Abstract:
Tube current modulation is one of the radiation dose reduction methods. Target SD is a part of AEC system designed by the manufacturer to select the image quality by fixing the image noise and be able to reduce the radiation dose. The purposes of this study are to determine the radiation dose and image quality when varying Target SD and beam pitch. Optimal protocols are obtained by the appropriate Target SD and beam pitch using lung man phantom with nodules.
The lung man chest phantom with five spheres simulated nodules of 12, 10, 8, 5 and 3 mm diameters at 100 HU were scanned with varying beam pitch of 0.637, 0.813, 1.388, kVp at 120 and 100, Target SD from 9 to 25 and mA of 10-400 for 320 MDCT. The radiation dose in term of C_VOL (mGy) and DLP (mGy.cm) were recorded from the CT monitor. The quantitative image quality was determined by the contrast to noise ratio (CNR) values, the qualitative image quality was evaluated in part of spatial resolution and nodule detection capability by two independent radiologists.
When varying Target SD and beam pitch for 320 MDCT using lung man phantom and nodules, C_VOL decreased from 5.9 to 0.7 mGy for 120 kVp and 5.3 to 0.8 mGy for 100 kVp. The C_VOL and DLP of 120 kVp were slightly higher than at 100 kVp in all pitch on Target SD 9 and 14 but for Target SD 20 and 25, C_VOL did not change in all pitch. The percent CNR is highest at pitch 0.637 when compared to other pitch. The percent CNR of lung window is higher than soft tissue window for all pitch and the best spatial resolution image was also obtained with lung window. The scoring on image quality by two radiologists was in good agreement.
Target SD and kVp affect radiation dose while Target SD, pitch and kVp affect the image quality. The selection Target SD depends on clinical applications. Optimal protocol for routine chest CT is Target SD 20, 10-400 mA at pitch 0.813 and 120 kVp.
