Measurement of the volume CT dose index on spiral CT scanning with a real-time ionization chamber

The measurement of computed tomography dose index 100 ( $CTD{I}_{100}$ ), which is feasible only through axial scanning, requires that the clinical spiral protocols be replaced with those for axial scanning. The real-time ionization chamber detects the integral of radiation dose rate profile, enabling the direct verification of the volume CTDI on spiral CT scanning ( $CTD{I}_{vol}^{Spiral}$ ).

This study aimed to develop a direct measurement technique for $CTD{I}_{vol}^{Spiral}$ and compare its accuracy with that measured using axial scanning ( $CTD{I}_{vol}^{Axial}$ ) or that displayed on the console ( $CTD{I}_{vol}^{Displayed}$ ).

A CTDI phantom with a real-time ionization chamber was placed on the headrest or examination table. $CTD{I}_{100}^{Axial}$ was measured with following parameters: tube voltage = 120 kV, effective mAs = 100, and rotation time = 1.00 s. The parameters for measuring $CTD{I}_{100}^{Spiral}$ were set identical to those used for axial scanning, except for rotation times = 0.33, 0.50, and 1.00, pitch = 0.35, 0.50, 0.75, 1.00, 1.25, and 1.50, and the scanning range = 15 cm. $CTD{I}_{100}^{Spiral}$ was extracted from the integral of radiation dose rate profile. $CTD{I}_{vol}^{Spiral}$ was subsequently calculated and compared with $CTD{I}_{vol}^{Axial}$ and $CTD{I}_{vol}^{Displayed}$ . Finally, $CTD{I}_{vol}^{Spiral}$ was measured for 10 clinical protocols and compared with $CTD{I}_{vol}^{Displayed}$ .

The differences between $CTD{I}_{vol}^{Axial}$ and $CTD{I}_{vol}^{Displayed}$ , $CTD{I}_{vol}^{Spiral}$ and $CTD{I}_{vol}^{Displayed}$ , and $CTD{I}_{vol}^{Axial}$ and $CTD{I}_{vol}^{Spiral}$ for the head and body phantoms were all < -2.0%. The differences between $CTD{I}_{vol}^{Displayed}$ and $CTD{I}_{vol}^{Spiral}$ for scans on clinical protocols with and without automatic exposure control were < 11.9% and < 10.5%, respectively; these large differences were observed in the dual-energy twin-beam protocol. Excluding this protocol yielded differences between the measurements with and without automatic exposure control of < 2.0% and < -3.9%, respectively.

The results showed an excellent agreement between $CTD{I}_{vol}^{Axial}$ and $CTD{I}_{vol}^{Spiral}$ , supporting the use of the clinical spiral CT scanning to verify $CTD{I}_{vol}^{Displayed}$ using a real-time ionization chamber.