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近年来,随着多层螺旋CT技术的飞速发展,CT血管成像(computed tomography angiography,CTA)以其检查的无创性、快捷性和准确性等优点得到了广泛应用[1-2],但伴随而来的较高辐射剂量也越来越引起人们的关注。2009年,第二代双源CT问世,它的时间分辨率达到75 ms,扫描速度45 cm/s,最大螺距3.4。双源CT的大螺距扫描模式,又称Flash spiral模式(Flash模式)。它实现了前瞻性螺旋扫描单个心动周期心脏成像,采集数据时间减少到0.25 s,明显降低了有效射线量。双源CT大螺距血管成像使得CT血管造影在很大程度上满足了临床的各种需要,使CT血管造影技术又达到了一个新高度。
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前瞻性心电门控技术、大螺距扫描模式、较快的进床速度以及0.28 s/圈的旋转时间促使两组球管-探测器在大螺距旋转扫描模式下能在1/4机架旋转时间内覆盖整个心脏容积范围,从而满足在一个心动周期内完成整个心脏的扫描。Achenbach等[3]对不同心率患者的双源CT冠状动脉图像质量进行评价发现,心率≤80次/min患者的最佳成像时间窗为RR间期的70%;Husmann等[4]研究认为,心率<60次/min最佳成像时间窗选择为RR间期的50%~60%。低心率患者采用回顾性心电门控模式时,利用冠状动脉舒张末期低速平台,采集时相选择为RR间期的70%~80%;采用Flash模式时,由于单个心动周期成像,扫描时间至少需要250 ms,如果在RR间期的70%~80%成像,则会落入下一周期的P波后产生伪影,因此,大螺距双源CT默认的扫描时间窗为RR间期的55%或60%[5]。
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Goetti等[6]对心率≥70次/min的患者采用大螺距前瞻性心电门控进行冠状动脉血管成像,结果发现,高心率患者在RR间期的30%成像,不可诊断的血管节段为2.8%,采用RR间期的60%采集数据成像,不可诊断的血管节段高达8.3%。Sun等[7]研究显示,高心率患者(平均心率90次/min)采用Flash模式在RR间期的20%~30%成像可获得较好的图像质量,不可诊断的冠状动脉节段仅为1.5%,这在以往的多层螺旋CT是难以达到的。另有研究表明,随着心率的增加,采用收缩期采集数据成像可降低伪影的发生[8]。
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对支架植入术后的评价,传统的多层螺旋CT的时间分辨率较低,支架的线束硬化伪影致使冠状动脉成像的图像质量较差,多层螺旋CT对支架内管腔观察受限,尤其对 < 3.5 mm的支架内再狭窄诊断受限。有研究发现,双源CT诊断支架内再狭窄的灵敏度为85.7%,特异度为95.7%,当支架直径>2.75 mm时,诊断支架内再狭窄的灵敏度与特异度均为100%[9]。另有研究显示,双源CT诊断支架直径≥3.50 mm、支架直径=3.00 mm和支架直径≤2.75 mm的再狭窄特异度分别为100%、80%和66%,阳性预测值分别为100%、95%和53%[10]。Wang等[11]研究结果显示,在173例支架植入术后的患者中,有156例获得了较好的图像质量,44例支架直径<3 mm的患者中,冠状动脉CTA发现有10例患者发生支架内再狭窄,而同样行冠状动脉血管造影仅发现9例。由此可见,双源CT诊断支架内再狭窄有较好的可行性,支架内径是影响支架内腔成像的关键因素。
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冠状动脉旁路移植术后5年内有25%的患者发生桥血管的狭窄或闭塞,随访复查至关重要。由于桥血管较长,因此,冠状动脉搭桥术后患者冠状动脉CT成像扫描范围较常规扫描范围大,辐射剂量高。Lee等[12]研究显示,使用Flash模式的辐射剂量可较回顾性心电门控扫描模式降低65%左右。Heye等[13]利用二代双源CT评价110条桥血管,诊断灵敏度和特异度分别为97.9%和100%,Goetti等[14]同样利用第二代双源CT评价125条桥血管的465个血管节段,不可诊断的血管比例仅为0.6%,有效剂量为2.3 mSv。由此可见,双源CT对桥血管狭窄或闭塞的诊断准确性优于多层螺旋CT。
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冠状动脉CT成像的主要局限性是对人体的电离辐射,一项评价冠状动脉辐射剂量的多中心临床研究显示,采用传统的回顾性心电门控平均有效剂量为12 mSv[15]。根据放射防护最优化合理使用剂量原则,在满足诊断需求的前提下,应尽可能降低辐射剂量。双源CT Flash模式其螺距可高达3.4,根据容积CT剂量指数(computed tomographic dose index,CTDIvol)的定义(CTDIvol=CTDIw/CT螺距因子)可知,螺距越大,剂量越低。双源CT结合低管电压及迭代重建方法,在保证冠状动脉图像质量的同时,应将辐射剂量降低到1 mSv以下[16-18]。Fuchs等[19]报道,冠状动脉CTA辐射剂量仅为0.21 mSv,接近普通X射线剂量。另有报道,低体重患者辐射剂量可低至0.1 mSv[20]。
双源CT冠状动脉CTA有效辐射剂量见表 1。
表 1 双源CT冠状动脉CT血管成像有效辐射剂量
Table 1. Dual-source CT coronary CT artery effective radiation dose
第二代双源CT大螺距模式低辐射剂量血管成像进展
Application of high-pitch and low-dose mode using dual-source CT by computed tomography angiography
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摘要: 双源CT应用临床以来,在CT血管造影方面越来越显示出其独特优势,其较高的时间和空间分辨率及较快的扫描速度使获得高质量的图像成为可能。双源CT大螺距扫描模式在血管成像方面具有图像质量好、有效射线剂量低、扫描时间快等优点。笔者就第二代双源CT大螺距模式在血管成像方面的进展及临床应用价值进行综述。
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关键词:
- 体层摄影术, 螺旋计算机 /
- 辐射剂量 /
- CT血管成像
Abstract: Given the unique advantages of dual-source computed tomography(DSCT), such as high temporary and spatial resolutions and fast scanning speed, its application in angiography has made possible for high-quality images to be obtained. In high-pitch scanning mode, DSCT can obtain high-quality angiography images with low effective radiation dose at a short scan time. This paper reviewed the progress and clinical applications of high-pitch mode DSCT in computed tomography angiography. -
表 1 双源CT冠状动脉CT血管成像有效辐射剂量
Table 1. Dual-source CT coronary CT artery effective radiation dose
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