Magnetic Resonance Imaging Notes on Retrotracheal Recess

Retrotracheal Recess

• Within the thorax, the right wall of the trachea is in contact with the mediastinal pleural reflections of the right upper lobe, creating the right paratracheal stripe seen on posteroanterior chest radiographs.
• Potential space, also known as the retrotracheal recess, exists between the posterior right half of the trachea and the posteriorly positioned right lateral wall of the esophagus
• Retrotracheal recess is frequently occupied by the lung
• Fair amount of normal variability in this space can create interfaces that stimulate pathology in conventional roentgenography

Anterior Junction Line

• Corresponds to the area where the two lungs come into close approximation anteriorly
• The pulmonary artery and the ascending aorta are the posterior boundary of this line
• The anterior boundary is the anterior chest wall

Posterior Junction Line

• The region where two lungs lie close to each other posterior to the heart and trachea
• The right lung invaginates behind the right hilar structures and heart to contact the pleura overlying the azygos vein and the esophagus
• Paraspinal area are contagious with the posterior junction
• Contains intercostal vessels and small lymph nodes

Diaphragm

• Musculotendinous structure
• Separates the thorax from the abdomen
• Chief muscle of respiration
• The anterior portion is attached to the posterior surfaces of the six lowermost coastal cartilages and the sternum
• The pericardium is attached to the upper surface of the central tendon of the diaphragm
• Superior diaphragmatic (cardiophrenic angle) lymph nodes are located extrapleurally

3 major passages in the diaphragm

1. for the inferior vena cava
2. for the esophagus and vagus nerves
3. for the aorta and thoracic duct

• these three openings are not visible in transaxial images

Diaphragm is very hard to image because of the following:

1. large-dome portion is not discretely visible in the CT
2. cardiac motion interferes with distinct imaging of the horizontal portion of the diaphragm

• the most anterior portion has a variable appearance on the CT
• the anterior portion often appears as a relatively smooth slightly undulating soft tissue arc concave posteriorly and continuous across the midline
• the posterior and caudal portion, especially the crural part, is generally well-defined on the CT
• the most cephalad section is at the level of the esophageal hiatus

Normal Variants

Situs Inversus Totalis

• situs is a reference position of the thoracic and the abdominal viscera within the body
• situs solitus, the most common type of situs, indicates a trilobed lung, right atrium and liver on the right side and a bilobed lung, stomach and spleen on the left side
• Situs inversus refers to the mirror image of the thoracic and the abdominal viscera
• 20% of the patients with situs inversus have Kartagener syndrome, subset of the ciliary dyskinesia syndrome

Mediastinal Vascular Anomalies

• Includes anomalies of the intrathoracic vessels, including the superior vena cava, the aorta and the pulmonary arteries and veins
• In some patients, these anomalies may have simulated a mediastinal mass on plain chest radiographs
• CT can serve as a relatively noninvasive procedure for definitive identification of these anomalies or variants
• Occasionally, anomalies of isolated partial pulmonary venous drainage are detected on CT
• Other anomalies of pulmonary venous drainage include the right inferior pulmonary vein cephalad into the azygos vein
• Rarely, other forms of anomalous venous return can be recognized on CT or MRI, particularly with bright blood 2D GRE sequences used for the latter
• Anomalies of the inferior vena cave can be associated with dilatation of the azygos and/or hemiazygos veins
• Careful examination of the serial CT scan should enable correct diagnosis from the characteristics of findings

Arterial Anomalies

Can be generally categorized into 3 groups:

1. left aortic arch
2. right aortic arch
3. double aortic arch

• most common congenital anomaly of the aorta is an aberrant right subclavian artery originating from an otherwise normal left aortic arch
• the anomalous right subclavian artery is seen on CT arising as the last branch from the distal portion of the aortic arch
• most frequent anomaly of the right aortic arch is with an aberrant left subclavian artery
• right aortic ach with mirror image branching has a high incidence of associated congenital heart disease
• double aortic arch is characterized by 2 arches arising from a single ascending aorta
• typically, the right arch is larger and situated slightly more cephalad than the left arch

Isolated Left Vertebral Artery

• rarely, the left vertebral artery may arise separately from the aortic arch and an additional great vessel will be seen at this level

Vascular-Aortic Arch

• Pseudocoarctation, or kinking of the aortic arch, is a rare anomaly that may be mistaken for true coarctation of the mediastinal mass on conventional chest radiography
• Because proximal descending aorta is located anteriorly, on CT lung can be seen interposed between the descending aorta and the spine