Does gantry rotation time influence accuracy of volume computed tomography dose index (CTDI vol ) in modern CT?

PURPOSE: The purpose of this study was to develop a gantry overrun corrected CTDIvol (cCTDIvol ) dosimetry and evaluate the differences between the displayed CTDIvol (dCTDIvol ), measured CTDIvol (mCTDIvol ), and the cCTDIvol .

METHODS AND MATERIALS: The each 8 rotation times between 275 and 1000ms of two CT scanners were investigated. Rotation time (Trot ) and the beam-on time (Tbeam ) in axial scanning were measured accurately to determine the gantry overrun time (Tover ) as Tbeam -Trot . Subsequently, mCTDIvol was measured by using a 100mm ionization chamber and CTDI phantoms. Furthermore, we introduced a gantry overrun correction factor (Co =Trot /Tbeam ) to obtain cCTDIvol . Upon completion of the data acquisition, the dCTDIvol and mCTDIvol were compared with the cCTDIvol .

RESULTS: The discrepancies of Trot were 0.2±0.2ms as compared to the preset rotation times, and Tover was machine-specific and almost constant (22.4±0.5ms or 45.1±0.3ms) irrespective of the preset rotation time. Both dCTDIvol and mCTDIvol were increasingly overestimated compared to cCTDIvol as the faster the preset rotation time was selected (1.7-23.5%).

CONCLUSION: The rotation time influences the accuracy of CTDIvol in modern CT, and should be taken into consideration when assessing the radiation output in modern CT.