Low-Dose Characterization of Kidney Stones Using Spectral Detector Computed Tomography: An Ex Vivo Study

Objectives The aim of this study was to investigate the feasibility of kidney stone composition analysis using spectral detector computed tomography scanner (SDCT) with normal- and low-dose imaging protocols. Methods A total of 154 stones harvested from nephrolithotripsy or nephrolithotomy with a known monocrystalline composition as determined by infrared spectroscopy were examined in a nonanthropomorphic phantom on an SDCT (IQon, Philips, Best, the Netherlands). Imaging was performed with 120 kVp and (a) 40 mAs and (b) 200 mAs, resulting in a computed tomography dose index (CTDIvol) of 2 and 10 mGy, respectively. Besides conventional CT images (CIs), SDCT enables reconstruction of virtual monoenergetic images (40–200 keV). Spectral coefficient images were calculated by performing a voxel-by-voxel combination of 40 and 200 keV images (Matlab R2017b, Mathworks Inc). All stones were semiautomatically 3D-segmented on CI using a threshold-based algorithm implemented in an offline DICOM viewer. Statistical assessment was performed using Steel-Dwass method to adjust for multiple comparisons. Results Ca-phosphate (n = 22), Ca-oxalate (n = 82), cysteine (n = 20), struvite (n = 3), uric acid (n = 18), and xanthine stones (n = 9) were included in the analysis. Stone diameter ranged from 3.0 to 13.5 mm. On CI, attenuation differed significantly between calcific and noncalcific stones only (P ≤ 0.05), the spectral coefficient differed significantly between (//): Ca-oxalate//Ca-phosphate//cystine//struvite//uric acid//xanthine in 10 mGy protocol (all P ≤ 0.05). The same results were found for the 2 mGy-protocol, except that differentiation of Ca-oxalate and Ca-phosphate as well as uric acid and xanthine was not possible (P ≥ 0.05). Conclusions Spectral detector CT allows for differentiation of kidney stones using semi-automatic segmentation and advanced image post-processing, even in low-dose imaging protocols. Received for publication January 25, 2018; and accepted for publication, after revision, February 18, 2018. Conflict of Interest: DM received honoraria for talks outside this specific project from Philips. All other authors declare that there is no conflict of interest. University Hospitals and Case Western Reserve University received institutional research support from Philips (not related to this article). Correspondence to: Nils Große Hokamp, MD, Department of Diagnostic and Interventional Radiology, University Hospital Cologne, Kerpener Str. 62, 50937 Cologne, Germany. E-mail: nils.grosse-hokamp@uk-koeln.de. Copyright © 2018 Wolters Kluwer Health, Inc. All rights reserved.

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