Chest X-ray – mediastinum anatomy

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:

Central mediastinum

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

CXR - middle mediastinum
Central mediastinum

Right mediastinum

Upper: Superior vena cava (SVC) – straight line

Middle: Right pulmonary artery (RPA) – round dime sized structure

Lower: Inferior vena cava (IVC) – not well visualized

CXR - right mediastinum
Right mediastinum

Left mediastinum

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

CXR - left mediastinum
Left mediastinum

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!

Thanks,

Ron

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I am an American Board of Radiology certified Diagnostic Radiologist with a certificate of additional qualification in Neuroradiology. I absolutely love my job. I have learned a tremendous amount about the human body, including its’ expected complications given our extraordinary complexity, and get to use that knowledge to help people. I personally have experienced emotional devastation and loss, which has made me compassionate and empathetic. I really care about people, and I hope I can use this site to help you. I graduated medical school in 2004 from New York Medical College. My wife and I had our first son in NY where we bought our 900 sf two bedroom one bath starter home. I stayed at Westchester Medical Center for radiology residency, where we had our second son. I obtained a neuroradiology fellowship position at Yale New Haven Medical Center commuting about an hour and a half each way for a year while we had our third boy. Upon graduation I was fortunate to return to Westchester Medical Center to work in Neuroradiology, Body Imaging, and Women’s Imaging departments, a rarity in academic medicine. Almost a decade later and I am living in a larger home in Reno working in a private practice community based outpatient radiology group which contracts with a rural hospital in Elko, Nevada. My wife and I now have four boys and gave up on the girl. We also have a male Bernese mountain Dog named Helmut and female Newfoundland named Lucy. Despite all its hardships and tremendous struggles, I have an absolutely amazing life, which is a gift I cherish. It’s down to my philosophy. I am a firm believer in teamwork. So much comes down to communication. When healthcare providers talk to each other one-on-one, the patient care is always improved, every time! We might not find an answer to your problem, but our collective knowledge sure can help improve your chances. At Yale the best conference I ever regularly attended was a head and neck tumor board. The head and neck surgeon (otolaryngologist = ENT) presented the patient’s clinical history, the radiologist showed the images, the oncologist discussed the tumor and chemotherapy options, and the patient came to the conference and we all did a physical exam looking into their mouth to directly visualize a tumor. It was incredible! Patients came from far and wide to see this highly trained, world-renowned, humble, Japanese, gentleman surgeon. It was also an epiphany. I found that when I see the patient, talk to them, listen to their story, examine them and discuss the patient with my colleagues, the patient felt better and I had a lot of fun. It’s weird but reminds me of seeing a comedy where it is always funnier in a packed theater with everyone laughing. Knowledge and compassion are both infectious and contagious. My greatest days are helping patients deal with some of their most difficult days, and I am good at it.

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