Evaluation of tube current modulation programms for the optimization of scan protocols in computed tomography

Computed tomography (CT) is commonly associated with relatively high patient doses. In order to keep the patient doses from the CT examinations on the acceptable level it is necessary to apply the principle of optimization. An essential part of optimization is the achievement of the compromise between the patient dose reduction and the maintenance of the image quality that provides accurate diagnostic information. The aim of the study was to determine the relations between the patient doses, CT image quality and the parameters of the tube current modulation program (Auto mA, CareDose and DoseRight) for the examination of the chest. The study was performed on the three most common modern tomographs in Russia: Ingenuity Core 128, Philips; Optima 64, General Electric; Definition AS, Siemens. The anthropomorphic phantom Lungman (Kyoto Kagaku CO., LTD) was used in the study. In order to assess the tube current modulation for the range of tube voltage (80–120 kV), the reference mA∙s (ref. mAs), Noise Index (NI) and Dose Right Index (DRI) were changed for the CareDose (Siemens), Auto mA (GE) and RightDose (Philips),  respectively. Estimation of the effective dose was performed using the method from Methodical guidance 2.6.1.2944-11 (MU 2.6.1.2944-11). In order to evaluate the image quality, the noise of a CT image in the mediastinum was selected as the most homogenous for chest region. It was estimated, that for the GE units with Auto mA, the noise of CT image had a liner relationship with NI; the patient dose decreased with the increase of NI. For the Siemens units with CareDose, the noise of CT image decreased with the ref.mAs for the range of tube voltage (80–120 kV); the patient dose was directly proportional to the ref.mAs and increased with the tube voltage. For the Philips units with DoseRight, the noise of CT image decreased with the DRI for the range of tube voltage (80–120 kV); the patient dose was directly proportional to the DRI and had no dependence on the tube voltage. The tube current modulations are proprietary for each manufacturer; it is necessary to consider  them for the protocol development. The obtained dependences could be useful for optimization of CT protocols.