 of those were kidney transplantations. Liver and heart transplantations were the next most common procedure. Although the frequency of renal transplantation has increased annually, large numbers of patients continue to wait for renal transplants. Long-term results of renal transplantation using kidneys from non-heartbeating donors were 50% in one large series, which was similar to the results of transplants from donors with a heartbeat at the time of transplantation. The use of kidneys from this group of potential donors should increase the availability of renal transplants.

Mortality/Morbidity

The long-term results of renal transplantation in children have been reported as 95% at 1 and 3 years following transplantation, with a graft survival rate of 88% at 10 years. Morbidity from renal transplantation is related to the surgical implantation of the kidney and the immunosuppression necessary to prevent organ rejection. This discussion is limited to the morbidity related to surgical complications.

The surgical site into which the transplanted kidney is placed represents a sectional plane created for the kidney. An incisional hernia was observed in 4% of transplants in one series. The presence of large polycystic kidneys, bladder outlet obstruction, and chronic lung disease was associated with the development of an incisional hernia. Both early and delayed incisional hernias were noted. Surgical repair was accomplished by primary facial approximation and polypropylene mesh reinforcement. Postoperative complications have been reported secondary to the use self-retaining retractors. Direct trauma may occur due to compression of large bowel, femoral nerve compression, or other soft tissue contusions related to the blade position.

Lymphocele collections may occur soon after renal transplantation or they may develop on a delayed basis. Whether its origin is from the recipient or donor is rarely identified. A lymphocele may result in urinary obstruction of the transplant or compromise blood flow. Treatment generally involves fenestration of the cavity into the abdominal space or, more recently, laparoscopic fenestration. The application of the laparoscopic technique demands a full understanding of the surgical anatomy before the procedure is performed. Urinoma or urinary fistulas may occur in any renal transplant patient but are more common in children in whom urinary tract anomalies are present.

The application of bladder augmentation techniques (ileum, ureter, sigmoid, stomach), incontinent urinary conduits (ileum, colon), and continent urinary reservoirs is associated with surgical complications, including stomal stenosis, stomal prolapse, renal artery stenosis, urine leaking, enterovesicular fistula, and wound dehiscence and urinary tract stones. Similar complications may occur in the older transplant patient who has undergone bladder or prostate surgery. In the period immediately after renal transplantation, hemorrhage may occur around the transplanted kidney or in the peritoneal or retroperitoneal spaces. Later, obstructive uropathy may occur because of stone formation, cross clamp stenosis, or ischemic stenosis of the ureter or renal pelvis.

Renal arterial and venous stenosis may result in hypertension or graft failure. The primary treatment opinion is renal arterial or venous angioplasty often with a stent placement. Surgical repair of transplant renal artery stenosis has been described using both direct vascular repairs as well as with preserved cadaveric iliac artery grafts. Vascular complications occur in up to 3% of renal transplants. Thrombotic, hemorrhagic, stenotic, and embolic complications related to the transplanted kidney have been reported.

Occasionally, masses may arise from within the transplanted kidney. Renal cell carcinoma, focal fungal disease, and B-cell lymphoma have been reportedly transmitted within renal transplants.

Race

The medical need for renal transplantation is increased in patients with diabetes and uncontrolled hypertension. Diabetes occurs more frequently among Hispanic adults, whereas severe essential hypertension is more common among blacks. The willingness to provide kidneys for donation is influenced by cultural and economic factors, which, technically, are not racial issues.

Sex

No important sex-related differences concern the complications of renal transplantation. Differences between male- and female-predominant malignancies remain after organ transplantation.

Age

As the experience with renal transplantation has grown, the age of patients who undergo renal and renal-pancreas transplantation has widened. In general, pediatric patients have a physical limitation in the size of the abdominal-pelvic spaces. Most children who receive renal transplants are small for their age because of chronic renal insufficiency. In older patients, the severity of vascular disease and the increased incidence of malignancy cause additional risks.

Anatomy

The anatomic basis for renal transplantation reflects the need to create an arterial inflow and venous return for the renal transplant. The transplanted kidney is generally placed in the upper lateral portion of the pelvis. The ureter from the transplant is attached into the urinary bladder via a submucosal tunnel.

The vascular supply to the ureter of the transplanted kidney is critical as well. The traditional surgical means of harvesting the transplant kidney allows the selection of either the right or left kidney. In most cases, the left kidney is favored because the left renal vein is longer than the right. The introduction of a laparoscopic approach for donor nephrectomy has further emphasized the selection of the left kidney in most cases. Single renal arteries and veins are most common; however, the donor kidney can have multiple renal arteries arising from the aorta from T12 to L4. Multiple renal arteries are noted in approximately 25-30% of renal grafts. Allografts with multiple renal arteries can be successful. The pretransplantation donor evaluation must include an accurate assessment of the number and the location of multiple renal arteries.

Pretransplantation donor evaluations must include adequate evaluation for accessory renal arteries beginning from just below the diaphragm and proceeding distally to include both external iliac arteries. Arterial assessment should include the possibility of early renal artery branching. Preoperative assessment is best performed using a multidetector CT scanner with extensive post processing using multiplanar reformatted images and 3D visualization.

Renal veins can be multiple and present frequent anomalies. The most common venous anomalies include the retroaortic position of the main left renal vein, multiple veins, early branching of renal veins, and anomalies of the anastomosis between the renal veins and the lumbar and gonadal veins. Anomalies in the potential renal donor may include urologic collecting system duplication, single kidneys, crossed-fused kidneys, and hydronephrosis. Determining the suitability of a kidney for transplantation is complex; the renal transplantation surgeon must make the decision whether to use a kidney after discussion among all interested parties.

The surgical placement of the donor kidney is subject to variation depending on the size of the kidney compared with that of the recipient, the number of renal arteries, and the presence of preexisting diseases such as polycystic renal disease and severe atherosclerotic disease of the pelvic arteries. Repeat renal transplantation is influenced by the location, condition, and size of the initial (failed) transplant. The primary arterial anastomosis may involve the external iliac artery or the hypogastric (internal iliac) artery. Dual pediatric donor kidneys are left connected to the donor aorta, which is then anastomosed to the external iliac artery.

Clinical Details

The renal transplant candidate is often treated with either hemodialysis or peritoneal dialysis for years before transplantation although there is an increased interest in transplantation before the patient in renal failure requires dialysis. Many renal transplant candidates have diabetes. Because of the patient's chronic pretransplantation disability, immunosuppression is a potential risk. Neoplasms, including renal cell carcinoma in the native kidneys and lymphoma, are more common among patients with chronic renal insufficiency (see Pathophysiology). Severe arteriosclerotic vascular disease (ASVD) and blindness may complicate the care of patients with diabetes. After successful renal transplantation, the immunosuppression needed to prevent organ rejection places the renal transplant recipient at increased risk for acute and chronic infections as well as malignancies.

Simultaneous heart and kidney transplantation has successfully broadened the treatment of patients with diabetes and chronic vascular disease. The simultaneous heart and kidney transplantation is associated with a 2-year survival rate of 67% in one series.

Although a full discussion of the potential infections in renal transplant patients is beyond the scope of this discussion, a careful review of chest radiographs, abdominal and pelvic sonograms, and CT scans must include a consideration of unusual fungal, protozoan, and bacterial infections. After the first few hours, a clinical milestone is the production of urine by the renal transplant. Poor function of the renal transplant may be the result of acute tubular necrosis (ATN). ATN is related to "warm" renal ischemia. Both the severity and duration of ATN is worse in cadaveric transplants. Because of the sensitivity of the injured renal transplant, nuclear medicine examinations are most commonly used to study ATN and to differentiate other causes of poor renal function in the immediate posttransplantation period.

After a patient recovers from ATN, both acute and chronic organ rejection become primary considerations. Allograft recipients are often treated with immunosuppressive medications, including, most recently, rapamycin. Infectious complications resulting from inhibition of the immune system may result in localized abscess formation or generalized infections. Hypertension, decreased renal function, and an increased arterial duplex resistive index are noted in renal rejection. Primary surgical complications include the development of fluid collections, including lymphoceles, hematoma, and abscesses. Diagnosis and intervention in these conditions are discussed below.

Preferred Examination

In the author's experience, the most common imaging procedure in the renal transplant recipient is US. Immediate complications related to surgery can be demonstrated. US enables suitable localization prior to biopsy in most patients. Duplex US also provides important information concerning the vascular status of the graft in cases of acute rejection. Delayed complications related to the renal transplant, such as lymphocele, may occur.

Concurrent with sonographic studies, radionuclide studies are frequently performed. The nuclear studies provide valuable information concerning functional status during the immediate postoperative period and during episodes of rejection. Correlation between renal sonographic and nuclear medicine findings helps to differentiate between purely functional disease, such as acute tubular necrosis or rejection, and abnormal fluid collections, such as hematomas, abscesses, and lymphoceles.

CT and MRI studies of the abdomen and pelvis offer similar information. In most cases, CT is preferred because CT-guided aspiration and drainage procedures may obviate surgical interventions. Angiographic study of the transplant may involve CT angiography, MR angiography, or intra-arterial catheter angiography if therapeutic procedures are to be performed. In general, the examination with the least risk should be selected for the diagnostic survey of the renal transplant.

The donor evaluation is most likely done with MRI(A) (magnetic resonance angiography) and CT(A) (computed tomographic angiography).

Limitations of Techniques

Conventional radiographs provide a limited amount of useful information about the complex clinical presentations of renal transplantation. Often, chest radiographic findings are nonspecific because chronic interstitial pulmonary disease is difficult to differentiate, whereas abdominal radiographs may fail to illustrate fluid collections.

US is safe. Generally, it should be the first cross-sectional examination in renal transplantation; however, sonograms may fail to differentiate gas versus calcifications, and bowel gas may obscure important findings. CT is not limited by bowel gas; however, the use of IV iodinated contrast material must be limited in many cases. Radionuclide studies offer important functional information but the findings are often nonspecific, and the images have relatively poor spatial resolution. Abdominal and pelvic MRI is valuable. Some patients object to the MRI experience, and others have direct contraindications such as cardiac pacemakers or cerebral aneurysm clips.

Although Doppler US and scintigraphy with Captopril, when the patient is hypertensive, are helpful, both tests have limitations.

Other Problems to be Considered

Abdominal abscess
Hydronephrosis
Lymphocele
Renal artery thrombosis
Perinephric hematoma
Urinoma
Urinary leak

Findings

Traditionally, the potential donor was examined with IVU and angiography. That no longer occurs. Although a number of pretransplantation urologic evaluations have been recommended in the literature, no one specific universally accepted protocol exists. The expense and potential risk of using IV contrast material should limit duplication of tests. In general, CT of the abdomen and kidneys can be performed at the time of the evaluation of the renal vasculature. If magnetic resonance angiography (MRA) is selected for the renal vascular evaluation, an IVU (as an MR Urogram) can follow with sufficient spatial resolution to obviate the need for a radio-opaque injection to perform a standard IVU.

Degree of Confidence

Donor anomalies related to the upper collecting system and duplicated ureters may make the kidney from a living related donor unacceptable. This is readily illustrated on CT and MRI examinations, replacing IVU. To confirm two kidneys with no gross abnormality, US can be used as a screening study. Conventional angiography was long considered the criterion standard for the evaluation of the renal arteries and veins of the living related donor candidate. Today, as noted before, CTA (with spiral multidetector and multiplanar reconstruction) and MRA have become the imaging techniques of choice.

Prior to transplantation, if there is the question of recipient kidney size or gross abnormality, US can be performed. If vascular status, severe arteriosclerosis, or venous abnormalities of the pelvic arteries is suspected that might complicate the creation of the renal artery anastomosis, MRA or CTA can be performed. Angioplasty or stent placement may be required with a subsequent interventional procedure.

False Positives/Negatives

As with all imaging techniques, anomalies and misleading images can imitate pathologic situations. The radiologist must be aware of absent or duplicated vessels, benign masses (cortical invagination), and physiologic phenomenon (diuresis) that can simulate hydronephrosis.

Findings

An increased interest in CT imaging has paralleled the improvement in the image quality and speed of the CT examination. With the introduction of spiral CT imaging, both the resolution and speed of the available examinations have improved. Concurrent with improved imaging, the expanded role of interventional radiologic procedures has created an important role for CT imaging in transplant recipients.

CT of the renal donor may reveal small renal cell cancers, cysts, upper collecting system duplications, and partial renal collecting system obstructions. Particularly important, the CT examination should be performed prior to the IV administration of contrast agent to look for renal stones, as well as immediately after administration of the bolus.

Early postinjection-phase images best demonstrate the renal cortex, while images obtained approximately 40 seconds after injection of the contrast agent bolus best show the combined enhancement within the renal medullary space and the renal pyramids. Nephrogram-phase images have been shown to be most sensitive in the overall detection of renal masses. By reexamining the kidneys, ureters, and bladder after a 2-3-minute delay, accurate evaluation of the renal collecting system, ureters, and urinary bladder is possible.

By carefully reviewing the axial CT images obtained during the first 20-30 seconds after the IV bolus, most renal artery duplications and many renal vein anomalies can be detected. Image collimation of 2-3 mm with an image index of 1-2 mm is necessary to separate small vessels, which are closely positioned. The patient must hold his or her breath during the arterial phase of CT scanning. Special imaging reformatting and reconstruction methods improve the general diagnostic accuracy of renal CT angiography and, in selected cases, the demonstration of small accessory arteries and venous anomalies that otherwise might not be detected. Such 3-dimensional (3D) imaging also provides the surgical team with an excellent overall review of the surgical anatomy.

CT scanning is useful in the evaluation of both immediate and delayed complications of renal transplantation. Multidetector CT urography has been shown superior to conventional IVP in the detection of urinary leaks and ureteral obstructions. In the immediate posttransplantation period, perigraft hemorrhage and urinary leaks can be detected with CT. In most cases, either US or functional nuclear medicine imaging should be performed first. CT scanning is the best means for surveying the most common intermediate and delayed surgical complications. CT findings are related to density differences between areas of hemorrhage, lymphoceles, and abscesses.

Degree of Confidence

Regarding the donor, after a careful review of the axial CT images with a cine technique on a workstation or picture archiving and communication system (PACS), the evaluation of maximum intensity projection images in multiple projections and variable slab thicknesses is useful. Coronal, curved, multiplanar, reformatted images best outline the full length of each renal artery.

Measurements of the renal arteries are obtained; these include the diameter of the renal artery origins and distance of each renal artery to a major branching point. The presence of fibromuscular hyperplasia or renal artery stenosis due to ASVD is assessed. Volume image sets displayed using 3D image rendering is useful in complex cases and in explaining surgical options to the renal transplantation team.

Immediate complications related to recipient renal transplantation is best evaluated with combined or independent renal US, functional renal nuclear medicine study, CT, and occasionally MRI. Acute perirenal collections with a high CT attenuation of 28 Hounsfield units (HU) or greater is most likely perinephric hemorrhage. Perirenal or pararenal fluid collections of 18-24 HU are most likely lymphoceles. Complex fluid collections anywhere in the abdomen or pelvis of more than 28 HU on a delayed basis may be abscess cavities or chronic (liquefying) hematoma. In general, the findings of perirenal transplant complications have a pattern similar to that of diseases of the abdomen in other immunocompromised patients. Multidetector CT urography is more complete and precise compared to more conventional diagnostic techniques. The overall diagnostic accuracy of CT urography has been reported to be 90%.

False Positives/Negatives

The process of organ transplantation begins with the evaluation of patients with end-stage kidney disease, which may require CT and/or MRI imaging depending on the clinical presentation. Chronic renal failure makes the potential transplant recipient more susceptible to a wide range of neoplasms, including carcinoma of the kidney. The significance of an atypical cyst or a small renal deformity is much greater in the face of chronic renal insufficiency. In a patient with a polycystic kidney, bleeding into a cyst may mimic a small cystic renal cell carcinoma. The bleeding associated with polycystic renal disease is not necessarily related to renal cell carcinoma.

A detailed surgical history is useful in the evaluation of the transplanted kidney. In cases in which native vessels are thrombosed, unusual alternate venous outflow patterns may result. In cases of IVC thrombosis, the transplanted kidney renal vein may drain into the mesenteric veins. This should not be mistaken for a pathological AVF or other primary venous anomaly.

Surgical complications after pancreas transplantation include difficulties in the management of organ rejection, as well as the potential for pancreatitis with or without ischemic necrosis.

Findings

Gadolinium-enhanced MRA is rapidly becoming the technique of choice for imaging the renal arteries. MRA has several advantages compared with conventional digital subtraction angiography (DSA). MRI and MRA are noninvasive and use no ionizing radiation. Gadolinium-based contrast media are safe to use in patients with renal insufficiency because the agents have minimal nephrotoxic effects. MRA provides true anatomic images and permits multiplanar reformatting. High-performance MR gradient systems allow the acquisition of 3D volumes in a single breath hold of shorter than 30 seconds. Imaging of the renal arteries is important in the examination of potential renal donors and of transplant patients with suspected renal artery stenosis.

MRI offers several advantages in the pretransplantation donor and recipient evaluation. MRI offers good spatial resolution with excellent tissue differentiation. No adverse effects related to radiation are known, and the gadolinium-based IV contrast agents can be safely used in patients with renal insufficiency. MRA with gadolinium enhancement has been improved by the development of fast 3D gradient-echo sequences. The T1 shortening effect of the IV injection of paramagnetic contrast material significantly improves vessel imaging by eliminating signal loss from saturation effects, minimizing dependence of the inflow phenomenon, and possibly decreasing intravoxel dephasing effects.

MRA demonstrates accessory renal arteries using gadolinium agents with the same degree of accuracy as CTA. MRA has been successfully applied to the diagnosis of most of the complications related to acute renal transplant vascular thrombosis. Gadopentetate dimeglumine (DTPA), when administered as an IV bolus during MRA, provides qualitative functional information concerning the renal transplant. It demonstrates the morphology in a manner similar to CT and is generally superior to diagnostic US.

Gadolinium-based contrast agents (gadopentetate dimeglumine [Magnevist], gadobenate dimeglumine [MultiHance], gadodiamide [Omniscan], gadoversetamide [OptiMARK], gadoteridol [ProHance]) have recently been linked to the development of nephrogenic systemic fibrosis (NSF) or nephrogenic fibrosing dermopathy (NFD). For more information, see the eMedicine topic Nephrogenic Fibrosing Dermopathy. The disease has occurred in patients with moderate to end-stage renal disease after being given a gadolinium-based contrast agent to enhance MRI or MRA scans.

As of late December 2006, the FDA had received reports of 90 such cases. Worldwide, over 200 cases have been reported, according to the FDA. NSF/NFD is a debilitating and sometimes fatal disease. Characteristics include red or dark patches on the skin; burning, itching, swelling, hardening, and tightening of the skin; yellow spots on the whites of the eyes; joint stiffness with troublemoving or straightening the arms, hands, legs, or feet; pain deep in the hip bones or ribs; and muscle weakness. For more information, see the FDA Public Health Advisory or Medscape.

Degree of Confidence

MRI has several limitations that should be considered in the choice of MRI, CT, US, or radionuclide-based imaging. The spatial resolution of MRI is generally less than that of CT, while the functional information available is generally qualitative compared with the quantitative data provided by nuclear renal scans. Patients with pacemakers, cochlear implants, most intracranial aneurysm clips, or severe claustrophobia cannot be safely examined using MRI. The use of conscious sedation in mildly claustrophobic patients and young children can be considered in MRI but this adds to the cost.

The layering of excreted gadolinium into the urinary bladder and ureters generates artifacts that are less easily managed at MRI than at CT. During MRI, changing the patient's position requires additional time in the scanner. In comparison, prone or decubitus positioning at CT allows the mixing of contrast layers, which enables a more complete evaluation of the urinary bladder, including the anterior bladder surfaces. In general, CT of the kidneys and urinary bladder is better in the detection of renal stones and of renal, ureter, and bladder anomalies. CT with CTA generally costs less than MRI. In most applications, MRI is reserved for examination of the transplant recipient, whereas CT and CTA are used in the potential donors.

False Positives/Negatives

False-positive MRI findings in the transplanted kidney and/or pancreas are commonly related to movement or susceptibility artifacts. Breathing or vascular pulsatile movement may cause a misregistration of the walls of a cyst; this may be mistaken for enhancement related to renal cell carcinoma in the wall of a cyst. Benign fat-containing tissue near the surface of a transplanted kidney or pancreas may have a suspicious appearance if movement occurs during the examination. MRI is insensitive to calcified renal stones or calcifications in the walls of cysts.

False-negative MRI results related to venous anomalies and the detection of incomplete venous thrombosis is more common in MRA than in CTA.

Findings

Diagnostic US, including duplex vascular study, is the most commonly performed examination for routine follow-up after renal transplantation. Typical renal transplant sonograms demonstrate the following: arcuate arteries, echo-poor medullary pyramids, hyperechoic renal sinus, absence of hydronephrosis, stable renal volume (formula: 0.5 x length x transverse x anteroposterior dimensions), normal position of renal transplant anterior to the external iliac vessels, and otherwise normal echogenicity in the parenchyma, collecting system, and surrounding tissues.

The measurement of the vascular resistive index (RI) is a part of the evaluation. The RI measures resistance to arterial flow within the renal vascular bed. RI is calculated from the peak diastolic arterial waveform. The normal RI is less than 0.7-0.8. An RI of greater than 0.9 indicates transplant dysfunction but not the cause. In some instances, a nonfunctioning renal allograft can have "normal" RI and pulsatility index (PI).

Sonographic evaluation of renal transplant complications is often successful because many of the potential peritransplant complications are associated with fluid collections. The superficial nature of a typical renal transplant allows good resolution with relatively few reverberation echoes. During the immediate postoperative transplant period, most fluid collections in and around the bed of the transplant are hematomas or urinomas. A hyperacute hematoma often presents as a hypoechoic fluid-filled mass near the surgical site. Hematomas may be stable in size but can enlarge rapidly. The enlarging hematoma may compress the kidney, resulting in a reduction of urine output. Later, a hematoma becomes complex with both hypoechoic and echoic areas.

A second possible cause for fluid around the transplant during the first 24 hours following transplantation is a urinoma. A urinoma results from a leak between the renal transplant ureter and the recipient's bladder. Such a leak may be the result of surgical failure or from necrosis of the distal portion of the transplant ureter. A urinoma enlarges more slowly and remains hypoechoic. If the urinoma becomes infected the quality of the fluid may become more complex as an abscess forms. After several weeks, a spontaneous hematoma or a urinoma becomes less likely.

A lymphocele may form anywhere in the general area of the renal transplant. While lymphoceles are more likely to occur soon after the transplant, they may develop long after the skin wound has healed. An uncomplicated lymphocele is hypoechoic and remains hypoechoic unless it becomes infected. While lymphoceles are slow to develop, a lymphocele can create significant pressure resulting in urinary or venous obstruction.

An abscess may develop at any time during the life history of a renal transplant recipient. Abscesses may be around the kidney, within the transplant, or in the abdominal cavity remote from the transplant. Most abscesses have complex sonographic qualities. An abscess can be diagnosed best by comparing the sonographic texture of a fluid collection to the white blood cell count and the patient's fever curve. In cases in which an abscess is considered likely, sonographically guided aspiration is recommended. Diagnostic ultrasound may both initially diagnose the fluid mass as well as guide the surgical or radiologically based drainage. If percutaneous drainage results in a recurrence of a lymphocele, laparoscopic drainage is indicated.

Sonographic findings include globular enlargement of the kidney, swelling and hypoechogenicity of the medullary pyramids, an indistinct cortical-medullary junction, and hypoechoic foci in the renal cortex. Among the early findings of acute renal transplant rejection is renal enlargement. The length of a renal transplant should not increase from the stabilized posttransplantation measurements.

Although acute transplant rejection must be considered in all cases of declining renal function after initially successful renal transplantation, other acute events may occur in the immediate postoperative period or later clinical follow-up. Perinephric hematoma and acute hydronephrosis are among the complications than can result in the acute loss of kidney function.

Perinephric hematoma appears as a peripheral, mixed, hypoechoic rim or ring surrounding the kidney. The bleeding may proceed away from the kidney. The extent of the bleeding may be difficult to evaluate at US because of overlying bowel gas. Acute or chronic hydronephrosis has a presentation that is generally similar to hydronephrosis in a native kidney. The central collecting system may be dilated and filled with hypoechoic fluid (urine), and a variable degree of dilatation is generally preset in the renal collecting system. Several diseases may affect the transplanted kidney simultaneously. Mild tissue rejection may cause transplant edema, and a mechanical obstruction may cause hydronephrosis.

Comparison with prior sonographic studies and communication between the radiologist and other members of the transplantation team are critical. Correlation with findings from functional examinations, including nuclear medicine study and dynamic MRA, should be performed whenever US cannot confirm the cause of the decreased renal function. Changes in native kidneys of renal transplant patients should include a survey to detect masses. Cysts frequently develop in the kidneys of patients with ESRD. New patterns of internal echoes or rapidly enlargement should be noted. Correlation with CT, MRI, or aspiration biopsy results may be necessary to exclude carcinoma or other pathology.

Degree of Confidence

If prior US results are compared and correlated with the clinical findings and if appropriate renal transplantation team members are consulted, US evaluation of renal transplants is highly accurate and clinically useful. The physical limitations of diagnostic US apply in transplantation as well.

Fluid within the peritoneal space is not a specific finding. Hypoechoic abdominal fluid may remain after peritoneal dialysis. Urine, bowel content, sepsis, and hemorrhage may have variable sonographic appearances. Recent collections should be aspirated for diagnosis by US guidance. Large fluid collections may be drained for symptomatic relief.

False Positives/Negatives

False-positive US results may occur if a full understanding of the surgical history of the patient is not available. In double (pediatric) renal transplantation, one of the kidneys may be much smaller than the other. The smaller kidney may be mistaken for a mass. Patients may undergo more than one renal transplant. The contralateral, or prior, transplant is generally smaller and can be mistaken for a mass.

Other false-positive results relate to the interpretation of duplex findings of renal artery stenosis and elevated RI. Torturous renal arteries may suggest renal artery stenosis in the absence of hemodynamically significant narrowing. Increases in the RI should be interpreted carefully. Variations in the location of the sampled arterial signal may suggest elevation of the RI. Whenever possible, examinations should be performed in a standard way using a departmental protocol.

False-negative US results are related to the limitations of the technique. Gas within the vascular or ductal systems may be interpreted as calcifications. Aneurysms and venous varices may be mistaken for lymphoceles unless duplex signals are routinely assessed. The degree of hydronephrosis may be underestimated in a patient with acute transplant rejection. Comparison with prior findings and correlation with the results of other examinations help to prevent most false-negative errors.

Findings

Nuclear medicine studies have a role in all stages of postrenal transplantation evaluation. Immediately after surgery, nuclear medicine studies can be used to evaluate diminished renal function. The most immediate cause of renal transplant failure may be renal artery occlusion. During the period immediately after the bolus IV injection of the technetium Tc 99m mercaptotriglycylglycine (MAG3) agent, both images and activity counts are collected from the area of the abdominal aorta and the renal transplant site. Visualization of the renal artery and the renal transplant should occur within 20-30 seconds, with no significant difference between the time of aortic and renal visualization.

The causes of diminished renal function include ATN, rejection, and drug nephrotoxicity. ATN is the most common cause of delayed graft function. ATN is associated with prolonged ischemia and reperfusion injury. ATN occurs after placement of most cadaveric grafts. Recovery is usually spontaneous within 2 weeks. ATN is less common in patients whose transplants are from living related donors.

Functional activity of the renal transplant is assessed using activity curves obtained from the renal transplant, aorta, and background. Orthoiodohippurate 131 and Tc 99m MAG3 studies demonstrate a delayed transit with a delayed time to maximum activity and a high ratio of findings at 20 minutes versus those at 3 minutes. Mechanical leaks from the renal pelvis, ureter, or urinary bladder can be detected as extrarenal nuclide collections.

Degree of Confidence

Although complete thrombosis and renal graft infarction is easily determined with nuclide studies, moderately severe transplant renal artery stenosis may not have a clear pattern on a renal scan. In dual (pediatric) renal transplants, the delay in the visualization of one of the kidneys may indicate partial thrombosis or stenosis of the arterial supply to that kidney. However, in most cases, only a single renal transplant is present, and comparing its activity with that of the other kidney is not possible. The degree of renal dysfunction must be compared with the known surgical history and the current laboratory value.

False Positives/Negatives

Renal scans and renograms of transplanted kidneys are subject to artifacts similar to those of other nuclear medicine tests of the abdomen and pelvis. The presence of barium used for other imaging tests may cause artifacts on frontal views.

Findings

Conventional angiography has been replaced by CTA or MRA in many diagnostic applications both in the pretransplantation and posttransplantation periods. Cost, potential morbidity, and possible nephrotoxic effects of iodinated contrast must be considered. In most cases, digital subtraction angiography (DSA) provides the most fully diagnostic evaluation of the renal artery, the recipient's vascular status, and the patency of the renal veins.

In select cases, renal transplant angiography may be preferred to CTA or MRA. In all cases requiring vascular intervention, DSA angiography immediately precedes the interventional procedure. The potential diagnoses best made with angiography include renal artery stenosis, renal vein thrombosis, aneurysms, and arteriovenous fistula. Except for a pelvic location, the diagnosis of renal transplant circulation disease is similar to the diagnosis of renal artery disease in the native kidney.

Degree of Confidence

Direct visualization of the lumen of a blood vessel is considered the most accurate means to determine the presence of arterial stenosis, venous thrombosis, and arteriovenous malformation (AVM) or arteriovenous fistula (AVF). Because angiography is generally performed just before therapy, other testing typically is not performed to confirm a catheter-proven lesion. Vascular flow gradients can be assessed at the time of the procedure to confirm a hemodynamically important lesion prior to treatment and to confirm the absence of a gradient after angioplasty with or without stent placement.

False Positives/Negatives

False-positive findings related to renal artery or iliac artery stenosis involve an overestimation of the degree of stenosis. Measurements made from two or more projections help to reduce errors. Direct gradient determination remains an important procedure.

Both false-positive and false-negative results can occur as a result of movement (breathing) during the angiography. DSA is particularly sensitive to misregistration artifacts. Barium in the GI tract from other tests must be avoided.

Many renal transplant patients undergo renal dialysis during the years prior to receiving the renal transplant. Interventional techniques used to treat thrombosed peripheral arteries have been applied to the removal of clots within arteriovenous (AV) shunts used for renal dialysis. At other times, double-lumen central venous access lines may be used to continue renal dialysis in the event of AV access failure. Interventional radiology plays an important role in the care and treatment of renal transplant recipients.

Historically, catheter angiography has been performed to evaluate the anatomy of the renal arterial and venous systems of potential renal donors. To lower costs and reduce risk, CTA or MRA has largely replaced conventional DSA in the preoperative phase of renal transplantation. However, image-guided interventional techniques play a vital role in the relief of mechanical urinary obstruction, drainage of abscess and cystic fluid collections, and aspiration biopsy of neoplastic and infectious processes.

The development of lymphoceles in the surgical site of a renal transplant is a diagnostic challenge. Occasionally, lymphoceles enlarge to cause compression of the renal transplant pelvis and hydronephrosis. Drainage of lymphoceles enables diagnosis, and it can relieve a mechanical obstruction; however, lymphoceles may recur after catheter drainage. Renal transplant patients' immunosuppression makes the development of opportunistic infections more likely. Evaluation of aspiration biopsy samples in spinal diskitis may reveal fungal or bacterial infections that are not often seen in most other patients.

Interventional techniques commonly used to treat vascular stenosis, including renal artery angioplasty and renal artery stent placement, may be applied to the management of renal artery stenosis. Improvements in the management of transplant rejection have made renal transplant arterial stenosis less common. Percutaneous nephrostomy may be applied to temporarily relieve a mechanical renal outlet obstruction.