[Helical CT for lung-cancer screening: third report: fundamental study for ultralow-dose CT by application of small tube current and filter].

In order to develop ultra-low-dose helical CT for lung cancer screening, the effect of reduction of the tube current to 20mA and application of a 10mm thick aluminium filter upon radiation dose and image quality was evaluated with a phantom. Exposure dose at the center of a gantry and absorbed dose at the center of an acrylic phantom at 20mA with the filter were 15% and 29% of the dose at 50mA without the filter, respectively. For reduction of absorbed dose, reduction of the tube current was more useful than application of the filter. Image noise at 20mA with the filter was double that at 50mA without the filter. Neither reduction of the tube current nor application of the filter changed full width at half maximum on section sensitivity of the Z-axis. Although reduction of the tube current did not affect the difference in CT values between an acrylic sphere and styroform, application of the filter caused a reduction of 4.5% in the difference in CT values. Neither reduction of the tube current nor application of the filter affected the contrast resolution of the high-contrast phantom; however, that of the low-contrast phantom deteriorated. Although improvement of the filter and evaluation of clinical images are necessary, reduction of the tube current to 20mA and application of the aluminium filter appear to be a promising method for ultra-low-dose helical CT of the lung.