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 pediatricand adult populations and represents a significant cause of potentially reversible renalimpairment. Persistent obstruction to urinary outflow may result in progressivedeterioration of renal function, recurrent urinary tract infections, nephrolithiasis,hypertension, and ultimately irreversible renal damage if diagnosis and appropriateintervention are delayed. Consequently, early identification of clinically significantobstruction is essential to preserve renal function and optimize patient outcomes.However, differentiating true functional obstruction from non‐obstructive dilatationremains one of the major diagnostic challenges in daily clinical practice, particularlyin patients presenting with hydronephrosis detected by ultrasonography or otheranatomical imaging modalities. (1‐3) Although ultrasonography, computed tomography (CT),and magnetic resonance urography provide excellent anatomical information regarding theurinary collecting system, they cannot always determine whether dilatation is associatedwith functionally significant obstruction. Hydronephrosis may occur in the absence ofobstruction due to physiological or congenital causes, including an extrarenal pelvis,non‐obstructive pelvicalyceal dilatation, vesicoureteral reflux, or postoperativechanges. Therefore, anatomical imaging alone is frequently insufficient for determiningthe need for surgical intervention, emphasizing the importance of functional assessmentof urinary drainage. (1‐3)Diuretic renography has become the established functional imaging technique forevaluating suspected urinary tract obstruction. It provides simultaneous assessment ofrenal perfusion, differential renal function, and urinary drainage afterpharmacologically induced diuresis, thereby enabling distinction between obstructive andnon‐obstructive dilatation. Among the radiopharmaceuticals used in renal scintigraphy,99mTc‐diethylenetriamine pentaacetic acid (99mTc‐DTPA) remains widely used because it iseliminated exclusively by glomerular filtration, allowing for the estimation ofglomerular filtration rate (GFR) in addition to assessing urinary drainage within asingle examination. (1,4)Interpretation of diuretic renography has traditionally relied on several qualitative andquantitative parameters, including visual assessment of the renogram curve, time to peakactivity (Tmax), half‐time of tracer clearance (T½), post‐diuretic washout, anddifferential renal function. Although these parameters remain fundamental components ofroutine clinical practice, each has recognized limitations. Factors such as impairedrenal function, severe pelvicalyceal dilatation, inadequate hydration, bladderdistension, patient motion, and variations in diuretic administration protocols maysignificantly influence tracer washout and reduce diagnostic accuracy. Consequently,equivocal or false‐positive interpretations may occur, particularly in markedly dilatedbut non‐obstructed collecting systems. (1,4,7)To overcome these limitations, several alternative quantitative indices have beenproposed to improve the assessment of urinary drainage. One of the most promisingparameters is Normalized Residual Activity (NORA), which represents the proportion ofresidual renal tracer activity at a specified time relative to the initial renal uptake.Unlike conventional washout measurements, NORA is less dependent on renal function andcollecting system compliance and provides a simple quantitative estimate of residualtracer retention. Previous investigations have demonstrated that NORA improvesdiscrimination 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 specificitycompared with conventional renographic interpretation, reducing the number of equivocalexaminations and minimizing unnecessary surgical intervention. Beatović et al. reportedthat post‐micturition NORA correctly reclassified the majority of indeterminate orfalse‐positive renographic studies while maintaining excellent diagnostic accuracy forurinary outflow obstruction. Such findings support the incorporation of quantitativedrainage parameters into routine renographic interpretation; however, the clinicalutility of NORA has not yet been universally adopted and remains insufficiently evaluatedacross different patient populations and imaging protocols. (5,7)Given the continuing reliance on conventional renographic parameters in many nuclearmedicine departments and the increasing interest in quantitative assessment of renaldrainage, further evaluation of NORA is warranted. Comparing NORA with establishedrenographic indices mayprovide additional evidence regarding its diagnostic value and its potential role inimproving the accuracy of diuretic renography for patients with suspected urinary tractobstruction. Therefore, the present study aims to compare the diagnostic performance ofNormalized Residual Activity with conventional renographic parameters in 99mTc‐DTPAdiuretic renography for the diagnosis of urinary tract obstruction.Additionally, Cortical Transit Time (CTT) is an effective metric for distinguishingbetween obstructed and non‐obstructed kidneys. A delayed CTT flags kidneys at risk offunctional deterioration if surgery is delayed, whereas a normal CTT signals that thekidney 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 diureticrenogram drainage pattern [1].