ABSTRACT
Objective To investigate the effects of different tube voltages on the dose of superficial radiation-sensitive organs and image quality when using organ dose modulation( ODM) in chest CT. Methods Based on clinical chest CT protocol with the sameother parameters, chest phantom was scanned using 140, 120, 100, and 80 kv ( 100 kV was the recommended by the CT system) without ODM ( ODM off ) or with ODM from the starting layer to the breast area ( ODM part ) . A long rod ionization chamber was placed iat a fixed position in front of the right breast area. The scans were repeated for 7 times with each group of scanning parameters and dose values were measured for each scanning, the CTDIvol and breast skin dose measurements( D) were recorded. Coronal images of 5 mm thickness for the lung and soft tissue algorithms were reformatted. The images were divided into 8 parts along the z axis direction, the contrast noise ratios( CNR) for every region were measured. For CTDIvol , D, CNR for different ODM and tube voltage scanning modes, two factor non-repeat test ANOVA was performed. LSD method was used for comparison among groups. Results The CTDIvol was lowest at 80 kV, and the breast skin dose measurement was lowest at 100 kV, CTDIvol decreased in turn from140 to 80 kV ( F=105. 5795, P<0. 05) . The breast skin dose measurement decreased in turn from140 to 100 kV, but increased instead at 80 kV. The difference was statistically significant(F=27. 736, P<0. 05). Compared with ODM off , the CTDIvol and D for ODM part both declined and the differences were statistically significant ( F=39. 732, 81. 961, P<0. 05). The CNRs of the lung and soft tissue images decreased at every tube voltage(F=12. 809, 11. 261, P<0. 05 ) . The CNRs decreased from140 to 100 kV, but there was no statistical difference( P>0. 05) , and the difference was significant at 80 kV( P<0. 05) . Compared with ODM off, the CNRs of lung and soft tissue algorithm images with ODM part decreased, withnot statistically significant differences ( P>0. 05 ) . Conclusions In clinical practice, with the tube voltage not less than the recommended(100 kV), the optimal reduction of breast radiation dose can be achieved by reducing kV and using ODM on the premise of resonable image quality.
ABSTRACT
Objective To explore the effects of detector width and pitch on radiation dose and image quality when using organ dose modulation (ODM) technology in a wide?area detector CT scanning. Methods Based on the clinical chest scan protocol,3 sets of scans of the chest phantom were performed using any combination of two detector width (40 mm and 80 mm) and pitch (0.500,1.000 and 1.375) with the same parameters:1 Do not use ODM technology (ODM off),2 open ODM (ODM part) 240 mm from scan start layer to breast area,3 open ODM (ODM all) in full 320 mm scan range. A long rod ionization chamber was placed in the fixed position in front of the right breast area. The scanning parameters of each group were measured 7 times, and the volume computed tomography dose index (CTDIvol) and breast skin dose measurement values D were recorded and the mean was calculated and recorded as Dav. The coronal 5 mm thick images of lung and soft tissue algorithms were reformed. It was divided into three parts in the Z?axis direction, and the contrast?to?noise ratio (CNR) and figure of merit (FOM) were measured separately. Independent sample t test was used for CTDI and breast skin doses D and CNR at both detector widths. ANOVA was used for dose and CNR of three sets of pitch (0.500, 1.000, and 1.375) and the three ODM techniques. Result The FOM factor was the largest when using an 80 mm detector with a pitch of 0.992 and partially turning on the ODM. The radiation dose of the three ODM groups decreased in turn, and the effect of ODM on CTDIvol (P=0.019) and breast skin dose (P=0.002) was statistically significant. The width of the detector increased and the dose was increased. The width of the detector was statistically significant for CTDIvol (t=-2.723, P=0.015). There was no statistically significant effect on the breast skin dose (t=-0.908, P=0.377). The effects of the pitch were not statistically significant for CTDIvol (P=0.254) and breast dose (P=0.146). The CNR of the three ODM groups decreased in turn, and the effect of ODM on the soft tissue image CNR was not statistically significant (P=0.146). The CNR of lung algorithm image (P=0.030) had significant effects. The multiple comparisons:only ODM all was significantly different from ODM off (P=0.009). With the increase of detector width,the values of CNR increased,the values of CNR (t=-4.128,P=0.001) of lung images were significantly affected. The effects on the soft tissue images were not statistically significant (P=0.187). There was no statistically significant difference in the effect of pitch on the CNR (P=0.660) of the lung images. The effects of the pitch on the values of CNR of soft tissue images (F=11.756,P=0.001) were statistically significant. By multiple comparisons, the difference of CNR between 0.500 (P=0.000) and 1.375 (P=0.013) was statistically significant compared with that when the pitch was 1.000. There was no significant difference among the three ODM modes (P>0.05) on the values of CNR of upper and middle parts of lung and soft tissue arithmetic images. The differences of CNR between ODM all and the other two groups were statistically significant (P<0.05) on the bottom part of images. Conclusion The changes of detector width and pitch will affect the organs dose modulation technique, and then affect the radiation dose and image quality. When using 80 mm detector with the pitch of 0.992 and partially turning on ODM in chest CT scan,achieving the optimized benefits of quality and dose.