MAGNETIC RESONANCE ANDCOMPUTED TOMOGRAPHY INPEDIATRIC UROLOGY

Atoosa Adibi MD. Isfahan University Of Medical scienses

Ultrasound (US) is the most widely used and

primary imaging modality for the urinary tract in children.

magnetic resonance (MR) imaging as second step, particularly in pediatric patients.

an advanced pediatric radiology unit : most routine uroradiologic examinations

can be performed with US and MR imaging

exception is:

following an US, additional diagnostic imaging for urolithiasis is needed

in the case of severe polytrauma,including blunt abdominal trauma.

computed tomography (CT)

MR IMAGING OF THE URINARY TRACT

Indications:

congenital anomalies,mainly pelvicaliectasis and/or ureterectasis,

renal and bladder tumors. Infections and vascular anomalies of

the urinary tract

precontrast, postcontrast, and dynamic postcontrast studies.

The precontrast sequences are optimal for depicting the urine-filled pelvicalyceal system and ureter and provide exquisite morphologic detail.

After administering intravenous (IV) contrast, performing dynamic sequences is a better choice for the kidneys, as it provides the information of a non dynamic contrast study in addition to functional information with depiction of the arterial, venous, nephrographic, and urographic phases.

The postcontrast dynamic study can be conductedas MR angiography (MRA).

Precontrast MR urography demonstrating bilateral UVJO.

Procedure Preparation:Hydration with IV fluid

administration starting a half hour before the scan

A bladder catheter is placed.( A distended bladder may have a negative effect on the excretion of urine)

The urine bag is placed below the level of the scanner table

Furosemide (Lasix) is administered IV at a dose of 1 mg/kg (maximum 20 mg), 10 minutes before the procedure.

It is best to place the patient in the prone position if we are evaluating the contrast excretion into the pelvicaliceal system.

Gadolinium-DTPA (Magnevist), has higher (1.208) specific gravity than urine (1.002–1.030) and settles in the dependent position.

a sagittal T2 sequence An axial T2 with fat saturation A 3D T2 with fat saturation The T1 fa tsaturated +post contrast

axial plane in T2 with fat saturation The sagittal T1sequence with fat saturation

Procedural and scan modifications1. Ectopic ureter: the precontrast

series maysuffice to depict the morphologic

findingsand the postcontrast part needs to beadded only if functional evaluations of

thekidneys are requested

2. Cyst versus diverticulum: Calyceal diverticulum fills with contrast in a retrograde manner later than the calyces or renal pelvis. The delay(sometimes needs to be 1 hour or longer)

CT OF THE URINARY TRACT: URO-CT

the main attractions for using uro-CT in pediatrics are availability, fast speed, less frequent/no need for sedation, and lesser cost.

CT may be used as a confirmatory secondary modality, as in the case of CT for urolithiasis.

try to find alternative modalities,completely avoiding potential radiation exposure.

uro-CT needs to be considered as a secondary option if US and/or MR imaging are inadequate, unavailable,or cannot be performed and the clinical suspicion

warrants further imaging clarification.

It is important to note that in blunt abdominal

trauma in children, renal lesions are more frequent.

than in adults because of a nonossified thoracic

cage, thin abdominal wall, and paucity of perirenal

fat. try not to overdo CT, even in the setting of

pediatric trauma

Procedure

preceded at least by USA multiphase study has rarely any

place in pediatric uro-CT

arterial phase: a bolus triggering , or a delay of 8 to 20 seconds

Nephrographic phase: delay of 70 to 100 seconds

the excretory phase: delay can be 5 to 15 minutes.

Additional CT angiography and/or urography are not routinely performed.

For renal trauma, mostly a nephrographic phase

acquisition will suffice.

splitting the contrast bolus and injecting at 2

different time points can produce both nephrographic and urographic phases simultaneously on one scan

Diagnostic Utility

In trauma cases, a meticulously conducted abdominal US and Doppler study is adequate to exclude major renal injury in children.

In the follow-up of traumatic renal findings,

US is also the imaging modality of choice

uroCT and urolithiasis

Suspected stone of the urinary tract when an US

with color Doppler does not depict a stone, but

secondary signs are present, or an US is inconclusive/ negative, and high clinical suspicion remains

a well-hydrated patient is optimal. The patient is placed in prone

position to be able to differentiate an impacted stone at the ureterovesical junction from that of a mobile bladder calculus

Assessment: ……

Diagnostic Utility: difference in usefulness between the 2 tests may not be clinically significant.

CT ANGIOGRAPHY

Indications:Renovascular hypertension traumatic renovascular injury other less common renovascular

disorders

For optimal power injection of the contrast, a suitable size of peripheral IV catheter is necessary

(neonate, 24 G; infant, 22/24 G; >1 year, 20/22 G).

The IV access is first tested with saline atthe same flow rate planned for contrast

injection.

The scan extends from the supraceliac aorta to the upper external iliac arteries

Alternatively, a split-bolus technique may be used, injecting one third to one-half of the contrast volume beforehand and the other two-thirds to one-half for an arterial phase scan. This allows a combination of an arterial and a urographic phase in one single acquisition.

Assessment

In renovascular hypertension, the focus is on

morphologic changes of the renal arteries (stenoses,aneurysms, beadings) and secondary signs

(poststenotic dilatation, collateral formation, focal parenchymal perfusion defects, asymmetric nephrogram, parenchymal scarring)

a 3-year-old patient with hypertension and neurofibromatosis type I demonstrates ashort-segment high-grade stenosis

CT CYSTOGRAPHY

Active contrast filling of the urinary bladder, to

detect extraluminal contrast, which is an indicator of rupture.

Direct CT cystography entails retrogradefilling of the bladder and indirect CT

cystographypassive antegrade filling of the bladder

afterIV contrast administration.

Indications

Bladder trauma with or without known pelvic

fracture and hematuria workup for suspected delayed

spontaneous rupture of augmented bladder

Procedure

For direct CT cystography before bladder catheterization: exclude urethral injuryAn age appropriate Foley catheter is placed; the balloon isnot inflated. A precontrast scan is performed fromthe diaphragm to the ischial tuberosity. A drip infusionis prepared with diluted (10%) water-solublecontrast (eg, 50 mL in 450 mL 0.9% NaCl solution).The bladder is filled until the patient startsto void or the maximal bladder capacity ([age 1, 2] 30, mL) is reached.Wait for about 5 minutes and rescan the abdomen andpelvis. If no contrast extravasation is visualized, itmay be necessary to perform further delayedscan of just the pelvis

Indirect CTcystography is performed after IV contrast

administrationand antegrade filling, particularly in thesetting of polytrauma. This includes occlusion ofthe Foley catheter, if present, when the patient arrivesin the CT suite and a delay of 5 to 10 minutesafter the IV contrast administration before rescanningthe abdomen and pelvis. However,the indirect cystography is much less reliablein the diagnosis of bladder rupture.

KEY POINTS: Ultrasound is the primary imaging modality for the pediatric

urinary tract. Magnetic resonance (MR) imaging needs to be the second

imaging option after ultrasound in children. Functional MR urography (fMRU) provides comprehensive

morphologic and functional information. Computed tomography (CT) is the imaging choice in children

only in the following circumstances: (1) inadequate ultrasound for urolithiasis, and (2) blunt

abdominal trauma in the setting of polytrauma. The choice of CT over MR for uroradiologic imaging is mainly

for ancillary reasons: availability, fast speed, no sedation, and low cost.

In children, CT angiography (CTA) of the urinary tract is primarily performed for evaluation of therenal arteries for suspected stenosis. Direct CT cystography may be necessary for evaluation of bladder rupture.

Diagnostic examination of the child with urolithiasis or nephrocalcinosis

Stones of all composition, with the exception of drugs (e.g. indinavir) and matrix (protein), have distinguishing characteristics of echogenicity and shadowing on ultrasonography.

Ultrasonography has the additional advantages

of wide availability, avoidance of ionizing radiation, ready detection of hydronephrosis, and ability to define some aspects of the anatomy of the urinary tract.

stones as small as only 1.5–2 mm in diameter can be visualized on ultrasonography (US), the success of this imaging method

clearly depends on inter-observer and intra-observer variability and skills.

For the detecting and monitoring of nephrocalcinosis,high-resolution ultrasonography is the optimal imaging

method .Nephrocalcinosis is classified according to the anatomic area involved.

Some pitfalls in the renal ultrasonography of neonates, and especially preterm infants, have to be noted:

Tamm–Horsfall protein (THP) deposits within the renal calyces may look like nephrocalcinosis .

THP deposition,however, disappears within 1–2 weeks, and follow-up will show completely normal kidneys.

the echogenicity of the renal cortex in neonates is physiologically increased, hence detection of cortical nephrocalcinosis can be difficult and may become evident only some weeks later when a rim of cortical calcification becomes visible.

Normal, still hyperechoic kidney of a preterm infant

Tamm–Horsfall kidney

medullary nephrocalcinosis (NC) grade 1

medullary NC grade II (mild increase of echogenicity at whole pyramid)

medullary NC grade III (more severe hyperechogenicity of entire pyramid);

Diffuse corticomedullary NC

vesicoureteral reflux in children Imaging studies are the basis of

diagnosis and management of VUR. The standard imaging tests include renal and bladder ultrasonography and voiding cystourethrography (VCUG).

Indications for imaging studies are as follows:

Imaging after the first UTI is indicated in all children younger than 5 years, children of any age with febrile UTI, and boys of any age with UTI

Children with prenatally identified hydronephrosis should be evaluated postnatally; however, ultrasonography performed during the first 3 days of life may have a high rate of false-negative results because of relative dehydration during the neonatal period

Although the traditional approach in children with UTI has been evaluation for VUR with VCUG or radionuclide cystography (RNC), some authorities now advocate that children with a history of febrile UTI undergo a dimercaptosuccinic acid (DMSA) renal scan, to assess for evidence of kidney involvement, kidney scarring, or both; if DMSA scan findings are positive, VCUG is recommended.

One approach is to perform RNC as the initial screening test in girls and then to perform standard VCUG when VUR is observed. Other clinicians use VCUG for the initial diagnosis and use RNC for follow-up studies.

VCUG is the criterion standard in diagnosis of VUR, providing precise anatomic detail and allows grading of the reflux. The International Classification System for VUR is as follows[3] :

Grade I - Reflux into nondilated ureter Grade II - Reflux into renal pelvis and calyces without

dilation Grade III - Reflux with mild to moderate dilation and

minimal blunting of fornices Grade IV - Reflux with moderate ureteral tortuosity

and dilation of pelvis and calyces Grade V - Reflux with gross dilation of ureter, pelvis,

and calyces, loss of papillary impressions, and ureteral tortuosity

In general, VCUG should be performed after the child has fully recovered from the UTI. However, some children demonstrate reflux only during an episode of cystitis.

Radionuclide cystography

Instillation of technetium-99m pertechnetate into the bladder and observation with a gamma camera is a highly sensitive test for VUR

Advantages include substantially lower radiation doses than with VCUG and the potential for increased sensitivity because of the ability to conduct prolonged periods of observation

Disadvantages primarily consist of the poor anatomic detail, especially of the male urethra

Grade I reflux is poorly detected by this study, because the distal ureters are commonly obscured by the bladder

Grading by nuclear cystography is limited to mild, moderate, and severe grades.

Pediatric Ureteropelvic Junction Obstruction Workup

Ultrasonography After the prenatal presumptive diagnosis UPJ

obstruction or other conditions causing hydronephrosis is made, the neonate should undergo ultrasonographic evaluation, but the timing is controversial. Transient neonatal dehydration occurs 48-36 hours after birth, so follow-up ultrasonography of mild-to-moderate cases of hydronephrosis should be performed after this period. In severe cases, such as very large renal pelvis, bilateral hydronephrosis, solitary kidney, or oligohydramnios, immediate evaluation within 48 hours must be performed; severe hydronephrosis in spite of the oliguric status of the child can suggest the need

Postnatal evaluation consists of a urinary tract study to determine whether the calyceal pelvic dilation with or without renal cortical thinning is present. The most widely used grading system of the severity of hydronephrosis on ultrasonography after birth is SFU system, rather than the anteroposterior diameter of the renal pelvis.

The SFU grading system for hydronephrosis is as follows[19] :

Grade 0 - No hydronephrosis, intact central renal complex seen on ultrasonography

Grade 1 - Only renal pelvis visualized, dilated pelvis on ultrasonography, no caliectasis

Grade 2 - Moderately dilated renal pelvis and a few calyces

Grade 3 - Hydronephrosis with nearly all calyces seen, large renal pelvis without parenchymal thinning

Grade 4 - Severe dilatation of renal pelvis and calyces with accompanying parenchymal atrophy or thinning

Voiding cystourethrogram (VCUG)

Vesicoureteral reflux (VUR) has been found in as many as 40% of affected children. The degree of reflux is often low grade, not contributing to upper urinary tract obstruction, and it is likely to spontaneously resolve. However, UPJ obstruction may also be seen with severe VUR when the tortuous dilated ureter develops a kink in the UPJ area, which is relatively fixed to surrounding structures, and may cause secondary obstruction.

Diuretic renography

Diuretic renography is the most widely used noninvasive technique to determine the severity and functional significance of UPJ obstruction. Various protocols and techniques have been developed, resulting in significant variability in the interpretive criteria and results.

Doppler ultrasonography [

The development of Doppler ultrasonography has become another useful diagnostic modality in the assessment of kidneys with ureteropelvic junction (UPJ) obstructions. With duplex Doppler ultrasonography, intrarenal vasculature can be assessed to determine the resistive index. Normal kidneys reliably demonstrate resistive indices less than 0.7, and obstructed kidneys show higher values. Administration of diuretics can aggravate the preexisting obstruction, thereby aiding the diagnosis by Doppler ultrasonography. It is especially reliable in the preoperative diagnosis of aberrant-accessory blood vessels associated with UPJ obstruction.

Intravenous pyelography (IVP)

IVP has been used to evaluate UPJ obstruction, but IVP may not provide adequate information to determine the true obstruction, and it is especially difficult to interpret in children. IVP provides information about the obstruction and contralateral side and especially facilitates operative planning; however, infant urograms are compromised by the immature renal function, which impedes adequate visualization of the collecting system. Bowel gas and underlying bony structures also make interpretation of the urogram difficult.