IMAGING

Urinary calculi composed of uric acid, cystine, and mineral salts: differentiation with dual-energy CT at a radiation dose comparable to that of intravenous pyelography
Thomas C, Heuschmid M, Schilling D, Ketelsen D, Tsiflikas I, Stenzl A, Claussen CD, Schlemmer HP
Department of Diagnostic and Interventional Radiology, University of Tübingen, Tübingen, Germany
Radiology. 2010; 257: 402-9

• Purpose: To retrospectively evaluate radiation dose, image quality, and the ability to differentiate urinary calculi of differing compositions by using low-dose dual-energy computed tomography (CT).
  Materials and Methods: The institutional review board approved this retrospective study; informed consent was waived. A low-dose dual-energy CT protocol (tube voltage and reference effective tube current-time product, 140 kV and 23 mAs and 80 kV and 105 mAs; collimation, 64 × 0.6 mm; pitch, 0.7) for the detection of urinary calculi was implemented into routine clinical care. All patients (n = 112) who were examined with this protocol from July 2008 to August 2009 were included. The composition of urinary calculi was assessed by using commercially available postprocessing software and was compared with results of the reference standard (ex vivo infrared spectroscopy) in 40 patients for whom the reference standard was available. Effective doses were calculated. Image quality was rated subjectively and objectively and was correlated with patient size expressed as body cross-sectional area at the level of acquisition by using Spearman correlation coefficients.
  Results: One calcified concrement in the distal ureter of an obese patient was mistakenly interpreted as mixed calcified and uric acid. One struvite calculus was falsely interpreted as cystine. All other uric acid, cystine, and calcium-containing calculi were correctly identified by using dual-energy CT. The mean radiation dose was 2.7 mSv. The average image quality was rated as acceptable, with a decrease in image quality in larger patients.
  Conclusion: Low-dose unenhanced dual-source dual-energy CT can help differentiate between calcified, uric acid, and cystine calculi at a radiation dose comparable to that of conventional intravenous pyelography. Because of decreased image quality in obese patients, only nonobese patients should be examined with this protocol.

• Editorial Comment
  Nowadays multi-detector computed tomography (MDCT) is used in attempt to determine the chemical composition of urinary tract stones. However, the attenuations values given in Hounsfield units of different types of calculi obtained with current technique overlap, making reliable distinction of chemical composition of urinary calculi very difficult. With the advent of new dual-source CT systems, CT scans are simultaneously and quickly obtained using two orthogonally positioned x-ray tubes and detector sets (double source at 80 and 140 kV); both helical acquisitions run simultaneously and are not limited by changes between the two scans in contrast enhancement or patient motion (1). Dual-energy CT may be used to distinguish pure uric acid, mixed uric acid, and calcified stones. However, dual-source CT imaging delivery a higher radiation dose to the patient than the currently recommended lower-dose MDCT protocols.
  The authors of this manuscript offer a great contribution to this subject by developing a low-dose unenhanced dual-source dual-energy CT protocol that can help differentiate between calcified, uric acid, and cystine calculi at a radiation dose comparable to that of conventional intravenous pyelography (mean 2.7 mSv). This protocol however was useful only in nonobese patients. New variation in dual-source CT protocols, are still in progress in an attempt to further decrease the radiation dose to the patients while keeping the ability to differentiate chemical composition of urinary tract calculi (1).

Reference

1. Ascenti G, Siragusa C, Racchiusa S, Ielo I, Privitera G, Midili F, et al.: Stone-targeted dual-energy CT: a new diagnostic approach to urinary calculosis. AJR Am J Roentgenol. 2010; 195: 953-8.

Dr. Adilson Prando
Head, Department of Radiology and
Diagnostic Imaging, Vera Cruz Hospital
Campinas, São Paulo, Brazil
E-mail: adilson.prando@gmail.com