Summary
Computed tomography (CT) technology is acquiring a key role in the diagnostic process of complex cardiac congenital anomalies. Recent advances and improvements in spatial and temporal resolution and radiation dose are encouraging the use of CT scanning in children. Paediatric cardiologists should have a good knowledge of the potential of CT techniques and their limitations to plan and properly perform CT examinations without forgetting radiation concerns. In this paper, we will discuss the principal indications for CT scans in newborns and children in our clinical practice. We will also outline the most-used strategies for dose reduction. Basic knowledge about the various CT techniques is crucial, not only to perform, but also to interpret CT results, thus helping the medical and surgical management of patients.
Résumé
Le scanner est devenu un examen diagnostique clé dans les cardiopathies congénitales complexes. L’amélioration de la résolution spatiale et temporelle et les techniques permettant une diminution des doses autorisent son utilisation dans la population pédiatrique. Les cardiopédiatres doivent connaître les bases techniques et les limites du scanner, sans oublier l’optimisation des doses, afin de le prescrire et le réaliser à bon escient. Dans cette revue, nous exposerons dans une première partie les principales indications du scanner cardiaque chez les nourrissons et les enfants en pratique clinique et dans une seconde partie, l’optimisation des doses. Ceci apparaît important afin de réaliser mais aussi d’interpréter les images pour la prise en charge médicale et chirurgicale des patients.
Introduction
The introduction of helical computed tomography (CT) about 30 years ago has changed the diagnostic medical approach around the world, with a considerable increase of the number of CT examinations in both paediatric and adult patients.
Transthoracic echocardiography (TTE) and cardiac catheterization serve as the mainstay modalities in the diagnostic process of complex congenital heart diseases (CHD). TTE with colour Doppler provides excellent definition of intracardiac anatomy, including haemodynamic evaluation. However, its performance may be lacking in terms of visualization of extracardiac structures such as the distal aorta, aortic arch branches, distal pulmonary arteries, pulmonary veins, or other associated vascular structures and airways abnormalities. Invasive catheter-directed cardiac angiography has been, for many years, the gold standard for the anatomical assessment of CHD in children, having the potential to couple anatomical and haemodynamic evaluation. However, its disadvantages include the risk of complications of an invasive procedure (vessel dissection, stroke, pseudoaneurysm formation, costs of hospital stay, patient discomfort) . Moreover, cardiac catheterization is not informative regarding associated airway pathology.
Consequently, cardiac electrocardiogram (ECG)-CT scanning, with all the recent advances and improvements in spatial and temporal resolution, is acquiring a key role in the diagnostic process of complex cardiac congenital anomalies. However, the increase in radiation exposure from CT in children has become a great concern. Although the link between medical radiation exposure and future cancer risk is still controversial, children have higher radiation sensitivity and a longer life expectancy. These factors justify a maximal effort to have scanning protocols specifically designed for the paediatric population and to restrict CT indication to specific cases when CT scan results are considered essential for patient management.
In the first part of this review, we will discuss the principal indications in clinical practice for paediatric CT scans. In the second part, we will discuss some optimization methods for reducing radiation dose exposure.
Principal indications for CT scanning in children
CT scanning is an accurate, non-invasive imaging modality to visualize cardiac, vascular, extracardiac and extravascular structures. After contrast injection, the heart and vessels can be evaluated in detail. With isotropic reformatted images created in multiple different planes, CT scans provide excellent anatomical detail of the cardiovascular system and any abnormalities. CT scans may provide accurate information to assess complex spatial relationships of vascular airway compression frequently associated with CHD . In the paediatric population, TTE contributes – in the majority of cases – to the definition of anatomical details and the relationships between the great vessels. In some complex cases, however, CT scanning is necessary to complete the diagnosis with precise anatomical elements, which are also required by surgeons to define the surgical strategy before intervention.
Previous data have suggested that CT scanning is a reliable modality for complex CHD in infants , but it is not yet recommended for routine evaluation in newborns and young infants. Use of CT varies according to the age of the patient for several technical reasons. Here, we will discuss the principal indications in clinical practice.
Systemic vessels
Aortic coarctation
In complex cases of aortic coarctation ( Video 1 ), CT scans may provide important anatomical details not shown by echocardiography, especially in distal lesions. CT scans can be performed at the time of diagnosis, as a complement to TTE, and during follow-up after surgical correction or catheter intervention when complications such as restenosis, residual stenosis, aneurysm or pseudoaneurysm should be assessed.
CT scanning allows measurement of the diameter and length of the stenosis and the distance between supra-aortic trunks. CT scans allow calculation of the diameter – in truly orthogonal planes – of the vessel axis to make the segments comparable. After endovascular treatment, CT scanning is suited to evaluate stent patency in children ( Fig. 1 ) . For patients suspected to have aortic arch hypoplasia, CT provides information about the exact location, shape and length of the hypoplastic segment, as well as the course of the collateral vessels.
Complex arch anomalies
Aortic arch anomalies account for 0.5–3% of the population . Corone and Vernant proposed a system that used a hypothetical embryonic double aortic arch to explain the various congenital anomalies of the aortic arch determined by the site of interruption or atresia of the embryonic arch system. In the majority of complex arch anomalies, CT is mandatory to diagnose the type of anomaly and the relationship between the trachea and oesophagus, which are surrounded by vascular structures ( Fig. 2 , Video 2 ).
Supravalvular aortic stenosis
Supravalvular aortic stenosis is a focal or diffuse narrowing of the aorta starting at the sinotubular junction and often involving the entire ascending aorta. CT allows visualization of the entire aorta and is a reliable modality to demonstrate the extent of the supravalvular aortic stenosis. CT scans are able to determine the permeability of the coronary ostia, especially in Williams syndrome, avoiding the risk of invasive coronary angiography. With an ECG-gated technique, myocardial hypertrophy and bicuspid valve can be depicted.
Aortopulmonary window
Aortopulmonary window is a communication between the ascending aorta and the pulmonary trunk or right pulmonary artery. It is a rare entity, representing less than 0.1% of CHD cases. Non-invasive evaluation with TTE may not demonstrate the communication in up to 37% of cases . CT scans can demonstrate the communication between the aorta and the pulmonary artery, as well as signs of pulmonary hypertension. CT can accurately identify the size and the exact location of the defect, and its relationship with the origin of the coronary arteries. CT scans can play an important role and be of significant help for the surgeon to plan the surgical strategy.
Pulmonary vessels and aortopulmonary collaterals
In all forms of pulmonary obstruction with suspicion of distal anomalies of pulmonary arteries – associated or not to the presence of major aortopulmonary collateral arteries (MAPCAS) – CT scans may define the distal anatomy of pulmonary branches and also precisely identify the anatomy of MAPCAS.
Invasive angiography is still performed in association with CT scans, especially to define the relationship between MAPCAS and native pulmonary branches and their eventual communication. CT scans corroborate invasive data and complement them with important anatomical details.
Three-dimensional (3D) reconstructions of vessels and the trachea permit understanding of their reciprocal relationship ( Video 3 ), allowing the surgeon to plan the surgical strategy. This kind of analysis should be performed before planning surgical interventions because anatomical details of pulmonary anatomy and collateral vessels are crucial for long-term prognosis. In case of other complex anomalies, such as retrotrachea pulmonary artery (pulmonary artery sling, absent left pulmonary artery), CT may provide details of distal anatomy and associated airway anomalies ( Fig. 3 ).
Pulmonary venous anomalies
In complex anomalies of pulmonary veins, such as subdiaphragmatic or mixed total anomalous pulmonary venous return or scimitar syndrome, CT can visualize and precisely identify the anatomy of all pulmonary vein connections.
When the anomalous pulmonary drainage is totally in coronary sinus or in superior vena cava/innominate vein, TTE is often sufficient to make the right diagnosis. When the anomalous drainage is subdiaphragmatic or of mixed type and the clinical status of the patient is not critical, CT is important to define the complete anatomy of the pulmonary veins ( Fig. 4 ).
