Standard-dose and 50%-reduced-dose chest CT: Comparing the effect on image quality

OBJECTIVE. CT accounts for considerable population-based radiation dose from radiographic diagnostic studies. The technical factors for CT examinations are not appropriately adjusted on the basis of patient size and anatomy. We hypothesized that radiation doses for routine chest CT can be reduced by 50% for the evaluation of normal structures without seriously jeopardizing image quality. SUBJECTS AND METHODS. After receiving institutional review board approval, we prospectively studied 24 patients with cancer who were 65 years old and older on a multidetector CT scanner. Each patient underwent imaging with four slices (centered at the carina) at the standard dose (220-280 mAs) and at 50%-reduced dose (110-140 mAs) at a constant 140 kVp. Single breath-hold scanning was performed with a 2.5-mm detector configuration, a tube rotation time of 0.8 sec, and a pitch of 6:1. Contiguous images were reconstructed at 5-mm intervals. Two subspecialty-trained chest radiologists who were unaware of the CT technique reviewed randomized images for overall image quality and anatomic detail of the structures in the lung, airway, mediastinum, and chest wall using a 5-point scale (1, worst; 2, suboptimal; 3, adequate; 4, very good; 5, excellent). The data were analyzed using the Wilcoxon's signed rank test. RESULTS. Although overall image quality was better with standard-dose CT, the quality of reduced-dose CT was acceptable. The differences in mean scores were statistically significant. There was a correlation of 0.59 between observers. The mean scores of standard-dose CT were always greater than or equivalent to those of low-dose CT for both observers. The assessment of great vessels and soft tissue of the chest wall contributed mainly to the differences in image quality. Both the central and peripheral lung parenchyma and the airway were adequately visualized on low-dose CT. Radiation doses (based on weighted-CT-dose index) from standard-dose CT and 50%-reduced-dose CT were 15.6-21.4 mSv and 7.8-10.7 mSv, respectively (from the manufacturer's data). CONCLUSION. Chest CT image quality appears to be acceptable for evaluating normal anatomic structures even with a 50% reduction in radiation dose.