Radiation anatomy of the thoracic aorta

Normal anatomy and topography

Morphologically, the aortic wall consists of the endothelial intima (intima), containing smooth muscle and elastic fibers of the media (media), as well as the connective tissue adventitia (adventitia), which contains the vasa vasorum (vasa vasorum).

Topographically, the thoracic aorta is divided into the following segments:

1. Aortic root (aortic root): originates from the left ventricle. Consists of the right and left coronary sinuses (giving rise to the respective arteries) and the non-coronary sinus (located posterolaterally). Situated within the pericardial cavity.
2. Ascending aorta (ascending aorta): has a length of about 5 cm, courses superiorly and slightly to the right. On a posteroanterior (PA) chest radiograph, it forms the right border of the mediastinum.
3. Aortic arch (aortic arch): arches posteriorly over the root of the left lung. At the junction of the arch with the pulmonary trunk, there is a fixation point — the ligamentum venosum (ligamentum venosum), which is a remnant of the embryonic ductus arteriosus. In 75% of cases, the brachiocephalic trunk, left common carotid artery, and left subclavian artery arise from the arch. Normal variants include: origin of the left carotid artery from the brachiocephalic trunk (20%) or direct origin of the left vertebral artery from the arch (6%).
4. Descending thoracic aorta (descending thoracic aorta): originates distal to the left subclavian artery and descends along the spine to the aortic hiatus of the diaphragm.

Instrumental imaging methods

Historically, assessing the thoracic aorta was a challenging task. Chest radiography shows only indirect signs (e.g., mediastinal widening) and cannot rule out pathology. Classical angiography provides information solely on the lumen (luminography), missing wall pathology.

Echocardiography (EchoCG) maintains a key role in evaluating valves, but has limitations:

• Transthoracic EchoCG (TTE): visualizes only proximal segments. Has a "blind spot" in the arch region due to overlying air in the right bronchus.
• Transesophageal EchoCG (TOE): ideal for intraoperative monitoring and unstable patients right at the bedside, but is invasive and also has "blind spots" (distal ascending aorta).

CT and MRI have become the foundation of modern diagnostics, enabling 2D and 3D reconstructions: multiplanar reformation (MPR), maximum intensity projection (MIP), and volume rendering (VR). Any diameter measurements on tomograms must be performed strictly perpendicular to the longitudinal axis of the vessel.

Computed tomography and magnetic resonance imaging

Multislice CT (MSCT) with ECG-gating is the method of choice in acute life-threatening conditions. It is performed within a single breath-hold (4–8 seconds), is widely available in intensive care units, and allows rapid exclusion of other causes of pain (PE, CAD). Main disadvantages: radiation dose and the risk of iodine contrast nephrotoxicity.

MRI does not use ionizing radiation, making it ideal for routine monitoring of young patients (e.g., with Marfan syndrome). Gradient-echo cine-MRI not only visualizes anatomy but also assesses aortic valve function in a single examination. Gadolinium-based agents are used for contrast enhancement, though new 3D SSFP sequences with navigator gating yield magnificent angiograms without any contrast at all.

Note: CT outperforms MRI in follow-up of patients with stents, as metal scaffolding causes artifacts on MRI that can obscure endoleaks.

Imaging findings in aortic dissection

Dissection is a tear of the intima allowing blood to track into the media, forming a false lumen (false lumen). The true and false channels are separated by an intimomedial flap (intimomedial flap).

On MRI using dark-blood sequences (BBFSE), the flap appears as a thin bright line against a black lumen. How to differentiate the lumina?

• False lumen: usually larger, located posteriorly. Due to slower blood flow, the MR signal within it is higher (especially distally). May contain the "cobweb" sign (cobwebs) — residual strands of torn media.
• True lumen: smaller, produces a homogeneous dark signal.

Treatment strategy depends on the classification. The Stanford system is the most practical: Type A involves the ascending aorta and requires emergency surgery (otherwise mortality is catastrophic). Type B is localized distal to the left subclavian artery and, in stable cases, is managed medically.

Class and exam questions

What else to study on this topic

• Clinical and imaging features of acute aortic syndrome (AAS)
• Differential diagnosis of intramural hematoma (IMH) and aortitis on MRI
• Limitations of transthoracic echocardiography in adult patients
• Intravascular ultrasound (IVUS) and its application in endovascular aortic surgery

Other topics in the «Chest Radiology» section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.