Cardiovascular Journal of Africa: Vol 27 No 1 (January/February 2016)

CARDIOVASCULAR JOURNAL OF AFRICA • Volume 27, No 1, January/February 2016

AFRICA

a crucial aspect of illustrating the detailed ring configuration

and to facilitate surgical planning.

Patent vascular channels

are evident on CTA as contrast-enhancing segments, and are

well visualised on reconstructed images (Figs 2, 3). Conversely,

atretic vascular segments and ligaments are not evident on

contrast-enhanced images (Figs 4, 5), but their presence can

be inferred from traction on associated vascular structures or

compression of the trachea.

The ‘four-artery sign’ is a useful

CTA radiographic sign which indicates an abnormal aortic

branching pattern and is suggestive of a vascular ring.

CTA allows multi-planar views to be obtained, with three-

dimensional reconstructions clearly illustrating vascular and

tracheal relationships. CTA also allows detailed evaluation

of the lung fields, particularly in patients with co-existing

pulmonary disease.

Inspiratory and expiratory CTA studies

allow the dynamic evaluation of tracheal calibre for narrowing

or traction, which is particularly important in patients with

associated tracheo- or bronchomalacia.

2,3,8

CTA is generally easily accessible and diagnostic interpretation

relatively straightforward.

CT scanning times are shorter than

MRI and therefore sedation is usually not necessary, a significant

advantage in a stridulous patient.

2,4

The principle disadvantages

of CTA are the need for intravenous contrast agents, and the

potential late consequences of radiation-dose exposure.

Vascular ring patients are only exposed to a single CTA, as

serial imaging is not indicated before or after surgery. In the

absence of basic investigations (CXR and CO) consistent with

a vascular ring, CTA should not be used as a screening tool to

exclude the diagnosis, except in a critically ill patient in whom the

diagnosis is considered.

Like CTA, MRI is a sensitive imaging tool for visualising

vascular ring configuration. Advantages of MRI over CTA

include the freedom from exposure to both radiation and

intravenous contrast, as well as the ability to undertake

haemodynamic studies in patients with intracardiac lesions.

The limitations of MRI include the longer scanning time

than CTA, the need for sedation in paediatric patients, and

limited accessibility and reporting expertise in the developing

world. Sedating patients with stridor resulting from a vascular

ring requires rigorous monitoring to avoid potential airway

obstruction, and is usually undertaken by specially trained

nursing staff and in some instances senior MRI specialists.

Endotracheal intubation is avoided to allow accurate tracheal

cross-sectional evaluation.

MRI demands more in terms of

human resources, expertise and time. Despite its availability at

our institution, CTA remains the favoured modality to obtain

cross-sectional imaging of both the vascular ring and the

trachea.

Echocardiography is used principally to investigate

intracardiac abnormalities that will be present in approximately

12.4% of vascular ring patients.

However, echocardiography is

a poor imaging tool to either establish or exclude the diagnosis

of a vascular ring due to poor acoustic windows, ligamentous

structures and hyperinflation of the lungs.

2,8

In the past, conventional catheter angiography (CCA) was

used, in conjunction with CXR and CO, to elucidate the exact

configuration of a vascular ring and thus plan surgery. In the

current era, non-invasive imaging modalities are preferred and

CCA is reserved for the investigation of concomitant intracardiac

lesions to obtain angiographic and haemodynamic data.

Fig. 3.

Double aortic arch with both arches patent. (A) The

plain chest radiograph demonstrates a widened supe-

rior mediastinal silhouette, with the presence of a right

aortic arch in this child with stridor and dysphagia.

(B) The reconstructed postero-anterior CTA image

illustrates the double aortic arch, with the LA and RA

patent and of similar calibre at the confluence with the

DA. Clearly illustrated are the head and neck vessels,

which arise individually from their respective aortic

arches, hence the ‘four-vessel sign’ used to aid diag-

nosing a vascular ring radiologically. LA: left arch; RA:

right arch; DA: descending aorta; LSA: left subclavian

artery; LCA: left carotid artery; RSA: right subclavian

artery; RCA: right carotid artery.