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Physiology of Renal Medullary Tip Hyperattenuation at Unenhanced CT: Urinary Specific Gravity and the NaCl Concentration Gradient¹

¹ From the Department of Radiology, University of California–San Francisco, 505 Parnassus Ave, Box 0628, C-324C, San Francisco, CA 94143-0628 (C.T.H., Z.J.W., R.G.G., B.N.J., A.Q., Y.F., R.S.B., F.V.C., B.M.Y.); and Division of Nephrology, Department of Medicine, University of Southern California Keck School of Medicine, Los Angeles, Calif (A.S.L.Y.). Received March 29, 2007; revision requested May 29; revision received July 1; accepted August 1; final version accepted September 26. Address correspondence to B.M.Y. (e-mail: ben.yeh{at}radiology.ucsf.edu).

Purpose: To retrospectively investigate the physiology of renal medullary tip hyperattenuation at unenhanced computed tomography (CT).

Materials and Methods: This retrospective single-institution study was IRB approved and HIPAA compliant. Informed consent was waived. One hundred consecutive patients (53 women, mean age, 52 years; 47 men, mean age, 48 years; P = .39) without and 34 (11 women, mean age, 49 years; 23 men, mean age, 45 years; P = .54) with unilateral ureteral obstruction underwent contemporaneous urinalysis and unenhanced CT. At CT, bladder urine attenuation was measured and two readers recorded the presence of renal medullary tip hyperattenuation. For obstructed kidneys (n = 34), renal pelvic urine attenuation was also recorded. The presence of medullary tip hyperattenuation was correlated with urinary specific gravity. To investigate the physiologic basis of medullary tip hyperattenuation, attenuations for NaCl and urea phantoms (range, 0–2000 mosm/kg) were recorded and correlated to solute concentrations by using linear regression.

Results: Patients with renal medullary tip hyperattenuation seen at CT had higher mean urinary specific gravity (1.023 and 1.022 for readers 1 and 2, respectively) than those without (1.015 and 1.016, respectively, both P < .05). The specific gravity correlated with higher urine attenuation (r = 0.40, P < .001). For the 34 patients with unilateral urinary obstruction, medullary tip hyperattenuation was less commonly seen in obstructed (two kidneys each for both readers) than nonobstructed (11 and 15 kidneys, respectively, both P < .005) kidneys and mean urine attenuation was lower in the obstructed renal pelvis (7.4 HU) than in the bladder (11.4 HU) (P < .005). Phantoms showed a 3.6-HU increase per 100-mosm/kg increase in NaCl concentration (r = 0.99, P < .001) but no change in attenuation with different urea concentrations.

Conclusion: Renal medullary tip hyperattenuation at unenhanced CT reflects increased urinary specific gravity, likely related to high medullary tip NaCl concentrations.

© RSNA, 2008