CT自动曝光性能的评价

Methods for Evaluation of Spatial Resolution and an Automatic Exposure Control of MSCT

目的用体模研究和评价CT的一种自动曝光(CARE Dose 4D)性能。材料用有机玻璃制作四个体模,分别为类椭圆26×17cm用于腹部检测;类椭圆36×16cm用于髋部检查;椭圆35×12cm用于肩部检测;直径从13cm-18cm的圆锥体模用于头部和腹部检测,四个体模长度均为10cm。西门子SOMATOM Sensation Cardiac 64螺旋CT。方法在自动曝光和手动固定曝光方式下。对圆锥体模用头部和腹部方案分别进行螺旋扫描,比较两种曝光方式噪声和有效mAs随体模直径的变化。在两种曝光方式下,对肩部、腹部和髋部体模采用相应的扫描方案进行螺旋扫描,分别检测两种曝光方式在图像噪声相同情况下,自动曝光有效mAs降低的幅度。结果对圆锥体模用头部和腹部方案扫描的结果是,自动曝光方式,剂量降低幅度依赖体模直径。自动曝光与手动固定曝光相比,腹部体模有效mAs降低在8.9%-5.9%之间。肩部体模有效mAs降低在38.1%-33.5%之间,髋部体模有效mAs降低在34.8%-27.1%之间。讨论在自动曝光(CARE Dose 4D)方式下,剂量降低幅度与体模的结构和大小有关,这种方法可作为CT自动曝光性能检测的一种手段。

Objective The purpose of the study was to evaluate an automatic exposure control （CARE Dose 4D） by phantoms. Materials Four phantoms were made of Plexiglas, one ellipse 35×12cm used for shoulder examination, one oval 26×17cm used for abdomen examination, one oval 36×16cm used for hip examination and a cone shaped phantom for head and abdomen examination and diameters of the cone gradually increase form 13cm to 18cm. Whole four phantoms was 10cm long. 64-slice spiral CT （SIEMENS, SOMATOM Sensation Cardiac 64 ） with an automatic exposure control that was based on both angular and z-axis tube current modulation.Methods The cone phantom was spirally scanning with and without the Automatic Exposure control by head and abdomen protocols. We measured and compared noise and effective mAs between automatic exposure and manual constant exposure controls with changes of the phantom diameter. The abdomen, shoulder and hip phantoms were respectively scanning with and without the automatic exposure control by their respective protocols. To compare effective mAs values between both exposure controls at the same noise level. Results The radiation dose reduction depended on the diameter of the cone phantom with the automatic exposure. At the same noise level, the automatic exposure could reduce dose between 8.9%-5.9% for the abdomen phantom, 38.1%-33.5% for the shoulder phantom, 38.1%-33.5% for the hip phantom. Discussion Patient centering was a critical factor in maintaining the image quality of scans with the automatic exposure and the radiation dose reduction depended on phantom geometry. The methods for evaluation of multi-slice CT could be a means for choice.