Iicongenital Anomalies Of The Aorta A Embryology

It is not necessary to thoroughly understand the embryological development of the aorta to diagnose the various arch anomalies, but it is useful to have a basic idea about how they arise as it makes the imaging findings easier to understand. In the classic theoretical embryological double arch proposed by Edwards (Fig. 1), there are right and left aortic arches that connect the ascending and descending portions of the aorta [1]. Each gives rise to a carotid and a subclavian artery. In normal development, the left arch persists and the part of the right arch distal to the origin of the right subclavian artery becomes

atretic. This results in three great vessels arising from the arch: a right brachiocephalic artery composed of the proximal remnant of the right arch and the right carotid and subclavian arteries; a left common carotid artery, and a left subclavian artery. The remnant of the ductus arteriosus, the ligamentum arte-riosum, connects the bottom of the transverse arch to the proximal left pulmonary artery. Essentially all of the developmental anomalies of the aortic arch branches can be explained by variations in which part of the embryological double arch becomes atretic.

B. Diagnosis of Congenital Aortic Anomalies and Vascular Rings

A vascular ring results from encircling of the trachea by a combination of the aorta and its branches and the ligamentum arteriosum. If the ring is tight enough, tracheal compression and respiratory compromise may result. It is important to remember that the mere presence of an arch anomaly does not mean that there is a vascular ring. The diagnosis of ring should be made on the basis of symptoms and anatomy, not anatomy alone. When presented with a patient with stridor, the evaluation should begin with a frontal and lateral chest radiographs and a barium esophagram. If these confirm the presence of airway compromise and suggest a vascular cause, further evaluation with cross-sectional imaging can be pursued. In this day and age, there is little, if any, indication for angiographic evaluation of vascular anomalies.

Magnetic resonance imaging (MRI) using spin-echo (black blood) techniques is an elegant method of delineating the mediastinal vascular anatomy in infants, children, and adults with suspected congenital aortic anomalies, particularly vascular rings and aortic coarctation [2]. The lack of need for intravenous contrast and the ability to obtain two- and three-dimensional reformatted images to optimize display of aortic and branch anomalies make MRI the modality of choice in infants and children.

Rapid scanning with contrast-enhanced spiral CT, particularly the recent advent of multidetector-row scanners capable of very rapid scanning with thin collimation and minimal respiratory motion artifact, provides information analogous to MRI [3]. The scanning parameters and contrast administration used for CT aortography in infants and children are tailored to the individual examination, but follow the basic principles of CT aortography as outlined in Table 1. The technique utilized to evaluate suspected congenital aortic anomalies with spiral CT is the same as for the evaluation of acquired aortic disease

Table 1 Helical CT Scanning Protocol for Thoracic Aortic Disease

Procedural element

Protocol

Anatomic cephalocaudal extent of scan

Apices ^ bases

Scan milliamp, kilovolt peak settings

220 mAs, 120 kVp

Duration of helical exposure

20-30 sec

Pitch

2.0:1 (single-detector CT)

6.0:1 (MDCT)

Table speed = 15 mm/rot.

Collimation

5 mm (infants = 3 mm) (single-detector

CT)

2.5 mm (MDCT)

Display field of view

Widest rib ^ widest rib from AP scout

Patient instructions during scanning

Single breath hold after three maximal

breaths

Precontrast scans

None

Contrast type/concentration/volume

Nonionic 300 mg% and 1 cc/kg (infants

and children), 150 cc (adults)

Contrast injection rate

1-3 cc/sec

Scan delay from start of injection to

40 sec

scan

Reconstruction algorithm

Standard

Reconstruction intervals

Contiguous for filming 3 mm for 3D

reconstructions (infants = 2 mm)

(single detector CT)

1.25 mm (MDCT)

in the adult population with the following exceptions: in neonates and children a lower exposure technique is utilized to limit radiation dosage, thinner colli-mation (3 mm) is utilized as less cephalocaudal coverage is needed, and the volume and rate of contrast administration is based on patient weight (2 mL of 300 mg% nonionic/kg injected at 2 mL/sec). Scans are reconstructed at overlapping (i.e., 2 mm) intervals for soft-copy interpretation and two- and three-dimensional reconstruction on a workstation.

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