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5. DYNAMIC RENAL SCINTIGRAPHY IN UTI
Dynamic renal scans belong to some of the commonest performed examinations in paediatrics and are indicated:
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 . 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 .
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 . 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 . 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 .
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 .
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:
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 .
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 .
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