Материал: Advanced Imaging of the Abdomen - Jovitas Skucas

KIDNEYS AND URETERS

Stones and hydronephrosis also develop in these patients.

The HIV-infected patients on indinavir sulfate therapy, a protease inhibitor used to treat HIV infection, develop ureteral obstruction due to nonopaque indinavir crystals.

An occasional AIDS patient develops a spontaneous perinephric hematoma.

Infection

Similar to other sites, renal infection with Pneumocystis carinii, aspergillus abscess, cytomegalovirus, and other unusual organisms develop in these patients. Some of these infections eventually evolve into multiple, punctate parenchymal calcifications. Focal mucormycosis mimics a renal neoplasm.

Tumor

Simple renal cysts are considerably more prevalent in children with AIDS that in normal children.

Renal involvement with lymphoma is common in AIDS. Even rarer lymphomas develop, such as a non-Hodgkin’s angiocentric renal lymphoma, which shows a propensity to invade and destroy small vessels.

Renal leiomyosarcomas develop in HIVinfected patients, including children.

Postoperative Changes

Kidney Transplantation

An uncommon indication for renal transplantation is bilateral, and synchronous renal cell cancers requiring bilateral nephrectomies. These patients undergo bilateral nephrectomies, a waiting period of several years on dialysis follows, and then they undergo renal transplantation. Such an approach is undoubtedly uncommon; most patients with early and low-grade renal cell carcinomas undergo partial nephrectomies.

An occasional patient receives both a kidney and a pancreas transplant; the consequences of the transplanted pancreas, with its exocrine secretions draining into the bladder, are discussed in Chapter 9. For a number of indications

rare patients have also received both kidney and bone marrow transplantation; these dual transplants are not discussed in this book.

Pretransplant Donor Evaluation

Preoperative donor kidney evaluation is performed to detect accessory renal arteries and other anomalies. In a setting of multiple renal arteries, donor renal artery reconstruction rather than ligation preserves renal mass. Although generally not affecting gross renal function,ligation of accessory renal arteries tends to be associated with focal renal infarction.

Horseshoe kidneys are used for transplantation. When possible, a horseshoe kidney is divided at the isthmus and the halves are transplanted into two recipients. Part of a living donor horseshoe kidney can be used.

Traditionally, renal vasculature was evaluated with angiography, a role partially supplanted by CT and now by MRA. Intraarterial DSA achieves over 95% sensitivity and specificity in detecting accessory renal arteries in kidney donors. Other abnormalities detected by arteriography include fibromuscular dysplasia and atherosclerosis, findings often affecting planned surgery.

Preoperative CT and MRA in living renal donors detect essentially the same number of renal arteries, with disagreement primarily among several-millimeter-diameter accessory vessels (134). Multislice helical CT with transverse, coronal, and 3D reconstructions detects most accessory arteries and early branching and defines other relevant anatomy, but these are rather technically complex studies requiring close attention to detail.

Gadolinium enhanced MRA correctly identifies the arterial supply to native kidneys (including accessory renal arteries), most proximal arterial branches and some anomalous draining renal veins (135,136). Addition of MIP, volume rendering and SSD algorithms may improve image presentation, but they do not improve accuracy. MRA can also evaluate renal size and fetal lobulations without the potential angiography complications. Reported sensitivities of detecting accessory renal arteries range from 75% to 100%, with the latter being rather optimistic. Also the ability of MRA to detect fibromuscular dysplasia is not established.

Reported Doppler US sensitivities in detecting accessory renal arteries range up to 100%,

but these studies are plagued by low specificities.

MR urography can potentially detect urinary collecting system abnormalities in living renal donors. Whether the technique is sufficiently accurate in actual practice to detect ureteric duplications and other relevant anomalies is not clear.

Transplantation

Whether a voiding cystourethrogram is performed in transplant recipients prior to renal transplantation varies between institutions. Some believe this study adds little value.

Some children requiring renal transplantation also have bladder dysfunction, such as posterior urethral valves, a neurogenic bladder, or vesicoureteral reflux, and augmentation cystoplasty is added either before or after transplantation.

With the exception of small children, the transplanted kidney is nearly always placed in the iliac fossa, extraperitoneal in location. Here the kidney is close to major vessels and bladder and is supported by surrounding structures. A side-to-end arterial anastomosis goes to the external iliac artery and an end-to-side venous anastomosis goes to the external iliac vein. The left iliac fossa is used if a second transplantation is necessary; third transplantations are high in the right iliac fossa.

Renal transplantation is feasible in patients infected with Schistosoma haematobium. Pretransplant antischistosomal chemotherapy controls posttransplant schistosomal infection; the increased risk for bladder cancer in these patients justifies close follow-up.

Posttransplant Evaluation

Clinical

Rough data from several sources suggest that first-year posttransplant mortality is about 4%, mostly due to infection; transplanted kidney loss ranges from 5% to 10%, mostly due to rejection, with a subsequent graft loss of about 5% per year. The half-life of transplanted kidneys varies widely, depending on donor compatibility and whether a living donor kidney or a cadaver kidney is used.

The most common direct surgical complication is ureteral obstruction.Other complications

ADVANCED IMAGING OF THE ABDOMEN

include ureteral or bladder fistulas, bladder outflow obstruction, ureteral stones, and lymphoceles. Transplantation complications can be divided into early and late. Early ones include rejection, urine leaks, and obstruction. The major late complication is renal artery stenosis.

Posttransplant drug therapy can result in hypersplenism and portal hypertension; these patients present with splenomegaly and thrombocytopenia.

Imaging

Because most transplanted kidneys are located rather superficially, gray-scale US can be performed with a higher resolution transducer than usual. As a result, better anatomic detail is achieved than is possible with a native kidney.

Regions of decreased color on color Doppler US scans in a transplanted kidney appear to be related to focal perfusion abnormalities. Such focal hypoperfusion regions include infections, arteriovenous fistulas, a kinked artery, and severed accessory arteries.

A color Doppler US finding of a significant decrease in interlobar artery blood flow, but with no flow changes in segmental arteries, suggests acute rejection. Decreased interlobar artery blood flow is also found in renal artery stenosis and interstitial edema.

One subset of patients consists of those with an oligoanuric allograft suspected to be due to either severe rejection or renal artery or vein thrombosis. Both Tc-99m-DTPA scintigraphy and color Duplex US can differentiate minimal and not perfused renal allografts.

Contrast-enhanced MRI can differentiate between graft kidneys with normal function, mild dysfunction, and severe dysfunction. Magnetic resonance imaging in transplant patients with normal renal function typically shows an expected postcontrast increase in signal intensity of the renal cortex and medulla, followed by a signal intensity decrease in the medulla. Patients with acute allograft rejection have less postcontrast increase in cortical signal intensity than those with normal allografts. Magnetic resonance imaging cannot, however, differentiate between normal, acute rejection and acute tubular necrosis, but MRA can evaluate renal artery anatomy (Fig. 10.35).

Scintigraphy evaluates function in a transplanted kidney. A Tc-99m-MAG3 renogram

Rejection

Differentiation between acute tubular necrosis and acute rejection is of obvious importance, but imaging provides only limited assistance, and a biopsy is generally necessary to evaluate graft dysfunction and distinguish between these two. Biopsy complication rates are low, although bleeding, pseudoaneurysm formation, or an arteriovenous fistula do form if an adjacent artery or vein is injured.

Acute tubular necrosis is due to transplanta- tion-associated ischemia and occurs within a day or so. Acute rejection occurs from the time of surgery to several months after transplantation. Rejection leads to renal swelling, a finding seen with other complications. Viral infections are often in the differential diagnosis; some, such as Epstein-Barr viral infections, are associated with lymphoproliferative disorders and are detected with a polymerase chain reaction.

Doppler US appears helpful in monitoring rejection. The pulsatility index, which is dependent on flow resistance, increases with rejection and in some hands when combined with peak arterial systolic velocity and acceleration index achieves a high sensitivity and specificity in differentiating a normally functioning transplant from a hypofunctioning one. Others, however, find that Doppler US cortical vascularity data do not correlate with rejection, and even severe transplant rejection can be associated with normal vascularity (138). In children with chronic rejection, power Doppler US using a high-frequency and high-resolution transducer (13MHz) found irregular and narrow interlobular vessels, in distinction to children with chronic rejection who have a pal- isade-like appearance (139); the authors ascribe their results to use of a high-frequency transducer, which was possible because of decreased tissue thickness in children.

Postcontrast MRI holds promise in differentiating transplant rejection from acute tubular necrosis. Magnetic resonance signal intensity data after contrast injection shows that in patients with transplant rejection the time to peak intensity in the renal cortex and medulla are longer than in patients with normal grafts; in patients with acute tubular necrosis, on the other hand, renal cortex and medulla times are similar to normal.

ADVANCED IMAGING OF THE ABDOMEN

Using surface coils, adiabatic excitation pulses and in vivo phosphorus MR spectroscopy, phosphorus-31 MR spectroscopy appears to have a role in detecting and possibly differentiating rejection and acute tubular necrosis. Patients with rejected kidneys have a higher inorganic phosphate-to–a-adenosine triphosphate ratio than controls and a reduced pH (140). Rejection and tubular necrosis could be differentiated from each other by pH. These findings need to be placed in a larger clinical perspective.

Scintigraphy in acute rejection reveals decreased perfusion, a finding also seen in other complications. An initial study shortly after the transplant is useful in establishing a baseline.

Xenon-CT is used to study cerebral perfusion; it can also measure regional blood flow in a transplanted kidney. Although differences in perfusion exist between normal kidney medulla and cortex, these perfusion differences are less evident in patients with chronic rejection.

Vascular Complications

Arterial Stenosis

Posttransplantation hypertension is common. In about one third or so of these hypertensive patients their hypertension is related to renal artery stenosis. The prevalence of renal artery stenosis is more common in cadaveric donor kidneys compared to living donor kidneys. This stenosis-associated hypertension can be treated medically, by angioplasty, or by surgical revascularization.

Stenoses occur at the anastomosis, distal to the anastomosis (donor artery), or, least common, proximal to the anastomosis in the recipient’s artery (Fig. 10.36). Stenosis may be related to surgery or may be part of rejection. It can recur after angioplasty.

Acceleration time, obtained with Doppler US, is prolonged with a significant proximal arterial stenosis, although the clinical accuracy in detecting arterial stenosis varies depending on the assumed threshold. Another measure is transplant artery peak systolic velocity, but here also results have been inconsistent. Peak systolic velocity varies considerably even without a stenosis, although use of the iliac artery as a standard is helpful.

Magnetic resonance indications in suspected renal artery stenosis continue to increase and gadolinium-enhanced MRA is assuming a major role in detecting transplanted renal artery stenosis, in some studies achieving sensitivities of >85% and specificities approaching 100% and in some patients even being superior to DSA. Depending on the sequences adopted and the type of reconstruction, transplant artery stenoses, renal vein thromboses, extrinsic compression, and perfusion deficits are identified with varying degrees of accuracy (Fig. 10.37). The relative roles for color Doppler US and MRA are not settled, but MRA appears to slightly overestimate renal artery stenosis while color Doppler US is more prone toward falsepositive results.

Posttransplant renal artery stenosis is amenable to percutaneous transluminal angioplasty, and in a number of institutions angioplasty is the procedure of choice for this complication. In experienced hands complication rates are low. Surgical revascularization is indicated if angioplasty is unsuccessful, although a restenosis after angioplasty can be treated with an endoluminal stent placed across the site of restenosis.

Arterial Thrombosis

The etiologies of renal artery thrombosis occurring shortly after transplantation include hypotension, renal artery stenosis, graft rejection with extensive arteriolar occlusions, and retrograde renal artery thrombosis, or it may be related to the surgical procedure.

Neither arterial nor venous flow is detected with Doppler US distal to a complete occlusion.

Vein Occlusion

The consequences of renal vein thrombosis are similar to those of renal artery thrombosis because the transplanted kidney has no additional collateral veins.

In the absence of proximal renal artery stenosis, Doppler US findings suggestive of partial venous occlusion after transplantation include the presence of small amplitude arterial waveforms but with diastolic flow still present; with complete renal vein obstruction no venous flow is evident and diastolic flow in the renal artery is reversed.

Vascular Fistula

The gold standard in detecting vascular fistulas is arteriography. Doppler US detects only some of these fistulas and early-phase contrast CT should be considered in a setting of normal Doppler US but clinical suspicion of a fistula. These fistulas are successfully occluded with transcatheter embolization; nevertheless, the complication rate is high even after successful embolization and includes renal artery occlusion and major hemorrhage necessitating a nephrectomy.

Ureter Complications

Renal pelvic and ureteral complications develop in roughly 10% to 15% of patients undergoing renal transplantation and include obstruction, necrosis, and urinary fistula. They are more common in a setting of multiple donor kidney renal arteries. Urinary obstruction occurs roughly equally in living-related donors and cadaveric donors, but leakage is more common in living donors. Ureter complications are managed with a percutaneous nephrostomy, transurethral bladder drainage, or drainage of fluid collections.