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32 Imaging
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Figure 3.7 Examples of urethral abnormalities demonstrated by MCUG. (a) Typical appearance in
posterior urethral valves with an abrupt change in calibre of the urethra at the level of the valve
leaet. (b) Heavily trabeculated bladder indicating long standing outow obstruction. (c) Urethral
stricture – best demonstrated with a simultaneous retrograde urethrogram. (d) MCUG alone does
not delineate the distal extent of the stricture.
DMSA scintigraphy (static renography) / Technical Aspects 33
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Figure 3.8 Ureterocele. (a) MCUG showing a large ureterocele as a lling defect at the bladder
base with (b) Associated reux into the dilated lower moiety of a duplex kidney. (c) US demonstrating the ureterocele at the bladder base. (d) MAG3 renogram demonstrating reux on the left and
the photopenic area of the ureterocele in the bladder.
DMSA SCINTIGRAPHY (STATIC
RENOGRAPHY)
Technical Aspects
Technetium-99m DMSA binds to the proximal
convoluted tubules with only 10% of the injected
dose being excreted in the urine.
e injected dose of Tc99m DMSA is calculated according to body surface area – which is
estimated from the child’s age and weight. Static
images are acquired approximately 2–3 hours
aer the injection of isotope tracer.
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Indications
A DMSA scan provides static images of functioning tissue and can be used to quantify relative (differential) function between two kidneys. Normal
values for dierential renal function are in the
range 45%–55%. It is important to note, however,
that values for dierential function are comparative rather than absolute measures of renal function. Despite signicant impairment of overall
renal function DMSA can therefore yield misleadingly “normal” values for dierential function if
both kidneys are equally aected by renal damage.
DMSA is a sensitive modality for documenting the presence and progression of renal scarring. It is also very useful for identifying small
and/or ectopic kidneys and cryptic or occult
duplication anomalies (Figures 3.9, 3.10). By
demonstrating an “isthmus” of functioning tissue crossing the midline, DMSA can play a useful role in the diagnosis of horseshoe kidneys
(Figure 3.11). It can also be used to conrm
the diagnosis of multicystic dysplastic kidney
(MCDK) by demonstrating that it is entirely
non functioning. During the acute phase of a
UTI, DMSA can demonstrate photopenic areas
Figure 3.9 DMSA images of ectopically located renal tissue. (a) Ectopic left pelvic kidney.
(b) Horseshoe kidney with functioning tissue crossing the midline. (c) Crossed fused ectopic kidney
with (d) US demonstrating the two fused moieties in this patient.
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Figure 3.10 Pelvic kidney. (a) MCUG showing reux into an abnormally sited kidney. (b) MAG3
demonstrating functioning parenchyma in an abnormal pelvic position with rising renogram curve.
Figure 3.11 Horseshoe kidney. (a) DMSA demonstrates the presence of functioning renal tissue
crossing the midline. (b) Coronal reconstruction from a CT angiogram for vascular anatomy pre
surgery also demonstrates the conguration of the kidney (a nephrostomy tube is in place in the
right moiety).
of renal parenchyma aected by pyelonephritis.
However, the changes seen during or shortly
aer acute pyelonephritis may be transient and
in order to demonstrate areas of permanent
renal scarring the scan should be delayed until
at least 8 weeks (preferably longer) aer the
infection has been eradicated. DMSA can also
be used to delineate areas of renal tissue which
are still functioning in cases of renal trauma
(Figure 3.6). Disadvantages include the requirement for venous cannulation, the limited yield
of anatomical information and the reduced
image quality in neonates with immature renal
function. DMSA also imposes a higher radiation
burden than dynamic renography (DR) with
Tc-99m labelled MAG3.
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Dynamic Renography
(MAG3 or DTPA)
Although Dynamic Renography (DR) is still being
used in the majority of centres it is likely to be
largely replaced by functional MRI. e principal
advantages of DR include its ability to quantify
dierential renal function and dierential drainage and the low radiation burden. Dynamic renography is of particular value in the investigation of
pelviureteric obstruction (Figure 3.12), congenital
drainage abnormalities (megaureters and other
structural or functional abnormalities of the renal
pelvis or ureters) and in duplex systems. However,
it is of more limited value in children with reduced
renal function and in those under 2 years of age –
especially infants under 6 months.
Technical Aspects
Dierential renal function is usually acquired at
about 1 to 2 minutes aer the injection of the isotope. Drainage from the kidney is estimated by
assessing the renogram curve, with the normal
pattern being an early peak followed by a rapidly
descending phase. A continually ascending curve
suggests a delay in excretion. In this situation the
administration of a diuretic may help to distinguish between stasis and obstruction. (See also
Chapter 7.)
e most commonly used isotope is Tc99m
dimercaptoacetyltriglycine (MAG3) (which relies
on tubular extraction) whereas the use of Tc99m
diethylentriaminepentaacetic acid (DTPA) - which
relies on ltration is declining.
Figure 3.12 Pelviureteric junction obstruction.
(a) US showing hydronephrosis (b) MAG 3 in
the same case demonstrating delayed transit
of isotope with poor drainage after micturition
and (c) Rising excretion curve in the right
kidney indicating accumulating isotope and
poor drainage.
DMSA scintigraphy (static renography) / Computed Tomography (CT) 37
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Cross-Sectional Imaging
is term is most commonly applied to computed
tomography (CT) and magnetic resonance imaging (MRI). CT plays only a limited role in children because of the high ionising radiation dose.
MRI does not involve any exposure to radiation
but has the disadvantage that sedation or general
anaesthesia are oen required to ensure the child
remains absolutely still during the scan.
Magnetic Resonance Imaging (MRI)
Unlike CT, MRI provides multiplanar images and
also oers superior delineation of dierent types
of tissue (both before and aer contrast media).
For imaging of the urinary tract MRI oers the
additional advantage of providing excellent
delineation of urine-containing structures.
Indications
MR is now the examination of choice for the
assessment of malignant and benign renal
tumours (Figures 3.13, 3.14). In this context, MR
angiography can yield particularly useful information on tumour vasculature prior to surgery.
e role of preoperative MR angiography is not
conned to malignant conditions and, for example, it can demonstrate the presence of aberrant
“crossing vessels” prior to a pyeloplasty or vascular
hitch procedure for pelviureteric junction (PUJ)
obstruction (Figure 3.15). Because many paediat-
ric urological procedures are now being performed
laparoscopically, MR is playing an increasingly
important role by providing the surgeon with anatomical information which could previously only
have been obtained at the time of open surgery.
MR is also a very valuable means of dening
complex upper tract anatomy – notably in duplex
systems with ectopic insertion of ureters.
MR angiography forms an important part of
the work-up for renal transplantation by delineating vasculature within the abdomen prior to
surgery (Figure 3.16).
the duration of the scan. Infants under 6 months
can be immobilised using a “feed and wrap” technique in which they sleep throughout the study.
Older infants can be scanned with sedation but
between the ages of 18 months and 5 years general anaesthesia is usually required.
e child must be well hydrated and venous
access is necessary for the administration of furosemide (to promote diuresis) and a gadoliniumbased contrast medium.
e term “sequence” is applied to a selected
group of dierent technical parameters which
determine the characteristics of the resulting
image. T1 weighted images provide anatomical
detail whereas T2 weighted images are a better
guide to pathology. On T2 images, water returns
a high signal (white) and therefore T2 weighted
sequences are very useful for imaging structures
such as the renal pelvis, ureters and bladder. e
use of gadolinium intravenous MRI contrast
medium is very useful for assessing tumour vascularity and in demonstrating kidney function
and excretion.
MR urography (MRU) is being increasingly
used to provide accurate anatomical evaluation
of abnormalities of both the upper and lower
urinary tract. Heavily weighted T2 sequences
are generally used for this purpose because of
their ability to clearly delineate urine-containing
structures. MRU has the added advantage of providing high resolution images of the renal parenchyma and collecting system during all phases of
kidney function – vascular, ltration, and excretion. It is capable of providing detailed images
of the ureters and any part of the urinary tract
which is dilated. e use of MRI is also being
extended to include the acquisition of the same
type of functional information which is provided
by a MAG3 or DMSA scan.
In summary, MR is rapidly becoming regarded
as a standard “mainstream” form of diagnostic
imaging in paediatric urology.
Computed Tomography (CT)
Technical Aspects
Scan times can be up to 30 minutes or even longer,
and the patient must remain absolutely still for
CT is not regarded as a “front line” investigation
in paediatric urology because many of the roles
fullled by CT in adults can be fullled by US in
children. Nevertheless, CT remains an important
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Figure 3.13 Renal tumours. (a) MRI with intravenous contrast. Coronal plane image showing bilat-
eral Wilms tumour in a horseshoe kidney. (b) DMSA in the same patient demonstrating the bilateral tumours as photopenic areas which do not contain normally functioning renal tissue. (c) CEUS
showing poor washout of a hypoechoic lesion at the upper pole of a lesion conrmed as mesoblastic nephroma.
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Technical Aspects
Intravenous access is required for the administration of contrast medium – with the exception of
scans being performed to visualise calcication
(calculi) for which contrast is not required. Because
modern multidetector CT scanners acquire images
very rapidly (typically in 5–10 seconds aer the
start of the scan) most children can be successfully
scanned without requiring sedation or anaesthesia.
Contrast is usually administered as a “split bolus”,
typically with two boluses given 30–40 seconds
apart. In this way both portal venous and arterial
enhancement are obtained in a single scan. For a
delayed scan in cases of trauma or suspected bladder rupture the two boluses are given 3–5 minutes
apart. Every eort is made to keep the radiation
dose to a minimum but even with ultramodern
CT scanners the radiation dose from a combined
abdominal and pelvic CT examination is signicantly higher than that of a chest X-ray.
Figure 3.14 Nephroblastomatosis. (a) CT with
intravenous contrast reconstructed in the
coronal plane showing extensive nephroblastomatosis in both kidneys (the misregistration at
the level of the diaphragm is due to the patient
taking a breath during the scan). (b) MRI in the
same patient demonstrates the same nding (in
the transverse plane).
investigation for the evaluation of major trauma
because of its high sensitivity for the detection
of contusions, lacerations, perinephric uid collections, areas of avascularity, bladder rupture,
extra or intra peritoneal leakage of urine, and
injury to other organs. CT also retains a role
in the investigation of stone disease (including colic) – particularly in children with physical features such as marked scoliosis or obesity
which limit the usefulness of US. MRI is a vastly
superior technique for assessing tumours and
has therefore superseded CT for this purpose.
However, chest CT still plays a role for the assessment of pulmonary metastases.
Abdominal X-Ray (AXR)
AXR is no longer regarded as a routine investigation in paediatric urology and its role is now
largely limited to the detection of urinary tract calculi, spinal anomalies, abdominal or pelvic mass
lesions and constipation. US can provide most of
the information previously sought by AXR.
Arteriography
Arteriography may be indicated when renal
artery stenosis is suspected. However, this is usually investigated by US (including Doppler) in
the rst instance and then by MRI. Conventional
arteriography does, however, have the advantage
that it can be combined with renal artery angioplasty under the same anaesthetic. Arteriography
is also useful in middle aortic syndrome, and
again angioplasty may be used. Some renal and
bladder arteriovenous malformations are treatable by embolisation.
Venography
Selective sampling of renal veins (for renin) and
the IVC may be indicated in children whose
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Figure 3.15 Pelviureteric junction obstruction. MRI in a patient with a ‘crossing vessel’ causing pel-
viureteric junction obstruction. (a) Heavily T2 weighted sequence showing marked hydronephrosis
of the right kidney. (b) T1 weighted sequence after intravenous gadolinium. This demonstrates
both the main right renal artery and the accessory artery ‘crossing’ the distended renal pelvis (and
causing the PUJ obstruction. This example demonstrates the ability of MRI to delineate both the
renal pelvic anatomy and the dynamic vascular anatomy in the same examination.
Figure 3.16 Pre-transplant “work-up” MR angi-
ography: coronal images following intravenous
gadolinium performed in the work-up for renal
transplant showing a normal aorta, iliac and
femoral arteries
hypertension is thought to arise from excessive renin production by a scarred or dysplastic kidney. Venography and embolisation are a
widely accepted technique for the treatment of
varicoceles.
Antegrade Pyelography
e principal indication lies in the investigation
of distal obstruction in the upper urinary tract.
Antegrade pyelography via percutaneous renal
puncture can be performed by an interventional
radiologist or by a urologist.
Percutaneous Nephrostomy
is is oen performed under ultrasound control
but may be combined with uoroscopy. e procedure is usually performed under general anaesthesia in children. Percutaneous nephrostomy
is generally preferred to open surgical nephrostomy and drainage. Its main use is in the decompression and drainage of acutely obstructed or
DMSA scintigraphy (static renography) / Renal and Tumour Biopsy 41
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infected hydronephrosis – usually due to PUJ
obstruction.
Renal and Tumour Biopsy
Although ultrasound guided procedures can
generally be performed using sedation and local
anaesthesia in compliant older children, general
anaesthesia is required for the younger age group.
Multiple cores are obtained. Whenever possible
these undergo immediate microscopic examination to conrm that adequate tissue has been
obtained so as to minimise the possible need for a
repeat procedure and further anaesthetic.
KEY POINTS
●
Congenital abnormalities of the
urinary tract account for many of the
conditions encountered in paediatric
urology. A combination of imaging techniques is usually required to
provide the degree of anatomical and
functional information needed to plan
open or minimally invasive surgical
management.
●
Ultrasound is invariably the initial
urological investigation of choice in
children. The scope of ultrasound is
being extended by the introduction
of contrast-enhanced ultrasound
(CEUS) using microbubble contrast
media.
●
Dynamic and/or static renography is
still widely used to assess functional
parameters across a broad range of paediatric urological conditions. However,
MR urography is being increasingly
used for this purpose.
●
MR provides excellent anatomical detail of normal and abnormal
anatomy of the urinary tract without
exposure to radiation. e principal
limitation is the requirement for sedation or general anaesthesia in younger
children.
●
e use of CT in children is limited by
the high radiation dosage. However, it
retains an important role in the evaluation of major abdominal trauma.
FURTHER READING
Dickerson EC, Dillman JR, Smith EA, DiPetro
MA, Lebowitz RL, Darge K. Paediatric
MR urography: indications, techniques,
and approach to review. Radiographics.
2015;35:1208–1230.
Duran C, Beltran VP, Gonzalez A, Gomez C,
del Riego J. Contrast-enhanced voiding
uro-sonography for vesicoureteral reux
diagnosis in children. Radiographics.
2017;37:1854–1869.
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