The function of Z-axis tube-current modulation technique with desired noise level to decrease radiation dose in MSCT chest scanning

Objective Retrospectively evaluate the effect of Z-axis tube-current modulation technique with desired noise level to improve image quality (image noise level) and decrease radiation doses of MSCT (16-slice CT) in chest scanning. Methods Consecutive two hundred patients whose CT scan projection radiographs showed no significant abnormal were randomly divided into two groups by the examination order: Z-axis tube-current modulation (ZTCM) group (odd number, test group) and constant tube-current (CTC) group (even number, contrast group). The desired noise level of ZTCM group was 10HU and the machine automatically set the dynamic tube-current in scanning according to attenuated information of chest acquired in scan projection radiographs, the tubo-current of CTC group was set at 200mA, while the other scan parameters remained totally the same. The maximum tube-current value,CTDIvol, DLP and the tube-current of the slice at the maximum breast level of female patients were recorded respectively. The noise of image at upper lung, aorta arch, left atrium and bottom lung level were measured and compared. The qualities of Images were classified in three levels (excellent, good, poor) with double blind method. Results The mean value of maximum mA, CTDIvol, DLP and mA of the slice at the maximum breast level of ZTCM group were (178.5±125.6) mA, (10.5±3.8) mGy, (231.6±24.3)mGy/cm and (116.0±22.5) mA, those of CTC were 200.0 mA, 12.8 mGy, (274.7±18.4)mGy/cm and 200.0 mA, ZTCM group decreased by 10.8%, 19.9%, 15.7% and 42.0%,respectively, as compared with CTC group. The image quality at upper lung and bottom lung level in ZTCM group was improved significantly (P < 0.05) and the cases of excellent images in ZTCM group was significantly higher than that of CTC group (P < 0.05). Conclusion ZTCM technique not only contributes to more rational distribution of radiation doses but also realizes individuation, decreases the total radiation doses and improves image quality in chest CT scanning. It is valuable and promising in chest CT scan. Key words: Tomography, X-ray computed;  Mass chest X-ray;  Noise;  Radiation dosage