Evaluation of tube current modulation programs for the optimization of chest computed tomography protocols

Computed tomography (CT) is commonly associated with relatively high patient doses. 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 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 CT of the chest. The study was performed on the three most common CT models in Russia: Ingenuity Core 128, Philips; Optima 64, General Electric; Definition AS, Siemens, using the anthropomorphic phantom Lungman (Kyoto Kagaku CO., LTD). 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. Signal-noise ratio (SNR) of a CT image in the mediastinum area was selected to evaluate the image quality. It was estimated, that for the Philips units with DoseRight program the patient dose is directly proportional to DRI; image noise decreases and SNR increasing with DRI. For GE unit with Auto mA program image noise proportional to NI, SNR inversely proportional to NI and patient dose decreases with increasing NI. For Siemens unit with CareDose program the patient dose is directly proportional to ref.mAs; image noise decreases with ref.mAs and SNR increases with ref.mAs 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.