A Comparative Study of Normalized Residual Activity and Standard Parameters in 99mTc-DTPA Diuretic Renography:

Urinary tract obstruction is a common clinical condition encountered in both pediatric and adult populations and represents a significant cause of potentially reversible renal impairment. Persistent obstruction to urinary outflow may result in progressive deterioration of renal function, recurrent urinary tract infections, nephrolithiasis, hypertension, and ultimately irreversible renal damage if diagnosis and appropriate intervention are delayed. Consequently, early identification of clinically significant obstruction is essential to preserve renal function and optimize patient outcomes. However, differentiating true functional obstruction from non-obstructive dilatation remains one of the major diagnostic challenges in daily clinical practice, particularly in patients presenting with hydronephrosis detected by ultrasonography or other anatomical imaging modalities. (1-3) Although ultrasonography, computed tomography (CT), and magnetic resonance urography provide excellent anatomical information regarding the urinary collecting system, they cannot always determine whether dilatation is associated with functionally significant obstruction. Hydronephrosis may occur in the absence of obstruction due to physiological or congenital causes, including an extrarenal pelvis, non-obstructive pelvicalyceal dilatation, vesicoureteral reflux, or postoperative changes. Therefore, anatomical imaging alone is frequently insufficient for determining the need for surgical intervention, emphasizing the importance of functional assessment of urinary drainage. (1-3)

Diuretic renography has become the established functional imaging technique for evaluating suspected urinary tract obstruction. It provides simultaneous assessment of renal perfusion, differential renal function, and urinary drainage after pharmacologically induced diuresis, thereby enabling distinction between obstructive and non-obstructive dilatation. Among the radiopharmaceuticals used in renal scintigraphy, 99mTc-diethylenetriamine pentaacetic acid (99mTc-DTPA) remains widely used because it is eliminated exclusively by glomerular filtration, allowing for the estimation of glomerular filtration rate (GFR) in addition to assessing urinary drainage within a single examination. (1,4)

Interpretation of diuretic renography has traditionally relied on several qualitative and quantitative parameters, including visual assessment of the renogram curve, time to peak activity (Tmax), half-time of tracer clearance (T½), post-diuretic washout, and differential renal function. Although these parameters remain fundamental components of routine clinical practice, each has recognized limitations. Factors such as impaired renal function, severe pelvicalyceal dilatation, inadequate hydration, bladder distension, patient motion, and variations in diuretic administration protocols may significantly influence tracer washout and reduce diagnostic accuracy. Consequently, equivocal or false-positive interpretations may occur, particularly in markedly dilated but non-obstructed collecting systems. (1,4,7)

To overcome these limitations, several alternative quantitative indices have been proposed to improve the assessment of urinary drainage. One of the most promising parameters is Normalized Residual Activity (NORA), which represents the proportion of residual renal tracer activity at a specified time relative to the initial renal uptake. Unlike conventional washout measurements, NORA is less dependent on renal function and collecting system compliance and provides a simple quantitative estimate of residual tracer retention. Previous investigations have demonstrated that NORA improves discrimination between true urinary tract obstruction and non-obstructive dilatation, particularly when measured on post-micturition images. (5-7)

Recent evidence has suggested that NORA demonstrates superior diagnostic specificity compared with conventional renographic interpretation, reducing the number of equivocal examinations and minimizing unnecessary surgical intervention. Beatović et al. reported that post-micturition NORA correctly reclassified the majority of indeterminate or false-positive renographic studies while maintaining excellent diagnostic accuracy for urinary outflow obstruction. Such findings support the incorporation of quantitative drainage parameters into routine renographic interpretation; however, the clinical utility of NORA has not yet been universally adopted and remains insufficiently evaluated across different patient populations and imaging protocols. (5,7)

Given the continuing reliance on conventional renographic parameters in many nuclear medicine departments and the increasing interest in quantitative assessment of renal drainage, further evaluation of NORA is warranted. Comparing NORA with established renographic indices may

provide additional evidence regarding its diagnostic value and its potential role in improving the accuracy of diuretic renography for patients with suspected urinary tract obstruction. Therefore, the present study aims to compare the diagnostic performance of Normalized Residual Activity with conventional renographic parameters in 99mTc-DTPA diuretic renography for the diagnosis of urinary tract obstruction.

Additionally, Cortical Transit Time (CTT) is an effective metric for distinguishing between obstructed and non-obstructed kidneys. A delayed CTT flags kidneys at risk of functional deterioration if surgery is delayed, whereas a normal CTT signals that the kidney is safe from decline, independent of T_(1/2) values or drainage curve patterns. Ultimately, a normal CTT rules out obstruction as effectively as a favorable diuretic renogram drainage pattern [1].