Septated hypoechoic perirenal fat on sonograms: a pitfall in renal ultrasonography

Abstract

Perirenal fluid collections on ultrasonography have been described as hypoechoic regions that include a fairly large number of pathologic conditions in the differential diagnosis, including most commonly abscesses, urinomas, and hematomas. 1In contrast, we describe a case in which this sonographic appearance was accounted for by a normal variant, hypoechoic septated perirenal fat. CASE REPORTAn 85 year old man with prostate cancer and continued elevated temperatures was referred for renal sonographic evaluation. This study demonstrated hypoechoic right perirenal collections both anteriorly and posteriorly (Fig. lA, lB). The patient had a CT scan 2 days later to further evaluate these areas, which were proved to be perirenal fat of a lower density than the surrounding perirenal retroperitoneal fat (Fig. lC).Received October 8, 1991, from the Departments of Radiology, National Naval Medical Center and Uniformed Services University of the Health Sciences (H.M.B), Bethesda, Maryland; U.S. Naval Hospital (W.S.S.), Okinawa, Japan; and Radiology Associates (P.R.L.), Thomasville, Georgia. Revised manuscript accepted for pub· lication January 22, 1992.Address correspondence and reprint requests to Harry M. Bram· mer, CDR, MC, USN, Department of Radiology, National Naval Medical Center, 8901 Wisconsin Avenue, Bethesda, MD 20889-5000.The opinions and assertions contained herein are the private views of the authors and are not to be construed as official or as reflecting the views of the Department of the Navy or the Department of Defense. DISCUSSIONHypoechoic collections around the kidney on sono· grams have been described as representing many pathologic conditions, including perirenal abscesses, urinomas, and hematomas. In addition, post-renal transplant seromas and lymphoceles have been found to have this appearance, as do large tracking pancreatic pseudocyst collections and infiltrating mantles of lym· phoma. We have found cases in which localized hypoechoic perirenal collections represent nonpathologic collections of fat.A previous report in the literature has described the shell of perirenal fat to be variable in both amount and echogenicity, from highly echogenic to relatively hy· poechoic. 2Our present case, however, is the first one in the literature to show that the perirenal fat can have both hypoechoic and hyperechoic appearances in the same perirenal space. We have also observed other, similar cases in which smaller, more localized central perirenal areas of hypoechoic fat are seen, surrounded by echogenic fat (Fig. 2). The fact that two areas of fat are separated in the perirenal space is explained by the bridging septa of the perinephric space described by Kunin with gross specimen and correlative CT scans." The variation in echogenicity of fat has been shown to be related to water content through experiments with solid margarine sticks. The margarine was studied prior to and after removal of 15 per cent of its water content by interval heating and cooling. Pure vegetable oil was found to be hypoechoic, where...

Retroperitoneal edema: sonographic mimic of retroperitoneal fluid collection

“…No known cause of perinephric collection could be identified in any of the 13 cases. We cannot exclude the possibility that one or more cases we describe represented hypoechoic retroperitoneal fat, an occasional normal variant [8], but this is not likely to have been so in all patients. We were also careful to distinguish between perirenal fluid and a subcapsular hypoechoic cortical rim that has rarely been observed [9].…”

Sonography of perinephric fluid collections: A pictorial essay

“…Perinephric fat typically presents as hyperechoic but it may become hypoechoic when the water content of the tissue is low, resulting in a hypoechoic rim that mimics perinephric fluid . A nonpathological perinephric hypoechoic fat pad has been observed in patients with renal atrophy but also in healthy individuals with normal renal function (Figures and ).…”