The Thoracic Aorta: Imaging in Health and Disease

The Normal Thoracic Aorta The thoracic aorta is the result of a succession of events relating to fusion of the paired primitive dorsal and ventral aortae and the persistence or regression of their connections to the paired arterial stlUctures of the six primitive branchial arches that develop in the embryonic stage. The thoracic aorta can be divided into five segments. The aortic root or bulb begins at the aortic valve, having three uniform cusps of equal size and shape. Each cusp subtends one of the three sinuses of Valsalva, named posterior or (noncoronary), left. The left coronary artery arises from the left sinus, whereas the right coronary artery arises from the right sinus. The sinus portion above the valve plane has a diameter of 3.6-3.9 cm. On a frontal chest radiography, this portion of the aorta is buried within the cardio-pericardial silhouette and is not visible. It is, however, readily distinguishable on aortography. The tubular portion begins above the sinuses at the sino-tubular junction with the ascending aorta, which is 4-5 cm long and of uniform diameter between 2.8-3.5 cm. Much of the ascending aorta is intrapericardial. On a frontal chest radiograph, the normal ascending aorta is not border forming. The transverse arch of the aorta is the remnant of the aortic sac proximally and the left fourth branchial arterial arch distally and courses posterolaterally from right to left in a parasagittal plane. It is approximately 5 cm long, 2.5-3 cm in diameter, and gives rise to the brachiocephalic or innominate artery, the left common carotid artery, and the left subclavian artery, in that order. The brachiocephalic artery branches further into the right subclavian artery and the right common carotid artery. Many variations of the branching pattern of the brachiocephalic vessels exist. On frontal chest radiography, the lateral margin of the left subclavian artery forms the superior left border of the mediastinum, and the lateral margin of the distal transverse arch is recognizable as the aortic knob. Occasionally, the superior intercostal vein appears as a nipple on the aortic knob on a frontal chest radiograph. The aortic isthmus is a distinct zone of narrowing beginning distal to the origin of the left subclavian artery and extending over a short variable distance. It is most apparent on aortography in the pediatric patient and disappears with age. It is probably the result of reduced flow in this region during fetal development, where the left ventricular outflow primarily serves the vessels of the head, neck, and upper extremities prOXimal to the isthmus, and the right ventricular outflow serves the descending thoracic aorta distal to the isthmus through shunting through the ductus arteriosus. A remnant of the infundibulum of the ductus arteriosus frequently can be seen even in adult aortae as the ductus bump along the anteromedial wall of the lesser curve of the aorta just distal to the left subclavian artery origin at the ligamentum arteriosum. Larger ones are called a ductus diverticulum. It is of prime importance to recognize this normal variant, especially in the patient who has suffered blunt chest trauma, because it can be mistaken for an aortic laceration. The descending thoracic aorta begins at the isthmus and courses distally slightly left of the vertebral column and approaches the midline at the aortic hiatus of the diaphragm. It has a diameter of 2.5-3 cm proXimally, or approximately two thirds the diameter of the ascending aorta and tapers distally to 2.4-2.7 cm diameter at the aortic hiatus. The descending thoracic aorta gives off from three to eight pairs of intercostal arteries, those on the left typically arising from the dorsal aspect of the aorta at the level of the vertebral pedicles. Although those on the right usually originate from the dorsal aspect of the aorta, the upper right intercostal arteries may arise from the right lateral or even the ventral surface of the descending thoracic aorta. The descending thoracic aorta also gives rise to the bronchial alteries. The most common pattern encountered is two left bronchial arteries and a single right intercostobronchial trunk. Although a large number of anatomic variation exists, each side usually has one or two bronchial arteries. The bronchial arteries typically arise from the ventral aspect of the descending thoracic aorta between T4 and T8, but can also arise from the undersurface or convexity of the aortic arch, the thyrocervical or costocervical tmnks and can derive collateral supply from many axillary and subclavian artery chest wall branches. It is important to be aware that radicular branches to the anterior spinal artery can arise in conjunction with intercostal or bronchial arteries, partiCUlarly if one is about to undertake embolization of these vessels. High quality, motion-free angiography with analog or digital subtraction is required if one is to recognize and avoid embolization of radiculomedullary feeders.