Radiography
Findings
On radiographs, the cardiac silhouette in patients with tetralogy of Fallot is normal in size; however, right ventricular hypertrophy can elevate the left ventricle. Combined with a small or absent main pulmonary artery segment, the heart can have the classic boot-shaped appearance (see Image 1). Most children with tetralogy of Fallot do not have boot-shaped heart (see Image 2).
Typically, the appearance of the vascularity of the pulmonary artery is reduced, but it can also be normal. Decreased pulmonary vascularity is frequently difficult for the general radiologist to appreciate. Large collaterals may give the appearance of normal vascularity.
The enlarged aorta in children with a right-sided arch can cause airway compression that can be identified on the chest radiograph (see Image 3). A right-sided arch is present in 25% of children with tetralogy of Fallot, and it can be identified by means of direct visualization, with displacement of the trachea to the left or with increased opacity of the spinal pedicles on the ipsilateral side of the aortic arch. The position of the aortic arch influences surgical planning because Blalock-Taussig shunts are more easily placed on the contralateral side of the aortic arch. Modified Blalock-Taussig shunts can be placed bilaterally.
Degree of Confidence
Confidence in conventional chest radiographic findings increases with the radiologist's reading experience. The use of echocardiography has reduced the importance of chest radiography in the initial diagnosis of congenital heart disease. Echocardiography should be used to confirm radiographic findings that are suggestive of tetralogy of Fallot.
False Positives/Negatives
The boot-shaped heart is overlabeled in neonates, who normally have right ventricular hypertrophy. If the chest radiograph shows lordosis, a normal heart may appear boot shaped.
A right-sided aortic arch in a child with congenital heart disease is most commonly associated with tetralogy of Fallot. Children with a large right-sided aortic arch may have a double aortic arch or an aberrant left subclavian artery without congenital heart disease. Other forms of cyanotic heart disease that are associated with a right-sided aortic arch are usually hypervascular and associated with cardiomegaly (eg, truncus arteriosus, transposition of the great arteries).
Although cyanosis and a right-sided aortic arch are associated with tetralogy of Fallot, the presence of cardiomegaly and increased pulmonary vascularity make an admixture lesion the more likely diagnosis. Transposition of the great vessels or truncus arteriosus is associated with increased pulmonary vascularity, cardiomegaly, and cyanosis, as well as a right-sided arch. Approximately one third of children with truncus arteriosus have a right-sided aortic arch. Only 5% of children with transposition of the great vessels have a right-sided aortic arch, but this is a more common form of congenital heart disease than truncus arteriosus.
Computed Tomography
Findings
Computed tomography (CT) scanning has an infrequent role in the evaluation of tetralogy of Fallot. This modality is useful for the evaluation of surgical complications such as infection or pseudoaneurysm formation. Helical CT scanning can be used to identify airway compression that is caused by a large ascending aorta that is associated with tetralogy of Fallot.
Magnetic Resonance Imaging
Findings
Spin-echo MRI can be used to identify the morphologic abnormalities of tetralogy of Fallot (see Image 4), which are as follows: right ventricular outflow tract obstruction, ventricular septal defect, right ventricular hypertrophy, and an overriding aorta.
The confluence, presence, and size of the branch pulmonary arteries can be identified (see Image 5). MRI measurements of the size of the pulmonary and branch pulmonary arteries are as accurate as angiographic measurements, and they can be used to calculate the McGoon ratio and the Nakata index (see Anatomy, Branch pulmonary arteries).
Postoperative evaluation of pulmonary artery stenoses is better with MRI than with echocardiography. Cine imaging can be used to identify pulmonary stenosis or regurgitation, which is depicted as flow voids. Right ventricular enlargement is best quantified with MRI. Flow-analysis quantification of pulmonary regurgitation is unique to MRI. Although gradients can be measured with echocardiography, only MR flow analysis enables the accurate cross-sectional measurement of flow.
Degree of Confidence
Experienced operators are required for a high-quality MRI evaluation. If properly performed, MRI can replace preoperative angiography, which is more invasive.
False Positives/Negatives
Oblique imaging with thin sections may be necessary to verify pulmonary arterial confluence and identify hypoplastic pulmonary arteries in neonates and infants. Flow analysis of pulmonary regurgitation is susceptible to aliasing if the velocity encoding is too low.
Ultrasonography
Findings
Echocardiography is the primary imaging method for examining a child in whom tetralogy of Fallot is suspected. Intracardiac anomalies, including pulmonary infundibular and valvular stenosis and the position of the aortic root overriding the ventricular septal defect, are identified with 2-dimensional echocardiography. The origins of the coronary arteries can also be identified.
Doppler ultrasonographic examination of the pulmonary outflow tract can be used to measure the velocity gradient in the right ventricular outflow tract and to differentiate severe stenosis from atresia. Continuity of the branch pulmonary artery with the main pulmonary artery can be identified, and the size of the branch pulmonary arteries can be measured. The initial placement of palliative shunts is likely in children who have small branch pulmonary arteries in order to allow the pulmonary arteries to grow before corrective surgery. The full length of the shunts may not be visible; however, Doppler ultrasonography can be used to verify shunt patency, even when the entire length of the shunt cannot be imaged.
Degree of Confidence
Intracardiac and central pulmonary artery abnormalities are identified in the presurgical patient with a high degree of confidence. Ultrasonographic windows in the older child and young adult limit the usefulness of echocardiography in postoperative follow-up imaging. Postoperative fibrosis in the mediastinum also reduces the effectiveness of echocardiography.
False Positives/Negatives
The pulmonary arteries and systemic-pulmonary arterial shunts are difficult to evaluate after surgery in children. Pulmonary regurgitation, a common complication of corrective surgery, is better evaluated with MRI.
Angiography
Findings
Angiography is the traditional criterion standard and best modality for the evaluation of the pulmonary and coronary arterial morphology, as well as the morphology of the systemic collateral arteries. The branch pulmonary arteries have a characteristic seagull appearance (see Image 6). Pulmonary arterial measurements for the calculation of the McGoon ratio and Nakata index are critical to surgical planning. An aortic root injection is used to evaluate the position and number of coronary arteries.
Degree of Confidence
Catheter studies remain the criterion standard for blood pressure measurements and morphologic imaging of the coronary and branch pulmonary arteries. Noninvasive techniques are preferred when they are available so that vascular access for potential future interventional procedures is preserved.
False Positives/Negatives
Unlike inherently tomographic studies such as echocardiography, MRI, and CT scanning, angiography may be limited by overlapping structures that may obscure other structures, despite the use of multiple planes.
More on Tetralogy of Fallot |
| Overview: Tetralogy of Fallot |
 Imaging: Tetralogy of Fallot |
| Follow-up: Tetralogy of Fallot |
| Multimedia: Tetralogy of Fallot |
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References
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Further Reading
Keywords
cyanotic congenital heart disease, right ventricular outflow tract obstruction, malalignment ventricular septal defect, overriding aorta, right ventricular hypertrophy
