Quantitative Assessment of Renal Function In Children with UPJO By Contrast-enhanced Ultrasound  A Pilot

Background: Contrast-enhanced ultrasonography (CEUS) is a new potential modality for the quantitative Objective: To prospectively evaluate the diagnostic value of CEUS in assessing renal function in patients with ureteropelvic junction obstruction (UPJO). Methods: The study protocol was approved by the ethics committee of Beijing Children’s Hospital (Beijing,China), and written informed consent was obtained from the patients’ parents or guardians. Ultrasonography (US), CEUS, and radioisotope renography were performed in 51 children (42 boys, 9 girls; mean age, 6.75 ± 4.14 years) with unilateral UPJO. The slope of the ascending curve (A), time to peak (TTP), peak intensity (PI), and area under the curve (AUC) were recorded during CEUS; the quantitative data were calculated by the QLAB system (semiautomated border tracking, Philips Healthcare) software. Sensitivity and specificity values were determined for CEUS and compared with radioisotope renography. Results: A total of 102 kidneys in 51 patients were depicted by CEUS and the perfusion time-intensity curve (TIC).CEUS depicted 102 kidneys in 51 patients, in whom the perfusion time-intensity curveTICwas determined. The TIC of renal cortical perfusion in all groups showed an asymmetrical single-peak curve, which could be clearly distinguished between the experimental group and the control group. Compared to the control group, the TTP was markedly prolonged but A was significantly decreased in the experimental group (P < 0.05). There was no significant correlation between AUC, PI and DFRbut the correlation coefficient between TTP, A and DFR remained significant (p<0.001).The receiver operating characteristic ROCcurve drawn to differentiate the differential renal function (DRF) using the TTP value provided an area under the ROC curve (AUROC) of 0.86. The diagnostic performance of contrast-enhanced US was better than that of US, as the sensitivity and specificity values were 92.86% and 76.14%, respectively. Conclusions: This preliminary experience represents the first report of evaluating the diagnostic value of CEUS AUC: area under the AUROC: an area under the ROC curve; BVF: blood volume fraction; CEUS: Contrast-enhanced ultrasonography; CKD: chronic kidney disease; DKD: diabetic kidney disease; DRF: the differential renal function; EFSUMB :European Federation of Societies Ultrasound Medicine GFR: glomerular filtration IVP: QLAB: Philips SPECT/CT: computed tomography/computed tomography; TTP: UPJO: US:


Background
Ureteropelvic junction obstruction (UPJO) is one of the most common causes of hydronephrosis and is usually detected by antenatal or neonatal ultrasound screening. The incidence of antenatal hydronephrosis (ANH) is approximately between 1% and 5% [1]. Diuretic renography (DRG) is widely regarded as the gold standard for evaluating differential renal function and upper urinary drainage in UPJO patients. Due to the disadvantage of overestimation, the DRG results are sometimes inconsistent with the actual degree of renal function [2]. Therefore, a more available option that can evaluate differential renal function would be an ideal choice for diagnosis in paediatric hydronephrosis.
With technical improvements, contrast-enhanced ultrasound (CEUS) has become widely used to investigate renal vessels and parenchymal abnormalities [3][4][5]. The contrast agent, microspheres, which are approximately the size of red blood cells and are not filtered or secreted by the kidneys, allow visualization of the renal parenchyma without interfering with the collecting system. It has been shown to be a cost-effective modality for evaluating renal function impairment (RFI) [6]. Studies have shown that contrast-enhanced US is almost as sensitive as radioisotope renography in the quantitative diagnosis of chronic renal insufficiency, and correspondingly, contrast-enhanced US could be a useful tool for the assessment of renal function in children with UPJO [7][8][9]. Ionizing radiation exposure is a known risk factor for patients with leukaemia and brain tumours, especially for children with a higher radiation sensitivity and longer life expectancy [10,11].The European Federation of Societies for Ultrasound in Medicine and Biology( EFSUMB)release guidelines state that intravenous CEUS is safe and effective in paediatric populations and can be used effectively to avoid ionizing radiation exposure [11].
Thus, the purpose of our study was to prospectively evaluate the diagnostic value of contrastenhanced US in the evaluation of renal function in children with UPJO.

Patients
A total of 51 patients with unilateral hydronephrosis (42 boys, 9 girls; mean age, 6.75 ±4.14 years; age range, 1-14 years) who presented to the department of paediatric urology between January and April 2018 were prospectively included. This study was approved by the ethics board of our institution. Written informed consent was obtained from patients. Inclusion criteria: (i) unilateral UPJO; (ii) normal functioning of contralateral kidney; (iii) no surgical treatment. Exclusion criteria: (i) bilateral UPJO; (ii) other urinary system malformations; (iii) history of hypersensitivity reactions to sulfur hexafluoride lipid microsphere components or to any of the inactive ingredients in SonoVue; (iv) patients with right-left shunt, severe pulmonary hypertension (pulmonary artery pressure> 90 mmHg), uncontrolled hypertension and in patients with adult respiratory distress syndrome; (v) for patients with contraindicated dobutamine who exhibit cardiovascular instability, SonoVue should not be used in combination with dobutamine. Urinary ultrasonography, intravenous pyelogram (IVP), and bilateral kidney CEUS were performed in all cases. The glomerular filtration rate (GFR) and DRF were determined by 99mTc DTPA renography.

US and contrast-enhanced US
US and CEUS were performed with a standard abdominal C5-1 convex probe (3.5-5.0 MHz) using an IU 22 system (Philips, Bothell, WA) by one experienced radiologist was blinded to the patients' clinical and pathological information. A 24-gauge cannula was used for the contrast injection. Each bolus dose of 0.03 ml/kg of microbubbles containing Sulphur hexafluoride (SonoVue, Bracco, Milan, Italy) was injected intravenously into an antecubital vein, followed by a flush with 5 ml of normal saline. The first injection was performed to evaluate the normal contralateral kidney. The contralateral kidney was evaluated after at least 15 minutes. At the time of examination, the younger children who were unable to cooperate were orally sedated with chloralhydrate (0.5 mL/kg). The recording was initiated at the start of the first contrast injection and at the start of the second injection, which were defined as "time-0" in all recorded. All time measurements started from these moments and concluded after 60 seconds. Raw images were stored digitally in DICOM format, and all patients underwent 30 minutes of clinical observation.

Image analysis
The affected kidney that met the inclusion criteria was selected as the experimental group, and compared to the opposite normal kidney in the same patient as control group. All video clips were stored digitally for subsequent analysis. An integrated computer workstation (QLab; Philips Ultrasound) was used to perform the perfusion TIC analysis. Ultrasound was performed to obtain the ratio of the depth of calyces to the thickness of the parenchyma (C/P ratio). For the TIC of the CEUS data, regions of interest were drawn located in the renal cortex using QLab software. The regions of interest (ROIs) were placed as close to the same depth as possible in accordance with greyscale images and contrast images. Five ROIs in the peripheral renal cortex were selected with the same size and shape (5 mm×5 mm square), and then the mean value of each perfusion parameter was obtained. TICs for the experimental group and the control group were generated (Figs. 1, 2). The following perfusion parameters of the TIC were recorded: slope of the ascending curve (A), area under the curve (AUC), peak intensity (PI) and time to peak (TTP). For each ROI, the analysis was repeated five times, and then the mean value of each perfusion parameter was obtained.

Statistical analysis
Statistical analysis was performed using SPSS statistical software (version 23.0, IBM, USA). A twosided P value of less than or equal to 0.05 was considered to indicate a significant difference.
Conventional CEUS quantitative parameters were analysed. One-way analysis of variance (ANOVA) and receiver operating characteristic (ROC) curves were used. ROC curves were constructed to determine the optimal cutoff value for evaluating the severity of kidney damage in UPJO patients.

Results
A total of 51 hydronephrotic kidneys matched with 51 contralateral kidneys were evaluated by US, DRG and CEUS. The APD of the hydronephrotic side was between 9 mm and 70 mm (mean: 34.2 ± 13.1). A total of 42 boys and 9 girls with a mean age (± standard deviation) of 6.75 years ± 4.14 (range 1-14 years) were included in this study. The image quality was clear enough for independent US and CEUS readings. A 14-year-old boy felt warmth at the injection site. No patients needed multiple applications. There were no serious adverse reactions to the ultrasound contrast agent application.

CEUS parameters analysis
Among the cortical perfusion parameters, the differences in the TTP and A between the control group and the experimental group were significant (P < 0.05) (Table 1). However, there was no significant difference in the AUC or PI between the two groups. There was no significant correlation between AUC, PI and DFR. The correlation coefficient between TTP, A and DFR remained significant (p<0.001).  Table 2 shows the observed AUROC, sensitivity, and specificity for the optimal cutoff values for each of the ROC curves.

Discussion
Ureteropelvic junction obstruction (UPJO) is the most common cause of hydronephrosis in the paediatric population and is characterized by impairment of urine drainage, leading to hydronephrosis and obstructive changes in the renal parenchyma and loss of differential renal function of the affected side. At present, the most sensitive imaging modality for evaluating DRF is single-photon emission computed tomography (SPECT)/computed tomography (CT).In children with hydronephrosis, 99mTc-DTPA diuretic renography is used to calculate the differential renal function (DRF) and to assess drainage. The DFR determined by 99mTc-DTPA is inaccurate because of the inability to assure quantitative emptying of the bladder and the overestimation caused by the selection of regions of interest, especially in children. Overestimated and supernormal DRF values are related to patient age, asymmetrical kidney size, renal insufficiency, and type of background correction method, especially for higher hydronephrosis grades or patients younger than 2 years old [2,12]. However, there is no dependable substitute for an accurately determined GFR, and diuretic renography is still the gold standard for evaluating the split differential renal function (sDRF) of children with hydronephrosis. The majority of cases only need conservative management or close follow up. For this reason, it is very important to differentiate between those with and without kidney damage without the use of ionizing radiation [13].
In normal physiological conditions, the renal cortical blood supply remains within a relatively constant range. Cortical perfusion gradually decreases with increasing renal insufficiency. Contrast-enhanced US has been used to assess ischaemic renal disorders, complex cysts, and indeterminate renal lesions, as well as for the follow-up examination of non-surgical renal lesions [14]. The major strength of contrast-enhanced US is that it can provide precise information about the renal cortical blood supply. It is able to depict the degree of enhancement compared with the surrounding tissues as well as the contrast distribution. While CEUS has been widely used in renal diseases, including vesicoureteral reflux, diabetic kidney disease (DKD), complex cysts, renal tumours, chronic kidney disease (CKD), ischaemic renal disorder and kidney transplantation, to provide information regarding cortical perfusion. To best of our knowledge, CEUS has not yet been use to evaluate the perfusion alterations in UPJO.As a tracer to assess tissue perfusion, microbubbles would be significant to quantitatively evaluate blood perfusion changes in the renal cortex to identify renal microcirculation perfusion changes, which can be used to diagnose renal damage early.
In the present study, enhanced and non-enhanced images of the renal cortex microvascular beds in both the control group and the experimental group were examined, and the TIC of renal perfusion could easily discriminate between the different levels of kidney injury (Fig. 5). The TIC could be used for the measurement of renal damage. Quantitative analysis showed significant increases in the TTP but decreases in A in patients in the experimental group compared with patients in the control group (P < 0.05). Unlike previous studies, no significant difference in the AUC or PI between the control group and the experimental group has been found in the present research (P > 0.05). Both an increase and a decrease in the AUC have been described in CKD, DKD and ischaemic renal disorder.
This may be explained by the fact that the AUC is influenced by the blood flow velocity, microbubble distribution volume, and interception time. To reduce the bias caused by the interception time, this study selected the first minute after the injection of the contrast agent as the interception time. Only the perfusion images could be obtained; contrast-reduced images and the slope of the descending curve could not be obtained. Therefore, the renal blood perfusion could not be evaluated overall.
More importantly, the TTP and GFR showed a very strong positive correlation, which strongly suggests that CEUS could accurately assess the microvascular perfusion and haemodynamic characteristics of the renal cortex. Quantitative analysis demonstrated that the change in A and the TTP in the UPJO group became more obvious as the disease progressed, which may be related to renal microvascular ischaemia and inhibited perfusion. The parenchymal thinning may be considered a limitation to CEUS, on account of the difficult selection of ROIs in some case.
In our study, we evaluated the diagnostic efficiency and value of CEUS parameters and found that the There was only one patient with contrast agent incompatibility (injection site warmth). No severe adverse effects were observed in our study. The relative rate of adverse reactions is reportedly much lower than in any other image diagnosis method [15]. A European survey confirmed that there were no adverse reactions in more than 4,000 intracavitary paediatric applications, and five of the nearly 1,000 intravenous medications had six minor adverse reactions [16]. A large study from Europe consisting of approximately 24,000 examinations reported a low rate of adverse events (3/23188, 0.013%) and severe adverse events (0.0086%) [17], which are significantly lower than the corresponding rate observed with CT contrast agents (1969/286087, 0.69%), with no risk of contrastrelated nephropathy or nephrogenic systemic fibrosis [18]. Their only contraindication is severe respiratory failure. Patients are not exposed to radiation or put at risk by nephrotoxic contrast agents.
Furthermore, they are not excreted through the kidneys, provide an excellent depiction of renal perfusion and are safe for patients with renal insufficiency [19]. Because the equipment required for the procedure is common, the procedure can be performed at the patient's bedside, e.g., in the intensive care unit.
CEUS has many important advantages, including serving as an ideal imaging modality for monitoring patients treated conservatively and reducing the number of follow-up diuretic renograms, especially in young (under 18 years) patients and women. The technique may represent a useful alternative to DMSA for follow-up examinations to spare nonoperative and postoperative paediatric from radiation exposure. Therefore, CEUS is effective and accurate in children, with the advantages of not requiring radiation exposure or the use of nephrotoxic contrast agents. This study was limited in that it was conducted at a single centre and enrolled a relatively small number of patients. There is a critical need for further exploration in future studies. We believe that future prospective studies are necessary to expand the clinical applications of CEUS in many areas in the paediatric population.

Conclusions
This preliminary experience shows a new method for measuring renal function noninvasively using ultrasound. Our study suggests that the method may represent a useful and accurate alternative to diuretic renography in evaluating renal function without radiation exposure.

Consent for publication
Not applicable.

Availability of data and materials
The datasets generated and analysed during the current study are available from the corresponding author on reasonable request.

Competing interests
The authors declare that they have no competing interests

Supplementary Files
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