Room: Exhibit Hall
Purpose: The volume computed tomography dose index (CTDIvol) is conventionally applied for measuring radiation output in modern CT systems. However, CTDIvol is inapplicable in wide-beam multi-detector row CT (MDCT) scanners because of the short length of the CTDI phantom and the chamber and so on. Thus, we developed a new method for measuring radiation output not involved in beam width in MDCT scanners.
Methods: Toshiba Aquilion ONE ViSION Edition CT scanner was employed. The scan parameters were: tube potential of 135 kVp; tube current of 50 mA; rotation time of 0.4 s; and total collimation width of 80, 120 and 160 mm. An ionization chamber with 6 cc was suspended free-in-air at the isocenter, and two sheets of lead (1 mm thickness) are placed to build apertures (40 - 80 mm width in increments of 8 mm) on the inner cover of the gantry. The air-kerma (Kair) rate profiles were measured with and without the apertures (Kw, Kw/o) for 4.8 seconds without table translation. Upon completion of the data acquisition, Kw rate were plotted as a function of the nominal beam fields (total collimation Ã— aperture width), and the second order least squares estimation was applied to obtain the extrapolated dose (K0) rates at zero-aperture.
Results: Means and standard errors of Kw/o rate with 80, 120 and 160 mm beam width were 14.85 Â± 0.02, 15.09 Â± 0.02, and 15.21 Â± 0.02 mGy/s (P<0.001), and K0 rates were reduced to 14.3 Â± 0.2, 14.1 Â± 0.2, and 14.1 Â± 0.1 mGy/s, respectively (no statistically significant differences).
Conclusion: Because our new technique could eliminate the scattered radiation, the K0 rate indicates the inherent Kair rate in the CT system. Thus, the K0 rate has a possibility of becoming a surrogate output in modern CT, unlike CTDIvol and its derivatives.
Scatter, Ionization Chamber, Quality Control