Radiolucent Calculi in Kidneys on Intravenous Urography: Red Color on Dual-Energy Computed Tomography Clinches the Final Diagnosis

A 1-year-old girl, follow-up case of bilateral hydronephrosis, underwent intravenous urography (IVU) to assess the function and drainage. The plain spot [Figure 1a] of the IVU study showed no radiopaque calculus. Subsequently, in 1-h post-intravenous contrast injection, mild bilateral hydronephrosis was present with two radiolucent filling defects seen, one each in the right and left renal pelvis [Figure 1b] suggestive of radiolucent calculi. Low-dose dual-energy computed tomography (DECT) was subsequently done for further delineation and characterization to decide upon the management. It showed bilateral nephrolithiasis with mild hydronephrosis [Figure 1c and d]. On dual energy evaluation, bilateral renal calculi were red and found to have a uric acid composition [Figure 1e and 1f].

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Figure 1:

Plain radiograph (a) shows no radiopaque calculi in the renal fossae on either side, while post-contrast injection 1-h spot (b) shows two radiolucent filling defects in the bilateral renal pelvis (black arrows) with mild hydronephrosis. Coronal (c) and axial (d) multiplanar reformatted images of dual-energy computed tomography (DECT) scan show bilateral renal calculi (black arrows in c and d) with mild hydronephrosis. Color overlay image (e) of DECT scan shows bilateral renal calculi, which are color-coded red. On dual-energy radiograph evaluation (f), left renal calculus (region of interest shown in e) is seen to have a uric acid composition (green circle in f).

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Radiolucent stones constitute only 10% of urolithiasis cases. These include uric acid, cysteine, and medication-induced (such as indinavir, triamterene, sulfonamides, and amorphous silica) calculi. DECT can help to differentiate uric acid stones from non-uric acid stones (such as calcium oxalate, calcium phosphate, struvite, cystine, and hydroxyapatite) due to their material decomposition ability.¹ Uric acid stones are color-coded red, while the non-uric acid stones are blue. This is important from a management point of view, as the uric acid calculi are medically managed using alkalizing agents. In contrast, non-uric acid stones are treated by lithotripsy or surgery.