CT, US and MRI of xanthine urinary stones: In-vitro and in-vivo analyses

Background: Xanthine urinary stones are a rare entity that may occur in patients with Lesch-Nyhan syndrome receiving allopurinol. There is little literature describing imaging characteristics of these stones, and the most appropriate approach to imaging these stones is therefore unclear. We performed in-vitro and in-vivo analyses of xanthine stones using computed tomography (CT) at different energy levels, ultrasound (US), and magnetic resonance imaging (MRI). Methods: Five pure xanthine stones from a child with Lesch-Nyhan were imaged in-vitro and in-vivo. CT of the stones was performed at 80 kVp, 100 kVp, 120 kVp and 140 kVp and CT numbers of the stones were recorded in Hounsfield units (HU). US of the stones was performed and echogenicity, acoustic shadowing and twinkle artifact were assessed. MRI of the stones was performed and included T2-weighted, ultrashort echo-time-weighted and T2/T1-weighted 3D bFFE sequences and signal was assessed. Results: In-vitro analysis on CT demonstrated that xanthine stones were radiodense and the average attenuation coefficient did not differ with varying kVp, measuring 331.0 ± 51.7 HU at 80 kVp, 321.4 ± 63.4 HU at 100 kVp, 329.7 ± 54.2 HU at 120 kVp and 328.4 ± 61.1 HU at 140 kVp. In-vivo analysis on CT resulted in an average attenuation of 354 ± 35 HU. On US, xanthine stones where echogenic with acoustic shadowing and twinkle artifact. On MRI, stones lacked signal on all tested sequences. Conclusion: Xanthine stone analyses, both in-vitro and in-vivo, demonstrate imaging characteristics typical of most urinary stones: Dense on CT, echogenic on US, and lacking signal on MRI. Therefore, the approach to imaging xanthine stones should be comparable to that of other urinary stones.