Optimization of the noise index setting in chest computed tomography scan with automatic tube current modulation and adaptive statistical iterative reconstruction

LU Rui; HE Jian; ZHOU Kefeng; QIN Guochu; HU Anning; YANG Shangwen; ZHU Bin

Abstract：Objective To study the effect of noise index (NI) in the chest computed tomography (CT) image quality with au-tomatic tube current modulation scanning and adaptive statistical iterative reconstruction ( ASIR) , and to explore the most appro-priate NI to reduce the radiation dose .Methods A total of 193 consecutive patients were prospectively enrolled .They underwent chest CT scan with the application of automatic tube current modulation and ASIR (50%weight).Those patients were randomly divided into 6 groups with different NI settings (15, 18, 21, 24, 27 and 30) and were further divided into 3 subgroups according to their body mass index (BMI):underweight and normal (BMI≤23.9), overweight (24≤BMI≤27.9) and obese (BMI≥28) subgroups.Two radiologists made the subjective score of the image quality blindly and independently (well, <13;critical, 13~15;unacceptable, >15).Image noise (fat), signal to noise ratio (SNR, aorta vs.fat), contrast to noise ratio (CNR, aor-ta vs.fat) were measured on both lung and medium reconstruction images .CT Dose Index volume ( CTDIvol), Dose Length Product (DLP) and Effective Dose (ED) of each scan were recorded.Additionally, the percentage of images obtained with a tube current lower than 50 mA among the total image slices in each patient was recorded .Results With the increase of the NI , the subjective score increased ( image quality decreased ) , the SNR and CNR decreased .The subjective score of chest CT images were lower than or equal to 15 with all NI settings, while CT images of 4 patients (4/34) were scored 13~15 when NI=30.The sub-jective score and SNR of chest CT images with NI =30 was significantly higher and lower than those with NI =15~27 respectively ( P <0.05).The radiation dose of scanning with NI =24~30 was significantly lower than that with NI =15~21 ( P <0.05). When NI was set at 27 or 30, the percentage of images obtained with a tube current lower than 50 mA among the total image slices decreased with the increase of BMI ( P <0.05).When NI was set at 30, the radiation dose significantly increased with the in-crease of BMI ( P <0.05).Conclusion The NI of chest CT scan can be set at 30 with the application of automatic tube current modulation and 50%ASIR to reduce the radiation dose dramatically with reserved image quality .