CT 검사에서 Gantry Aperture의 Isocenter와 피사체 중심의 불 일치 시 선량과 화질의 평가

Evaluation of Dose and Image Quality in Case of Discordance Between Isocenter of Gantry Aperture and Object Center in CT Examination

Purpose To recognize thar exact adjustment of the table’s vertical center is important in CT examination by analyzing the dose effect and image quality in case of discordance between isocenter of gantry aperture and object center in CT examination. Materials and methods 32 cm body phantom with pencil type CT ionization chamber was used to measure CTDI, and Rando phantom with glass dosimetry was used to measure organ dose. Also, ACR phantom was used to analyze image quality at GE LightSpeed VCT XT 64 MDCT. Dose and image quality were checked at off-centering of each 30 mm, 60 mm lower in the vertical direction, compared with center in the isocenter of gantry aperture. CTDI was measured at 4 position(12, 9, 6, 3 o’clock) of same plane in each vertical direction levels. And organ doses were measured ar both chest CT and abdomen CT with and without AEC. Measured organ were thyroid, breast, heart, lung in chest CT and liver, kidney, uterus, gonad in abdominal CT. Analysis of image quality was done by evaluating objective noise, low contrast resolution, high contrast resolution and uniformity under the condition of clinical abdominal study protocol. Results 1. DTDI value ar die position of 12 o’clock in the phantom was increased according to lower object, while CTDI values at die position of 3, 6, 9 o’clock were decreased. 2. When object was positioned at lower levels in chest CT and abdomen CT with AEC, all organ doses were decreased. 3. Breast and heart doses were increased at lower levels dian gantry isocenter in chest CT without AEC. 4. CT No. of water, low contrast resolution, high contrast resolution, uniformity were our of standard range according to off-centering of object. Conclusion Incorrect patient’s position can affect dose and image quality due to varied noise index value presetted by bow-tie filter in using AEC technology and increase surface dose to sensitive organ such breast. Therefore, it is always need for centering object to isocenter of gantry aperture precision with the greatest care.