Correlation between MRI features and metal compositions of gallstones

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Objective To analyze the correlation between the metal components and MRI signal intensities of gallstones, to investigate the causes of gallstone signal features on MRI. Methods The MRI data and the metal components of the gallstones in vivo and in vitro of 30 patients were retrospectively analyzed. The MR sequences, namely 3D fast spoiled gradient-echo with fat saturation T1-weighted imaging (3D-FSPGR-FS T1WI), fast spoiled gradient-echo with fat saturation T1-weighted imaging (FSPGR-FS T1WI), fast imaging employing steady-state acquisition (FIESTA) and fast spin-echo T2-weighted imaging with fat saturation (FSE-FS T2WI), were performed on the gallstones in vivo and in vitro. According to the characteristics of the surface and cross section, gallstones were divided into pigment gallstones(n=16) and cholesterol gallstones(n=14). The MR signal characteristics of the gallstones were observed and the signal intensity rates of the gallstones were calculated. Preoperatively, the signal intensity rates of cholesterol and pigment stones of each MR sequence were compared by using t test and Mann-Whiteney U test. Postoperatively, the signal intensity rates of the fresh, dried and re-soaked gallstones were compared by using paired t test and Wilcoxon test. The correlation between the signal intensity rates of gallstones on the 3D-FSPGR-FS sequence and their metal components was analyzed by using Linear Regression analysis. Results The pigment gallstones showed high signal intensity on the 3D-FSPGR-FS T1WI. The signal intensity rates of the pigment gallstones were higher than the rates of the cholesterol gallstones on the 3D-FSPGR-FS T1WI, which were 2.02±0.53 and 0.51±0.24 (t=10.26, P< 0.01), respectively. On the 3D-FSPGR-FS T1WI, the signal intensity rates of the drying pigment stones were significantly lower than the rates of the fresh ones, the rates of the two states of the pigment gallstones were 0.21±0.06 and 1.42±0.35(t= 13.49, P<0.01),respectively. The signal intensity rates of pigment gallstones showed significant rebound after re-soaking, the rates of the two states of the pigment gallstones were 0.21±0.06 and 1.68±0.86(t=-6.63, P< 0.01),respectively. The metal components of pigment gallstones were significantly higher than the cholesterol stones. In the pigment gallstones and cholesterol gallstones, the medians of the Calcium were 28.186 and 2.347 mg/g (Z=-4.66, P< 0.01), respectively. For pigment gallstones, there was a significant correlation between the calcium and the signal intensity rate on 3D-FSPGR-FS T1WI. The regression equation of linear regression analysis was SI= 65.40 logCa-166.67. Conclusions The pigment gallstones containing much more water and metal showed high signal intensity on 3D-FSPGR-FS T1WI. The Calcium in the pigment gallstones may be the main cause for this MR appearance. Key words: Cholelithiasis; Magnetic resonance imaging; Comparative study