两种双能CT技术虚拟单能谱图像的噪声比较研究

Image quality evaluation of virtual monoenergetic spectral images obtained in two types of dual energy CT based on noise level

目的探讨在相同辐射剂量下，两种双能CT技术虚拟单能谱（VMS）图像噪声指标的异同。方法将12只5ml针筒装含碘15mg／ml对比剂和12只5ml装生理盐水（碘水对）分别放置于仿真体模头、胸、腹部的上、下、左、右4个方位。对仿真体模分别行双源CT和能谱CT扫描，在相同剂量下（容积CT剂量指数为6．52mGy）经后处理分别获得40～140keV共7个能级虚拟单能谱图像（VMS双型和VMS管电压）。测量不同能级VMS双源和VMS管电压、图像的碘CT值、背景噪声、碘信噪比（SNR）和碘对比噪声比（CNR）。采用配对t检验比较相同能级VMS双型和VMS管电压的碘CT值、背景噪声、碘SNR和碘CNR，采用单因素方差分析比较同一成像方式相同能级不同部位上述参数的差异。结果VMS双源和VMS管电压图像的碘CT值均随着能级升高逐渐下降，且各部位各能级的VMS双源图像碘CT值均高于VMS管电压图像，差异有统计学意义（P〈0．05）。VMS管电压的CT值在低于100keV时从头、胸、腹部依次降低，而VM双源多数能级的CT值在胸部较高，差异有统计学意义（P〈0．05）。VMS管电压和VMS双源图像的背景噪声均在40keV时最高，在头、胸、腹部各能级均依次升高。VMS管电压背景噪声随着能级增高逐渐降低，在70keV达低点后，80keV有所升高，而后继续降低；VMS双源背景噪声在70或80keV降至低点后，而后逐渐升高。除头部40、100keV和胸部100keV的VMS管电压和VMS双源背景噪声差异无统计学意义外，其余差异均有统计学意义，且在低于100keV时VMS双源背景噪声低于VMS管电压，高于100keV时VMS双源背景噪声高于VMS管电压（P〈0．05）。VMS管电压和VMS双源的碘SNR在40或60keV最高，除40keV头、腹部外，VMS双源的碘SNR均高于VMS管电压，差异有统计学意义（P〈0．05）。VMS管电压和VMS双源的碘CNR均在60keV最高，除40keV头部和120、140keV腹部外，VMS双源的碘CNR均高于VMS管电压，差异有统计学意义（P〈0．05）。SNR和CNR在各能级从头、胸、腹部均依次降低。结论两种双能CT技术VMS图像噪声指标在不同能级、部位表现各不相同，但二者均可在约70keV获得较低噪声的图像，在60keV获得最佳SNR和CNR的图像。

Objective To investigate the image quality difference of the virtual monochromatic spectral（VMS） images synthesized from fast kilovohage switching and dual-source dual-energy CT for a given radiation dose. Methods A plurality of disposable syringes containing 15 mg/ml iodine contrast agent and saline were placed on the surface of the male anthropomorphic phantom consisting ofhead neck and torso, GE HD750 gemstone spectral imaging and Siemens Somatom Definition Flash dual energy CT protocols were performed on the phantom for a same dose（volume CT dose index 6.52 mGy）, and VMS images （40, 60, 70, 80, 100, 120, 140 keV） were reconstructed and obtained（VMSkV switching and VMSdual-source）, respectively. The objective image noise, iodine signal noise ratio（SNR）, iodine contrast noise ratio （CNR） and CT values were measured. The results were analyzed using the paired t test and ANOVA. Results All VMS iodine CT values were gradually decreased with the increasing of keV, and iodine CT values on VMSdual-source were greater than on VMSkV switching images（P〈0.05）, VMSkV switching iodine CT values in descending order from the head, chest, abdomen at less than 100 keV,while most VMSdual-source iodine CT values were highest on chest（P〈0.05）. VMSdual-source and VMSkV switching image noise were highest at 40 keV and successively raised from the head, chest , abdomen. VMSkV switching image noise gradually decreased with the increase of keV in the range of 40 to 70 keV , gradually increased up to the trough after 70 keV from 80 keV gradually decreased, while VMSdual-source noise was lowest at 70 keV or 80 keV , and then with the keV increased gradually increased. Image noise on VnSkV switching images in the range of 40 to 100 keV were higher than that on VMSdual-source images and lower in the range of 100 to 140 keV at most part（P〈0.05）. VMSdual-source and VMSkV switching iodine SNR were highest at 40 keV or 60 keV, SNR on VMSdual-source are greater than on VMSkV switching images except 40 keV images on head and abdomen（P〈0.05） . VMSdual-source and VMSkV switching iodine CNR were highest at 60 keV, CNR on VMSdual-source images are greater than on VMSkV switching images except 40 keV images on head 100 keV and 120 keV on abdomen（P〈0.05）.SNR and CNR decreased from the head, chest, abdomen （P〈0.05）. Conclusion VMS images synthesized from fast kilovohage switching and dual-source dual-energy CT have different image quality performance at most keV and body parts, but VMS imaging at approximately 70 keV yielded lower image noise, and at approximately 60 keV yielded highest SNR and CNR.