Normal Anatomy and Anatomic Variations

The aortic arch and great vessel origins are best shown on the CE MRA. They are also seen to relatively good advantage on the 2D TOF images although "stairstep" artifact is usually quite noticeable in this region due to a combination of vessel pulsation and respiratory motion at the level of the aortic arch. This motion degradation is minimized with CE MRA due to the rapid acquisition and the averaging of pulsatile motion across the entire 3D data acquisition set.

The common carotid arteries bifurcate into internal and external carotid arteries and are also well shown on neck MRA (Figs. 1,2). The common carotid artery bifurcations most commonly occur at approximately the level of C4 (Fig. 3). Occasionally, one or both carotid bifurcations occur either higher or lower in the neck. This variation can present a problem in visualization when using a targeted 3D TOF technique to provide better vessel detail over a limited length of coverage. However, this is not a problem for 2D TOF or for contrast enhanced mRa since the entire length of the vessels is typically defined with high detail.

Both vertebral arteries throughout their length are also well shown with CE MRA. However, depending on the thickness of coverage of the coronal 3D slab for contrast enhanced MRA and the degree of cervical lordosis present, there may be a slight cutoff of the most posterior portion of the distal loop of the vertebral arteries at C1-2 just prior to their entry into the foramen magnum at the skull base.

The aortic arch generally gives rise to three separate origins of the great vessels of the neck (Fig. 4). These include the right innominate artery, the left common carotid artery, and the left subcla-vian artery. The right innominate artery courses slightly superiorly and laterally prior to giving rise to the right common carotid artery. Just a few mil limeters distal to this origin is the origin of the right vertebral artery. The left common carotid artery most often arises as a separate branch from the aortic arch and courses superiorly and slightly leftward through the base of the neck. It gives off no branches until it reaches the common carotid bifurcation. The left subclavian artery courses superiorly and turns laterally. Generally in the region at or just proximal to the lateral turn of the left subclavian artery one can find the origin of the left vertebral artery.

Subclavian Anatomy Vertebral Artery

Fig. 1. Normal anatomy of the head and neck vessels

I Brachiocephalic trunk (innominate artery)

II Right common carotid artery

III Right vertebral artery

IV Right subclavian artery

V Right internal Carotid artery

VI Right external Carotid artery

Fig. 1. Normal anatomy of the head and neck vessels

I Brachiocephalic trunk (innominate artery)

II Right common carotid artery

III Right vertebral artery

IV Right subclavian artery

V Right internal Carotid artery

VI Right external Carotid artery

1 Right mammarian artery

2 Right thyrocervical trunk a Right superior Thyroid artery b Right lingual artery c Right facial artery d Right ascending Pharyngeal artery e Right maxillary artery f Right occipital artery g Right posterior Auricular artery h Right superficial Temporal artery

Anatomy The Vessels The Neck Tof
Fig. 2. Normal anatomy of the head and neck vessels

A Brachiocephalic trunc (innominate artery)

B Right common carotid artery

C Right subclavian artery D Left common Carotid artery

E Left subclavian artery

F Vertebral arteries

G Internal carotid arteries

H External carotid artery

I Basilary artery

1 Internal mammarian arteries (internal thoracic artery)

2 Thyrocervical trunk

3 Costocervical trunk a Posterior inferior cerebellar artery b Anterior inferior cerebellar artery c Superior cerebellar artery

Fig. 3. Normal common carotid artery bifurcations are well demonstrated on CE MRA. Note the excellent contrast together with smooth vessel outlines without stairstep artifact

Aortic Arch Four Vessels

Fig. 4. Normal antomy of the aortic arch

I Ascending aorta

II Aortic arch

III Descending aorta

Fig. 4. Normal antomy of the aortic arch

I Ascending aorta

II Aortic arch

III Descending aorta

A Brachiocephalic trunk (innominate artery)

B Right common carotid artery

C Right subclavian artery

D Left common carotid artery

E Left subclavian artery

Normal Variations

There are a number of common variations in the anatomy of the great vessels that generally occur at the level of the origins from the aortic arch (Figs. 5, 6). A common anomaly is that of the so-called bovine arch. In this anomaly the right common carotid artery and the left common carotid artery essentially arise together and branch just distal to their origin from the aortic arch. In this anomaly, there is a common ostium for the right inominate artery and the left common carotid artery. The right inominate artery and the left common carotid artery arise together and bifurcate immediately above the aorta (Fig. 7a-c). This is a rather common finding and is seen in approximately 30% of normal individuals.

In another commonly seen variation, the left vertebral artery arises as a separate branch from the aortic arch rather than as a branch of the left subclavian artery. This occurs in approximately 10% of normal individuals. In these cases, the separate origin of the left vertebral artery arises between the ostea of the left common carotid artery and the left subclavian artery (Fig. 8).

Additionally, the right common carotid artery may have an anomalous origin from the descending aorta. In these cases, rather than arising as a branch of the right innominate artery, the anomalous right common carotid artery is the last branch of the neck vessels arising separately from the descending aortic arch just distal and posterior to the left subclavian artery origin. The anomalous right common carotid artery then courses right-

ward and crosses the midline behind the trachea and the esophagus prior to turning superiorly and ascending in the right neck. This anomalous configuration forms a partial vascular ring around the trachea and esophagus. Similarly in case of a luso-rian artery the right subclavian artery arises separately from the descending aortic arch distal to the left subclavian artery origin with the right common carotid artery branching directly from the aorta as the first vessel of the aortic arch (Fig. 9).

The size of the vertebral arteries between right and left is often asymmetric. In approximately 70% of cases the left vertebral artery is dominant in size relative to the right vertebral artery. There is a wide variation in the caliber of the vertebral arteries from symmetrical left and right vertebral artery size (approximately 10 % of population) to that of an extremely hypoplastic right vertebral artery and a large dominate left vertebral artery. In approximately 20% of cases the right vertebral artery is the dominate vessel. A distinction must be made between naturally occurring asymmetry due to normal anatomic variation and pathologic narrowing of a vertebral artery due to atherosclerosis or vessel dissection. In the case of normal variations cited above, the hypoplastic vertebral artery has a uniform and diffuse narrow size from origin to skull base. In pathologic cases due to atherosclerotic stenosis or narrowing from vessel dissection, there is generally focal reduction(s) in caliber between the proximal vertebral artery and that of the vertebral artery distal to the site of stenosis or dissection.

Vertebral Artery

Fig. 5. Common variations of the supraaortic vessels and the arch

Bovine Anomaly Left Subclavian

Fig. 6. Common variations of the supraaortic vessels and the arch

I Left aortic arch

II Right aortic arch

Fig. 5. Common variations of the supraaortic vessels and the arch

A Right common carotid artery

B Left common carotid artery

C Right subclavian artery D Left subclavian artery

Fig. 6. Common variations of the supraaortic vessels and the arch

I Left aortic arch

II Right aortic arch

A Right common carotid artery

B Left common carotid artery

C Right subclavian artery

D Left subclavian artery

Bovine Arch

Fig. 7a-c. Bovine type aortic arch is seen in approximately 30% of normal individuals. a Incidental finding of a bovine type arch in a 14-year old boy. Note the common ostium (arrow) of the right innominate artery and the left common carotid artery [Image courtesy of Dr. G. Schneider]. b Bovine type aortic arch in an elder patient with arteriosclerotic disease. The right innominate artery and the left common carotid artery arise from a common ostium. Due to vessel elongation in arteriosclerosis the left common carotid artery courses almost horizontally leftward across the mediastinum (arrow) before ascending along the left side of the neck. Note there is also a mild narrowing of the proximal left common carotid artery (arrowhead). c Targeted MIP image of the left common carotid artery confirms narrowing of the proximal left common carotid artery (arrows)

Fig. 7a-c. Bovine type aortic arch is seen in approximately 30% of normal individuals. a Incidental finding of a bovine type arch in a 14-year old boy. Note the common ostium (arrow) of the right innominate artery and the left common carotid artery [Image courtesy of Dr. G. Schneider]. b Bovine type aortic arch in an elder patient with arteriosclerotic disease. The right innominate artery and the left common carotid artery arise from a common ostium. Due to vessel elongation in arteriosclerosis the left common carotid artery courses almost horizontally leftward across the mediastinum (arrow) before ascending along the left side of the neck. Note there is also a mild narrowing of the proximal left common carotid artery (arrowhead). c Targeted MIP image of the left common carotid artery confirms narrowing of the proximal left common carotid artery (arrows)

Bovine Arch NormalBovine Aortic Arch Normal Aortic Arch
Fig. 8. Normal variation illustrating separate origin of the left vertebral artery, which arises directly from the aortic arch (arrow) rather than from the left subclavian artery. This is seen in approximately 10% of normal individuals
Distal Aorta Anatomy
Fig. 9. Lusorian artery. The right subclavian artery arises separately from the descending aortic arch (arrow) distal to the left subclavian artery origin with the right common carotid artery branching directly from the aorta as the first vessel of the aortic arch (arrowhead)
Essentials of Human Physiology

Essentials of Human Physiology

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Responses

  • Alice
    Why left superficial temporal artery larger than right?
    5 years ago
  • jamie-leigh
    Why is there anatomical variation?
    5 years ago
  • retu
    What are the bones in the neck?
    5 years ago
  • Brhane
    What are vertebral arteries?
    5 years ago

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