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Objective: This study aims to investigate the consequences on dose and image quality of the choices of different combinations of NI and adaptive statistical iterative reconstruction (ASIR) percentage, the image quality parameters of GE CT equipment., Methods: An anthropomorphic phantom was used to simulate the chest and upper abdomen of a standard weight patient. Images were acquired with tube current modulation and different values of noise index, in the range 10-22 for a slice thickness of 5mm and a tube voltage of 120 kV. For each selected noise index, several image series were reconstructed using different percentages of ASIR (0, 40, 50, 60, 70, 100). Quantitative noise was assessed at different phantom locations. Computed tomography dose index (CTDI) and dose length products (DLP) were recorded. Three radiologists reviewed the images in a blinded and randomized manner and assessed the subjective image quality by comparing the image series with the one acquired with the reference protocol (noise index 14, ASIR 40%). The perceived noise, contrast, edge sharpness and overall quality were graded on a scale from -2 (much worse) to +2 (much better)., Results: A repeatable trend of noise reduction versus the percentage of ASIR was observed for different noise levels and phantom locations. The different combinations of noise index and percentage of ASIR to obtain a desired dose reduction were assessed. The subjective image quality evaluation evidenced a possible dose reduction between 24 and 40% as a consequence of an increment of ASIR percentage to 50 or 70%, respectively., Conclusion: These results highlighted that the same patient dose reduction can be obtained with several combinations of noise index and percentages of ASIR, providing a model with which to choose these acquisition parameters in future optimization studies, with the aim of reducing patient dose by maintaining image quality in diagnostic levels., (Copyright © 2012 Elsevier Ireland Ltd. All rights reserved.)

The purpose of this study was to evaluate the consequences of different choices of acquisition parameters on the actual image noise and on the patient dose with an automatic tube current modulation system. The CT investigated was a GE Lightspeed 16-slice and an anthropomorphic phantom was used to simulate the patient. Several acquisitions were made varying noise index (NI), kilovoltage and pitch values. Tube current values were compared for the different acquisitions. Patient dose was evaluated in terms of volumetric computed tomography dose index (CTDI(vol)) and also as effective dose. The noise actually present in the images was analyzed by a region of interest analysis considering representatively phantom sections in the regions of the shoulders, of the lungs and of the abdomen. The obtained results generally evidenced a good agreement between the noise index and the measured noise for the abdomen sections, whereas for the shoulders and the lungs sections the measured noise was respectively greater and lower of the NI. Varying the kV the automatic current modulation system provided images with a substantially constancy of the actual noise and of the patient dose. An increase of the pitch generally decreased the patient dose, whereas the noise was slightly greater for the lowest pitch and almost constant for the other pitch values. This study outlines some important relationships between an automatic tube current modulation system and other CT acquisition parameters, providing useful informations for the choice requested by radiologists in the task of optimization of the CT acquisition protocols. Unless there are other considerations in place, pixel pitches below 1.375 should be avoided, and kVp settings can be changed with no real impact on dose or image noise.