[Thin-section computerized tomography of the kidney in the differential diagnosis of small tumor and cystic hypodensities].

Abstract Small hypodense renal lesions with a round shape are frequently detected on CT scans of the upper abdomen after contrast medium administration. In nearly all cases these round hypodensities are simple small cysts with no clinical significance. However, the fluid density of these cysts cannot be always defined, due to the partial volume averaging which occurs on CT when 10-mm-thick slices and contrast enhancement are used. Therefore, a malignant tumor--i.e., small renal tumors or metastatic lesions--cannot be ruled out in some cases. Since the limitations of CT are related to partial volume averaging, the authors used both 5-mm and 10-mm slices to reduce this artifact. Forty-eight small hypodensities (< 15 mm) were studied after contrast agent administration: 42 of them were simple cysts and 5 were tumoral lesions--i.e., 3 renal cell carcinomas and 2 lymphomatous lesions. The results showed that, with 5-mm slices, the density of the fluid hypodensities decreased in nearly all cases and in 81.3% of cases it was below 30 HU. The difference in densities between 10-mm and 5-mm slices was about 50 HU (75 +/- 30 to 21 +/- 16 HU). In the 5 tumoral hypodensities, lesion density was still in the soft tissue range also with 5-mm slices, with no major decrease. These results show that this technique is simple and effective in the differential diagnosis of small renal hypodensities since it requires only a short additional examination time.