作者:Frederik Vandenbroucke
作者单位:比利时 布鲁塞尔自由大学 放射科
本文发表于:Insights Imaging(2015) 6:363–379
翻译整理:张雅茜
内容提要
GE的能谱CT能为肝脏射频消融后的评估带来许多传统CT无法提供的诊断价值,碘基图上肿瘤残余与肝实质对比度增高,低keV上碘的衰减增大,都是肿瘤残余的重要征象,从而更好的辅助临床诊断和治疗。
Liver ablation 肝脏消融
热损伤造成的坏死以缺乏血流灌注为特点,表现为增强CT上的无强化区域(Table 1)。由于缺乏内部的血管,碘基图可以很好的区分出治疗后的区域和正常肝脏实质(Figs. 1 and 2a)。双能量CT可以在增强图像中分离出碘,获得水基图,这样就可以免去平扫,如Fig. 3a所示。毋庸置疑,在减少辐射剂量方面,这是一个相当大的优势。但缺点是目前DECT技术提供的水基图,信噪比无法和真正平扫一样。
Necrosiscaused by thermal damage is characterised by an absence of blood perfusion,resulting in a non-enhancing area on contrast-enhancedCT (Table 1). Due to this lack of internal vascularity, iodine maps are well suitedfor delineating the treated area from the liver parenchyma (Figs. 1 and 2a).Dual-energy CT has the possibility to extract iodine from the enhanced images tocreate water map images and thus potentially skip true unenhanced images, asshown in Fig. 3a. This undoubtedly offers a considerable advantage in limiting radiationdose exposure. However, a drawback is that current DECT techniques do notproduce water map images with the same contrast-to-noise ratio as true unenhancedimages.
Table 1 肝脏病灶RF消融后成功和失败的早期和远期改变及DECT技术的附加诊断价值
Fig. 1 乏血供肝脏转移瘤。53岁女性直肠癌患者,术后接受了肝脏局部病灶的RF消融术。8个月后,局部肿瘤进展(第一列),消融区头侧可见以中等密度(小箭号)。在这个简单的病例中,肿瘤组织在40和70keV的灰阶碘基图上均显示清晰。然而,40keV图像的噪声更为明显。根据碘浓度的递增,颜色分别由红(渐变为黄)到蓝,彩色编码使用彩虹色阶模板进行碘基图的编码。中间过渡的颜色代表了肿瘤(小箭号)。在PET/CT上可见明显的FDG摄取(箭头)。RF消融后24小时(第二列),可见边界明显的消融区。在所有的重建图像上,可以清楚地显示消融区无血供的特点。尤其是在彩色编码碘基图上,消融区与周围肝脏实质对比最为明显。在 PET/CT未见FDG摄取。3个月后的PET/CT(未提供图像)也无肿瘤活性征象。
Fig. 2 富血供转移瘤。63岁男性肾细胞癌消融后4年,进行随访的CECT扫描。在动脉期发现肝脏VII段一26mm富血供结节,病理证实为RCC转移瘤。遂行RF消融。a 消融后24小时行DECT检查。水基图显示消融区中心高密度影(箭号),为治疗区域烧焦及脱水的组织。注意这一高密度影在碘基图上显示较差。动脉期图像未显示消融区周围任何局灶性的富血供组织。未见环形强化。由于肝实质的碘含量更高,整个消融区在门脉-静脉期的灰阶及彩色编码碘基图上均显示清晰。b 在一年后的CECT随访中,消融区边缘光滑,证实了消融技术上的成功。中心仍可见高密度影(箭头)。
平扫图像上可以观察到消融区中心常常有高密度影。我们认为这一高密度区是由于消融区域中心细胞被明显破坏,据研究报道,其原因为高度脱水凝固的病灶被烧焦造成。通常它会在下一次随访中消失,但需要一段较长时间 (Fig. 2b)。这一高密度影在水基图和平扫图像上均可观察到。尽管没有碘,它仍能在碘基图上显示,这是由于在物质分离后高密度区代表了水和碘的结合 (Fig. 2a)。仔细比较水基图和碘基图可以区分静脉注射造影剂后RF消融后中心高密度影和强化的结构。然而阅片者应该注意,对比真正的平扫图像,水基图图像质量较差(Fig. 3a),这一点在双源DECT也有报道。与Lee等的研究类似,我们也观察到在碘基图上消融区域有明显的边界。然而,不同于他们在双源DECT系统观察到的结果,我们在单源DECT图像上的经验并不能证实碘基图上消融区域内部的同质性。
Frequently,a hyperattenuation centrally within the ablation zone can be seen on unenhancedimages. It is thought that this area of high density correlates to a region ofgreater cellular disruption in the centre of the ablation zone, reportedlycaused by the intense charring of the severely desiccated coagulated lesion. Ithas usually disappeared by the time the next follow-up CT examination takesplace, but can persist for a longer time (Fig. 2b). This hyperattenuation isobservable on both the water map and true unenhanced images. Despite theabsence of iodine, this can also be seen on the iodine-coded images, since thehyperattentuated area is represented as a combination of both water and iodineafter material decomposition (Fig. 2a). Meticulous comparison of water andiodine-coded images allows for differentiation of this central hyperattenuationafter RF ablation and structures enhancing after IV contrast administration. Thereader should however be aware of the lower image quality of water images withrespect to true unenhanced images (Fig. 3a), as also reported on dual-sourceDECT. Similar to the study of Lee et al., we observed sharp depiction of theedge of the ablation zone of the iodine images. Contrary to their findingsobtained using a dual-source DECT system, our experience with single-sourceDECT images did not prove an internal homogeneity of the ablation zone on theiodine images.
Fig. 3 肝细胞癌。43岁肝硬化男性,增强MRI示VII段典型的肝细胞癌表现,并经病理证实。a RF消融后24小时,病人接受了常规的平扫CT扫描和增强的DECT扫描。平扫CT显示消融区中央高密度影(箭头),但在水基图上显示较差。中心高密度影在单能量40keV图像上显示较好,但在70keV图像显示较差,在碘基图上难以辨别。在40和 70keV单能量图上,消融区可以显示为周围高密度环(白色箭号)的结节影。对比单能量图像,在灰阶和彩色编码碘基图上我们可以更好地辨别病灶后缘富血供的局部增厚。b 出于对a图所见的局部增厚的性质的怀疑,缩短了进行随访复查的时间,于是在RF消融后5周行DECT检查。在70keV图像上可见以局灶性富血供结节,尽管在40keV图像上显示更佳,但需要再次强调的是,在灰阶和彩色编码碘基图上更为显著。局部活检后证实为HCC。c 第二次消融后24小时,灰阶和彩色编码图显示动脉期(箭头)消融区周围明显的碘摄取。因此,这一局灶性富血供区仍然无法鉴别。这一环形区在静脉期未见显示(未提供图像)。10周后MRI显示无复发的征象。
在消融后短时间内进行DECT扫描,我们常常可以发现消融区域周围高密度的晕影,这是由于热损伤后毛细血管渗漏造成动脉灌注增高。这种周围强化主要出现在动脉期,甚至在某些病例中非常明显 (Fig. 3c),影响了肿瘤残余的判别。边缘不规则或结节状表明有肿瘤残余,但良性改变也可出现这些征象。但若消融区域边缘局限性增厚并变得模糊,建议进行短期随访[32]。过往的研究表明,术后1个月,89%的病例仍然可见富血供边缘,该征象在术后1-3个月占56%,3-6个月后仍有22%病例可见。理想状态下,热坏死的范围应在每一个断面均超过转移病灶范围1cm。肿瘤残余在消融区域周围会一直维持局灶性结节状强化 (Fig. 3b)。对于双能量CT,低keV(例如,40keV左右)图像上碘的衰减增加更有利于检测消融区域周围细小的密度差异 (Fig. 3b)。再加上彩色编码图,图像视觉上的对比差异更为明显。
Immediatelyafter ablation, it is common to find a hyper-attenuating halo surrounding theablation zone on CECT, correlated to an increase in arterial perfusion due tocapillary leakage from thermal damage. The circumferential enhancement ispredominantly visible on the arterial phase and can be very prominent in somecases (Fig. 3c), obscuring the interpretation of residual tumour. An irregularor nodular rim can indicate the presence of residual tumour, although benign variationsdo occur. When focal thickening at the border of the ablation zone is unclear,a shorter follow-up period is recommended. Previous studies have shown that ahypervascular rim remains visible in 89 % of cases after 1 month, in 56 % ofcases between 1 and 3 months, and in 22 % of cases between 3 and 6 months.Ideally, the volume of thermal necrosis exceeds the limits of the metastasis by1 cm in all dimensions. Residual tumour will persist as a focal nodularenhancement in the periphery of the ablation zone (Fig. 3b). For dual-energy CT, the increased attenuation of iodine on the low-keV (e.g. around 40 keV)images is better suited for detecting any subtle density differences around theablation zone (Fig. 3b). Adding colour-coding enhances the visibility of thecontrast in the image.
水碘物质分离能凸显出含碘化对比区域。碘基图较单能量图在造影剂吸收的量化方面更为重要。因此,对比单能量图,更高的碘流入的富血供结节在碘基图上显示更佳 (Fig. 2b)。
Thewater-iodine material decomposition can highlight areas containing iodinatedcontrast. Iodine maps are superior to synthesised monochromatic images for thequalification of contrast uptake. Consequently, a hypervascular nodule with ahigher inflow of iodine is better depicted on the iodine maps when compared tosynthesised monochromatic images (Fig. 2b).
附
GE公司的宝石能谱CT,采用单源瞬时切换千伏峰值转换技术,是目前双能量CT应用最为广泛的技术之一,本文所有病例均为宝石能谱CT采集。
【CT临床研究】精彩回顾:
【CT临床研究】肝脏、肾脏及肺部结节射频消融后的双能量CT特点述评(连载一)
【CT临床研究】肝脏、肾脏及肺部结节射频消融后的双能量CT特点述评(连载二)
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