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BACKGROUNDS AND OBJRCTIVESUrinary stone disease is a world-wide problem that has become increasingly prevalent and has a high rate of recurrence.Determination of stone composition not only dictates treatment options by the urologists’decision between conservative and surgical intervention, but also provides the basis for prevention of recurrence and cause analysis. For example, uric acid calculi may be treated with urinary alkalinization as a first-line treatment, and struvite calculi are known to be removed with external shockwave lithotripsy (ESWL).X-ray diffraction, polarization microscopy and infrared spectroscopy are common techniques for in vitro stone analysis, but they are costly and time consuming, especially when chemical analysis of the stones is performed after extraction. The analysis of stone composition before treatment is more in-line with clinical needs. Inquiries based on clinical history, urine crystals, pH values of urine and kidney, ureter, and bladder (KUB) have a certain value forecast, but components of the stones still cannot be accurately analyzed.Since its introduction in the early 1990s, unenhanced computed tomography (CT) has become the gold standard for the evaluation of urinary stone diseases at many centers. The speed, safety, high sensitivity and accuracy of unenhanced helical CT make it the method of choice for the assessment of patients with suspected urinary tract calculi . Helical CT can also provide helpful information on the exact localizations of calculi, as well as size and stone composition. Dual-energy CT, by facilitating low and high-energy scanning during a single acquisition, has inherent capabilities to help differentiate between materials that have similar electron densities, but varying photon absorption.The purpose of this study was to assess the capability of determining the chemical composition of urinary stones using spectral imaging (SI) mode on spectral CT in vitro.METHODSForty four freshly excised pig kidneys with 264 extracted human Urinary Calculi in them which were immersed in a 10cm-deep water tank underwent CT(Discovery CT750 HD)scans with GSI mode and conventional polychromatic imaging (CPI,120kVp) mode, respectively. All GSI data were transferred to Workstation (AW4.4, GE Healthcare) to acquire monochromatic images of 50keV,Effective Atomic Number(Eff-Z) mapping images, Water (Calcium)-based images and Calcium(Water)-based images with GSI Viewer. CT numbers of stones were measured and compared at 50keV monochromatic images and 120kVp polychromatic images, the mean Eff-Z, Calcium density and Water density were measured at Effective Atomic Number(Eff-Z) mapping images, Calcium(Water)-based images and Water (Calcium)-based images, respectively. The mean Eff-Z , Spectral Hu Curve Slope and Calcium Water Ratio(CWR) were compared. (CWR was defined as the ratio of calcium density to water density and Spectral Hu Curve Slope was defined as: Slope = (HU40keV-HU100keV) / 60, where HU40keV was the CT value of calcium at 40keV image and HU100keV was the CT value of calcium at 100keV image)RESULTSThe composition of Urinary Calculi was determined by infrared spectrometer after CT examination,according to the result of stone composition determined by infrared spectroscopy, 174 pure kidney stones were divided into five groups: Uric Acid stones(UA,n=26), Struvite stones(STR,n=31),Cystine stones(CYS,n=14), Calcium phosphate(CaP,n=36),Calcium oxalate (COX, n=67). The mean Eff-Z , CWR, Spectral Hu Curve Slope and the mean CT numbers at 50keV images and 120kVp images of each group were as below:UA7.39±0.45、0.0327±0.0509、-0.74(-1.62-12.03)、503.08±167.59HU、461.85±103.73HU);STR(12.05±1.01、0.1859±0.0718、18.45(12.24-22.79)、1055.98±290.09HU、764.96±221.72HU);CYS(11.16±0.59、0.1253±0.0297、12.79(9.3-14.42)、739.75±172.22HU、564.89±129.09HU);CaP(15.98±0.45、0.6921±0.1085、37.36(35.77-38.79)、2567.44±177.62 HU、1601.89±200.14HU);COX(15.43±0.52、0.5798±0.0984、36.23(34.53-37.87)、2267.47±384.92 HU、1483.19±286.24HU). There were significant differences the mean Eff-Z, CRW, Spectral Hu Curve Slope ,the mean CT numbers at both 50keV and 120kVp among groups(F=633.860、352.195、242.931、196.006、116.372,P=0.000). the differences in the mean Eff-Z ,CRW, Spectral Hu Curve Slope, the mean CT numbers at both 50keV and 120kVp among groups were statistically significant by Binary comparison (P<0.05),While there were no significant differences in CT numbers at 120kVp images between UA and CYS(P=0.240),CaP and COX(P=0.068)..there were also no significant differences in Spectral Hu Curve Slope between CaP and COX(P=0.276).CONCLUSIONSSpectral imaging provides a novel method to better characterize pure urinary stones using the mean Eff-Z, CD, CWR and the CT numbers at 50keV. .
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