[Methods and Materials]
:The measurement of volume CT dose index (CTDI100) was carried out using 64-row multidetector CT (TOSHIBA Medical Systems),
a pencil ion chamber,
and a polymethyl methacrylate phantom with a diameter of 32 cm (See Slide No.1 and 2).
:From the central and peripheral values,
a weighted CTDI was calculated.
Then,
CTDIvol was calculated (See Appendix 1.1 to 1.4).
:The measurement parameters were kept constant: tube voltage,
tube rotation time,
radiation profile width (RPW) ,
beam pitch,
and scan FOV were 120 kV,
0.5 second,
32 mm(0.5×64 mm),
1:0.83,
and 350 mm,
respectively.
Tube current was in increment from 50 mA to 500 mA.
These parameters were the same as the current standard clinical chest CT protocol used in our institution (See Slide No.3).
:The displayed DLP was recorded for 24 patients who underwent chest CT examinations with tube current modulation techniques in our institution.
DLP of each patient was also calculated from the CTDIvol and the tube current displayed on DICOM headers of CT data set using in-house software developed by MATLAB.
In addition,
the amount of DLP due to over-scanning (DLP0) was estimated using extrapolation method and added to the calculated DLP (See Slide No.4 to 7).
:In TOSHIBA CT scanner,
the focus size is determined according to the tube current.
If the current value is over 300 mA,
the large focus size is selected,
and otherwise the small focus is selected.
:Our in-house software,
firstly,
determined the focal size from the information on DICOM headers of CT data.
Secondly,
the DLP values in scan length for each patient was calculated from each slice of CT data set using equation of the relationship between tube current and CTDIvol.
Finally,
DLP0 was added to the DLP values calculated.