Differential density of normal renal parenchyma on nonenhanced CT: A new observation

Purpose: Nonenhanced CT scans through the kidneys commonly show a difference in radiopacity between the inner peripelvic portion of the renal parenchyma and the more peripheral regions of the renal parenchyma. This normal observation has not been described. Method: We reviewed 50 nonenhanced CT scans. Renal parenchyma was evaluated by both visual inspection and density measurements of inner and outer aspects of the kidneys in 38 and visual inspection only in 12. A narrower window was used to better show density differences (100 W, 35 L). We recorded the indication for the CT, history, and recent blood chemistries. Results: The inner parenchyma was of greater radiopacity than the outer parenchyma in 35 of 50 cases by visual inspection and in all densitometry cases (38 of 38). There was no correlation with clinical history or blood chemistry. Conclusion: A difference in density between the inner and outer portions of the renal parenchyma is very common and will often be seen if one looks for it. We have not shown an etiology for this difference, and indeed, it may be physiological. One must be aware of this finding to avoid mistaking it for pathology.