Truncus arteriosus is a congenital heart disease characterized by a single great artery that leaves the base of the heart, giving rise to the coronary, pulmonary, and systemic arteries. Chest radiography usually is the initial investigation performed in the neonatal period. Cardiomegaly is frequently present at birth. Chest radiographic findings usually reveal the increase in pulmonary arterial blood flow manifesting as increased pulmonary vascular markings. The combination of a right-sided aortic arch, cardiomegaly, and increased pulmonary vascularity strongly suggests truncus arteriosus; however, further diagnostic investigations are always needed to confirm the diagnosis. 
Echocardiography is the modality of choice for diagnosing truncus arteriosus. The other investigations noted in this section are complementary. Echocardiography has markedly changed the evaluation, diagnosis, and management of congenital heart disease. Echocardiographic findings, usually diagnostic, demonstrate the origin and configuration of the pulmonary arteries, the ventricular septal defect, the truncus arteriosus, and the aortic arch, as well as the status of the truncal valve. Complete echocardiographic examination can be performed to establish goal-directed treatment, to almost eliminate the need for confirmatory cardiac catheterization, and to provide a cost-effective, feasible tool for close follow-up observation of patients after surgery. [2, 3, 4]
Computed tomography (CT) scanning is another imaging modality that can be used to evaluate infants with complex congenital heart disease. [5, 6, 7, 8] Standard CT scans are useful for evaluating suggested anomalies of the aortic arch, a double aortic arch, and retroesophageal vascular structures indicating an anomalous origin of the subclavian artery. Electron-beam CT (EBCT) scanning accurately defines systemic and pulmonary venous connections, demonstrating other anomalies associated with truncus arteriosus, including abnormalities of the pulmonary artery.
Contrast-enhanced EBCT and magnetic resonance imaging (MRI) are the noninvasive procedures of choice for the diagnosis and exclusion of anomalies of the origin and course of the coronary arteries. Standard CT, EBCT, and MRI are useful, noninvasive techniques for visualizing the cardiovascular anatomy in patients with congenital heart disease. EBCT and MRI can also enable assessment of cardiovascular function. MRI appears to be the most suitable of these techniques for assessing congenital heart disease. [9, 10]
By comparing angiographic and 2-dimensional (2D) echocardiograms, MRI enables accurate anatomic diagnosis of complex congenital heart diseases. In some instances, MRI can replace the need for invasive cardiac catheterization or reduce the number of catheterizations required in the care of patients with complex congenital heart disease.
MRI of complex congenital heart diseases is necessary for preoperative assessment in adults and in infants, and the results influence surgical planning by providing information about the anatomic topography of the vascular malformation and its relation to the bronchial system. MRI is reliable in classifying truncus arteriosus by showing the anatomy of the pulmonary artery. The size of the pulmonary artery and its branches can be measured in the transverse, coronal, and sagittal planes. 
Cardiac catheterization is usually performed to confirm anatomic details and to obtain physiologic data regarding pulmonary vasculature and to accurately calculate pulmonary vascular resistance.  Cardiac catheterization is important in helping make decisions regarding the time and type of surgery (palliative vs corrective)
The role of nuclear imaging in the diagnosis of truncus arteriosus is not well established. However, all of the other modalities, including cardiac angiography, are the standard of care for diagnosing this complex congenital cardiac disease with all of its associated anomalies.
Although imaging modalities have improved tremendously, some limitations remain. One fairly common pitfall with imaging techniques is the suggestion of the presence of a partially formed aorticopulmonary septum and, therefore, the presence of a main pulmonary segment. At surgery, however, the branch pulmonary artery orifices may be found adjacent to one another in the left posterolateral aspect of the common arterial trunk. The surgeon may be unable to excise a main pulmonary artery segment from the common arterial trunk, even when the segment was depicted on images, because it may have no actual length. 
The images below demonstrate the radiographic characteristics of truncus arteriosus.
In truncus arteriosus, 4 anatomic types are recognized on the basis of the anatomic origin of the pulmonary arteries, according to Collette and Edwards.  Types I-III are demonstrated in the image below.
In type I, a common form, a short pulmonary trunk arises from the truncus arteriosus, giving rise to both pulmonary arteries.
In type II, each pulmonary artery arises separate from, but close to, the other, from the posterior aspect of the truncus.
In type III, each pulmonary artery arises from the lateral aspect of the truncus.
Type IV, pseudotruncus (seen in the image below), is currently considered to represent a form of pulmonary atresia with ventricular septal defect, that is, a severe form of tetralogy of Fallot, rather than truncus arteriosus. 
In 1965, Van Praagh introduced a new classification with 4 subtypes, which are demonstrated in the images below.
Type 1 is similar to the type I described by Collette and Edwards. 
Type 2 is made up mostly of types II and III in the Collette and Edwards classification, in which the proximity of the origin of the pulmonary arteries is not specified.
In type 3, 1 pulmonary artery branch does not arise from the common pulmonary trunk, instead originating from the ductus arteriosus or directly from the aorta.
In type 4, the aortic arch is hypoplastic or interrupted, and a large patent ductus arteriosus is present.
The Van Praagh classification specifies the presence (subtype A) or absence (subtype B) of ventricular septal defect. Each case is accordingly assigned a nomenclature that includes a letter and a number. 
Although both classifications have found wide application in clinical cardiology and cardiac surgery, each has limitations. Collette and Edwards type IV is probably a misnomer, because it describes a separate entity with different therapeutic and prognostic implications. In addition, cardiothoracic surgeons often refer to a type 1½, which is similar to type I but with a shallow aorticopulmonary segment. This fairly common entity is not included in either classification. 
The term hemitruncus has fallen out of use, but it refers to a rare anomaly in which 1 pulmonary artery branch, usually the right, arises from the ascending aorta just above the aortic sinuses, while the main pulmonary artery and the other pulmonary artery branch arise in their normal positions.
Chest radiography is usually the initial investigation performed in the neonatal period.
Cardiomegaly (seen in the image below) is frequently present at birth.
As pulmonary vascular resistance decreases, usually after the second or third day of life, the increase in pulmonary arterial blood flow is considerable and manifests as increased pulmonary vascular markings.
In 50% of patients, the left atrium enlarges to accommodate the increased pulmonary venous return. This enlargement is best identified beneath the left bronchus on a lateral image. In addition, volume overload of the left side of the heart results in dilatation of the left ventricle. Late, the pulmonary vascular pattern shows evidence of venous congestion due to left ventricular failure with cardiomegaly. Enlargement of the right ventricle and right atrium emerges with the development of congestive heart failure or when the right ventricle selectively receives regurgitant blood flow across the truncal valve. 
One third of patients have a right-sided aortic arch. A dilated truncal root (large aortic shadow) is not uncommon.
A superiorly located proximal left pulmonary artery, as seen in type I truncus arteriosus, can be identified on frontal chest radiographs in 10% of patients. The right hilum is elevated in 30% (waterfall or hilar comma sign). The hilar comma sign is especially evident on the opposite side of the aortic arch. 
In truncus arteriosus with an absent pulmonary artery,  the pulmonary vascular markings are diminished on the side of the absent pulmonary artery, which usually coincides with the side of the aortic arch. The result is a concave pulmonary segment that is best appreciated on the right anterior oblique view. This finding is seen in 50% of patients when separate pulmonary arterial branches arise directly from the truncus. 
In late survivors with high pulmonary vascular resistance, the lungs are oligemic, the main pulmonary artery and the right and left branches increase in prominence with pruning of the peripheral vascular tree, and the size of the left ventricle is almost normal unless clinically significant truncal valve regurgitation or stenosis occurs. 
The combination of right-sided aortic arch, cardiomegaly, and increased pulmonary vascularity strongly suggests truncus arteriosus; however, further diagnostic investigations are always needed to confirm the diagnosis. 
CT scanning is another imaging modality that can be used to evaluate the heart. Although CT has some disadvantages compared with MRI, such as a lower contrast resolution, an inability to image in multiple planes, the use of ionizing radiation, and often the use of an iodinated contrast agent, CT has advantages, such as a relatively fast imaging time and the ability to depict calcification.
Standard CT scans are useful for the evaluation of suggested anomalies of the aortic arch.  Contrast-enhanced CT is usually required to demonstrate the vascular tissue surrounding the trachea in the presence of a double aortic arch and for evaluating the retroesophageal vascular structure, which indicates an anomalous origin of the subclavian artery. The presence of 4 paratracheal vessels arranged symmetrically at the cervicothoracic junction suggest a double aortic arch. 
EBCT scans accurately define systemic and pulmonary venous connections and demonstrate atrial and ventricular septal defects. Normal and abnormal atrioventricular valves can be demonstrated by using EBCT. EBCT scans obtained at the base of the heart effectively show congenital anomalies of the arteries. 
EBCT effectively demonstrates other anomalies associated with truncus arteriosus, including abnormalities of the pulmonary artery (eg, congenital absence of the artery, as seen in the images below), peripheral coarctations, and hypoplasia. However, multiplanar MRI is the most effective technique for assessing pulmonary arterial anomalies. 
Contrast-enhanced EBCT and MRI are probably the noninvasive procedures of choice for the diagnosis and exclusion of anomalies of the origin and course of coronary arteries. These studies are especially important for showing a course in the ventricular septum or between the base of the aorta and the pulmonary artery when the left anterior descending coronary artery arises from the right coronary artery.
CT scanning plays an important supplementary role in the evaluation of patients with truncus arteriosus. Fast multisection spiral CT with high-quality 2D and 3-dimensional (3D) multiplanar reformatted images can be created to accurately and systematically evaluate the mediastinal vessels, cardiac chambers and ventricular-arterial connections, and coronary artery and valves in a step-by-step approach. 
Contrast-enhanced CT of anomalies of the mediastinal vessels has an accuracy of greater than 90%. CT demonstration of abnormalities of the great vessels, such as positional anomalies, atresias, and hypoplasias, is equivalent to angiocardiographic depiction, and CT is usually superior to 2D echocardiography.
Magnetic Resonance Imaging
MRI scans, based on proton-density and proton-relaxation dynamics, differ from images produced by x-rays, which are associated with the absorption of x-ray energy. The MRI dynamics vary according to the tissue under examination and reflect the tissue's physical and chemical properties.
MRI has evolved sufficiently to be recognized as a useful noninvasive method that is complementary to echocardiography in the evaluation of congenital heart disease. In some cases, MRI is superior to other imaging modalities, particularly in the evaluation of thoracic aortic anomalies and in defining the anatomy of central pulmonary arteries and characterizing the morphology of the truncus. (The MRI characteristics of truncus arteriosus are demonstrated in the images below.)
In addition, MRI is the procedure of choice for the postoperative follow-up observation of patients with congenital heart disease. Technologic advances now permit not only morphologic evaluation (with spin-echo and magnetic resonance angiographic techniques) but also the collection of functional and flow information (with fast cine gradient-echo and velocity-encoded sequences). As a result, pediatric cardiologists and cardiac surgeons recognize MRI as an essential technique for the preoperative and postoperative evaluation of some congenital heart diseases. 
The volumes of shunts, valvular function, and pressure gradients across valves and conduits can be estimated by using velocity-encoded cine MRI (velocity-flow mapping). However, the widespread application of echocardiography and Doppler techniques for many of the same purposes influence the clinical use of MRI's capabilities. As a consequence, the current clinical role of MRI is to supplement information acquired by using echocardiography.
Degree of confidence
MRI can be used with high diagnostic accuracy in the assessment of the morphologic and functional features of congenital heart disease.  MRI is particularly valuable for imaging the heart and great vessels, because flowing blood produces a unique signal. Therefore, no contrast medium is required to define the cardiac chambers and the lumina and locations of the great vessels. Cardiac evaluation requires either electrocardiogram (ECG)-gated MRI or cine MRI.
Several centers have reported effectiveness ratings of MRI for the evaluation of congenital heart disease in children and adults. [22, 24] In several studies in which the results of MRI were corroborated with angiography and/or 2D echocardiography, accurate anatomic diagnosis of anomalies was achieved with MRI in more than 90% of patients. [11, 25] Diagnostic accuracy of MRI exceeds 90% for abnormalities of atrioventricular connections.
After congenital heart disease is surgically corrected, patients must be monitored for extended periods, because morphologic and functional abnormalities may remain or develop. Therefore, a noninvasive imaging tool is mandatory for the timely detection of such abnormalities. Echocardiography may be hampered in these patients, because scar tissue and thoracic deformities limit the acoustic window. MRI is advantageous in the follow-up imaging of postsurgical patients, and, with the use of several techniques, morphologic and functional abnormalities can be evaluated and followed over time. [11, 24]
Echocardiography is one of the most frequently used techniques for diagnosing cardiovascular diseases.  This diagnostic modality facilitates comprehensive evaluation of the cardiovascular system. The standard echocardiographic views (long and short axes, 2- and 4-chamber views) are usually obtained in the parasternal, apical, and subcostal positions. Extended echocardiographic examination with more views can be performed as necessary.
Use of 2D echocardiography and Doppler echocardiography, including color-flow techniques, has greatly revolutionized the clinician's ability to accurately determine the cardiac anatomy and, in some patients, the hemodynamics in malformations of the conotruncus. [27, 28, 29] In some centers, if echocardiography reveals straightforward anatomy, the patient undergoes repair without the need for angiocardiography. 
Echocardiography demonstrates the origin and configuration of pulmonary arteries. It also helps in determining the relationship of the truncus to the left and right ventricles, identifying the ventricular septal defect, defining the morphology and functional derangement of the truncal valve, and assessing the physiologic consequences.  (See the image below.)
When visualized from the parasternal view, 3 defects with relatively similar echocardiographic appearances can be confused: truncus arteriosus, pulmonary atresia with ventricular septal defect, and tetralogy of Fallot. However, from a high parasternal short-axis view obtained by scanning superiorly from the semilunar valve, direct visualization of the origin of the pulmonary arteries usually helps in differentiating the 3 lesions and reveals the origin of the pulmonary artery arising directly from the truncal root in truncus arteriosus. 
Color-flow imaging further delineates the truncus and pulmonary arterial arrangements. Visualization of the truncal origin of the truncal artery or its branches is a major requirement for the echocardiographic diagnosis of truncus arteriosus. The truncal valve and its leaflet morphology can be interrogated by using the short-axis view. Color-flow imaging in the long-axis or 4-chamber view is assessed to establish the presence and degree of truncal valve regurgitation.
Continuous-wave Doppler helps to determine the presence and the degree of truncal valve stenosis.
Two-dimensional imaging provides information on biventricular function. 
High-resolution echocardiography can be used to diagnose truncus arteriosus in utero. This allows prenatal counseling and planning of pregnancy, delivery, and prenatal care. 
As a result of its superb visualization of cardiovascular structures, transesophageal echocardiography (TEE) is increasingly used in the diagnosis of congenital heart disease (CHD).  With a small probe, TEE can be performed in infants and young children. However, TEE is required less often in pediatric patients than in adults. General anesthesia is usually needed to perform TEE in children younger than 9 years. 
Proper technique and cognitive skills are required for the optimal application of echocardiography and the interpretation of its results. Echocardiography is an operator-dependent modality, even more so than other cardiovascular techniques.
Nuclear medicine studies have limited usefulness in the diagnosis and treatment of truncus arteriosus; at times, however, nuclear medicine comes into play. In truncus arteriosus, patients may have a discrepancy in the pulmonary blood pressures between the arteries because of ostial stenosis or previous pulmonary artery banding, and radioisotope lung scanning can help in determining the selective pulmonary arterial resistance.
This resistance cannot be separately estimated in each lung by using an angiocardiogram unless the blood flow to each lung is determined. Furthermore, when the pulmonary artery is absent, perfusion lung scanning can be done to confirm the absence of 1 of the pulmonary arteries and to ascertain the status of peripheral pulmonary arterial flow. 
Echocardiographic findings are usually diagnostic. The images demonstrate the origin and configuration of the pulmonary arteries, the ventricular septal defect, the truncus arteriosus, and the aortic arch, as well as the status of the truncal valve.
Cardiac catheterization (demonstrated in the images below) is usually performed to confirm anatomic details, to obtain physiologic data regarding the pulmonary vasculature, and to accurately calculate pulmonary vascular resistance.  Cardiac catheterization must be performed with the patient in stable condition, particularly in terms of the acid-base balance, if meaningful data are to be obtained.
Since the introduction of the flow-directed balloon catheters (Swan-Ganz catheters) that have greatly facilitated entrance to the pulmonary arteries into the truncal root to reach the aortic arch and descending aorta, all of the necessary information can be obtained by using the venous approach alone. However, the retrograde arterial approach is occasionally chosen, particularly if angiocardiography of the truncal root is used to assess insufficiency of the truncal valve. 
The cranially angulated angiocardiographic view facilitates visualization of the proximal pulmonary arteries.
In patients in whom previous operations were performed and in whom pericardial space entered, epicardial adhesions may obscure direct visualization of coronary arteries at the time of surgery. In these patients, preoperative selective coronary angiography should be performed if truncal root injection does not satisfactorily provide the answer. 
In patients with an absent pulmonary artery, a pulmonary wedge injection or selective injection in the systemic collateral arteries can be used to identify the pulmonary arterial tree on the affected side. 
In patients with interrupted aortic arch, identification of the exact site of interruption in relation to the aortic branches is important for planning the appropriate corrective procedure. 
The use of oximetry has allowed clinicians to identify cardiac shunts at different levels and to accurately calculate the severity of the shunt.
Cardiac catheterization is important in making decisions regarding the time and type of surgery (palliative vs corrective).
In patients beyond early infancy, pulmonary vascular resistance must be assessed accurately for the proper selection of corrective surgery.  This resistance can be calculated indirectly by dividing the mean driving pressure across the pulmonary bed by the total pulmonary flow index.
Patients with truncus arteriosus who have 2 pulmonary arteries and pulmonary arterial resistance greater than 8 units/m2 have operative and postoperative mortality risks higher than those of patients with low resistance. [33, 34] This difference is due to progression of pulmonary vascular obstructive disease with secondary severe pulmonary hypertension and right ventricular failure.
Angiocardiography tends to cause underestimation of the severity of truncal valve insufficiency. Because the pulmonary arteries arise from the truncal root, preferential runoff into the pulmonary circulation may mask insufficiency of the truncal valve, leading to difficulty in assessment.