Abstract
Computed tomography in suspected urolithiasis provides information about the presence, location and size of stones. Particularly stone size is a key parameter in treatment decision; however, data on impact of reformatation and measurement strategies is sparse. This study aimed to investigate the influence of different image reformatations, slice thicknesses and window settings on stone size measurements. Reference stone sizes of 47 kidney stones representative for clinically encountered compositions were measured manually using a digital caliper (Man-M). Afterwards stones were placed in a 3D-printed, semi-anthropomorphic phantom, and scanned using a low dose protocol (CTDIvol 2 mGy). Images were reconstructed using hybrid-iterative and model-based iterative reconstruction algorithms (HIR, MBIR) with different slice thicknesses. Two independent readers measured largest stone diameter on axial (2 mm and 5 mm) and multiplanar reformatations (based upon 0.67 mm reconstructions) using different window settings (soft-tissue and bone). Statistics were conducted using ANOVA ± correction for multiple comparisons. Overall stone size in CT was underestimated compared to Man-M (8.8 ± 2.9 vs. 7.7 ± 2.7 mm, p < 0.05), yet closely correlated (r = 0.70). Reconstruction algorithm and slice thickness did not significantly impact measurements (p > 0.05), while image reformatations and window settings did (p < 0.05). CT measurements using multiplanar reformatation with a bone window setting showed closest agreement with Man-M (8.7 ± 3.1 vs. 8.8 ± 2.9 mm, p < 0.05, r = 0.83). Manual CT-based stone size measurements are most accurate using multiplanar image reformatation with a bone window setting, while measurements on axial planes with different slice thicknesses underestimate true stone size. Therefore, this procedure is recommended when impacting treatment decision.
Highlights
Computed tomography in suspected urolithiasis provides information about the presence, location and size of stones
As a consequence, filtered back-projection (FBP) has gradually been replaced by hybridand model-based iterative reconstruction algorithms (HIR and MBIR, respectively), which enable a reduction of radiation dose while maintaining or improving image quality and diagnostic accuracy[4,5,6,7]
Stone size as determined using a digital caliper served as reference standard with an average stone size of 8.8 ± 2.9 mm ranging from 4 to 15 mm, while computed tomography (CT)-based measurements systematically underestimated stone size (7.7 ± 2.7 mm; when averaging all measurements; p < 0.05); yet, Man-M and CT-based measurements showed a good correlation (p < 0.05, r = 0.70), (Table 2)
Summary
Computed tomography in suspected urolithiasis provides information about the presence, location and size of stones. This study aimed to investigate the influence of different image reformatations, slice thicknesses and window settings on stone size measurements. Reconstruction algorithm and slice thickness did not significantly impact measurements (p > 0.05), while image reformatations and window settings did (p < 0.05). Manual CT-based stone size measurements are most accurate using multiplanar image reformatation with a bone window setting, while measurements on axial planes with different slice thicknesses underestimate true stone size. This procedure is recommended when impacting treatment decision. A few studies demonstrated size and volume measurements of kidney stones to be unaffected by radiation dose, comparing normal-dose and low-dose protocols as low as 2 mGy8,9
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