Computed tomography of urinary calculi.

Computed tomography (CT) was used in the evaluation of nine patients with nonopaque calculi in the upper urinary tract. In each case the calculus was identified as a very high density object (370-586 Hounsfield units) with calcium oxalate and cystine stones having somewhat higher attenuation values than uric acid or xanthine stones. The differentiation between calculi and other "radiolucent" filling defects was readily made since calculi had much higher attenuation values than blood clot or neoplasm. CT of the upper urinary tract may obviate the need for more invasive procedures such as retrograde pyelography when nonopaque filling defects require differentiation.