Introduction

CT is increasingly used in patients with congenital heart disease. Faster gantry rotation speed and longer z-axis coverage of multi-slice CT as well as ECG-synchronized data acquisition are essential imaging techniques that contribute to this increased clinical utility of cardiac CT [1]. Imaging findings on cardiac CT in patients with congenital heart disease reported in the literature have usually focused on morphological features of cardiovascular structures [2–10] and airways [11–15] but not greatly on haemodynamic features of cardiovascular defects. CT findings of cardiothoracic shunt lesions have been rarely described in adults [16, 17]. Various haemodynamic features on cardiac CT may be influenced by intravenous injection methods as well as patient factors such as cardiac output, central blood volume and body weight. Even though we try to achieve uniform cardiovascular enhancement by using various intravenous injection methods [1], the result may not be always predictable particularly in patients with congenital heart disease. Such findings on cardiac CT may be overlooked unless we are familiar with various haemodynamic findings caused by various cardiovascular defects. This article comprehensively describes the normal haemodynamic phenomena of cardiovascular structures and various abnormal haemodynamic findings with their mechanisms and clinical significance on cardiac CT in patients with congenital heart disease. This pictorial review was based on approximately 1,700 cardiac multi-slice CT examinations performed at our institution over a 10 year period.

Normal haemodynamic phenomena

Normally, contrast agent administered intravenously through an arm vein using a power injector may be regurgitated into the tributaries of the thoracic veins (Fig. 1). Likewise, a mild degree of regurgitation of contrast agent from the right atrium to the hepatic veins may be observed when a leg vein is used for the intravenous administration of contrast agent (Fig. 1). As blood flow from the superior vena cava is normally mixed with that from the inferior vena cava in the right cardiac chambers and only one extremity vein is usually used for contrast enhancement of cardiac CT, gradual mixing between opacified blood and nonopacified blood is commonly seen in the right cardiac chambers (Fig. 1). A right-to-left shunt through the patent foramen ovale (PFO), one of the fetal circulations, may be seen in newborns and young infants before it closes within the first 3 months of life (Fig. 1). Because of the oblique shape of the PFO, the contrast agent jet from the right atrium to the left atrium may be seen only when contrast agent is intravenously injected via a leg vein.

Fig. 1
figure 1

Normal haemodynamic phenomena on cardiac CT. a Axial image shows regurgitation of contrast agent into the tributaries of the thoracic veins (arrows) in the posterior portion of the left chest wall. A right-sided aortic arch is noted. b Axial CT image shows regurgitated contrast agent in the hepatic veins when a leg vein is used for the intravenous administration of contrast agent. This mild degree of normal regurgitation of contrast agent is more pronounced with the use of a power injector. c Oblique coronal CT image shows a column (arrows) of opacified blood from the superior vena cava (S) in the superior half of the right atrium. d Axial image shows the right atrial appendage (RAA) is filled with opacified blood from the superior vena cava. e Conversely, on coronal oblique view the inferior half of the right atrium is filled with opacified blood (arrows) from the inferior vena cava that is gradually mixed with unopacified blood from the superior vena cava (S) in the right ventricle. f Coronal oblique view shows a shunt (arrows) from the right atrium (RA) to the left atrium through a PFO (asterisk) that is commonly seen during the first 3 months of life when contrast agent is injected through a leg vein. RV, right ventricle; LA, left atrium; LAA, left atrial appendage; A, aorta; P, pulmonary trunk

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Abnormal haemodynamic findings associated with congenital heart disease

Shunt via atrial septal defect or PFO

An atrial septal defect (ASD) is classified as a secundum defect, a primum defect or a sinus venosus defect depending on the location of a hole or holes in the interatrial septum. The secundum defect is the most common type of ASD and the sinus venosus defect is frequently associated with partial anomalous pulmonary venous connection. Depending on the pressure difference between the left atrium and the right atrium throughout the cardiac cycle, an ASD may cause a left-to-right, right-to-left, or bidirectional shunt. A shunt via the ASD can be seen as a jet on cardiac CT if a substantial interatrial difference in contrast enhancement is present (Fig. 2). The jet via an ASD (Fig. 2) tends to be more perpendicular to the interatrial septum than that via a PFO (Fig. 3), which is mainly caused by the different shapes of these two interatrial defects [18]. It should be noted that a large amount of right-to-left shunt via an ASD results in poor opacification of the right ventricle and pulmonary vessels on cardiac CT (Fig. 2). In a study, gradual pulmonary artery enhancement on bolus-tracking CT images was suggested to be a sign of septal defects [19]. However, such assessment using mostly 23-phase images with 120 kVp and 20 mAs would result in substantial radiation exposure, at least 2- or 3-times higher than that of low-dose cardiac CT itself. Therefore, it is impractical and is not recommended, particularly for children, even though the haemodynamic CT finding may suggest the presence of septal defects.

Fig. 2
figure 2

Shunt via an ASD. a Four-chamber CT image shows a sharply delineated jet (arrows) of less opacified blood from the left atrium to the right atrium through a small ASD. b In contrast, a left-to-right shunt through a larger ASD (asterisk) is less well-defined on four-chamber CT image. c, d A hyperdense jet (arrows) from the left atrium to the right atrium through an ASD is seen on four-chamber (c) and short-axis (d) CT images acquired at later phases. e Short-axis CT image shows a right-to-left shunt via an ASD (asterisk). f The intravenously injected contrast agent preferentially fills the left ventricle due to a considerable right-to-left shunt (arrow) through a large ASD on axial CT image acquired during bolus tracking. Notably, the severely enlarged right ventricle and smaller left atrium are almost completely unopacified, while the descending thoracic aorta (A) shows a high degree of contrast enhancement. RA, right atrium; LV, left ventricle

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Fig. 3
figure 3

Shunt via a PFO. a In an 8-year-old boy with a moderate degree of tricuspid regurgitation, short-axis CT image shows a narrow contrast jet (arrow) from the left atrium to the enlarged right atrium through a PFO. b Short-axis CT image shows a narrow contrast jet (arrow) from the right atrium to the left atrium through a PFO. The pressures of the right cardiac chambers were higher than the left cardiac chambers in a 4-year-old girl with pulmonary atresia, VSD and major aortopulmonary collateral arteries. A shunt via a PFO beyond the first 3 months of life is regarded pathological

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As in ASD, PFO may frequently cause interatrial shunt, seen in 25–35% of the general population based on autopsy findings. PFO may cause ischaemic stroke or other types of paradoxical embolism, systemic arterial desaturation, migraine and decompression sickness. In addition PFO and, less frequently, ASD may lead to poor pulmonary arterial enhancement on CT performed for the diagnosis of pulmonary embolism during deep inspiration [20]. PFO with a left-to-right shunt is frequently associated with shorter tunnel length and septal aneurysm [21]. A right-to-left shunt via a PFO may occur transiently during Valsalva maneuver or persistently in congenital heart diseases with elevated right atrial pressures (Fig. 3).

Shunt via ventricular septal defect

In a commonly used classification system after modifying the originally proposed system [22], a ventricular septal defect (VSD) is classified as perimembranous, juxta-tricuspid and nonperimembranous, doubly committed juxta-arterial, or a muscular defect (Fig. 4). The perimembranous defect is the most common type involving the membranous part of the interventricular septum with variable extension of the adjacent muscular septum, e.g. inlet extension, trabecular extension, outlet extension, or a combination of the above. According to the direction of a left-to-right shunt via a perimembranous VSD across the tricuspid valve, its outlet or inlet extension can be distinguished from the VSD involving the membranous part only on short-axis CT images (Fig. 4). A few cases showing a left-to-right shunt across a VSD on CT were previously reported [23, 24].

Fig. 4
figure 4

Shunt via a VSD. a Diagram superimposed on coronal oblique CT image shows a modified classification of a VSD. b On short-axis image, a left-to-right shunt through a perimembranous VSD with outlet extension (short arrows) runs along the superior portion of the tricuspid annulus in a 47-year-old woman with TOF. A shunt via a defect with inlet extension (arrowheads) runs along the inferior portion of the tricuspid annulus. On the other hand, a shunt via a defect involving the membranous part only (long arrow) runs across the tricuspid valve. c Coronal oblique CT in a 7-month-old girl with pulmonary valve atresia and VSD shows that deoxygenated blood from the right ventricle and oxygenated blood from the left ventricle make separate flow columns (arrowheads) in the overriding aorta. d In a 1-month-old girl with complete transposition of the great arteries, axial CT image acquired during bolus tracking shows a right-to-left shunt (arrows) through a muscular VSD (asterisk). e Oblique coronal CT image shows that a shunt (arrows) from the left ventricle to the aorta arising from the right ventricle is seen through a large subaortic VSD (asterisk) in a 24-year-old man with double outlet right ventricle, subaortic VSD, bilateral superior vena cavae and dextrocardia. Of note, the right superior vena cava (S) is anomalously connected to the left atrium, which contributes to systemic arterial desaturation of the patient. PM, perimembranous VSD; OE, outlet extension; TE, trabecular extension; IE, inlet extension; JT, juxtatricuspid and nonperimembranous VSD; JA, doubly committed juxta-arterial VSD; M, muscular VSD

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In tetralogy of Fallot (TOF), the most common cyanotic congenital heart disease, the anterosuperior deviation of the outlet septum leads to right ventricular outflow obstruction, a large anterior malalignment type of VSD and overriding of the aorta. As a result of this right ventricular outflow obstruction, a right-to-left shunt through the VSD and a column of deoxygenated blood in the ascending aorta arising from the right ventricle are commonly seen (Fig. 4). It is conceivable that the degree of cyanosis is paralleled by the size of this right-to-left shunt. A right-to-left shunt via a VSD is also caused by a right ventricle with supra-systemic pressure including complete transposition of the great arteries (TGA) (Fig. 4).

In double outlet right ventricle (DORV), the shunt direction through a VSD is an important physiological determinant of this anomaly (Fig. 4). Accordingly, DORV may be physiologically equivalent to VSD, TOF or complete TGA.

Shunt via an atrioventricular septal defect

The atrioventricular septal defect (AVSD) involving the atrioventricular septum is divided into partial and complete forms depending on a ventricular inlet defect in addition to a primum ASD. A shunt through a small AVSD may be seen on cardiac CT (Fig. 5). However, a large shunt through an AVSD (that is usual for this defect) leads to rapid intracardiac mixing of the blood, which results in a very low chance of the contrast jet via the AVSD being visible on cardiac CT (Fig. 5). For the same reason, a larger quantity of contrast agent is needed to achieve sufficient cardiovascular enhancement at CT and patients with AVSD are vulnerable to pulmonary vascular disease.

Fig. 5
figure 5

Shunt via an AVSD. a Four-chamber CT image shows right-to-left shunts via an AVSD (arrows) and a secundum ASD (arrowheads) in a 1-month-old boy. b Four-chamber CT image demonstrates extensive mixing between more opacified blood from the left atrium and less opacified blood from the right atrium via a huge AVSD in a 1-year-old boy

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Shunt via patent ductus arteriosus

A shunt via a patent ductus arteriosus (PDA) may be seen on cardiac CT when there is a substantial difference in vascular enhancement between the aorta and the pulmonary artery (Fig. 6) [25]. In addition, the jet through a PDA indicates a major direction of the shunt flow, i.e. left-to-right or right-to-left. Of note, the course of a PDA may reflect the haemodynamics of the fetal circulation in utero and thus may imply the pathophysiology of underlying cardiac defects. The horizontal course of a PDA connecting to the distal aortic arch is seen in left-side obstructive lesions as well as uncomplicated PDA, whereas the vertical course of a PDA connecting to the transverse aortic arch is seen in right ventricular outflow obstruction [2].

Fig. 6
figure 6

Shunt via a PDA in a 5-day-old boy with coarctation of the aorta. a, b Oblique sagittal CT images show a PDA (short arrows), tubular arch hypoplasia (arrowheads) and a posterior shelf (long arrow). The degree of contrast enhancement of the pulmonary arteries, PDA and descending aorta is higher than that of the ascending aorta and aortic arch, which indicates a right-to-left shunt via the PDA. PA, pulmonary artery; DA, descending aorta

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Shunt via anomalous pulmonary venous connections

In total or partial pulmonary venous connection, the draining site such as the right atrium, coronary sinus, systemic vein, or portal vein is generally enlarged on cardiac CT. A shunt by these anomalies may be demonstrated as a difference in contrast enhancement between the pulmonary vein and the draining site.

Shunt via fenestration of extracardiac Fontan conduit

In the fenestrated Fontan operation, a fenestration is created between a Fontan conduit and the right atrium and acts as a “pop-off” valve to prevent rapid volume overload to the lungs. A jet through this fenestration may be seen on cardiac CT and indicates that it is working normally (Fig. 7).

Fig. 7
figure 7

Shunt via a fenestration surgically created to prevent rapid volume overload to the lungs, of an extracardiac Fontan conduit in a 24-year-old man with functional single ventricle. Oblique axial (a) and oblique coronal (b) CT images show a right-to-left shunt via the fenestration (arrow) toward the left atrium (LA). Layering of undiluted contrast agent (arrowhead) is noted in the dependent portion of the conduit. Temporary pacing wires are also noted

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Cardiac valvular abnormalities

Valvular thickening, doming and calcification are CT findings of cardiac valvular stenosis. Stenosis of the semilunar valve may cause poststenotic dilatation of the corresponding great artery and hypertrophy of the corresponding ventricle. The severity of valvular stenosis or regurgitation is usually assessed by means of valvular area planimetry [26]. A jet caused by valvular stenosis or regurgitation is very rarely seen on cardiac CT (Fig. 8). Valvular atresia may be diagnosed on cardiac CT when there is a considerable difference in contrast enhancement between the great artery and ventricular outflow tract (Fig. 8) or between the corresponding atrium and ventricle.

Fig. 8
figure 8

Cardiac valvular abnormalities. a Oblique coronal CT image shows a contrast jet (arrows) caused by pulmonary regurgitation in a 24-year-old man with double-inlet left ventricle, DORV and VSD who underwent pulsatile bidirectional cavopulmonary shunt. b Oblique axial CT image demonstrates that the degree of contrast enhancement of the ascending aorta (A) is comparable to that of the right ventricle and is higher than that of the smaller left atrium and left ventricle, suggesting no communication between the aorta and the LV, i.e. aortic valve atresia

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Poor enhancement caused by obstructive cardiovascular lesions

Cardiovascular structures may show a low level of contrast enhancement in proximal or distal obstructive cardiovascular lesions such as pulmonary artery stenosis (Fig. 9), pulmonary artery thromboembolism (Fig. 10), hypoxia-induced vasoconstriction of pulmonary arteries secondary to air trapping (Fig. 11) and obstructive cor triatriatum sinister (Fig. 12). This poor contrast enhancement is caused by reduced blood supply as well as delayed transit time. A study reported a decrease in size of the left atrium and pulmonary veins reflecting decreased pulmonary venous return caused by massive pulmonary embolism [27]. However, decreased contrast enhancement of these structures has not been described. Hypoxic pulmonary vasoconstriction occurs in regions with impaired ventilation, leading to reduction of regional pulmonary blood flow [28]. In cor triatriatum sinister, differential opacification of the two left atrial chambers has been described on contrast echocardiography [29] as well as contrast-enhanced ECG-triggered electron beam CT [30].

Fig. 9
figure 9

Poor contrast enhancement of the pulmonary vein and left atrium caused by pulmonary artery stenosis in a 4-year-old girl. a Four-chamber CT image shows poor enhancement (arrows) in the right-half of the left atrium and right pulmonary vein. b Oblique axial CT image shows a severe stenosis (arrow) of the right pulmonary artery compressed by the enlarged ascending aorta (A)

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Fig. 10
figure 10

Poor contrast enhancement of the pulmonary vein caused by pulmonary thromboembolism in a 17-year-old adolescent after total correction of TOF and pulmonary valve replacement. a Oblique coronal CT image shows poor enhancement (arrows) of the slightly smaller right lower pulmonary vein. b Oblique coronal CT image shows a total occlusion (arrowheads) of the descending branch of the right pulmonary artery (RPA) causing poor enhancement of the right lower pulmonary vein (arrows)

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Fig. 11
figure 11

Poor contrast enhancement of the pulmonary vein caused by hypoxic pulmonary vasoconstriction in a 6-year-old girl after total correction of TOF. a Oblique coronal CT image shows poor enhancement (arrows) of the right lower pulmonary vein. The right pulmonary artery (RPA) is unobstructed and slightly enlarged instead. b Oblique coronal minimum-intensity projection CT image with a lung window setting shows complete occlusion (arrowheads) of the right main bronchus causing air trapping of the right lower lobe (arrows) responsible for this hypoxic pulmonary vasoconstriction

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Fig. 12
figure 12

Poor contrast enhancement of the posterior chamber of the left atrium caused by obstructive cor triatriatum sinister in a 2-day-old boy. Four-chamber (a) and volume-rendered (b) CT images show a low level of contrast enhancement of the posterior chamber (asterisk) of cor triatriatum sinister. The pulmonary veins are connected to this posterior chamber, while the anterior chamber of the left atrium is connected to the left atrial appendage (LAA) and the mitral valve. The anterior chamber and the left ventricle show a high level of contrast enhancement due to a right-to-left shunt (arrows) via an unrestrictive ASD on short-axis CT image (c). The pulmonary veins and the posterior chamber of the left atrium are not dilated because of the presence of a descending vein between this chamber and the portal vein (not shown)

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Systemic arterial collaterals to the lung

Systemic arterial collaterals to the lung frequently develop in patients with cyanotic congenital heart disease, particularly in the lung segments poorly supplied by pulmonary arteries. A study [31] demonstrated that CT showed a high detection rate (77%) of these collaterals and an excellent correlation with conventional catheter angiography in the measurement of their diameters in children with cyanotic congenital heart disease. However, accurate discrimination between normal and abnormal systemic arteries to the lung may not be achieved on the basis of visibility of these arteries on current CT images with submillimetre spatial resolution. In fact, normal bronchial arteries are commonly seen on cardiac CT. Nevertheless, most CT studies have used size criteria, e.g. 1.5 mm, to assess enlarged bronchial and nonbronchial systemic arteries [32]. Intense enhancement of a pulmonary vein branch may occasionally be seen in the lung portion supplied by systemic arterial collaterals on cardiac CT (Fig. 13) and this haemodynamic CT finding would be a more confident clue than just the arterial size to the presence of abnormal systemic arterial collaterals.

Fig. 13
figure 13

Systemic arterial collateral to the lung in a 6-month-old boy with hypoplastic left heart syndrome after bidirectional cavopulmonary connection and arch repair. a Oblique coronal CT image shows unusually high contrast enhancement in the upper branches of the right pulmonary vein (arrows), comparable to that of the aorta. The superior cavopulmonary connection (arrowheads) is noted. b Coronal CT image shows the enlarged bronchial artery (arrows) supplying the right upper lobe

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Intrathoracic venovenous collaterals

A systemic to pulmonary venous shunt in superior vena cava obstruction has been demonstrated on CT in a few case reports [33–35]. This right-to-left shunt also occasionally develops in patients after the Fontan operation (Fig. 14). Coil embolization is necessary for a larger shunt lesion leading to arterial desaturation or symptoms.

Fig. 14
figure 14

Systemic to pulmonary venous collaterals. Oblique coronal (a) and coronal full-slab maximum-intensity projection (b) CT images show a small collateral vein (arrows) filled with undiluted contrast agent between the inferior vena cava (I) and the right lower pulmonary vein in an 18-year-old man with functional single ventricle after Fontan operation

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The azygos-hemiazygos venous system is an important collateral pathway in venous obstruction or elevated systemic venous pressures [36]. The flow direction of this collateral pathway can be determined at CT on the basis of a gradient in contrast enhancement in the azygos vein and the superior vena cava as well as intravenous injection site of contrast agent (Fig. 15). The azygos vein is usually ligated at the bidirectional cavopulmonary connection to prevent the development of systemic venous collateral pathway to the inferior vena cava via the azygos-hemiazygos venous system.

Fig. 15
figure 15

The azygos vein as an intrathoracic venous collateral pathway. Coronal (a) and sagittal (b) CT images demonstrate the development of collateral veins in the right upper thorax connecting to the dilated azygos vein (asterisks) with craniocaudal flow direction in a 1-year-old boy with pulmonary atresia and intact ventricular septum after bidirectional cavopulmonary connection. Of note, the ligated azygos venous arch (arrow) is well preserved

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Blood flow in superior or total cavopulmonary connections

Preferential pulmonary blood flow from each vena cava was demonstrated on contrast-enhanced time-resolved MR angiography in patients after superior or total cavopulmonary connection [37]. Similarly, CT may show this preferential pulmonary blood flow in such patients (Fig. 16). A disorganized attenuation pattern due to incomplete mixing of contrast agent is commonly seen in the extracardiac conduit of Fontan pathway on CT (Fig. 16). In addition, layering of iodinated contrast agent is often seen in the dependent portion of this extracardiac conduit (Fig. 7).

Fig. 16
figure 16

Blood flow in superior or total cavopulmonary connections. a Oblique coronal CT image shows preferential blood flow from the right superior vena cava (short arrow) to the right pulmonary artery (long arrow) in a 9-year-old boy with functional single ventricle after bilateral bidirectional cavopulmonary connections. b, c Oblique coronal CT images show preferential blood flow from the right superior and inferior vena cavae (short arrows) to the right pulmonary artery (long arrows) in a 5-year-old boy with functional single ventricle after bilateral bidirectional cavopulmonary connections. The faintly opacified left superior vena cava (asterisks) shows preferential blood flow to the left pulmonary artery, which contributes to a lower level of contrast enhancement of the left pulmonary circulation. d Oblique coronal CT image in an 11-year-old girl with functional single ventricle after the extracardiac conduit type of Fontan operation reveals a complex pattern of contrast enhancement in the conduit (arrows) that is typically seen after this type of surgery and makes the accurate assessment of the Fontan pathway patency difficult. In contrast, other cardiovascular structures including the right pulmonary artery (arrowheads), the aorta and the left ventricle show uniform contrast enhancement

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Contrast regurgitation into hepatic veins

In a study, tricuspid regurgitation, pulmonary hypertension and right ventricular systolic dysfunction were independent predictors of retrograde inferior vena cava or hepatic vein opacification [38]. In contrast to normal contrast medium regurgitation into hepatic veins (Fig. 1), abnormal retrograde opacification extends to more peripheral or nondependent portions of hepatic veins (Fig. 17). This retrograde opacification is almost always combined with various degrees of dilatation of the inferior vena cava and hepatic veins.

Fig. 17
figure 17

Contrast regurgitation into the hepatic veins. Oblique axial thin-slab maximum-intensity projection CT image shows substantial contrast regurgitation into the dilated hepatic veins (arrows) in a 10-year-old girl with tricuspid atresia after the atriopulmonary connection type of Fontan operation. The inferior vena cava (I) is also dilated

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Septal configuration

Ventricular septal motion has been evaluated with cine images using cardiac MRI and multidetector CT [39, 40]. Cardiac septal configuration is determined by a pressure difference between the corresponding right and left cardiac chambers [27]. In a study using cardiac MRI [41], right ventricular systolic pressure could be estimated from the left ventricular septal-to-free wall curvature ratio measured at cardiac MRI. Therefore, the ventricular septum is flat or bows to the left when right ventricular pressures are equivalent to or higher than left ventricular pressures on cardiac CT acquired during the systolic phase (Fig. 18). Likewise, leftward bowing of the interatrial septum suggests elevated right atrial pressures above that of the left atrium (Fig. 18).

Fig. 18
figure 18

Septal configuration indicating elevated pressures of the right cardiac chambers. a Short-axis CT image shows ventricular septal bowing (asterisks) toward the left ventricle at end-systole. The right ventricle is severely hypertrophic. b Four-chamber CT image demonstrates leftward bowing (arrows) of the interatrial septum suggesting the elevated pressure in the right atrium is higher than that in the left atrium. A ventricular inlet defect (asterisk) is noted

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Conclusion

Various haemodynamic findings on cardiac CT illustrated in this article help us to understand normal and abnormal flow dynamics in patients with congenital heart disease and enrich our interpretation of various cardiac defects.