Objective: The aim of our study was to evaluate CT scanning protocols to determine how best to minimize patient exposure to ionizing radiation while maintaining sufficient image quality to detect pulmonary diseases.
Subject and methods: The CT dose index (CTDI) was determined by scanning an acrylic phantom at various tube current-time products (7.5-115 mAs). Image quality was evaluated by comparing the homogeneity and noise level of CT scans obtained in the acrylic phantom with those obtained in a water-equivalent phantom. The CT scans obtained at various milliampere-second settings in patients with nodules or diffuse opacifications were assessed. The relationships between the CTDI and the image quality of the CT scans (noise level and artifacts) were established.
Results: The reduction of a conventional tube current-time product (115 mAs) by 65%, 78%, or 93.5% can decrease the CTDI by 60%, 70%, or 85%, respectively. In correlating the image quality of each CT scan to the milliampere-second settings used to obtain it, we found that homogeneity decreased as milliampere-second settings decreased, whereas the noise level increased as milliampere-second settings decreased. For both 8- and 3-mm slice thicknesses, the homogeneity of CT scans acquired at 7.5 mAs or greater was within the acceptable range (< 4 H). However, the noise level of CT scans remained within the acceptable range (< 0.35%) when 25 mAs or greater was used. The evaluation of the image quality of the patients' CT scans indicated no statistical significance in image quality rating between the scans obtained at 25 mAs and those obtained at 115 mAs (p > 0.01).
Conclusion: Low-dose (i.e., 40 or 25 mAs) helical chest CT produced satisfactory image quality and reduced the CTDI, thereby maximally protecting patients from radiation exposure.