5. DYNAMIC RENAL SCINTIGRAPHY IN UTI

Dynamic renal scans belong to some of the commonest performed examinations in paediatrics and are indicated:

• Whenever there is dilatation of the collecting system, then both differential renal function and drainage can be assessed.
• Whenever surgery on the renal pelvis or ureter is planned.
• When indirect radionuclide cystography is required.
• Follow up after renal transplantation when blood flow must be assessed.
• If differential function is required for any other reason.
• A diuretic is required whenever there is dilatation of any part of the collecting system.
• When investigating a child with sustained systemic hypertensioni [12].

The only time a dynamic renogram may provide inadequate differential function is in the presence of very poor renal function and when there is an ectopic kidney present. In these circumstances DMSA is isotope of choice [12]. In patient with unilateral hydronephrosis dynamic renal scintigraphy in 25% has shown 'supranormal' renal function . The explanation for this finding is unclear, but in most cases it is caused by technical problem, likely the inadequate background subtraction of mercaptoacetyltriglycine in the liver [13].

Dynamic renal scintigraphy plays a role in investigation of child with UTI when IRC is required and diuretic renography is useful tool for the investigation of the various causes of hydronephrosis of that of obstruction [4,15]. Hydronephrosis (HN) and hydroureteronephrosis (HUN) is a common finding in the workup of patients with UTI. There are multiple causes for HN and HUN, including vesicoureteral reflux, UTI, previous obstruction (urethral valves), congenital malformations (prune-belly, megacalyces/ megaureter), noncompliant bladder, and urinary tract obstruction(congenital stenosis, tumour, lithiasis). The commonest hydronephrosis seen is that of the dilated renal pelvis, usually referred to as uretero pelvic junction, in which the question of obstruction is raised, since a dilated PUJ does not require surgery [15]. The definition of obstruction is generally taken as a failure of drainage following a diuretic stimulus, yet more attention is now paid to the combination of the function of the kidney as assessed in the first 1-3 min, the degree of dilatation and washout following diuresis. In paediatrics dynamic renal scans are generally carried out in the supine position, and thus failure of drainage may be related to the absence of the effect of the gravity. However, the other causes for poor drainage include a full bladder, massive dilatation and very poor renal function [5]. Another important factor affecting the outcome of diuretic renography is the patient's state of hydration. The diuresis response is dependent on the availability of fluid within the tissues to produce urine and respond to the diuretic stimulus [14].

The differential function is calculated using the Patlak-Rulland plot which is a simple robust technique, avoiding the detrimental effect of blood background.

Some investigators advocate the use of mean transit times as a further quantitative parameter useful in the diagnosis of obstruction. However, its use in paediatrics is limited due to the poor statistical information gained from the small cortex in children which renders deconvolution unfeasible [14].

Of the 99mTc labelled compounds either DTPA or MAG3 can be used. If one wishes to achieve high reliability and reproducibility in the analysis of the renogram, especially in the infants then there are good reasons to choose MAG3:

• The biodistribution of MAG3 is mainly intravascualr. MAG3 is bound to plasma protein. This is very important especially in children under 2 years of age because of the relatively large extracelular space. Using DTPA, which is freely diffusable from the intra to extravascular space (DTPA is a small molecule, not bound to plasma protein crosses the capillary membrane easily), the amount in the intravascular space is small and so the kidney is presented with small concentration of isotope compared to MAG3.
• The rate of maturation of the glomeruli and tubules is different. The GFR rises steadily over the first two years from approximately 30ml/min/1.73m2 at birlh to 80% of the adult value by that stage. The tubules, on the other hand, mature more rapidly and have reached 90% of the adult value in the handling of MAG3 by 6 months of age. MAG3 is excreted by the proximal tubules of the kidney, while DTPA is excreted via glomerular filtration. This results in the extraction fraction of MAGS from the kidney being much greater than DTPA [5,12].
• If indirect radionuclide cystography is undertaken, the higher renal extraction and subsequent low background make MAGS the tracer of choice. It is rapidly cleared from the blood stream by tubular secretion and is not retained in the parenchyma of normal kidneys, leading to high target-to-background ratio and excellent imaging characteristics [13,15].
• If diuretic renography is undertaken MAG3 has several advantages that include a rapid renal clearance and primary excretion by tubules on which furosemide acts. In the presence of obstruction or poor renal function, the absorbed radiation dose to the patient will be less compared with other renal radiopharmaceuticals [15].

Patients undergoing diuresis renography should be at least 1 month old to reduce the likelihood of immature renal function significantly affecting results. As above mentioned newboms have a lower glomerular filtration rate (GFR) then older children. The 'immaturity' of the kidney may alter the renogram pattern, and could affect diuretic response. Premature infants should be older than 1 month before the initial study, since their tubular function is even less likely to adequately respond to a diuretic stimulus [15].

A diuretic renogram in any child, including an infant, may be undertaken with child supine on the gamma camera face with diuretic(furosemide Img/kg) usually given at 18-20 minutes after the isotope. Post-micturition views at 40 min are possible if the child is allowed to go to the toilet after 30-40 min. Infants and toddlers may simple be held on parent's shoulders for 7-8 min, during which time micturition will occur in the vast majority of cases [5].