Spectral CT imaging in the evaluation of composition of kidney stones

Objective To evaluate the feasibility of determining the chemical composition of kidney stones using gemstone spectral imaging ( GSI ).Methods One hundred and sixty eight extracted human kidney stones immersed in a 10 cm deep water tank underwent CT (Discovery CT750 HD) scans with GSI mode and conventional polychromatic imaging ( CPI,120 kVp) mode.All GSI data were transferred to Workstation AW 4.4 to acquire monochromatic images of 50 keY,effective atomic number (Zeff) mapping images,water (calcium)-based images and calcium (Water)-based images with GSI Viewer.CT numbers of stones were measured and compared at 50 keV monochromatic images and 120 kVp polychromatic images,the mean Zeff,calcium density and water density were measured at Zeff mapping images,Calcium (Water) -based images and Water (Calcium)-based images,respectively.The mean Zeff,spectral HU curve slope and calcium water ratio (CWR) were compared with ANOVA and Wilcoxon test.The composition of kidney stones was determined by infrared spectrometer after CT examination.According to the result of stone composition determined by infrared spectroscopy,108 pure kidney stones were divided into five groups:Uric acid stones ( UA,n = 13 ),struvite stones ( STR,n = 24),cystine stones ( CYS,n = 14),calcium phosphate stones ( CaP,n = 18),and calcium oxalate stones ( COX,n = 39).Results The mean Zeff,CWR,the mean CT numbers at 50 keV images,120 kVp images and spectral HU curve slope of each group were listed as the following:UA [ 7.4 ± 0.4,0.0085 ± 0.0021,( 503 ± 168 ) HU,(495 ± 106 ) HU and - 0.77 ] ; STR [ 11.8 ± 0.9,0.1743 ± 0.0677,( 1056 ± 290 ) HU,( 799 ± 165 ) HU and 18.72 ] ; CYS [ 11.2 ± 0.6,0.1253 ± 0.0297,( 740 ± 172 ) HU,( 565 ± 129 ) HU and 12.79 ] ; CaP [ 16.0 ± 0.4,0.6781 ± 0.0952,( 2567 ±178 ) HU,( 1602 ± 200 ) HU and 37.14 ] ; COX [ 15.4 ± 0.4,0.5683 ± 0.0759,( 2267 ± 385 ) HU,( 1489 ±284) HU and 36.36 ],there were significant differences among groups ( P ＜ 0.01 ).The differences in the mean Zeff,CRW,spectral HU curve slope were statistically significant among the five groups ( P ＜ 0.05 ).Conclusion Spectral CT imaging provides a new method to characterize the kidney stones with the information orovided by mean Zeff,CRW and the CT numbers at 50 keV. Key words: kidney calculi;  Tomography; X-ray computed;  Compative study