Does gantry rotation time influence accuracy of volume computed tomography dose index (CTDIvol) in modern CT?
Atsushi Fukuda1, Nao Ichikawa2, Yoshiharu Fujita2
1Division of PET Imaging, Shiga Medical Center Research Institute, 5-4-30 Moriyama, Moriyama, Shiga 524-8524, Japan.
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.
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