Current status of imaging diagnosis in the transplanted kidney. A review of the literature with a special focus on contrast-enhanced ultrasonography

Abstract

Objectives: Ultrasonographic scanning is currently the most widespread imaging diagnostic procedure. The method provides real-time morphological, vascular and elastographic information in a non-invasive manner. In recent years, harmonic vascular examination has become accessible using intravenous contrast agents. In urological pathology, this procedure is used in the detection and evaluation of vascular and ischemic complications, in the classification of complex cysts according to the Bosniak system, also in the renal lesions with uncertain etiology and in acute pyelonephritis for the detection of abscesses. The contrast agent (SonoVue) is angiospecific and can be used in patients transplanted immediately after surgery without adverse effects or impaired renal function. Thus, it is desirable to be used in the nephrological pathology of the renal graft and to develop diagnostic models based on the evaluation of renal microvascularization, as well as the quantitative data resulting from the graphical representation of the specific parameters. The purpose of this review is to evaluate the current state of the literature regarding the place and role of contrast substance ultrasound in the early diagnosis of acute renal graft dysfunction and to make a differential diagnosis of this pathological entity.

Method: This review quantifies the role of contrast ultrasound in the diagnosis of acute complications of the renal graft. The research was conducted based on the databases PubMed, MedScape, Cochrane, according to the search criteria such as contrast-enhanced ultrasound + kidney transplant, "time intensity curves" + "kidney transplant", filtered for the period 2004-2018.

Results: In the nephrological pathology of the renal graft, contrast-enhanced ultrasound is a valuable tool, superior to Doppler ultrasound in predicting the evolution of the renal graft, identifying very small early defects in renal microvascularization. A number of studies succeeded in identifying acute graft dysfunction, some of which establish its etiology - humoral rejection versus acute tubular necrosis. On the other hand, the contrast-enhanced ultrasound parameters do not have the ability to distinguish between cellular and humoral rejection.

Conclusions: If, at present, the histopathological examination is the only one that can differentiate with certainty the cause of acute renal graft dysfunction, we consider that contrast-enhanced ultrasound, as a non-invasive imaging technique, opens a favorable perspective for increasing the survival of the renal graft and decreasing the complications in the renal transplant. The combination of other ultrasound techniques, together with contrast-enhanced ultrasound, could lead to the development of new diagnostic models.

Keywords: acute humoral rejection; acute rejection; acute tubular necrosis; contrast-enhanced ultrasound; imaging diagnosis; kidney transplantation; time-intensity curve.

Figure 1

Grey-scale ultrasonographic appearance of a renal graft located in the right iliac fossa. The image quality is superior to the ultrasonographic appearance of the kidney in its native (lumbar) position due to the superficial position of the organ. The kidney appears as an oval structure where in the periphery there is the parenchyma (with more hypo-echogenic areas corresponding to the medulla) and in the center the hyper-echogenic appearance of the pyelocalyceal system.

Figure 2

Ultrasonographic appearance in color Doppler mode of a transplanted kidney. The presence of the circulatory signal starting from the renal hilum and ending with the arch arteries, in subcapsular position, is noticed. The red and blue coloring of the signal is a convention; the color being directly correlated with the direction of movement of the blood column relative to the transducer.

Figure 3

Spectral Doppler ultrasound aspect of a blood flow passing through main artery of a kidney graft. The systolic-diastolic waveform may be noticed – where the minimum speed is upper to the 0 line – suggesting a low resistance circulatory bed. The laminar flow can be seen by placing the velocity vectors in the upper area of the curve. The distribution of vectors above line 0 suggests oriented flow transducer.

Figure 4a

Early arterial phase (<20 sec). On the left: the image created by fundamental echoes. On the right: the harmonic, vascular image. To notice the enhance at the level of the segmental arteries followed by the arcuate arteries.

Figure 4b

Delayed arterial phase (or cortical phase) - lasts approx. 20 – 40 seconds. On the right image it is noticed the full enhancement of the cortical parenchyma; Accentuated hypoechoic aspect of the medulla may suggest the wrong appearance of renal cyst.

Figure 4c

The medullary phase (also called nephrographic) can be followed for 40 – 120 sec after the contrast administration. In this phase, the contrast agent is uniformly captured in the renal parenchyma.

Figure 5

Quantitative analysis of the contrast passage mode in different anatomical regions at of the renal graft.

Figure 6

Parametric, semi-quantitative analysis of the contrast passage mode at the level of the circulatory bed in a limited time. Each moment of penetration of the contrast agent has a different color representation.

Figure 7

Time intensity curve (TIC) in case of acute rejection renal parenchyma. A delay in the wash-in and wash-out can be observed.

Figure 8

Time intensity curve (TIC) in case of normal renal graft parenchyma. A fast wash-in and wash-out can be observed (the outflow slope is “sharp”).