Detection of renal calculi on late arterial phase computed tomography images: are noncontrast scans always needed to detect renal calculi?

OBJECTIVE

Renal calculi are sometimes incidentally detected on contrast-enhanced computed tomography (CT) scans. The purpose of this study was to evaluate the detectability of renal calculi on arterial phase CT.

METHODS

One hundred thirty-three kidneys of 67 consecutive patients (one had prior unilateral nephrectomy) who underwent multidetector CT including noncontrast and arterial phase (acquired at 25 seconds from the start of 120 mL of intravenous contrast injection at a rate of 3 mL/s) scans were retrospectively evaluated. The detectability of renal calculi was determined by comparison of the noncontrast and contrast-enhanced scans.

RESULTS

Sixty-three calculi in 25 kidneys were detected in 17 patients on noncontrast CT. Forty-seven (75%) of the 63 calculi were also detected on the arterial phase scans. The size and highest attenuation of calculi (measured on noncontrast CT) detected on arterial phase scans were 4.3 +/- 3.0 mm and 556 +/- 353 Hounsfield units, and those detected only on noncontrast CT were 2.8 +/- 0.8 mm (P = 0.053) and 181 +/- 47 Hounsfield units (P < 0.0001), respectively. All calculi larger than 5 mm were detected on arterial phase scans.

CONCLUSIONS

On arterial phase scans, 75% of the renal calculi and allrenal calculi larger than 5 mm were detected; the attenuation of the calculi had a significant correlation to detectability.