Both series of images demonstrate a large vessel arising from the posterior aspect of the aortic arch, running to the right side posterior to the oesophagus and then cranial to the right supraclavicular region. This is an incidental left-sided aortic arch with an aberrant right subclavian artery configuration. The aberrant right subclavian artery is arteriomegalic. Technically this is not a diverticulum of Kommerrel, from which these can arise as a focal aneurysmal dilatation of the origin, but rather a vessel that is enlarged throughout its length. Patients with a left -sided aortic arch and aberrant right subclavian artery do not have a complete vascular ring, however, as in this case, may have compression of the oesophagus in this region, sometimes referred to as "dysphagia lusoria".
One would expect that with the aberrant right subclavian artery instead of a conventional brachiocephalic artery there would be 4 vessels arising from the aortic arch (in turn: right common carotid, left common carotid, left subclavian, aberrant right subclavian). There are indeed 4 vessels arising off the aortic arch on the 2-D time-of-flight images. However, on the left side both extend into the neck. There is indeed no flow within the left subclavian artery at any level. What is the fourth vessel? This is a direct origin of the left vertebral artery from the aorta. This occurs in approximately 5% in patients instead of origin from the proximal left subclavian artery.
However, the MRA contrast enhanced images, including the maximum intensity projection, do demonstrate enhancement of 5 arch vessels including the more peripheral left subclavian artery with a more proximal occlusion of this vessel, over approximately 1.5 cm. There are asymmetric tortuous collateral vessels in the left side of the neck collateralising to the subclavian artery.
How can we account for the lack of left subclavian artery visualisation on the 2D time of flight sequence but visualisation, albeit incomplete, on the MRA study? What does this tell us? Well the 2-D time-of-flight images are important as they indicate directionality of flow. Note that only cephalad directed arterial flow is recorded. As every slice is acquired sequentially, a saturation pulse is sequentially applied above the next imaged slice to eliminate in-flow signal from structures flowing from above the slice into the slice at the time of acquisition (ie venous blood), allowing only in-flow signal from flowing structures below the slice (arterial). If we wanted insted to visualise venous flow without arterial contamination we would place the saturation pulse below every slice in the 2D time of flight.
So this indicates that there is no cephalad flow in the left subclavian vein. But since there is opacifaction on the contrast enhanced MRA acquisition, the vessel must be partly patent. However, the flow must be retrograde (ie caudally directed). This is a not uncommon situation in vertebral steal phenomenon. A stenosis of the origin of the left subclavian results in poor left subclavian arterial perfusion. During periods of increased left subclavian arterial demand (exercise), the left subclavian artery fills retrogradely via the left vertebral artery (steal) which anastomoses across the circle of Willis and then connects to the subclavian artery distal to the obstruction. In this case, however, as the vertebral artery arises directly of the aorta at this collateral pathway is not available.
We can see on the MRA examination that there is an additional tortuous vessel in the left neck anastomosing distally to the left subclavian artery. This vessel is not visualised on the 2-D time-of-flight acquisitions, indicating that this is downard flow supplying the distal left subclavian artery. Therefore, in combination these findings indicate that there is only retrograde filling of the left subclavian artery from these left-sided cervical branches. This is comparable to subclavian steal, but because of separate origin of the vertebral artery the retrograde filling of the left subclavian is not occurring from the vertebral itself.
There is a complex case with two congenital variations of which one, the left vertebral direct aortic origin, is clinically pertinent. Analyzing the difference between 2-D time-of-flight and MRA images assists in determining directionality of flow, and in this case clinical physiology.