The Transplanted Kidney

, Nicola Carboni¹, Daniela Dabbene¹, Giuliano Peta¹, Paola Piccinni¹, Anna Clara Renzi¹ and Gian Marco Giuseppetti¹

(1)

University Politecnica delle Marche, Az. Ospedali Riuniti, Ancona, Italy

Ultrasound evaluation of the transplanted kidney is fairly simple because the graft is commonly placed in the retroperitoneal space of the right iliac fossa in a superficial position. The vessels are in the medial side of the organ and are connected with the recipient’s common or external iliac vessels. After identifying the transplanted kidney, images are acquired in the transverse and sagittal planes, and the renal size is recorded. The adjacent soft tissues are scanned to identify any perirenal fluid collections. After that, color or power Doppler is applied to evaluate the parenchymal flow and to identify the renal hilum and the anastomoses. Flow quantification can be measured by the resistivity index (RI), pulsatility index (PI), and systolic/diastolic ratio.

The study is normally performed with a convex transducer with variable frequency between 3.5 and 5 MHz (Fig. 8.1). The healthy graft has comparable ultrasound features to the healthy native kidney; however, a more detailed two-dimensional image is obtained as the transplant is usually located more superficially. Graft sizes are similar to native kidneys; however, gradual increase of its dimensions can be seen over the first few weeks by up to 32 % of the initial length by the first month. The size of the vessels can depend on whether the donor is living or not which, in the first case, are smaller.

Fig. 8.1

US: B-mode shows regular transplanted kidney with bipolar diameter of 11.2 cm

The Doppler ultrasound is performed by positioning the color box in the area of study (Fig. 8.2). The pulse repetition frequency (PRF) is set to 1–1.5 KHz and wall filter to 100 Hz, and the gain is set in order to optimize the image without having aliasing or color bleed. The spectral analysis is executed with the sample volume positioned in the vessel lumen of the interlobar artery.

Fig. 8.2

CDUS: normal Doppler spectra (RI 0.73) of transplanted kidney with regular morphology

The RI is used as a measure of the resistances to the blood flow of the arteries of the transplanted kidney. An RI between 0.70 and 0.80 is considered normal. A value higher than 0.80 is an expression of graft dysfunction (Fig. 8.3).

Fig. 8.3

CDUS: transplanted kidney with regular morphology and Doppler spectra increased (RI 0.85)

The US Doppler plays an important role in the differential diagnosis of vascular, urological, and surgical complications. It has less important impact in the parenchymal complications because of the lack of specific signs. Various elements can affect the RI such as age, heart rate, arterial pressure, and pharmacological therapy, so it has low diagnostic value in the parenchymal complications, especially if not repeated.

The diagnostic impact increases if the measurements are repeated over time and correlated with laboratory parameters like creatinine and azotemia [1–3].

8.1 Complications

The ultrasound is the first, easy to apply and to repeat and easily accessible in posttransplant surgery evaluation and its complications.

The complications can be divided into immediate (within the first week), early (between the first and twelfth week), and late (after the twelfth week of transplant) [4–6].

Aside from this division, it can be classified depending on the anatomical district:

Parenchymal: acute tubular necrosis, hyperacute/acute transplant rejection, chronic rejection, pharmacological toxicity, and infections
Vascular: renal artery stenosis, renal infarct, renal vein thrombosis, arteriovenous fistulas, and pseudoaneurysms
Urinary collector system: urinomas, urinary obstruction, and kidney stones
Liquid collections: hematomas, lymphoceles, and abscesses
Neoplasms and recurrent renal diseases

8.1.1 Parenchymal Complications

8.1.1.1 Rejection

It can be hyperacute, acute, or chronic.

Hyperacute rejection is a rare eventuality that occurs in the immediate posttransplant period (from a few minutes to a few hours after transplant surgery), and it is caused by the presence of preformed antibodies in the receiver’s serum. In a few minutes, the graft undergoes necrosis. The US features are nonspecific and similar with those of the acute rejection or the acute tubular necrosis.

Acute rejection is more common (10–37 %) and occurs in the first 3 weeks after transplantation (20–30 % of nonliving donors). An episode of rejection in high-dose immunosuppressive therapy is an adverse long-term prognostic indicator. Clinically, the patient could be asymptomatic or can have flu-like symptoms. The US and Doppler features appear to be nonspecific:

Kidney enlargement with globular aspect due to edema.
Hypoechoic and enlarged pyramids.
The cortical can be both hypoechoic and thinned or hyperechoic and thickened.
Enlargement of the collector system with obliteration and hyperechoic appearance.
Perinephric fluid collections due to necrosis/hemorrhage.
IR > 0.8. The diastolic arterial flow is lowered in all arterial branches. An inversion of diastolic flow can be seen in the main renal artery.

These findings are similar to those of the acute tubular necrosis; the main difference is the time when they occur since the acute rejection doesn’t usually happen in the first 48 h after transplant surgery.

Chronic rejection is the common cause of late transplant failure. To be classified as chronic, it has to take place at least 12 weeks after transplant surgery (Fig. 8.4). Various factors can contribute to the chronic rejection such as acute tubular necrosis, drug toxicity, and donor age. A progressive renal deterioration is seen. Hyperechoic parenchymal appearance, cortical thinning, reduced number of intrarenal vessels, and mild hydronephrosis could be detected on US examination [5, 7, 8].

Fig. 8.4

CDUS: sampling of interlobar artery of transplanted kidney with abnormal Doppler spectrum, RI mildly increased (0.82). Chronic rejection

8.1.1.2 Acute Tubular Necrosis

Acute tubular necrosis is a more common condition in cadaveric donors than in living donors, due to ischemia and a reperfusion injury. Other causes could be donor hypotension, the hot (>30 min) and cold (>24 h) ischemic period, and age and weight of the donor and receiver. It usually resolves in 2 weeks.

US reveals nonspecific signs such as renal enlargement, reduced echogenicity of parenchyma and pyramids, and low cortical-medullary differentiation. Other authors have reported a thickening of the cortex and increase of its echogenicity with pyramidal prominence (present also in acute rejection).

Renal sinus can be compressed or obliterated from the edema, while the intraparenchymal RI can be increased (>0.8). Doppler evaluation of severe cases can be characterized by the absence of flow during all the diastolic phase [9, 10].

8.1.1.3 Drug Toxicity

The immunosuppressive therapy used in the posttransplant, such as cyclosporine and tacrolimus, is nephrotoxic. It can cause the afferent artery vasoconstriction and interstitial fibrosis. Recently, it has been suggested that the polyomavirus is reactivated by immunosuppression and causes a nephropathy indistinguishable from acute tubular necrosis. From the point of view of US-ECD, you can notice an increase of the indices of resistance that should be correlated with the serum values of the drugs [11].

8.1.1.4 Infections

More than 80 % of transplant patients can present an infectious episode in the first 6 months. Opportunistic infections are facilitated by immunosuppressive drugs, catheters, and urine. Patients present with fever and pain or may be completely asymptomatic.

Ultrasound examination will show focal or diffuse areas of alteration and thickening of the cortical echogenicity. A dilatation in renal chalices and pelvis system associated with an increase of echogenicity may indicate a pyonephrosis (Figs. 8.5 , 8.6 , and 8.7 ), while a rounded focal area with a slight shadow cone is typical of fungal infections (fungus balls).

Fig. 8.5

CEUS of transplanted kidney: renal abscess with a size of 3.5 cm in the lower pole

Fig. 8.6

US B-mode of transplanted kidney: hypo-anechoic lesion in the lower pole with a size of 2.49 cm abscess

Fig. 8.7

Renal abscess studied with CDUS

In acute pyelonephritis, transplanted kidney can appear with an increased volume for acute inflammation, with focal areas of increased or decreased echogenicity in its interior, while the renal sinus appears reduced in volume. The power Doppler, for the ability to detect slow flows in the arteries, is more sensitive, than the simple ultrasound B-mode.

Even the enhanced US has a key role in the diagnosis of acute pyelonephritis posttransplant.

In emphysematous pyelonephritis, the presence of gas content in the parenchyma is due to reverberation artifacts.

Abscesses may look nonspecific, sometimes complex, like cystic, with mixed echogenicity and are usually treated by drainage [12, 13].

8.1.2 Vascular Complications

Vascular complications occur in less than 10 % of patients, but are a major cause of graft dysfunction and are associated with high morbidity and mortality. Once identified, vascular lesions can be easily treated by interventional radiology.

Despite that angiographic MRI is the gold standard for diagnosis of vascular complications, the color Doppler is an excellent noninvasive technique for the investigation of vascular lesions [3, 11].

8.1.2.1 Renal Artery Stenosis

Stenosis of renal arteries is the most common vascular complication. It usually occurs in the first 3 months after transplantation. The segments most affected are the iliac artery proximal to the anastomotic site (atherosclerosis of the donor, by clamping surgical lesion), the anastomosis itself (due to the surgical technique), or proximal to the renal artery (intimal ischemia). The type of anastomosis that most frequently undergoes this type of complication is the T-T anastomosis because they have a possibility of stenosis greater than in those T-L.

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