· 第351期 ·
本文为美国放射学会旗下杂志《Radiographic》的SA-CME LEARNING OBJECTIVES（自我评价式继续医学教育读物）；完成阅读后，达到以下要求——
■ Describe the various causes of a mosaic attenuation pattern on thoracic CT. /能描述胸部CT马赛克征的不同原因
■ Discuss differentiation of airway causes of mosaic attenuation from vascular causes. / 学会鉴别气道病变和血管病变所致的马赛克征
■ Identify ancillary findings of mosaic attenuation on thoracic CT to help narrow the differential diagnosis. /能在胸部CT中发现有助于鉴别马赛克征的辅助表现，以缩小鉴别诊断的范围
缩略语对照表
CTEPH
chronic thromboembolic pulmonary hypertension / 慢性血栓栓塞性肺动脉高压
DIPNECH
diffuse idiopathic pulmonary neuroendocrine cell hyperplasia / 弥漫性特发性肺神经内分泌细胞增生
H-E
hematoxylin-eosin / 苏木精-伊红
PAH
pulmonary arterial hypertension / 肺动脉高压
PCH
pulmonary capillary hemangiomatosis / 肺毛细血管多发性血管瘤
PVOD
pulmonary veno-occlusive disease / 肺静脉阻塞性疾病
Mosaic attenuation is an imaging pattern on computed tomography (CT) of the chest that is defined as variable lung attenuation that results in a heterogeneous appearance of the parenchyma. It is important to remember that mosaic attenuation is a finding that implies a large differential diagnosis and is not a diagnosis in itself. Although the differential diagnosis is broad, mosaic attenuation most commonly occurs in diseases that affect the small airways, pulmonary vasculature, alveoli, and interstitium, alone or in combination.
马赛克征（超链接戳蓝色字）是胸部CT的一种影像征象，其定义是肺实质的异质性表现导致肺密度出现不同变化。非常重要的一点是牢记马赛克征本身并不是一种诊断，它只是一种提示很多诊断不相同的病变的一种影像表现。虽然需要鉴别诊断的疾病很广泛，但马赛克征最常见于累及小气道、肺脉管系统，肺泡，和间质的疾病，这些疾病可单独发病或合并发病。
Small airways disease can be a primary disorder, such as respiratory bronchiolitis or constrictive bronchiolitis, or be part of parenchymal lung disease, such as hypersensitivity pneumonitis, or large airways disease, such as bronchiectasis and asthma (1). Although the predominant cause of mosaic attenuation may vary by institution, parenchymal lung diseases contribute to approximately one-half of cases of mosaic attenuation, whereas diseases of the small airways are the underlying cause in approximately one-third of cases (2,3). Diseases of the pulmonary vasculature contribute to the remaining cases.
小气道疾病可能是主要的病变，如呼吸性细支气管炎或缩窄性毛细支气管炎，或者是肺实质疾病的一个组成部分（如过敏性肺炎）；或者是大气道疾病的一个组成部分（如支气管扩张和哮喘）。虽然马赛克征的主要原因在不同的研究单位会有不同，但肺实质病变占了将近一半的病例，而小气道病变则在约三分之一病例中是基础病因。肺脉管疾病占了余下的病例。
The major difficulty with this pattern lies in the fact that it can be difficult for a radiologist to determine which areas of lung attenuation are normal and which are abnormal. In some instances, the areas of lower attenuation are abnormal, while in other instances the areas of higher attenuation are abnormal. In certain diseases, areas of both high and low attenuation are abnormal, making it difficult to determine if normal lung parenchyma exists.
对于放射科医生来说，对马赛克征的认识最大的困难是，难以判断哪些区域的肺的密度是正常的，哪些是异常的。在某些情况下，低密度区域是异常的。在一些特定的疾病，高密度和低密度飞区域都是异常的，这导致要确定是否有正常肺实质存在变得十分困难。
The purpose of this article is to review the causes of a mosaic attenuation pattern and highlight distinguishing features that can help one determine the underlying pathologic process.
本文的目的，是综述马赛克征的病因，总结出有助于鉴别的特点，使放射科医生能判断马赛克征的基本病理过程。
Mosaic Attenuation and Airtrapping:
How Much Is Normal?
马赛克征和空气潴留：有多少是正常的？
Some degree of parenchymal heterogeneity can be seen in normal individuals. In general, the most dependent portions of the lung are of slightly higher attenuation than the less-dependent lung. However, discontinuity of this gradient can be seen at the level of the fissures, where the posterior aspects of the upper lobe often have higher attenuation than the superior segments of the lower lobes (4). In addition, perfusion gradients exist axially, with an increased degree of perfusion centrally than peripherally. However, even when these physiologic gradients are taken into account, mild mosaic attenuation at inspiration can be seen in up to 20% of normal patients (5). This degree of heterogeneity may be further accentuated at imaging performed below total lung capacity.
正常人的肺实质可存在一定程度的密度不均匀。总体上看，血流灌注重力依赖最多的区域的肺野密度比非依赖区稍高。这种不连续的渐变可以在叶间裂的附近看到，肺上叶的后部密度常较下叶上段的密度高。此外，灌注梯度呈轴向性，中央的灌注量超过外周灌注量。然而，即使把这些生理梯度考虑进去，多达20%的正常病例在吸气时可以出现轻微的马赛克表现。在CT扫描时如果把全肺总容量考虑进去，肺野的不均匀性可能更明显。
为了搞清楚什么叫most dependent portions和less-dependent lung，Miss 巫找了上面这张图。点比较密集的地方就是血流灌注多的区域，点少的地方反之。这是肺在重力作用下产生的血流梯度改变。（很多肺的生理都忘记得差不多了，恶恶恶恶补一下）
Teaching point （敲黑板，记重点）
One of the best methods to differentiate between causes of mosaic attenuation is to perform expiratory imaging.
In patients with small airways disease, air cannot readily escape in the regions where the small airways are obstructed. Because of this, the attenuation of the involved segments remains relatively unchanged in comparison with that at inspiratory imaging. With air normally conducting through the noninvolved areas, the difference in attenuation between the normal and abnormal areas becomes much more pronounced and airtrapping can be diagnosed (Fig 1). At expiratory CT in patients without small airways disease, the lungs should show a relatively diffuse increase in attenuation and appear grayer (Fig 2).
教学点！！！
鉴别马赛克征病因最好的方法之一：进行呼气相扫描。
患小气道病变的患者，空气难以从小气道阻塞的区域呼出。由于这个原因，与吸气相获得的影像比较发现，受累的肺段在呼气相时密度几乎维持不变。而没有受累的区域，气体可以正常呼出，正常与异常肺野在密度上的区别变得更加明显，同时气体潴留的诊断成立（图1）。没有小气道病变的患者，在呼气相CT上，两肺的密度应该呈相对弥漫的增加，显得更灰一些（图2）。
图1a
图1b
Figure1 Airtrapping due to constrictive bronchiolitis in a 43-year-old woman with rheumatoid arthritis. (a) Axial CT image at the level of the carina shows mosaic attenuation, with geographic areas of decreased attenuation (*) adjacent to normal lung. Note the relative decrease in vascularity in the hypoattenuated regions, a finding seen in both small airways disease and vascular causes of mosaic attenuation. (b) Expiratory CT image at a similar level shows that the areas of hypoattenuated lung in a have not changed in attenuation due to airtrapping (*). The adjacent areas of higher-attenuation lung represent the expected increase in attenuation at expiratory imaging. Bowing of the posterior wall of the trachea (arrow) signifies good expiratory effort. The presence of airtrapping confirms a small airways disease cause of the mosaic attenuation.
图1. 43岁女性类风湿性关节炎患者，因阻塞性细支气管炎出现空气潴留。（a）CT轴扫在隆突水平显示马赛克征，邻近正常肺野的区域伴有地图样密度降低（*）。注意低密度区内血管影相对减少，这种表现同时可见于小气道疾病和血管原因的马赛克征。（b）呼气相CT，同一层面显示低密度肺野由于空气潴留而没有发生密度改变（*）。密度较高肺野的邻近区域在呼气相则再出现了预期的密度降低。气管后壁出现弯曲说明患者努力呼气。空气潴留的表现证实了马赛克征为小气道疾病所致。
图2a
图2b
Figure 2 Inspiratory and expiratory CT in a 55-year-old woman with organizing pneumonia. (a) Axial CT image during inspiration shows mosaic attenuation, with slightly increased attenuation most prominent in the left upper lobe (*). (b) Expiratory CT image at the same level shows a diffuse increase in attenuation in both the areas of relatively increased attenuation and the areas of relatively decreased attenuation. This finding allows exclusion of airtrapping, and the similar size of the pulmonary vasculature throughout the lung suggests that the area of increased attenuation is abnormal. Subsequent biopsy of this region demonstrated organizing pneumonia.
图2 55岁女性机化性肺炎患者，吸气相及呼气相CT。（a）吸气相CT显示马赛克征，以左上肺叶为主（*），密度稍升高。（b）呼气相，同层面显示原密度相对较高的区域和密度相对减低的区域均显示弥漫的密度升高。这种征象排除了空气潴留，肺内肺血管直径没有变化提示密度增加区域为异常改变，后续活检该区域证实机化性肺炎。
As with inspiratory imaging, a normal gradient exists at expiratory imaging, with the dependent lung having slightly higher attenuation than the nondependent lung (6). However, beyond this physiologic gradient, areas of lobular airtrapping occur in 40%–80% of normal patients on both qualitative and quantitative CT (Fig 3) (4,5,7–9). In a study by Park et al (5), mild (total area < three adjacent lobules) and moderate (area between three lobules and a pulmonary segment) airtrapping occurred to a similar degree in normal individuals with normal pulmonary function test results and patients with asthma.
正如吸气相影像，呼气相时也存在正常的梯度，重力依赖肺密度会轻微增加，高于非依赖肺。然而，除此生理梯度外，在定性和定量的CT扫描中，空气潴留的肺小叶可以在40%-80%的正常病例中观察到（图3）.Park 等的研究中，轻度（总区域＜三个相邻肺小叶）和中度（总区域介于三个肺小叶和一个肺段之间）的空气潴留在肺功能正常的人中同样存在，程度与哮喘病人相似。
图3a
图3b
Figure 3 Normal variation in lung attenuation at expiration in a 73-year-old woman with a history of ulcerative colitis and worsening shortness of breath. (a)Inspiratory CT image shows normal lung attenuation. (b) High-resolution CT image during expiration shows the normal physiologic gradient, with the anterior portion of the lung (*) having lower attenuation than the posterior portion. The dependent portion of the lung demonstrates heterogeneous parenchyma at expiration, although almost all of the lung has increased in attenuation. In addition, there are a few dependent lobules that remain lucent (arrows). This variation at expiratory imaging is seen in a large percentage of patients and is considered to be nonpathologic.
图3. 73岁女性，有溃疡性结肠炎和气促加剧病史。呼气相呈正常肺的密度变化。（a）吸气相CT显示正常肺密度；（b）高分辨CT呼气相显示正常生理梯度表现，肺前部（*）的密度较后部低。虽然全肺的密度都有所减低，但肺的依赖部分在呼气相表现出肺实质不均匀改变。此外，尚有一些依赖肺小叶保持其透亮度（箭）。有比例很高的患者在呼气相出现这种影像变化，这种改变目前认为是正常的。
In addition, airtrapping involving an entire pulmonary segment can occasionally be seen in a small percentage of normal individuals, and the presence of airtrapping increases with age and is more pronounced in smokers (4,10,11). Therefore, various degrees of mosaicism and airtrapping may be an incidental finding and not related to the underlying symptoms of the patient. In general, mosaic attenuation or airtrapping involving more than a pulmonary segment often has an underlying pathologic basis for which a cause should be sought.
此外，空气潴留累及整个肺段也偶然会在正常人中观察到，但比例不高，且空气潴留随年龄增高并更多多见于吸烟者。因此，不同程度的马赛克表现和空气潴留可能是一个偶然的表现，而与病人的基础疾病无关。一般来说，马赛克征或空气潴留征累及超过一个肺段，常需要去找出导致病变的原因。
未完待续
资料来源 | http://pubs.rsna.org/doi/full/10.1148/rg.2015140308
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