[Optimizing dosage in thoracic computerized tomography]

Radiation exposure in computed tomography (CT) by far exceeds radiation dose in chest radiography. Dose requirements in CT of the chest, however, are much smaller than for the abdomen because of low x-ray absorption in the lungs. This article describes scanner parameters that influence patient exposure and image quality. Suitable compromises will be explained that allow for dose reduction in chest CT without jeopardising image quality. Dose reduction in chest CT should be performed depending on the clinical indication and requires active reduction of mAs settings. For helical CT, a pitch of 1.5 to 2 should be employed. Dedicated low-dose techniques for screening of bronchogenic carcinoma are described. Dose reduction decreases image quality but the detrimental effects can be reduced by applying proper parameters for scanning and image reconstruction. Thus, images of the mediastinum should be reconstructed with a smoothing filter, while a higher resolution filter should be used for the lungs. In multislice CT, reconstruction of thicker axial or multiplanar sections retains spatial resolution but keeps image noise and thus dose requirements low. The effective, weighted CT dose index (CTDIw,eff) can serve as a rough estimate of the effective patient dose (E) in helical or multislice CT of the whole chest: for most scanners and age groups the conversion factors are 0.5 mSv/mGy for females and 0.4 mSv/mGy for males.