A phantom study of the effect of deviation from isocentric points on CT image quality
10.3760/cma.j.cn112149-20220710-00596
- VernacularTitle:偏离等中心点对CT图像质量影响的体模研究
- Author:
Lingming ZENG
1
;
Han DENG
;
Qin LYU
;
Tao HUANG
;
Liyi HE
;
Libo CAO
;
Zhenlin LI
Author Information
1. 四川大学华西医院放射科，成都 610041
- Keywords:
Tomography, X-ray computed;
Isocenter scan;
Image quality
- From:
Chinese Journal of Radiology
2022;56(11):1237-1241
- CountryChina
- Language:Chinese
-
Abstract:
Objective:To investigate the effect of deviation from the isocenter point on the quality of CT images at the same radiation dose.Methods:A 160-layer CT scanner was used to scan the phantom at isocenter and deviations of 3, 6, 9, 12 and 15 cm. CT was performed with the following parameters: 120 kVp; 400 mAs; slice thickness, 1 mm; and slice increment, 1 mm. Images were reconstructed using the filtered back projection algorithm. Noise power spectrum (NPS), task transfer function (TTF) and detectability index (d′) were measured. NPS peak was used to quantify the noise magnitude and TTF 50% was used to quantify the spatial resolution. NPS, TTF and d′ were compared using one-way ANOVA. Results:The NPS average spatial frequency, spatial resolution and d′ values gradually decreased as the offset distance increased and the amount of noise increased. NPS peak at isocenter and deviations of 3 cm, 6 cm, 9 cm, 12 cm and 15 cm were (94.31±1.48), (104.25±1.46), (131.44±1.96), (171.86±1.91), (224.05±1.37), (286.51±2.09)HU 2·mm 2, respectively ( F=37 241.91, P<0.001). And d′ values of 2 mm low-contrast lesions were 3.51±0.06, 3.31±0.04, 3.01±0.04, 2.59±0.06, 2.21±0.03, 1.88±0.03, respectively. The reduction in spatial resolution was more pronounced for high contrast, and the d′ values decreased to a similar extent for various types of lesions. The noise was increased by about 82%, the high contrast spatial resolution was decreased by about 12%, and the d′ value was decreased by about 26% at 9 cm from the isocenter point (all P<0.05). The noise was increased by about 204%, the high contrast spatial resolution was decreased by about 27%, and the d′ value was decreased by about 45% at 15 cm from the isocenter (all P<0.05). Conclusion:The CT image quality was decreased with the increase of the offset distance from the CT isocenter point. The image quality was severely compromised at offset distances greater than 9 cm.
