不同水模体直径和不同重建层厚对CT图像噪声的影响

Effect of different water phantom diameters and reconstruction slice thicknesses on CT image noise

目的研究不同水模体直径和重建层厚等因素对CT图像噪声的影响,探讨CT质量控制检测中水模体合理的直径范围,为完善我国CT质量控制检测规范提供数据支持。方法采用GE公司的Revolution型CT机,在两档剂量水平,即CTDI_W分别为30. 20 m Gy(120 kV、200 m As)和49. 82 m Gy(120 kV、330 m As)、图像重建层厚为5 mm和10 mm的条件下,分别对直径为16、18、20、22、24 cm的圆柱型均质水模体轴向扫描一圈,测量其CT值(水)和噪声等指标。比较不同水模体直径、不同层厚和不同剂量等条件下噪声测量值的差异。结果噪声测量值随水模体直径增大而增大,随剂量增大而减小,在大小两档剂量水平下水模体直径分别为24、22 cm时的噪声值已超过现有检测标准;噪声随重建层厚的增大而减小。结论噪声的测量结果与剂量、水模体直径、重建层厚等因素有关,在进行CT质量控制噪声项目的检测时,需规定所选择的剂量水平和层厚大小,并规定所选择水模体的直径大小。本研究结果提示CTDI_W在接近50 m Gy的条件下,18～22 cm可能是比较合理的水模体直径范围。

Objective To study the influence of different water phantom diameters,reconstruction slice thicknesses and doses on CT image noise,discuss the reasonable diameter range of the water phantom in the CT quality control testing,and provide data to improve the standard of CT quality control testing in China. Methods The axial scanning mode of the CT of Revolution type produced by GE company was used. Under the conditions of different doses of CTDI_W30. 20 m Gy( 120 kV,200 m As) and 49. 82 m Gy( 120 kV,330 m As),and different reconstruction slice thicknesses of 5 mm and 10 mm,the water phantom with the diameters of 16,18,20,22 and 24 cm was scanned and its CT values and image noises were measured. The differences of the image noises were compared under different water phantom diameters,thicknesses and doses. Results The value of image noise amplified with the increase of the diameter of the water phantom,and decreased with the increase of the dose. The noise values with the water phantom diameters of 24 cm and 22 cm exceeded the existing testing standard. The noise decreased with the increase of the thickness of the slice thickness. Conclusion The measurement results of image noise are related to factors such as the dose,the diameter of the water phantom,the reconstruction thickness,etc. In the noise measurement of CT quality control testing,the dose,slice thickness,and the size of the water phantom are required to be selected.The results show that the range between 18 cm and 22 cm may be a reasonable value for the diameter of water phantom on a dose of close to 50 m Gy.