RENAL MEDICAL IMAGING

Ultrasound is the first line of investigation in both antenatally detected abnormalities and in symptomatic children.
● The bladder should be scanned first, as voiding may often occur during the examination.
● Measurements of both kidneys, either length or renal volume, should be taken to highlight any difference in size and to provide a baseline for further growth comparison.
● A variety of planes can be used to view the kidneys in children. Often a posterior approach is best for obtaining an accurate bipolar length.
● Ensure that renal pelvic dilatation is not physiological, by rescanning postmicturition.
● Measure the anteroposterior diameter of any renal pelvic dilatation in transverse section through the renal hilum.
● Always scan the bladder immediately after micturition, paying attention to the ureteric orifice and looking for any ureteric or renal dilatation which may suggest reflux. Measure any residual volume.
● Colour Doppler may be helpful in identifying the ureteric orifice, by locating the jets of urine entering the bladder

Normal appearances
After birth the renal cortex is relatively hyperechoic compared to the adult kidney, in strong contrast to the hypoechoic medullary pyramids. The outline of the kidney is often lobulated due to a persistent fetal lobulation. The renal pelvis is relatively hypoechoic, as the fat deposition seen in the adult is not yet present.

Gradually the cortex becomes less hyperechoic with age, the corticomedullary differentiation lessens and fat deposition in the renal sinus becomes more evident. The outline becomes smooth, although fetal lobulations do persist in some adult kidneys.

Normal postnatal growth of the kidneys, in terms of length and volume, is closely related to the height, weight and age of the child. Charts giving normal age- and weight-related values should routinely be referred to. Errors do occur in measurements of renal length with a potential error in the order of 1 year’s growth. Thus follow-up measurements for renal growth should not be undertaken at intervals of less than 1 year.

Anatomical variants and pathology
The duplex system
The duplex system is one of the more common congenital anomalies, occurring in up to 9% of referrals. It stems from aberrant budding of the Wolffian duct in utero, and can take a variety of forms, from complete duplication with two kidneys, each with a separate ureter, to a partial duplication involving the kidney only.

Complete duplication predisposes to reflux, particularly into the lower moiety and subsequently to infection. The upper pole moiety of a duplex kidney is more prone to obstruction either secondary to a ureterocoele or, less commonly, with an obstructed ectopic ureter. In the former case the obstructed upper moiety may be associated with a dilated ureter which can be followed to the bladder where a ureterocoele, that is, a cystic dilatation of the distal ureter, may be seen within the bladder at the ipsilateral vesicoureteric junction. The ureterocoele may extend into the urethra, causing bladder outlet obstruction which, if severe, may result in bilateral hydronephrosis.

In the absence of any dilatation, it may be difficult to demonstrate the duplex kidney on ultrasound. Generally, the kidney is longer than normal and two discrete, hyperechoic sinus echoes can be seen. Ectopic insertion of the upper pole ureter in a duplex system is a cause of urinary incontinence in girls. It may not be possible to follow an ectopic ureter to its distal end, even when dilated, but one may be able to demonstrate that the ureter is passing distally to the bladder.

When there is a strong clinical suspicion of an ectopic ureter an intravenous urogram or MR urogram will be required to identify a duplex kidney and site of ureteric insertion.

Renal fusion and ectopia
The horseshoe kidney is the most common form of renal fusion, in which the lower poles of the kidneys are fused with a central isthmus or ‘bridge’ across the front of the spine. The isthmus frequently lies behind gas-filled bowel and can be difficult to detect. The sonographer should be suspicious of a horseshoe kidney when the lower poles of the kidneys cannot be clearly outlined, particularly when both kidneys look a little smaller than expected for age. Always ensure you see the outline of the lower poles clearly by turning the child prone or by scanning coronally through the side if necessary.

A dimercaptosuccinic acid (DMSA) scan may demonstrate the isthmus or bridge of renal tissue (when the ultrasound scan is equivocal) but only if it is functioning. In some cases the bridge is composed of non-functioning, fibrous tissue. Fusion can take other forms, including an L shape, where one kidney lies horizontally across the midline; crossed ectopia, where both kidneys lie on the same side; H-shaped fusion of the hilar regions; and complete fusion to form a ‘cake’- shaped solitary kidney.

Ectopic kidneys occur most frequently in the pelvis. In rare cases the kidney may be situated in the thorax. Ectopic and horseshoe kidneys are often associated with a degree of malrotation of the kidney. This can be associated with a degree of obstruction at the pelviureteric junction, and predispose to the development of renal calculi.

Renal agenesis
The kidneys form from the ureteric bud, which arises from the pelvic area during the fifth to sixth week of gestation. The bud undergoes numerous divisions, forming the ureters, renal pelvis, calyces and renal tubules. Any interruption of this process may cause renal agenesis or ectopia. Bilateral renal agenesis is lethal and is usually diagnosed prenatally. The incidence of unilateral renal agenesis is about 1:450 live births and is usually prenatally detected. Ultrasound is useful in confirming the prenatal diagnosis and excluding the presence of an ectopic kidney. A DMSA scan confirms the diagnosis. Renal agenesis is associated with VATER syndrome and with ipsilateral gynaecological anomalies in girls.

Polycystic disease of the kidneys
Autosomal recessive polycystic disease of the kidney (ARPCDK: infantile) may be diagnosed prenatally. Both kidneys are abnormal, being large and hyperechoic, with loss of corticomedullary differentiation. There is a spectrum of severity of disease and in some cases it may present later in childhood with the milder, juvenile form of the disease. Prenatally the less severe forms appear normal on ultrasound. ARPCDK is associated with hepatic fibrosis and portal hypertension.

Autosomal dominant polycystic disease of the kidney (ADPCDK: adult) also has a wide spectrum of severity. Although it tends to present later in life, the more severe forms can present in childhood and can occasionally be diagnosed prenatally. Frank cysts can usually be demonstrated on ultrasound, but may not be detected until the second or third decade of life. The disease is also associated with cysts in the liver and pancreas, and with intracranial berry aneurysms.

Fungal infection
Candidiasis is a fungal infection which is most commonly seen in infants who are acutely ill or immunocompromised or in sick ventilated neonates. Fungal balls dilate and may obstruct the collecting system of the kidney. Ultrasound is particularly useful in making the diagnosis by demonstrating the hyperechoic fungal balls within the dilated collecting system. Fungal infection may also undergo haematogenous spread to the spleen and liver, where it can result in multifocal abscess formation.

Xanthogranulomatous pyelonephritis
Xanthogranulomatous pyelonephritis results from chronic infection in an obstructed kidney and children present with a history of general malaise, lowgrade fever and flank pain and may be found to be anaemic. The finding of a palpable abdominal mass on examination often leads to an early diagnosis of a possible Wilms’ tumour. On sonography the kidney is diffusely enlarged, with loss of the normal corticomedullary differentiation. The presence of calyceal dilatation with debris and calculi in the collecting system and confirmation of urinary infection in addition to the generalized involvement of the kidney helps to differentiate this condition from Wilms’ tumour. Occasionally CT scanning may also be helpful. The kidney will usually be found to be non-functioning on a DMSA scan and nephrectomy is required.

Renal vein thrombosis (RVT)
RVT primarily occurs in the neonatal period but may occur in the older child, particularly in association with renal malignancy and amyloidosis. Classically the sick neonate is noted to develop gross haematuria in association with a palpable abdominal mass. RVT is usually unilateral but may be bilateral and is associated with acute adrenal haemorrhage when left-sided. Sonographically the affected kidney is enlarged and globular and develops an inhomogeneous echogenicity of the renal parenchyma with areas of increased echogenicity due to haemorrhage.

Thrombus may be detected in the ipsilateral renal vein and IVC and Doppler sonography shows reduced or absent blood flow in the renal vein and loss of the normal variation in the renal vein waveform. Arterial flow is also decreased. On follow-up the kidney may completely recover due to the development of collateral blood flow or early recanalization of the renal vein, but in severe cases the kidney may atrophy and calcify.