Vertical off-centering affects organ dose in chest CT: Evidence from Monte Carlo simulations in anthropomorphic phantoms.

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The objective of our study was to assess the effect of patient vertical off-centering on organ dose in chest CT with tube current modulation. For this purpose, anthropomorphic phantoms representing adult male, female, and overweight male were scanned on 192-slice CT scanner at 11 different vertical positions (maximal off-centering ± 5 cm). Monte Carlo simulations were performed for each of the investigated setup, using tube current values extracted from the raw data, in order to obtain 3D dose distributions. Organ doses were calculated as a function of vertical off-centering and compared with the reference values, calculated for the phantoms positioned in the gantry isocenter. Image noise was also calculated as a function of phantoms vertical position using few circular regions of interest. Pearson statistical analysis was used to determine the correlation coefficient between image noise and organ dose values with vertical off-centering. Results of our study showed a significant difference in tube currents applied by the CT scanner when the phantom was scanned in off-centered vertical positions compared to those obtained when the phantom was positioned in the gantry isocenter (P < 0.005). For all investigated phantom configurations the vertical off-centering below 20 mm in both directions resulted in relative organ dose differences below 7%, while the off-centering above 40 mm was associated with higher organ dose changes of about 20%. The highest relative dose difference of 38% was observed for the thyroid gland at the lowest table positions. A significant correlation between organ doses for breasts, heart, lungs, thyroid, and liver, and vertical off-centering (R2 = 0.909-0.998, P < 0.005) was found. The relative dose increase associated with lower table position was more pronounced in peripheral organs: breast and thyroid gland. Image noise behaved opposite to the tube current and organ doses and increased at higher table positions. Strong vertical off-centering in chest CT with tube current modulation results in misoperation of the TCM function affecting both radiation dose and image noise. Therefore, special attention must be paid to a correct patient positioning in order to optimize organ doses and image quality of the respective CT examination.