Thoracic Soft Tissue and Lung
Thoracic CT scans may be performed with or without the intravenous injection of iodinated contrast. Contrast-enhanced thoracic CT scanning is usually performed following injection into an upper extremity vein, with imaging started at 30 to 40 seconds after the injection is begun, depending on the specific application. In this patient, contrast was injected via the right upper extremity, resulting in dense enhancement of the right subclavian vein (and the right brachiocephalic vein on Axial 5 and subsequent images); the left subclavian vein (and left brachiocephalic vein on Axial 5 and subsequent images) remains unenhanced in this patient because imaging was begun before injected contrast had sufficient time to circulate throughout the body and reach the left subclavian vein.
At this level, in lung windows, a small apical-posterior segment left upper lobe bronchus and artery are visible adjacent to one another. This relationship aids in the identification of small, peripheral pulmonary vessels—arteries course adjacent to bronchi, whereas pulmonary veins course separately. Note this relationship on subsequent images and in the peripheral aspects of the lower lobes.
The composition of the anterior mediastinum varies substantially with age. In younger patients (particularly infants and children), abundant soft tissue, representing thymus residing within anterior mediastinal fat, is often readily visible on thoracic CT. As individuals age, the thymus normally involutes and becomes more fatty replaced, and progressively less soft tissue is visible within the anterior mediastinal fat. The pace at which thymic involution occurs is variable, and a fairly wide variation in the appearance of “normal thymus” at various ages is recognized.
At this level, the segmental bronchi within the right upper lobe are now visible. Segmental bronchi are commonly named for the anatomic segment they supply (e.g., apical segment right upper lobe bronchus). Another nomenclature system for segmental bronchi may occasionally be encountered, as described by the Federative Committee on Anatomical Terminology in Terminologia Anatomica. This terminology identifies the segmental bronchi according to numbers (e.g., B1 = apical segment right upper lobe bronchus, B1+2 = apical-posterior segment left upper lobe bronchus; see Axial 19). Both nomenclature systems are illustrated in this text.
A number of variations in the branching pattern of the pulmonary arteries are recognized. In particular, variations in the branching pattern of the left upper lobe pulmonary artery are commonly encountered in clinical practice. In the most common pattern, the left upper lobe pulmonary artery gives rise to apical-posterior and anterior segmental branches in a manner analogous to the bronchial branching pattern. Less commonly, as in this patient, a posterior left upper lobe subsegmental artery arises directly from the left pulmonary artery. The left upper lobe pulmonary artery can be seen originating from the left pulmonary artery on Axials 17 and 18.
On this and a number of cranially and caudally located images displayed in lung windows, the oblique fissures are visible as thin, hypovascular planes extending transversely across the lungs bilaterally. The oblique fissures sweep anteriorly from the posterior toward the anterior chest wall. The oblique fissures are also often commonly referred to as the major fissures.