Figure 3: A 42-year-old male with a history of renal calculi presenting with left flank pain. Noncontrast CT for the assessment of renal calculi in axial (a) and coronal (b) reformats. These images clearly show a large calculus in the left renal pelvis extending into the upper ureter as well as a second large calculus in the lower pole of the left kidney (arrows). Axial dual energy noncontrast CT images in the same patient showing iodine- (c) and water- (d) based attenuation. The calculus is visible on the iodine-attenuated image (c) and is unchanged when compared to the standard CT. However, the water-attenuated image (d) shows “dropout,” and the calculus is no longer visible. This indicates the presence of low-molecular-weight elements. Post removal, this stone proved to be predominantly composed of urate.