Radiopacity and Hounsfield Attenuation of Cystine Urolithiasis: Case Series and Review of the Literature

Abstract

Objectives:

Given the high recurrence rate of cystine urolithiasis, understanding of the radiographic stone characteristics is important in following cystine stone formers over their lifetime. However, due to their infrequent incidence, in vivo radiographic properties of cystine stones have not been well characterized. The purpose of our study is to characterize the in vivo radiographic properties of cystine urolithiasis.

Methods:

Patients with a cystine stone analysis and noncontrast computed tomography (NCCT) were extracted from our stone clinic database. Stone attenuation in Hounsfield units (HU) was measured for each stone and plain abdominal films (kidney, ureter, and bladder radiograph [KUB]) within 30 days of the NCCT prior to any intervention were reviewed by a blinded radiologist to assess whether urolithiasis could be visualized.

Results:

Twenty patients met our study inclusion criteria. When plotted by attenuation, two distinct groups of stone attenuation were noted for cystine stone formers (p<0.001). The largest group (n=16) had an attenuation of <550 HU (424±106 HU), while a distinct second group (n=4) was >850 HU (972±134 HU). Sixteen patients had a KUB, with 88% of the stones being visualized by a blinded radiologist. Stone size and attenuation were not significantly different between visualized and nonvisualized stones via KUB, however, the body mass index was significantly higher in the nonvisualized group (34.4 vs 26.9 kg/m², p=0.03).

Conclusions:

Cystine stones were visualized by KUB, which has implications in post-treatment follow-up imaging. Though most cystine stones had an attenuation of <550 HU, a second distinct group of cystine stones were noted to have a high attenuation of >850 HU. HU measurements alone are not sufficient to differentiate cystine stones from other stone compositions.