Ultrasound nephrology transplantation …. 28.07.2010 18:16 1

EFSUMB – European Course Book Editor: Christoph F. Dietrich

Nephrology, Transplantation and Shunts

FM Drudi, F Malpassini, N Di Leo

Corresponding Author Prof. F. M. Drudi FrancescoM.Drudi@uniroma1.it

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Content

Content ....................................................................................................................................... 2 Introduction ................................................................................................................................ 2 Evaluation of kidney transplants using gray-scale sonography and color Doppler ................... 2 The usefulness of the resistive index.......................................................................................... 4 Arteriovenous fistula as a complication of percutaneous graft biopsy ...................................... 6 US contrast agents...................................................................................................................... 6 Magnetic resonance imaging...................................................................................................... 7 The use of ultrasound in the assessment of arteriovenous fistulae for hemodialysis access ..... 8 References ................................................................................................................................ 11

Introduction

Renal transplantation is an established cost-effective treatment in patients with end-stage renal

disease [1]. However, after the first year, graft survival curves show an exponential decline in

numbers of functioning grafts. Many causes of renal graft dysfunction are treatable, thus

making prompt detection and diagnosis of complications mandatory. Acute tubular necrosis

(ATN) may cause immediate oliguria and may follow an initial short period of graft function

as well as acute rejection. Finally, there is a possibility of cyclosporine or tacrolimus toxicity.

Evaluation of kidney transplants using gray-scale sonography and color

Doppler

Renal graft measurements (length, width) and anatomical characteristics (corticomedullary

differentiation, the existence of hydronephrosis affecting the graft, perinephric fluid

collections or masses) [Mov.1] as well as vascular flow [3,4] [Mov.2, Fig.1] can be defined

using gray-scale sonography and color Doppler.

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Figure 1 Color Doppler in a normal patient shows blood flow and vascular

morphology (renal artery and renal vein are both visible)

During the first 24 hours after surgery, the graft must therefore be explored using gray-scale

and color Doppler as the first imaging technique according to the protocol or clinical

indication. Absence of perfusion in the graft is a sign of renal artery occlusion. Reverse

diastolic flow in the arteries due to retrograde blood flow during diastole [Fig.2] is a sign of

complete allograft vein thrombosis or acute rejection. Incomplete vein thrombosis is more

difficult to identify. When color Doppler cannot completely rule out arterial occlusion,

stenosis or vein thombosis, CT scans or even arteriography is required [7].

Figure 2 Acute rejection. Typical Color Doppler aspect. The flow under the baseline

indicates the retrograde arterial blood flow during diastole.

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Infarctions can be diagnosed at Doppler ultrasound or power Doppler examination by

demonstrating a lack of blood flow to the infarcted region of the parenchyma. In these cases,

exploration using ultrasound contrast agent is the best option.

Sonographic exploration cannot differentiate between ATN and acute rejection, so ultrasound

guided allograft biopsy is required.

Renal artery stenosis occurs at a rate of 1% to 10%, usually at the site of the surgical

anastomosis. Diagnosis is made by demonstration of a focal and segmental region of flow

abnormality characterized by elevated peak systolic velocity (PSV; normal value 250 cm/s)

with associated turbulence at an adequate insonation angle. The ratio of renal artery PSV

compared to that of the iliac artery can be more useful, because PSV may be variable in the

allograft artery. In addition to this, tardus-parvus waveform abnormalities can be observed in

the renal parenchyma. After sonographic diagnosis of possible renal artery stenosis, magnetic

resonance imaging or computed tomography angiography may be indicated to confirm the

diagnosis before percutaneous transluminal angioplasty is performed.

The usefulness of the resistive index

Resistive index (RI) is usually determined as a standard practice in clinical monitoring. The

RI value depends on the graft vessels, but it is possible that it is even more influenced by the

recipient's vessels and their elasticity. An isolated elevated RI has limited value and is

nonspecific. In the postoperative days, elevated RI values (0.9) [Fig. 3] can be found in

several types of graft dysfunction such as acute rejection, calcineurin inhibitor toxicity, severe

ATN [Fig.4], renal vein obstruction, uretheral obstruction and pyelonephritis. Periodic RI

measurement is therefore useful in patients affected by these complications to help monitor

the graft function [8] [Fig.5].

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Figure 3 RI results 0.90; this is a value which might indicate acute pathology

Figure 4 Sample volume used to calculate Resistive Index is positioned at the level of

the interlobar artery.

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Figure 5 Sample volume used to calculate Resistive Index is positioned at the level of

the arcuate artery.

In chronic allograft function, RI has limited value, because it is not a particularly sensitive

marker of chronic graft pathology and cannot be used in clinical decision-making.

However, it has been shown that an elevated RI value (0.8), measured 3 months

postoperatively, is a predictor of subsequent poor graft function and/or death [9].

Arteriovenous fistula as a complication of percutaneous graft biopsy

Arteriovenous fistula is a possible complication after percutaneous graft biopsy. Doppler

ultrasound depicts arteriovenous fistulae as a localized area of disorganized color that extends

outside the confines of the normal vessel caused by perifistula aliasing [12]. Waveform

analysis demonstrates a high-velocity, low-resistance flow in the supplying artery and a high

pulsatility flow in the draining vein (arterialization).

US contrast agents

Recently, the introduction of second generation contrast agents has given new possibilities

and perspectives to US imaging and quantification of renal blood flow as well as

microvascular tissue perfusion [Mov.3]. CEUS has overcome the limitations of color Doppler

ultrasonography and permits depiction of parenchymal blood perfusion.

CEUS provides the best visualization of perfusion deficit indicating its extension and

allowing the characterization of indeterminate renal lesions, atypical cystic lesions, and the

identification of acute pyelonephritis [17] [Mov.4, Fig.6a, 6b].

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Some authors have concluded that renal perfusion patterns of normal and abnormal tissue can

be visualized using contrast-enhanced phase-inversion ultrasound imaging.

New diagnostic possibilities of CEUS include evaluation of both cortical and medullar vessels

as well as functional evaluation of renal perfusion. Measuring the microbubble transit time is

useful in the diagnosis of renal artery stenosis and in the differential diagnosis between ATN

and acute rejection in transplanted kidneys [13].

Magnetic resonance imaging

Magnetic resonance imaging (MRI) has emerged as an alternative imaging modality in renal

transplant graft assessment [Fig.6c]. MRI is promising due to its multiplanar capabilities and

lack of ionizing radiation, invasiveness, and contrast medium-induced nephrotoxicity. Recent

studies have shown the utility of MRI in the evaluation of the renal graft and peritransplant

region, but its role is not yet firmly established [5]. At MRI, loss of corticomedullary

differentiation on T1-weighted images is the most useful finding indicating rejection.

Figure 6 In the upper part of this image, CEUS clearly shows the parenchymal region

with a perfusion deficit (a). Transplanted patient with infarction in the

central portion of the kidney; CEUS shows a parenchymal perfusion defect

(b). MRI using contrast agent confirms the presence of a hypovascular area

(c).

a

b

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c

The use of ultrasound in the assessment of arteriovenous fistulae for

hemodialysis access

Color Doppler ultrasonography has proved to be effective in the evaluation of arteriovenous

fistulae for hemodialysis access both in the preoperative phase, for the assessment of

anatomical vascular features, and in the postoperative phase, to detect clinically presumed

arteriovenous fistula complications [Fig.7].

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Figure 7 An arteriovenous fistula for hemodialysis access assessed using color Doppler

ultrasonography.

Color Doppler ultrasonographic vein mapping allows the identification of veins that are not

clinically evident. The inability of clinical examinatio8 alone to predict adequacy of venous

outflow causes the use of suboptimal veins and arteries, and this results in a high failure rate

of arteriovenous fistulae and in an increasing use of arteriovenous grafts [11].

Selection criteria for arteriovenous fistula insertion should be the following [12, 13]:

1. Cephalic or basilic veins measuring at least 2 mm in diameter in continuity with the deep

system.

2. An appropriate artery measuring at least 2 mm in diameter with peak systolic velocity of

50 cm/s or greater.

3. Patent ipsilateral subclavian vein.

The arteriovenous fistula complications that occur most frequently are thrombosis-correlated

stenosis and aneurysm [14, 15, 16].

Hemodynamical data with higher diagnostic significance are the mean flow volume (1204

ml/min), the mean maximum velocity in anastomosis (2.7 m/s), and the mean maximum

velocity in the brachial artery (1.35 m/s) [15].

Turbulent blood flow causes extensive vessel wall and perivascular tissue vibration. This

localized tissue vibration causes artifactual color assignment of the perivascular soft tissues,

which precludes adequate visualization of the venous anastomosis [14] [Fig.8].

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Figure 8 Tissue vibration due to turbulent blood flow causes artifactual color

assignment to the perivascular soft tissues.

Diagnostic criteria indicating stenosis at color Doppler ultrasonography are visible narrowing

of the lumen, increased velocity of flow greater than 100% compared with that of an adjacent

normal segment in color Doppler study [17, 18].

Arteriovenous fistula waveform is arterial with systolic velocities of 100-400 cm/sec [19] but

demonstrates high end-diastolic flow because of the low-resistance runoff in the draining

veins [20].

An increase of mean maximum velocity in anastomosis (4.35 m/s) indicates narrowing of the

vessels with the degree of stenosis related to the increase in velocity [20, 21]. On the contrary,

both the mean flow volume and the mean maximum velocity in the brachial artery are found

to be lower in fistulae with venous stenosis [15].

In more than 3% of hemodialysis vascular accesses a false aneurysm is present, and in more

than 50% of cases there is an aneurysm (vein diameter >6 mm). Almost 100% of these

alterations are located at the puncture site [15].

Available data indicate that the mean flow volume is significantly higher in the fistulae with

aneurysms. A strong correlation has been found between aneurysm and calcifications and

aneurysm and fistula age [15].

Increased flow rates, more than 1500 ml/min, are associated with steal syndrome or venous

hypertension.

The Doppler finding noted in steal syndrome was marked reduction of flow in the distal

artery, while high flow rates reaching 600 ml/min in the distal venous limb of the

arteriovenous fistula are associated with venous hypertension [14].

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References

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