British Journal of Radiology (2005) 78, 961-966
© 2005 British Institute of Radiology
doi: 10.1259/bjr/20210494
MR angiography of aberrant left subclavian artery arising from right-sided thoracic aortic arch
J Salanitri, FRANZCR
Department of Radiology, Northwestern Memorial Hospital, 448 East Ontario Street, Suite 300, Chicago, Illinois 60611, USA
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Abstract
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Right-sided thoracic aortic arch with aberrant left subclavian artery is an uncommon entity that is usually asymptomatic, but may present in adults with symptoms of dysphagia. Cross-sectional imaging such as CT or MR is often required to conclusively diagnose this condition, which may be suggested by chest radiograph or barium swallow. MRI including TRICKS (time-resolved imaging of contrast kinetics) magnetic resonance angiography (MRA) was utilized in the evaluation of a 40-year-old man presenting with dysphagia and abnormal barium swallow study. TRICKS MRA is a recently developed MRA technique, which can provide multiple high-resolution images of the thoracic vessels with a high temporal resolution without the need for timing doses of gadolinium contrast or multiple injections.
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Introduction
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Right-sided thoracic aortic arch with an aberrant left subclavian artery is a rare anomaly, occurring in 0.05% of the general population, which may initially present in adulthood with symptoms of dysphagia. While the diagnosis may be initially suggested by the chest radiographic and barium swallow findings, cross-sectional imaging, e.g. CT or MRI, is usually needed to provide the definitive diagnosis. The barium swallow, MRI and magnetic resonance angiography (MRA) findings of a case of right-sided aortic arch with aberrant left subclavian artery are reported.
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Case report
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A barium swallow was performed on a 40-year-old man with a 6 month history of worsening gastro-oesophageal reflux and dysphagia for solids. In addition to severe gastro-oesophageal reflux extending to the level of thoracic inlet (Figure 1a
), a smoothly marginated 3 cm x 1.2 cm mass was demonstrated indenting the posterior oesophagus at the T4-5 level (Figure 1b,c), which did not obstruct the passage of either liquid barium bolus or a 12 mm diameter barium impregnated tablet. A differential diagnosis of a leiomyoma or an extrinsic mass was given on the basis of these findings.
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Figure 1. Barium swallow images demonstrating (a) severe reflux of barium on the supine lateral view with anterior displacement of the upper oesophagus (arrow) and an obliquely orientated impression (arrows) upon the upper thoracic oesophagus at the level of the 4th thoracic vertebra on (b) the anteroposterior (AP) and (c) right anterior oblique (RAO) views.
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Chest MRI and MRA was subsequently performed on a 1.5 Tesla Siemens Avanto scanner (Siemens, Erlangen, Germany). Electrocardiogram (ECG)-triggered axial (Figure 2a,b) and coronal (Figure 2c) single-shot balanced gradient echo (True-FISP) sequences (repetition time (TR)/echo time (TE)=2.36 ms/1.18 ms, field of view 420 mm, matrix 178 x 256, slice thickness 5 mm, flip angle 80°), demonstrated a right-sided aortic arch with aberrant origin of the left subclavian artery from a diverticulum of Kommerell located posterior to the oesophagus at the level of the fourth thoracic vertebra. There was no evidence of an oesophageal mass lesion. The descending thoracic aorta was located in the right hemithorax.
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Figure 2. Axial single-shot balanced gradient echo (True-FISP) MR images demonstrating: (a) the right-sided aortic arch with origin of the aberrant left subclavian artery from a diverticulum of Kommerell (arrow) located posterior to the oesophagus and trachea and (b) the descending thoracic aorta (arrow) to the right of the vertebral column. (c) Coronal True-FISP MR image shows the diverticulum of Kommerell (arrow) in relation to the right-sided aortic arch and descending aorta (arrowheads).
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A temporally resolved three-dimensional contrast enhanced spoiled gradient echo (Fast low angle shot – FLASH) MRA using variable k-space sampling (time-resolved imaging of contrast kinetics – TRICKS) was performed (Figure 3a). Sequence parameters were: TE 0.7 ms, TR 1.8 ms, slice thickness 2 mm, number of slices 64, field of view 335 mm x 380 mm, matrix size 135 x 256, flip angle 10°, spatial resolution 2.5 mm x 1.5 mm x 2 mm. 20 ml of gadolinium contrast agent was injected at a rate of 4 ml s–1 by power injector followed by a 20 ml saline flush. 14 measurements (including a mask), each taking 2.54 s to acquire, were obtained following the onset of the contrast bolus injection. Automatic in-line subtraction derived maximal intensity projection (MIP) images from each measurement for display as a cine loop. Source images from the measurement best corresponding to the aortic phase were further post-processed on dedicated workstations to yield volume-rendered (VR) images that elegantly delineated the anatomy of the right-sided aortic arch and great vessels (Figure 3b,c).
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Figure 3. Maximal intensity projection (MIP) image from the aortic phase measurement of time-resolved imaging of contrast kinetics (TRICKS) contrast enhanced magnetic resonance angiography (MRA) (time to acquire data=2.54 s). The aberrant left subclavian artery and diverticulum of Kommerell are demonstrated clearly (arrow). (b) Anterior and (c) posterior projected volume rendered reformats of the source data from (a) illustrate the relations of the vessels arising from the right-sided thoracic arch.
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On the basis of the barium swallow findings, a trial of prokinetic and antireflux medications was initiated in this patient, which resulted in good symptomatic relief. The management plan for the Kommerell diverticulum was for periodic follow-up with MRA and to only intervene surgically if there was definite evidence of progressive aneurysm dilatation or if the patient's symptoms became no longer responsive to medication.
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Discussion
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Right-sided thoracic aortic arch is an anatomical variant present in approximately 0.1% of the general adult population [1, 2]. In half of these cases, there is also an aberrant origin of the left subclavian artery, which may either arise as the fourth (last) branch of the right-sided aortic arch or from a diverticulum (of Kommerell) [3].
Developmentally, this anomaly is a result of abnormal development of the primitive aortic arches. Early in the embryonic stage of development, blood leaves the heart via the truncus arteriosus. This is divided by the aorticopulmonary septum into the ventral aortas, which later become the aortic sac and the pulmonary trunk. Six symmetrical branchial vessels (the aortic arches) connect the paired dorsal aortas with the ventral aortas. These aortic arches are not all present at the same time during embryological development, instead appearing in a sequential fashion. As the embryo develops, patterned regression and persistence of these primitive vessels ultimately results in the modification of the arterial pattern to the mature form present at birth (see Table 1) [4].
Normally, the left fourth aortic arch persists to form the adult thoracic aortic arch. The brachiocephalic trunk and right subclavian artery are formed from portions of the right fourth aortic arch while the left subclavian artery is formed from the left fourth aortic arch and the anlagen of the 7th dorsal intercostal artery [2, 4, 5].
Right-sided aortic arch with aberrant origin of the left subclavian artery (Edward's type III pattern) is the most common form of right-sided thoracic aortic arch [6] and is caused by involution of the left fourth aortic arch with concomitant persistence of the right fourth aortic arch which is interrupted between the left common carotid and left subclavian arteries [2, 7].
In right-sided aortic arch with aberrant left subclavian artery, the order of branch vessels is from proximal to distal; left common carotid artery, right common carotid artery, right subclavian artery and left subclavian artery. The aberrant left subclavian artery passes from the right into the left hemithorax, posterior to the oesophagus and trachea, and together with the ligamentum arteriosum forms a vascular ring. A remnant of the left fourth aortic arch may manifest as Kommerell's diverticulum at the origin of the aberrant left subclavian artery. However, in adults, an atherosclerotic aneurysm of the aberrant left subclavian artery may have an identical appearance to these congenital diverticula [2].
The descending thoracic aorta may be located in either the right or left hemithorax, depending on whether the left or right dorsal aorta involutes, respectively. There is a low incidence of associated congenital cardiac anomalies, generally reported as 5–10% [1, 5].
The majority of patients with right-sided aortic arches with aberrant left subclavian arteries are asymptomatic, with diagnosis being a fortuitous discovery on chest radiograph or CT performed for other indications [2, 7, 8]. Occasionally, adults may present with symptoms and signs including stridor, respiratory embarrassment, recurrent respiratory infections or dysphagia [6, 8]. Postulated mechanisms responsible for the development of dysphagia in adults include mechanical compression of the oesophagus by an elongated thoracic aorta or by a left subclavian artery aneurysm (especially at the site of Kommerell diverticulum), or the elongated aorta and/or increased rigidity of the oesophageal wall [7].
However, as many patients with right-sided aortic arches and aberrant left subclavian arteries respond to pharmacological therapy (prokinetic or antireflux drugs), coexisting oesophageal disease, e.g. hiatal hernia or reflux, may well be the aetiology in most cases rather than mechanical compression of the oesophagus [7]. In the adult patients who do not respond to conservative drug treatment, division of the ligamentum arteriosum by a left thoracotomy may be curative [5]. Division of the aberrant left subclavian artery with re-implantation onto the aortic arch may be performed when there is a potential risk of rupture of the Kommerell diverticulum/aneurysm [5].
Imaging findings of right-sided aortic arch with aberrant left subclavian artery includes widening of the right superior mediastinum [3] with absence of the normal left aortic arch contour on frontal chest radiograph and a well defined area of increased opacity in the retrotracheal space at the level of the upper aortic arch on the lateral view [6]. Barium swallow demonstrates anterior displacement of the oesophagus [3] with a characteristic diagonal impression on the posterior oesophagus at the level of the fourth thoracic vertebra, due to the obliquely coursing aberrant left subclavian artery [1, 8]. Arch aortography, CT or MR can be used to confirm the diagnosis, demonstrate any coexisting cardiac or vascular anomalies and to delineate the exact anatomy [2, 6, 7], with MRI having the advantages of being non-invasive and not utilizing ionizing radiation or iodinated contrast medium.
TRICKS MRA is a recently developed technique that allows the acquisition of multiple high-resolution contrast-enhanced three-dimensional MR angiograms at a high temporal resolution (effective frame rate being 2–5 s) [9]. This technique, in essence, over samples central k-space, which is responsible for contrast resolution, relative to peripheral k-space (which encodes higher spatial frequencies) and then combines appropriately weighted elements of central and peripheral k-space (temporal interpolation) to produce, after Fourier transform, multiple high-resolution MRAs of the same anatomical region separated by short time periods [8]. This form of MRA differs from the technique of ultra-fast (or sub-second) contrast-enhanced MRA (with standard k-space acquisition strategies but significantly reduced number of phase encoding steps and slices) in that spatial resolution is not sacrificed in order to yield improved temporal resolution.
With the TRICKS MRA technique, a timing bolus of gadolinium is no longer required to determine maximal aortic enhancement as there is an increased likelihood that a "pure" arterial-only phase image will be obtained. The resulting temporally defined images are similar in appearance to digital subtraction angiograms, except that they represent three-dimensional volumes and are not two-dimensional projection images. The source images from each individual MRA (or measurement) can be sent to workstations for further processing into maximum intensity projected or volume rendered images, which may be displayed as cine loops. Furthermore, the amount of gadolinium and time spent on the MR scanner can be substantially reduced compared with conventional high-resolution contrast-enhanced MRA techniques.
In conclusion, MRA has utility for the diagnosis and follow-up of aberrant left subclavian artery associated with right-sided thoracic aortic arch, being a non-invasive imaging modality with no ionizing radiation or exposure to iodinated contrast media. TRICKS MRA is a promising new technique for imaging the central thoracic vessels in multiple phases with obtained images having both high spatial and temporal resolution.
Received for publication September 13, 2004.
Revision received January 6, 2005.
Accepted for publication April 15, 2005.
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Abstract
Introduction
