Research Progress on Reducing Metal Artifacts in CT Imaging
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摘要:
颅内动脉瘤栓塞术、口腔金属置入物、脊柱内固定物以及髋关节置换术等金属置入物在临床中的应用日益增加。采用CT图像对金属置入物术后患者进行评估时产生的金属伪影会使CT图像无法清晰显示金属-骨界面及邻近的组织结构，影响医生诊断的精准性。随着CT的发展，去金属伪影技术和虚拟单能量成像技术等手段有助于减少CT图像中的金属伪影，深度学习重建算法和光子计数CT的出现为金属置入物术后的精准评估提供更加可靠的依据。本文就减少CT图像中金属伪影的研究进展作一综述。
Abstract:Intracranial aneurysm embolization, oral metal implants, spinal internal fixation, hip replacement, and other metal implants are increasingly used in clinical practice. Metal artifacts generated by computed tomography (CT) images during the evaluation of patients after placement of metal implants will prevent CT images from clearly showing the metal–bone interface and the adjacent tissue structure, which affects the accuracy of doctors' diagnosis. With the development of CT, metal artifact reduction technology and virtual monoenergetic imaging technology are helpful to reduce metal artifacts in CT imaging. The emergence of deep learning reconstruction algorithms and photon counting CT provides a more reliable basis for the accurate evaluation of metal implants after surgery. This paper reviews the progress of research on reducing metal artifacts in CT imaging.
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Keywords:
- deep learning /
- CT imaging /
- metal artifact /
- monoenergetic imaging /
- photon counting CT
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随着人口老龄化，冠心病患者日益增多，如何快速、准确评判冠状动脉病变至关重要[1-2]。当前冠状动脉CT血管造影（coronary CT angiography, CCTA）已成为诊断冠心病的重要手段之一，具有无创、便捷、经济等优势，但是其所带来的电离辐射危害及碘对比剂引起的不良反应不容忽视[3]。研究表明[4-5]CTA根据患者体质量指数（body mass index, BMI）不同，采用个体化方案能有效降低辐射剂量及碘摄入量。
本文通过对不同BMI患者以低管电压低剂量对比剂行64排CCTA检查进行前瞻性研究，从而探讨其临床应用价值。
1. 资料与方法
1.1 一般资料
前瞻性选择2022年1月至2024年3月上海市金山区中西医结合医院临床疑诊冠心病而行CCTA检查的160例患者为研究对象，其中男97例，女63例，年龄27~85岁，平均（62.17±12.69）岁。病例纳入标准：具有胸痛、心前区不适、胸闷等可疑冠心病患者；BMI介于（18.5~28）kg/m2且能主动配合CCTA检查技术要求；心率齐且在50~80次/分之间。排除标准：碘对比剂过敏；心肝肾功能不全、甲状腺功能异常等禁忌症；有金属伪影、冠状动脉支架植入术、心脏搭桥手术者。
将160例按照BMI分为A组（18.5<BMI<24）和B组（24≤BMI<28），各80例，每组按照随机原则分对照组（A1/B1组）和试验组（A2/B2组），各40例。
本研究经医院伦理委员会审查并通过，所有患者均知情同意。
1.2 检查方法
采用Siemens Definition 64排螺旋CT机扫描，回顾性心电门控下采集数据，患者双手抱头，取仰卧位，足先进的进床方式，扫描范围从气管分叉至心底下缘水平。扫描前去除体表异物，训练屏气，告知相关检查注意事项。
管电流采用自动调节技术，准直64×0.6 mm，转速0.33 s/r，螺距0.25，扫描层厚3 mm，FOV 180~220 mm，矩阵512×512，重建期相67％ R-R间期。
重建层厚0.75 mm，间隔0.6 mm，重建函数B26 f。对照组均采用120 kV管电压扫描，A1、B1两组分别采用60 mL对比剂+40 mL生理盐水、80 mL对比剂+20 mL生理盐水注射。
试验组均采用双低剂量方式，A2组80 kV管电压，40 mL对比剂+60 mL生理盐水，B2组100 kV管电压，60 mL对比剂+40 mL生理盐水。所用对比剂均为碘海醇注射液（北陆药业，浓度350 mgI/mL），采用双筒高压注射器，经肘正中静脉以4.5 mL/s流率先后注射碘海醇及生理盐水。将升主动脉根部气管分叉下约1~2 cm水平作为触发层面，感兴趣区（ROI）置于中央位置，当CT阈值达110 HU时延迟6 s进行容积扫描。
1.3 图像后处理
将螺旋扫描所得原始数据上传至工作站，对冠状动脉各支血管进行图像后处理，包括多平面重组（multiplanar reconstruction, MPR）、最大密度投影（maximum intensity projection, MIP）、容积再现（volume rendering, VR）、曲面重建（curved planar reconstruction, CPR），分析图像质量。
1.4 图像质量评价
1.4.1 图像主观评分
按照美国心脏协会冠状动脉分段法，从血管边缘锐利度、细小分支显示、管腔内对比剂清晰度、有无伪影或错层4个方面评价，采用5分法评分[6]。5分：血管边缘锐利，细小分支多、管腔对比剂清晰，图像无运动伪影和错层；4分：血管边缘较锐利，分支显示较多、管腔对比剂较清晰，图像有轻度运动伪影或错层；3分：血管边缘略毛糙，分支显示尚可，图像错层较大，存在较多的运动伪影，但不影响评分；2分：血管边缘模糊，分支少，管腔对比剂模糊，图像错层大、运动伪影较严重，影响评分；1分：血管显示不清，分支不显示，存在大量运动伪影或错层，管腔无法评价。
由两名经验丰富CTA诊断医师对所有图像进行双盲法评分。将评分3 分及以上定义为合格，能满足诊断要求，4分及以上为优良，2分及以下为不合格，不符合诊断要求，统计每组图像的优良率及合格率。
1.4.2 图像客观评价
选择升主动脉根部（冠状动脉最先出现的层面）作为测量层面，将ROI放置于血管腔中间位置，避开管壁钙化和斑块，测量其CT值，ROI面积为（100~120） mm2，每例测量5次并求平均值，标记为CT1。将同层面皮下脂肪组织作为对比信号，避开钙化及乳腺组织，测量其CT值，ROI面积为30~40 mm2，每例测量5次并求平均值，标记为CT2，取CT2的标准差作为图像噪声（standard deviation, SD）。
计算图像信噪比（signal to noise ratio, SNR）和对比噪声比（contrast noise ratio, CNR），SNR= CT1/SD，CNR=（CT1−CT2）/SD。
1.5 辐射剂量及碘摄入量
根据设备自动生成的辐射剂量报告记录患者的容积CT剂量指数（CTDIvol）和剂量长度乘积（DLP），并计算有效剂量（ED），ED =DLP×k，其中k取0.014。碘摄入量=对比剂浓度×对比剂用量[7]。
1.6 统计学方法
利用SPSS 25.0软件进行统计学处理，计量资料均符合正态分布，以（
$\bar{x}\pm s $ ）表示，采用独立样本t检验。计数资料以百分比（％）表示，行x2检验。采用Kappa检验对两名医师评分的一致性进行检验。P < 0.05为差异有统计学意义。
2. 结果
2.1 一般资料比较
所有病例符合要求，均顺利完成CCTA检查，经统计分析，A1、A2两组及B1、B2两组的患者年龄、性别、心率、身高、体重、BMI无统计学差异，具有可比性（表1）。
表 1 一般资料比较Table 1. Comparison of basic data组别 例数 年龄/岁 性别比/（男/女） 心率/（次/分） 身高/cm 体重/kg BMI/（kg/m2） A1组 40 62.18±14.14 27/13 69.45±5.38 167.98±6.57 63.95±5.13 22.66±1.22 A2组 40 64.85±11.04 24/16 70.15±5.81 168.33±5.81 62.93±4.91 22.20±1.26 B1组 40 60.05±13.01 25/15 66.80±5.97 162.35±5.70 68.95±4.49 26.15±0.95 B2组 40 61.60±12.37 21/19 67.28±5.44 163.00±5.82 68.85±4.63 25.90±0.80 P 0.348＃ 0.485＃ 0.578＃ 0.801＃ 0.364＃ 0.103＃ 0.587△ 0.366△ 0.711△ 0.615△ 0.922△ 0.204△ 注：＃表示A1、A2两组比较；△表示B1、B2两组比较。 2.2 图像质量评价
两名医师的图像评分一致性好（A、B两组Kappa值分别为0.848、0.809）。经比较，A1和A2组图像合格率、优良率、SNR及CNR均无统计学差异，A2组的主动脉CT值、图像噪声高于A1组，有统计学差异（表2、表3、图1）。
表 2 图像质量主观评分比较Table 2. Comparison of subjective scores of image quality组别 例数 得分（例） 合格率/％ 优良率/％ 5分 4分 3分 2分 1分 A1组 40 15 12 7 4 2 85.0 67.5 A2组 40 12 13 10 2 3 87.5 62.5 B1组 40 16 10 3 7 4 72.5 65.0 B2组 40 18 16 2 3 1 90.0 85.0 P 0.745＃ 0.639＃ 0.045△ 0.039△ 注：＃表示A1、A2两组比较；△表示B1、B2两组比较。 表 3 图像质量客观比较Table 3. Comparison of objective comparison of image quality组别 例数 主动脉CT值/HU 同层面胸壁脂肪CT值/HU SD SNR CNR A1组 40 394.49±32.75 −103.86±8.42 16.81±1.97 23.79±3.50 29.04±2.95 A2组 40 559.65±37.61 −104.39±8.84 25.01±2.03 22.92±1.75 28.19±2.00 B1组 40 377.29±28.89 −102.75±8.45 18.30±1.45 20.76±2.49 26.41±2.98 B2组 40 512.90±24.50 −103.64±7.69 18.73±1.55 27.58±2.86 22.53±1.90 P 0.000＃ 0.783＃ 0.000＃ 0.164＃ 0.137＃ 0.000△ 0.625△ 0.202△ 0.000△ 0.000△ 注：＃表示A1、A2两组比较；△表示B1、B2两组比较。 ![]()
图 1 A组患者冠状动脉横断面及曲面重建图像注：（a）~（b）对照组患者，管电压120 kV扫描，使用60 mL对比剂+40 mL生理盐水注射，主动脉CT值402.95 HU，图像评分4分；（c）~（d）试验组患者，管电压80 kV扫描，使用40 mL对比剂+60 mL生理盐水注射，主动脉CT值602.73 HU，图像评分5分。Figure 1. Coronary artery cross-sectional and curved planar reconstruction images in group A patientsB2组图像合格率、优良率、主动脉CT值、SNR及CNR均高于B1组，图像噪声与B1组无差别（表2、表3、图2）。
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图 2 B组患者冠状动脉横断面及曲面重建图像注：（a）~（b）对照组患者，管电压120 kV扫描，使用80 mL对比剂+20 mL生理盐水注射，主动脉CT值362.91 HU，图像评分4分；（c）~（d）试验组患者，管电压100 kV扫描，使用60 mL对比剂+40 mL生理盐水注射，主动脉CT值519.73 HU，图像评分4分。Figure 2. Coronary artery cross-sectional and curved planar reconstruction images in group B patients2.3 辐射剂量及碘摄入量比较
A2组CTDIvol、DLP、ED均低于A1组，差异有统计学意义，ED较A1组减少68.3％（表4）。经计算，A2组碘摄入量较A1组减少33.3％。
表 4 患者辐射剂量及碘摄入量比较Table 4. Comparison of radiation dose and iodine intake组别 例数 曝光长度/cm CTDIvol/mGy DLP/（mGy·cm） ED/mSv A1组 40 157.10±5.22 14.29±1.01 225.30±24.15 3.15±0.34 A2组 40 156.63±6.15 4.85±0.85 71.56±12.16 1.00±0.17 B1组 40 158.53±4.78 15.20±1.09 235.07±22.66 3.29±3.17 B2组 40 157.85±5.02 7.82±1.29 137.38±11.61 1.92±1.63 P 0.710＃ 0.000＃ 0.000＃ 0.000＃ 0.540△ 0.000△ 0.000△ 0.000△ 注：＃表示A1、A2两组比较；△表示B1、B2两组比较。 B2组CTDIvol、DLP、ED均低于B1组，ED较B1组减少41.6％，碘摄入量较B1组减少25％（表4）。
3. 讨论
近年来，由于CCTA的广泛应用，其所导致的较高辐射剂量及碘负荷所致的对比剂肾病问题备受人们关注[8-10]。BMI是评定人群肥胖程度的标准，CCTA扫描参数选择和对比剂用量与之相关[7,11]。目前基于BMI的个性化CCTA研究报道较少，本研究针对不同BMI患者，与常规方法相比，采用双低剂量行64排CCTA检查，探讨其对图像质量的影响。
影响CT辐射剂量的主要因素包括扫描模式、管电压、管电流及螺距，降低管电压可使受检者辐射剂量呈指数下降[12]。本研究18.5< BMI<24、24≤ BMI<28试验组中，管电压分别采用80 kV、100 kV患者所受ED较120 kV管电压分别减少68.3％、41.6％，与文献[13-14]报道降幅更大。
降低管电压图像噪声将增加，但由于光电效应使得血管CT值升高，增强与周围软组织对比度，从而补偿低电压导致的高噪声，改善图像质量[15]。本研究18.5<BMI<24试验组中，80 kV管电压图像噪声虽增加，但主动脉CT值也增高，SNR、CNR与对照组比较无统计学差异，两组图像优良率、合格率比较均无差异，因此证明80 kV低管电压图像能满足诊断要求，在正常BMI患者行CCTA是可行的，与陈惠娟等[16]研究相符。24≤BMI<28试验组100 kV图像噪声与对照组比较无统计学差异，但主动脉CT值、SNR、CNR、图像优良率及合格率均高于对照组，图像质量好于对照组。
目前CCTA所需对比剂剂量多以mL/kg固定值计算[11,17-18]，但对于较大BMI患者容易造成对比剂过量而增加肾脏负担。本研究试验组分别采用40 mL、60 mL低量对比剂+适量生理盐水注射，与对照组比较图像质量影响不大，但碘摄入量分别减少33.3％、25％，从而降低了对比剂毒性。
本研究的不足：①样本量较少；②所入组患者BMI限于18.5~28 kg/m2，有一定局限性；③对CCTA检查适应症要求相对严格，研究结果可能存在一定偏倚，有待今后进一步深入研究。
综上所述，对于不同BMI患者采用低管电压低剂量对比剂行64排CCTA检查是可行的，既能保证图像质量，又能明显降低辐射剂量并减少碘摄入量，具有安全可靠的临床价值。
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