Purpose: To investigate the potential of projection angle-dependent anatomical adaptation of tube current to reduce dose in spiral CT examinations of the thorax without loss in detectability of pulmonary nodules and image quality.
Patients and methods: Dose can be reduced for non-circular patient cross-sections without an increase in noise if the tube current is reduced at those angular positions where the patient diameter and, consequently, attenuation are small. The examinations were dose with SOMATOM Plus 4 (Siemens AG). CT projection data were analysed to determine the optimum tube current for each projection angle in real time. We compared image quality, done, and detection rates of pulmonary nodules for thoracic spiral CT examinations with and without online anatomically adapted tube current control in a group of 38 patients. Three radiologists counted all intrapulmonary nodules in consensus separated in three different groups (< 5 mm, 5-10 mm, > 10 mm). Image quality was evaluated in a scale from 1-3 (1 = very good, 2 = good, 3 = worse).
Results: On average, the dose was reduced by 21% (15-34%). With a constant tube current, 704 pulmonary nodules were detected, with tube current modulation 707 pulmonary nodules. For three patients we saw more pulmonary nodules (five) with tube current modulation, for two patients we saw less pulmonary nodules (two). In a direct comparison, the missed intrapulmonary nodules were also detected in the respective method. Thus, the detection rate of intrapulmonary nodules was uninfluenced. In general, no deterioration of image quality was observed.
Conclusion: On average, 21% dose reduction was achieved by an anatomically adapted tube current modulation in spiral CT examinations of the thorax without a loss in detectability of pulmonary nodules and image quality. This method is an important contribution to dose reduction in spiral CT.