The mediastinum describes the central structures seen on a frontal chest radiograph. Remember, left and right are “flipped” with sidedness determined by the patient’s perspective (their right and left).
In the center of the chest is the heart and mediastinum. The mediastinal structures consist of the thymus, trachea/airway, esophagus/food pipe, lymph nodes and the large blood vessels (arteries and veins) connected to the heart. In normal healthy individuals, half of these structures are not even visible on the frontal chest radiograph making our jobs that much simpler.
The thymus is present in children, shrinks during adolescence and disappears in adulthood. Lymph nodes are only seen on chest X-ray when they become quite large in people who are sick with serious diseases like cancer (lymphoma, lung cancer) and sarcoidosis (a chronic lung disease) and therefore are often not visible. The esophagus is also pretty much invisible on chest X-ray in normal patients.
In the central mediastinum, the trachea is seen as a “negative image” since it is low density containing air (click here for a review of X-ray densities). The trachea splits near the mid-chest at what is called the carina into right mainstem bronchus (RMSB) and left mainstem bronchus (LMSB). Okay, so that leaves us with only the big blood vessels. Easy!
Before getting to the big blood vessels we need to quickly simplify the heart. The heart has a left and right side separated by a wall/septum. The left and right sides are each again divided into an input chamber/atria and an output chamber/ventricle. Veins empty into the heart and arteries leave the heart. That’s it!
The right heart input veins are the superior vena cava (SVC) and inferior vena cava (IVC) that receives low oxygen blood from the upper and lower half of your body, respectively. These are both located on the patient’s right side of the heart with the SVC being visible as a relatively straight line connected to the heart. The IVC goes through the liver and plugs into the bottom right side of the heart and is not clearly visible on chest X-ray. These two large vessels input low oxygen blood to the right atrium. This goes through the tricuspid valve into the right ventricle. The right ventricle then outputs this low oxygen blood to the lungs where it is replenished with fresh air. The output is the main pulmonary artery which quickly branches into a right pulmonary artery (RPA) and left pulmonary artery (LPA) visible as two round markings on either side of the heart with the left higher than the right.
The left heart input is the pulmonary veins, again paired into left and right coming from each lung. There is also a superior/upper and inferior/lower pulmonary vein on each side for a total of four main pulmonary veins. These four main pulmonary veins input directly into the left atrium and are also not well-visualized on chest X-ray. Blood then goes through the mitral valve into the left ventricle. The left ventricle outputs high oxygen blood to the aorta, our bodies largest artery. It comes off the heart pointing in the direction of the patient’s right shoulder. It then immediately arches around toward the left shoulder generally giving off three major blood vessels at the top of the loop to supply our brain and arms before making the final turn down to the lower body. This effectively breaks the aorta into an ascending, arch, and descending thoracic/chest aorta.
Okay, ready for a quick review:
Trachea – “negative image” tube at the midline
Carina – “inverted V-shaped” structure at the midline where the trachea splits into a right mainstem bronchus (RMSB) and left mainstem bronchus (LMSB).
Esophagus – midline not well seen
Upper: Superior vena cava (SVC) – straight line
Middle: Right pulmonary artery (RPA) – round dime sized structure
Lower: Inferior vena cava (IVC) – not well visualized
Upper: Aortic arch – sometimes called the aortic knob on chest X-ray where the vessels curves from going toward the head to going toward the feet.
Middle: Left pulmonary artery (LPA)
Lower: Descending thoracic aorta
That’s a descent basic introduction to the mediastinum for now. Sadly, it is a bit more complicated than I led you to believe, but not by much. We will cover it when we get to mediastinal pathology. Next, I will go over heart/cardiac anatomy on chest X-ray and then we will do the bones. Finally, we will dive into some pathology and have a corollary discussion for each chest X-ray anatomy section.
This was an exciting week for me to learn that I am getting traction with these posts. Hearing feedback from you all out there is highly encouraging. Hopefully, you find this useful, helpful, informative, and not to difficult to digest. I received some feedback from non-medical friends this week that the posts were a little hard to comprehend. I apologize. I am trying hard to make these articles readable to the layperson, but it is very difficult. My medical educational training took 10 years beyond college, so I’m a bit of a disconnected nerd at this point.
Let me know your thoughts on the readability of these posts!