The Effect of Adjusting Computed Tomography Scanner Couch Height for a Chest Phantom Scan Using Automatic Tube Current Modulation System: Radiation Dose, Image Quality and Thyroid Dose
Keywords:Radiation dose, Computed tomography scanner, Thyroid, Automatic tube current modulation, Patient mis-positioning
Background : Automatic tube current modulation (ATCM) system plays an important role for computed tomography (CT) examinations. Thyroid is one of the more radio-sensitive organs.Objective : The aims of this research were to measure the thyroid dose received from a chest CT scan using ATCM system and to examine the impact of patient’s vertical mis - centering to the radiation dose and image quality.Method : The experiments were performed using a Philips Brilliance 64 CT scanner. A chest phantom was positioned at the isocenter of CT gantry, a surview image was taken at 180 degree angle tube position and the phantom was scanned. The couch heights were then raised and lowered by 2, 4, 6 and 8 cm, the surview images and scans of the phantom were repeated for individual couch heights. Skin doses at the thyroid were measured using Nano dot dosimeters. Volume computed tomography dose index (CTDIvol) and dose length product (DLP) were recorded from the scanner’s display and values of image noise along the scan length were measured using Image J software.Result : The surview images were magnified when the couch heights were lowered (or nearer the X-ray tube). The average mAs/ slice and CTDIvol for different couch heights were within ±7.4% compared to those for the isocenter, they were relatively increased and decreased when the couch were higher and lower than the isocenter position, respectively. These were opposite to the sizes of surview image as mentioned earlier. This might be because the ATCM software allows accurate estimation of the phantom attenuation. The average values of image noise measured for different couch heights were not significant differences from that for the isocenter, except for that when the couch was 8 cm lowered. Doses for thyroid decreased when the couch were moved away from the isocenter due to the more radiation attenuation by the edge of bow-tie filter.Conclusion : A well-centered patient positioning was very important for use of CT ATCM system, in order to achieve appropriate radiation dose and image quality levels.
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