Introduction

This e-Anatomy module is dedicated to the anatomy of the thorax (lungs, pleura, heart, aorta, thoracic lymph nodes and other relevant anatomical structures) using a normal thorax CT.  

It has been designed to help radiologists in their daily practice (for example, special focus has been made on topics such as the segmental anatomy of the lungs, the IASLC lymph node stations for the pretreatment stage classification of lung cancer, or the thoracic aortic measurements for TAVR planning or thoracic aneurysm follow-up). But this module is also an educational tool to learn about the radiological anatomy of the thorax, especially for medical students, or radiology, pneumology, radiation oncology or thoracic surgery residents.

Material and methods

The thorax computed tomography was performed by Antoine Micheau MD (Radiologist), Imaios France, on a normal 43 year old male patient, using a Siemens Somatom Definition Edge, with the following technical specifications:

Iodinated contrast agent (Omnipaque [Iohexol] 350 mg I/ mL) was injected with an electrocardiographic (ECG) gating.

We used images in parenchymal and mediastinal window, with multiplanar (MPR) reconstructions in frontal (coronal) and sagittal planes.
We created 3D-volume rendering images of the thorax, including general anatomy, surface anatomy, thoracic bones, thoracic muscles, trachea and bronchi, lungs, thoracic vessels, thoracic aorta, pulmonary arteries, pulmonary veins, and superior vena cava.

The anatomical labeling was carried out by Antoine Micheau MD, using the Terminologia Anatomica 2.
During this labeling, we discovered some inevitable anatomical variations that are listed at the end of this page.

Anatomical variants and notes from the author about the anatomical labeling of the thorax CT:

• In the lower lobe of the left lung, there is an inconstant subsuperior pulmonary segment that is seen in approximately 30% of individuals, located between the superior and basal segments of the lower lobe. For pedagogical purposes, we included it in the superior segment of right lower lung, but we termed his bronchus as subpsuperior bronchus (S*), and his artery as subsuperior artery (A*).
• The lower lobe of the right lung has two superior segmental bronchi, with separate origins from the right inferior lobar bronchus.
• In the lower left lobe, it is not clear if there is a common basal anteromedial segment, or if these segments are separated (this is the option we chose for labeling, but consequently, the anterior segment appears to be slightly lateral and the lateral segment appears to be slightly posterior….).
• Segmental and subsegmental pulmonary arteries are generally parallel to segmental and subsegmental bronchi and run alongside them. This is in contrast to the course of most pulmonary veins, which run independently of bronchi within interlobular septa. In this module, the segmental arteries were named according to the bronchopulmonary segments that they feed, following the Terminologia Anatomica 2 (we just added the terms interlobar arteries, used in daily practice). However, the proximal portions of the arteries can run independently of their respective bronchi for short segments. Frequently, there are also accessory arteries from neighboring segments, particularly in the right upper lobe. Segmental and subsegmental pulmonary arteries vary considerably in the location of their origins, in whether they arise as common trunks with other arteries or as separate arteries, and in their number. Here are the variations encountered in this e-Anatomy module:
  - There is an additional segmental middle lobar artery, draining the upper lateral part of middle lobe, and connected directly to the left lower lobe artery.
  - There are two separate superior and subsuperior segmental arteries in the right lung.
  - There are at least 3 separate superior segmental arteries in the left lung.
• In our case, the brachiocephalic artery shares a common origin with the left common carotid artery forming a bovine arch, most common variant of the aortic arch (near 15% population).
• For the measurements of aortic diameters, we put colored lines as overlay on axial, coronal, sagittal and 3VR, however these landmarks are inevitably imprecise because the measurements of the diameters of the aorta must imperatively be measured on multiplanar reconstructions perpendicular to the aorta (blood flow) and not directly on axial, coronal or frontal images.
• Some structures like the phrenic nerve along with the pericardiacophrenic artery and vein, or some lymph nodes of the thorax cannot be clearly seen on this CT. But for educational purposes, we put anatomical labels on the presumed place of these structures. 
• The IASLC lymph node map provides a reproducible and consistent set of definitions for the discussion of regional lymphadenopathy in patients with lung cancer. However, because of its comprehensiveness and text-based presentation, it may be challenging to grasp, remember, and apply during daily practice especially on thorax CTs. More importantly, ambiguities may emerge when its definitions are rigorously applied during interpretation of CT images. That’s why we used the article of El-Sherief et al. (Radiographics) for the delimitations of the thoracic lymph node area. The main changes concern the lower boundary of station 1 (thoracic inlet with sagittal oblique plane preferred to the clavicles) and the lower boundary of the station 4 (it should be on the right side of the inferior border of the azygos vein, and on the left side of the superior border of the left main pulmonary artery, but the result is unclear for the precarinal space that is included in station 4 during daily practice).