The second step in interpretation of a chest radiograph

obtained in a new patient is to ascertain visceral situs, the

heart position, and the position of the aortic arch relative

to the trachea. For example, the radiographic report may

start with this sentence: “The radiograph shows situs solitus,

levocardia and a left aortic arch.”

■ Visceral Situs

Visceral situs refers to the pattern of arrangement of the

body organs relative to the midline. There are four types

of visceral situs: situs solitus, situs inversus, heterotaxy

with thoracic right isomerism, and heterotaxy with

thoracic left isomerism (Fig. 7.1).1–4 Visceral heterotaxy

has also been described as situs ambiguus, which means

uncertain situs. This term should be abandoned because

the situs is not uncertain but rather needs more words to

describe it precisely. The structures that are helpful in de-

termination of the situs include (1) the gastric air bubble,

(2) the larger lobe of the liver, (3) the tip of the spleen, (4)

the bronchi and pulmonary arteries, (5) the minor fissure,

and (6) the azygos vein. The basic concepts regarding vis-

ceral situs is discussed in detail in Chapter 2.

In situs solitus, the gastric air bubble is on the left side,

and the larger lobe of the liver is on the right side (Fig. 7.2).

The splenic tip can often be identified when the stomach

and adjacent bowel loops are filled with air. When the

bronchial air column can be traced, an asymmetric

bronchial branching pattern with a short right and a long

left main bronchus can be appreciated on the frontal radi-

ograph. Normally the left main bronchus is 1.5 to 2 times

longer than the right main bronchus. At the pulmonary

hilum, the left pulmonary artery is seen slightly higher

than the right pulmonary artery. The left pulmonary

artery is seen above the left upper lobe bronchus,

whereas the right pulmonary artery (in fact, its descend-

ing branch) is seen below the right upper lobe bronchus.

An asymmetric branching pattern of the bronchi and pul-

monary arteries is also evident on the lateral radiograph.

As the upper lobe bronchi normally have horizontal

courses, they are seen as two round lucencies arranged

superoinferiorly in the middle mediastinum. The right

upper lobe bronchus is located approximately one verte-

bral height above the left upper lobe bronchus. The right

Visceral Situs, Heart Position, and Aortic Arch Position7

68

Fig. 7.1 Four types of visceral situs. GB, gallbladder; IVC, inferior vena cava; PA, pulmonary artery; SVC, superior vena cava.

pulmonary artery lies anterior and slightly inferior to the

right upper lobe bronchus. The left pulmonary artery lies

posterior and superior to the left upper lobe bronchus. In

short, the right upper lobe bronchus lies higher than the

left, and the left pulmonary artery lies higher than and

posterior to the right pulmonary artery. The minor fissure

may cast a horizontal linear shadow over the midzone of

a trilobed right lung.

Situs inversus is characterized by a mirror-image

arrangement of the visceral organs (Fig. 7.3). The gastric

bubble is on the right and the larger lobe of the liver on

the left. However, it is important to understand that the

situs should not be called “situs inversus” solely because

of the inverted positions of the gastric bubble and hepatic

shadow. Plain film diagnosis of situs inversus can only be

made when an inverted bronchial and pulmonary arterial

branching pattern is clear. A lateral radiograph is helpful

in differentiating the lateralized situs (i.e., situs solitus

and inversus) from the symmetric situs (i.e., right iso-

merism or left isomerism). Normal hilar arrangement of

the upper lobe bronchi and branch pulmonary arteries on

the lateral view with a right-sided gastric bubble on the

frontal view is highly suggestive of situs inversus even if

the splenic shadow is not identified.

In right isomerism, the hepatic shadow usually extends

across the upper abdomen (Fig. 7.4). The gastric bubble

can be seen on either side but tends to be closer to the

midline. Interestingly, about 15% of patients with right

isomerism have a hiatal hernia (Fig. 7.5).5 Hiatal hernia

can be regarded as a manifestation of visceral heterotaxy.

Bilaterally short bronchi can be appreciated in a well-

taken frontal radiograph (Fig. 7.6). However, this feature

is often unclear in infants. On the lateral radiograph,

the end-on shadows of the upper lobe bronchi are seen at

the same or similar horizontal level, and the pulmonary

arterial shadow is seen mostly in front of the bronchi

(Fig. 7.4). The presence of bilateral minor fissures indicates

right isomerism (Fig. 7.7). In fact the diagnosis of right

isomerism can be entertained when the minor fissure is

present on the same side as the stomach even when a minor

fissure is identified only on one side (Fig. 7.4, left panel).

This is because the minor fissure and stomach cannot be

7 Visceral Situs, Heart Position, and Aortic Arch Position 69

Fig. 7.2 Situs solitus. Magnified views of

the hilar anatomy (lower panels). Asterisks on

the magnified frontal view mark the upper

lobe bronchi. The left main bronchus is twice

as long as the right main bronchus in this

individual (solid lines).

on the same side in either situs solitus or situs inversus,

and no minor fissure is present in either lung in left iso-

merism. However, a potential pitfall is the presence of an

accessory fissure, which can simulate a horizontal fissure

(Fig. 7.8).6,7 Right isomerism is associated with complex

congenital heart disease in almost all cases. We have not

seen any single case of right isomerism without complex

congenital heart disease. As pulmonary atresia or stenosis

is present in the majority of cases, the pulmonary vascu-

larity is usually reduced (Figs. 7.4, 7.5, 7.6, and 7.7).

70 II Systematic Approach to Chest Radiographs

Fig. 7.3 Situs inversus. The frontal view shows mirror-image arrange-

ment of the bronchial and pulmonary arterial trees. Asterisks mark the

upper lobe bronchi.The lateral view appears identical to that seen in

situs solitus (Fig. 7.2) but note the difference in labeling. This patient

had Kartagener’s syndrome with subtle pulmonary infiltrates in the

right lower lung.

Fig. 7.4 Heterotaxy with thoracic right isomerism. The hepatic silhou-

ette extends symmetrically across the upper abdomen. The stomach

with a nasogastric tube in place is on the right side, lying somewhat

close to the midline. A minor fissure is visible in the right lung. The

presence of a minor fissure on the same side of the stomach is conclusive

evidence of right isomerism. On the lateral view, the similar length of

the right and left main bronchi results in the upper lobe bronchi being

projected at a similar horizontal level. The right and left pulmonary

arteries are projected mostly in front of the upper lobe bronchi.

Left isomerism is also characterized by a symmetric

visceral arrangement. However, the hepatic shadow in

left isomerism is usually less symmetric than in right

isomerism, and it is not uncommon for the hepatic

shadow to be indistinguishable from that seen in situs

solitus or inversus (Fig. 7.9). The gastric bubble can be

on either side. The splenic shadow can be identified

along the greater curvature of the stomach. On the

frontal radiograph, bilaterally long bronchi can often be

appreciated (Fig. 7.9, right panels). On the lateral radi-

ograph, the upper lobe bronchi cast end-on shadows at

the same or similar horizontal level, and the pulmonary

arterial shadow is mostly seen behind the bronchi. The

presence of a minor fissure in either lung excludes the

7 Visceral Situs, Heart Position, and Aortic Arch Position 71

Fig. 7.5 Ectopic location of a part of the stomach in the thorax, which is often described as hiatal hernia, in a neonate with abdominal heterotaxy

and thoracic right isomerism.

Fig. 7.6 Symmetrically short main bronchi are shown in this frontal

radiograph in a child with right isomerism. This feature is not usually

as apparent in neonates and small children. Asterisks indicate the origins

of the upper lobar bronchi.

Fig. 7.7 Bilateral minor fissures (arrows) in a neonate with right

isomerism.

diagnosis of left isomerism. In approximately 80% of

cases, left isomerism is associated with interruption of

the inferior vena cava with azygos or hemiazygos venous

continuation.8 In this situation, the dilated azygos vein

can be identified in the area of the tracheobronchial

angle (Fig. 7.9, left panels). Absence of the inferior vena

caval shadow on the lateral radiograph has also been

described in patients with an interrupted inferior vena

cava. We find this feature unreliable. As interruption

involves the postrenal prehepatic segment of the infe-

rior vena cava, the posthepatic segment of the inferior

vena cava is present and collects the hepatic veins.

Therefore, a web-like shadow, albeit somewhat smaller

than normal, can be identified in the lateral radiograph.

In addition, the inferior vena caval shadow can be hid-

den by the posteriorly enlarged heart. In contrast to

right isomerism, left isomerism is not always associated

with congenital heart disease. Left isomerism may be

seen as an incidental finding in otherwise normal indi-

viduals or in patients with an arrhythmia or biliary

atresia.9

It should be remembered that there are exceptions,

although rare, to these rules, as discussed in Chapter 2.

72 II Systematic Approach to Chest Radiographs

Fig. 7.8 Accessory left anomalous fissure simulating the appearance

of bilateral minor fissures in an infant with situs solitus and hypoplas-

tic left heart syndrome. Right isomerism was suspected, but there

was a situs solitus arrangement of the other organs. An accessory left

minor fissure is not an uncommon normal variant.6,7

Fig. 7.9 Heterotaxy with left iso-

merism. Magnified views of the hilar

anatomy (lower panels). The hepatic

silhouette is not as asymmetric as it is

in situs solitus or situs inversus, but is

not as symmetric as it is in right iso-

merism. The splenic silhouette is

barely visible lateral to the stomach

(arrow). Symmetric bronchial branch-

ing can be appreciated on the frontal

radiograph. The dilated azygos vein is

seen above the right tracheobronchial

angle. The trachea is mildly bent to

the right by the left aortic arch. The

lateral view shows the upper lobe

bronchi at a similar horizontal level

because of a similar length of the

right and left main bronchi. The right

and left pulmonary arteries cast a

shadow mostly behind and above the

upper lobe bronchi. The inferior vena

caval shadow is not obvious on the

lateral view.

7 Visceral Situs, Heart Position, and Aortic Arch Position 73

Fig. 7.10 Three cardiac positions defined by the location of the main part of the heart relative to the midline.

Fig. 7.11 (A) Rightward displacement of the heart due to right lung

hypoplasia in a patient with absence of the right pulmonary artery in

the mediastinum and (B) in another patient with scimitar syndrome.

The displaced cardiac position in scimitar syndrome is often called

dextroposition, which is a confusing term.

■ Heart Position

The cardiac position is described by using the terms levo-

cardia, dextrocardia, and mesocardia (Fig. 7.10), which

describe where the main part of the heart is located

within the thorax. These terms are not used when the

heart is displaced to one or the other side due to extracar-

diac pathology, such as hypoplasia of a lung or deformity

of the thoracic cage (Fig. 7.11). When the heart is partially

or completely outside the thorax, it is called ectopia

cordis. Usually the cardiac apex points toward the side

where the main part of the heart is positioned. However,

there are exceptions where the cardiac position and the

base-apex orientation are not matched.

When defining the position of the heart on a chest

radiograph, it is important to check whether there is any

obliquity of radiographic projection. A subtle obliquity

may bring the cardiac silhouette to the other side of the

thorax (see Fig. 6.2). In general, the cardiac silhouette is

brought to the right when the radiograph is obtained in a

left anterior or right posterior oblique projection, whereas

it is brought to the left when the radiograph is obtained in

a right anterior or left posterior oblique projection.

Although there are many exceptions, levocardia tends

to have the right-sided right ventricle and the left-sided left

ventricle (so-called D-loop ventricles), whereas dextro-

cardia tends to have the left ventricle on the right and the

right ventricle on the left (so-called L-loop ventricles). This

tendency explains why congenitally corrected transposition

74 II Systematic Approach to Chest Radiographs

Fig. 7.12 Discordant visceral situs and heart position. Situs solitus and

dextrocardia (left panel). Situs inversus and levocardia (right panel).

Both cases had congenitally corrected transposition of the great arteries.

Ao, ascending aorta; LA, left atrium; LV, left ventricle; RA, right atrium;

RV, right ventricle.

Fig. 7.13 Aortic arch positions relative to the trachea. The trachea nor-

mally shows indentation on the side of the aortic arch. The most proxi-

mal part of the descending aorta (arrows) can be traced on the same

side of the aortic arch. Bilateral indentation is typically seen in double

aortic arch but other forms of vascular ring cannot be excluded.

of the great arteries is the most common diagnosis when

there is discordance between the heart position and vis-

ceral situs, such as dextrocardia with situs solitus (Fig. 7.12,left panel) and situs inversus with levocardia (Fig. 7.12,right panel).

■ Position of the Aortic Arch

The reference structures used for the determination of the

left- or right-sidedness of the aortic arch are the trachea and

main bronchi.10 The left aortic arch courses backward on the

left side of the trachea and above the left main bronchus,

whereas the right aortic arch courses backward on the right

side of the trachea and above the right main bronchus.

Chest radiographic determination of the aortic arch

position depends largely on the indentation of the tracheal

air column. A left aortic arch indents and bends the tra-

chea to the right side, whereas a right arch indents and

bends the trachea to the left (Fig. 7.13). In almost all cases,

the aortic arch continues with the proximal descending

aorta on the same side, which is usually identifiable as a

vertical stripe along the spine. The rare exceptions are the

so-called circumflex retroesophageal aortic arches in

which the distal segment of the aortic arch crosses the

midline behind the esophagus to connect to the descend-

ing aorta on the opposite side10,11 (Fig. 7.14). When there

Fig. 7.14 Circumflex retroesophageal right aortic arch (right aortic

arch with a left descending aorta). The proximal aortic arch is on the

right side, whereas the distal arch has a retroesophageal course to con-

nect to the descending aorta on the left side of the spine. The tracheal

indentation is on the right side with the descending aorta seen on the

left (arrows).

Fig. 7.15 Cervical left aortic arch in a 14-year-old patient. (A) Chest

radiograph and coronal magnetic resonance (MR) image show the

aortic arch (arrows) reaching the upper thorax above the level of the

clavicle (C). (B) Contrast-enhanced MR angiogram seen from the left

side shows a tortuous aortic arch, aneurysmal dilatation (asterisks) in

the distal aortic arch and left subclavian artery (LSA) and severe

long segment narrowing of the proximal descending aorta. Dilated

intercostal arteries are seen as collateral channels. As an incidental

finding the left innominate vein (LIV) takes a retroaortic course. Ao,

ascending aorta; LV, left ventricle.

A

B

is double aortic arch, the trachea is bent to neither side or

bent only slightly to the side of the smaller aortic arch.

The trachea may show bilateral indentations with con-

centric narrowing of its lumen (Fig. 7.14). The descending

aorta can be seen on either side, although a left-sided

descending aorta is more frequent. The aortic arch can be

unusually high, its apex reaching above the level of the

clavicle (Fig. 7.15). This condition is called a cervical aor-

tic arch.10,12 It occurs more commonly with a right aortic

arch, often taking a retroesophageal course. It is com-

monly associated with tortuosity, aneurysmal dilatation,

and narrowing.

76 II Systematic Approach to Chest Radiographs

■■ The situs can be accurately defined when the

bronchial branching pattern is clearly shown.

■■ A right-sided gastric air bubble is not a specific sign

for situs inversus, as it can also be seen with right

and left isomerism. A right-sided gastric air bubble

simply means that the situs is not normal.

■■ The presence of a minor fissure in both lungs is a

definitive sign for right isomerism. However, its

absence means nothing.

■■ The presence of a minor fissure on the same side of

the stomach is a definitive sign of right isomerism.

■■ Symmetry of the right and left lobes of the liver is

highly suggestive of right isomerism, especially

when the heart is not enlarged and the pulmonary

vascularity is diminished.

■■ A dilated azygos vein on either side is highly

suggestive of left isomerism, but can be seen in

other conditions such as superior or inferior vena

caval obstruction.

■■ Cardiac position has nothing to do with the type

of visceral and atrial situs.

■■ Levocardia tends to occur with D-loop ventricles

(right ventricle on the right), and dextrocardia

with L-loop ventricles (right ventricle on the

left), but there are many exceptions.

■■ A left aortic arch indents the left side of the

trachea, and a right aortic arch indents the right

side of the trachea.

■■ The stripe of the descending aorta is seen on the

same side as the aortic arch in most cases.

■■ In double aortic arch, the trachea bends to

neither side if the arches are symmetric in size.

When the arches are asymmetric in size, the

trachea may bend to the side of the smaller

arch. The descending aorta can be seen on

either side.

Pearls

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