By rule of thumb, the diameter of intrapulmonary subsegmental bronchi is increased if larger than the adjacent pulmonary artery at cross-sectional imaging such as computed tomography (CT), coining the definition of bronchiectasis. This definition, however, is limited by the fact that the artery itself is subject to diameter changes in airways and vascular pulmonary diseases, which also affect the bronchus-to-artery-ratio [1]. In respiratory medicine, there is the dogma that bronchiectasis is generally irreversible. However, there are different morphological forms of bronchiectasis, which can ultimately grow into bizarre morphology with cystic destruction of whole lung lobes. At cross-sectional clinical imaging, the intrapulmonary airway has a limited range of possibilities to respond to noxious stimuli. The bronchus lumen may either be filled with mucus, its wall may be thickened, or it may increase in diameter. In many cases a combination of these processes will be present, and their differentiation may be also be difficult, as a mucus layer covering the inner airway wall will look the same as mere wall thickening on CT. Most of the intrapulmonary airways smaller than 1 mm in diameter are invisible at the present settings of CT, and increased visibility is a sign of dilatation thereby crossing the threshold of CT resolution. This is often appreciated as the tree-in-bud pattern when present in groups [2].

根据经验，如果在计算机断层扫描 (CT) 等横截面成像中肺内亚段支气管的直径大于相邻肺动脉，则其直径就会增加，从而形成了支气管扩张的定义。然而，这一定义受到以下事实的限制：动脉本身会受到气道直径变化和血管性肺部疾病的影响，这也会影响支气管与动脉的比率[1]。在呼吸系统医学中，有一种教条认为支气管扩张通常是不可逆转的。然而，支气管扩张有不同的形态，最终会发展成奇怪的形态，导致整个肺叶囊性破坏。在横断面临床成像中，肺内气道对有害刺激做出反应的可能性有限。支气管腔可能充满粘液，其壁可能增厚，或者直径可能增加。在许多情况下，这些过程会同时出现，而且它们的区分也可能很困难，因为覆盖内气道壁的粘液层在 CT 上看起来与单纯的气道壁增厚相同。大多数直径小于 1 毫米的肺内气道在目前的 CT 设置下是不可见的，可见度的增加是扩张的标志，从而超过了 CT 分辨率的阈值。当成群出现时，这通常被认为是树芽模式[2]。