Protocol analysis of dual-energy CT for optimization of kidney stone detection in virtual non-contrast reconstructions.

Abstract

Previous studies have shown that split-bolus protocols in virtual non-contrast (VNC) reconstructions of dual-energy computed tomography (DE-CT) significantly decrease radiation dose in patients with urinary stone disease. To evaluate the impact on kidney stone detection rate of stone composition, size, tube voltage, and iodine concentration for VNC reconstructions of DE-CT. In this prospective study, 16 kidney stones of different sizes (1.2-4.5mm) and compositions (struvite, cystine, whewellite, brushite) were placed within a kidney phantom. Seventy-two scans with nine different iodine contrast agents/saline solutions with increasing attenuation (0-1400HU) and different kilovoltage settings (70kV/150kV; 80kV/150kV; 90kV/150kV; 100kV/150kV) were performed. Two experienced radiologists independently rated the images for the presence and absence of stones. Multivariate classification tree analysis and descriptive statistics were used to evaluate the diagnostic performance. Classification tree analysis revealed a higher detection rate of renal calculi > 2mm in size compared with that of renal calculi < 2mm (84.7%; 12.7%; p < 0.001). For stones with a diameter > 2mm, the best results were found at 70kV/Sn 150kV and 80kV/Sn 150kV in scans with contrast media attenuation of 600HU or less, with sensitivity of 99.6% and 96.0%, respectively. A higher luminal attenuation (> 600HU) resulted in a significantly decreased detection rate (91.8%, 0-600HU; 70.7%, 900-1400HU; p < 0.001). In our study setup, the detection rates were best for cystine stones. Scan protocols in DE-CT with lower tube current and lower contrast medium attenuation show excellent results in VNC for stones larger than 2mm but have limitations for small stones. • The detection rate of virtual non-contrast reconstructions is highly dependent on the surrounding contrast medium attenuation at the renal pelvis and should be kept as low as possible, as at an attenuation higher than 600HU the VNC reconstructions are susceptible to masking ureteral stones. • Protocols with lower tube voltages (70kV/Sn 150kV and 80kV/Sn 150kV) improve the detection rate of kidney stones in VNC reconstructions. • The visibility of renal stones in virtual non-contrast of dual-energy CT is highly associated with the size, and results in a significantly lower detection rate in stones below 2mm.