The high-riding truncus brachiocephalicus is upwardly shifted in the lower neck, with the bifurcation lying close to the thyroid gland. This variant probably develops as a consequence of anomalous regression of the IV arch [14] (Fig. 9).
The caudal part of the thyroid gland can be supplied by a thyroid ima artery, a variant often associated with the absence of inferior thyroid arteries. A thyroid ima artery may arise from the aortic arch, the truncus brachiocephalicus, right common carotid artery, or internal thoracic artery and reaches the thyroid bed coursing along the anterior surface of the trachea [15] (Fig. 10).
Both of these variant vessels may pose a surgical threat, principally when tracheostomy or thyroid/parathyroid surgery is planned; identification on cross-sectional imaging requires careful assessment of vascular structures in the peritracheal soft tissues.
A retropharyngeal carotid artery relates to the relatively common medial shift of the ICA (less frequently the common or external carotid artery); it can be bilateral, a condition referred to as “kissing carotids” and is prevalently seen at the oropharyngeal and hypopharyngeal level [16] (Fig. 11).
The incidence of this variant is linearly related to patient age; thus, it is possibly explained by increased tortuosity and atherosclerotic changes or by hypertension. Severe complications may be generated by retropharyngeal carotid artery injury, even during routine surgery such as tonsillectomy or peritonsillar abscess drainage. Interestingly, a change in position (from and to retropharyngeal) on MDCT examinations acquired at different time points has been described in 6.3% of cases [17].
In the temporal bone, the vertical portion of the petrous internal carotid artery may be undeveloped and bypassed by hypertrophied inferior tympanic and caroticotympanic arteries, coursing in the hypotympanum: this condition is named an aberrant ICA [18] (Fig. 12). It may manifest as pulsatile tinnitus and mimic a vascular mass on otoscopy or may remain asymptomatic. CBCT/MDCT may indicate the absence of the vertical portion of the ICA, presence of a hypotympanic soft tissue mass, enlargement of the inferior canaliculus, and absence of bone coverage on the intratympanic segment of the vessel. As the hypotympanic soft tissue mass may be obscured by diffuse inflammatory opacification of the middle ear, awareness of this condition is crucial. On MRI, time-of-flight (TOF) angiography shows a pinched contour at the intersection of the vertical and horizontal segments of the ICA.
Rarely, hemorrhage during middle ear surgery may be produced by injury to a persistent stapedial artery. During fetal life, the stapedial artery provides a connection between branches of the external and internal carotid artery; in about 0.05% of cases, the vessel does not regress and may be seen in its entire course, arising from the petrous ICA [19]. The persistent stapedial artery crosses the antero-medial hypotympanum, courses between the crura of the stapes to reach the facial nerve canal, and follows retrogradely a short segment of its tympanic portion up to the geniculate ganglion, where it enters the extradural space in the middle cranial fossa [20]. When the stapedial artery persists, the middle meningeal artery arises from it, and thus MDCT and MRI images show the absence of the foramen spinosum; in addition, a small vascular canal may be seen along the cochlear promontory and the facial nerve canal will have an abnormally large diameter. High-resolution submillimetric (i.e., 0.9 mm or less isotropic voxel) MRI sequences with gadolinium will show the vessel, along with an abnormal enhancement along the second segment of the petrous facial nerve (Fig. 13).