Objective
To evaluate the applicability of 3-dimensional evaluation of renal vascularization for predicting postnatal renal function in fetuses with suspected urinary obstruction.
Study Design
Fetuses were evaluated by 3-dimensional power-Doppler histogram, and vascular indices were estimated. Depth between the probe and the renal cortex was also evaluated. Postnatal follow-up was obtained in all cases and the main outcome was renal impairment.
Results
Twenty-three fetuses with urinary dilatation (cases) and 73 with normal renal morphology (controls) were included in the current study. Five (21.7%) cases developed renal impairment. Vascularization index and vascularization and flow index were significantly lower in fetuses that developed renal impairment compared with those with normal renal function ( P = .009 and P = .036 , respectively). The 3 vascular indexes correlated with depth. Percentage of depth-corrected vascularization index and vascularization flow index were lower in fetuses developing postnatal renal failure.
Conclusion
Fetal renal vascularity (vascularization index and vascularization and flow index) was significantly lower in fetuses that developed renal impairment.
Obstructive uropathy is a major cause of chronic renal failure during childhood, accounting for 20% of cases of end-stage renal failure. Around 80% of children with urinary tract dilatation are diagnosed in utero. In such cases, parental counseling is of major importance. When dilatation is detected during fetal life, counseling is based mainly on ultrasound evaluation, and sometimes on biochemical analysis of fetal urine or blood. Several ultrasound parameters have been proposed as predictors of long-term renal function, such as amniotic fluid index and renal parenchyma evaluation. However, studies evaluating these parameters have not yielded sufficiently high sensitivities and specificities to allow for accurate counseling using ultrasound alone. The study of fetal or urine biochemistry has been considered to increase the precision of the prenatal evaluation of fetal renal function, but more recent studies have questioned their role in the evaluation of fetuses with urinary obstruction, and many centers do not consider biochemical evaluation sufficiently accurate to allow its routine use in this setting.
Three-dimensional (3D) quantification of blood flow using power Doppler and virtual organ computer-aided program (VOCAL) provides an estimation of 3 vascular indexes: vascularization index (VI), flow index (FI), and vascularization and flow index (VFI). A recent study performed by Morel et al observed a linear correlation between vascular indexes and real blood flow in placental cotyledons in an animal model. Furthermore, these indexes have also been studied in some fetal organs, particularly placenta and lung, to predict postnatal prognosis in fetuses displaying diaphragmatic hernia and intrauterine growth restriction.
Because it is known that fetuses with renal failure caused by obstruction have low-glomerular count, the current study aimed to evaluate renal vascularization using 3D renal power Doppler to verify possible correlation of vascular indexes with postnatal prognosis.
Materials and Methods
A prospective case-control study was conducted at the Necker-Enfants Malades Hospital (Paris, France) between May 2008 and July 2009. Inclusion criteria were singleton pregnancy, no maternal disease, gestational age between 20 and 40 weeks and:
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Cases: fetuses referred to the prenatal care center for bilateral hydronephrosis, defined by an anteroposterior measurement of the renal pelvis greater than 5 mm at the second trimester, or greater than 7 mm at the third trimester, or sustained dilatation of the bladder.
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Controls: fetuses with normal evaluation of morphology at the second-trimester scan followed at the low-risk pregnancy clinic of our center.
Exclusion criteria were patient withdrawal from the study and/or unavailability for follow-up.
Each fetus underwent 3D evaluation of both kidneys, to study renal vascularization, using a Voluson 730-expert ultrasound machine with 4-8 MHz transducer (Kretztechnik, Zipf, Austria). For all cases, power-Doppler settings were preestablished before volumetric acquisition of the image: Persistence-2; Line filter-Low; Contrast-7; Enhance-2; Reject-0; Quality-normal; Harmonic frequency-middle; Smooth-5/7; Frequency-high; Ensemble-13; Flow resistance-Middle 1; Line density-5; PD Map-5; Balance gray->225; Artifact-on; Line filter-off; Quality-normal; WMF-low 1; PRF-0.3 Khz. Three-dimensional acquisition was performed for each kidney separately. Kidney was acquired longitudinally with the renal artery placed center screen visualized by power Doppler ( Figure 1 ) , using a variable angle according to gestational age and kidney dilatation. Immediately after 3D acquisition of each kidney, the shortest distance between the mothers’ skin and the renal cortex was measured (depth) ( Figure 2 ) . Fetal renal volumes were measured separately for each fetal kidney using the VOCAL. Subsequently, the 3D-power-Doppler histogram ( Figure 3 ) was used to calculate the vascular indices: the VI, FI, and VFI ( Figure 4 ) . The formula used to calculate vascular indexes are as follows : VI = color voxels/(total voxels/background voxels); VFI 1 = weighted color voxels/(total voxels/background voxels) (Background voxels being the anechoic space in the volume).
