CT of Posterior . .

.

,

i:

.

Mediastmal Akira

Kawasbima,

MD MD MA

This article presents tenor mediastinal remains the study firm the presence tion and extent of involvement.

esophageal

cysts,

Masses MD

Elliot K Fishman, Janet E. Kuhlman, Matthew S. Nixon,

vascular

1

.

an algorithmic approach to the evaluation of posmasses seen with computed tomography (CT). CT ofchoice, since it not only can be used to help conof these masses, but it also helps define the (a) locathe lesion, (b) adjacent organ involvement, or (c) Causes ofposterior mediastinal masses include

lesions,

congenital

or

acquired

vascular

lesions,

foregut

intrathoracic adenopathy,

goiters, mediastinal pseudocysts, fat-containing neurogenic tumors, infectious spondylitis, and vertebral tumors. From the CT appearance of the lesion, one can often distinguish among the various masses and identify their origin and cause. This information enables patient triage and therapy to be expedited and, in most cases of posterior mediastinal masses, allows a correct diagnosis to be made solely on the basis of the CT examination. tumors,

U INTRODUCTION The evaluation of suspected

ologist.

Chest

suspected

thoracic

detected encountered. confirming

the

known

its presence,

(CT)

can

help

adjacent organ, or vascular involvement. the capability to distinguish fat, water, CT appearance of the lesion, one can identify

their

origin

In most cases netic resonance planar

imaging

Index plasms,

terms: 675.30

RadloGrapb.ica the

From

Wolfe April

and

NC

Abbreviation:

Russell

St. Baltimore, 16 and received

cause

better

= inferior

Mediastinum,

1991;

and

of mediastinal (MR) imaging

vena anatomy

ofwhether remained

mediastinal

define

soft often

is a challenge

examination

has clinical

has

or suspected

CT

initial

question

abnormality

tomography

ofa

masses

used as the

However,

mediastinal

Computed

evaluation

mediastinal

is widely

disease.

or suspected

for the

posterior

radiography

the

location

for the

radi-

in patients

with

a radiographically is frequently examination of choice

significance the

mass

(1-3).

In addition

and

extent

of the

to

lesion,

With measurement of attenuation, CT has tissue, and calcium. On the basis of the distinguish among various lesions and

(1-8).

disease, the is comparable

contrast

anatomic to that

resolution

information provided provided by CT. Direct

are

additional

121 1

#{149} Mediastinum,

advantages

by magmulti-

of MR

imag-

cava #{149} Mediastinum,

CT, 675.

cysts,

675.31

#{149} Mediastinum,

neo-

11:1045-1067

H. Morgan

Department

MD 21205. From July 1 ; accepted

ofRadiology

and

the 1990 RSNA scientific July 5. AK. is a Winthrop

Radiological

Science,

TheJohns

assembly. ReceivedJanuary Pharmaceuticals fellow.

Hopkins 16, Address

Hospital,

600

N

1991; revision requested reprint requests to

E.K.F. C

RSNA,

1991

1045

ing.

MR

imaging

can

allow

the

differentiation

of vascular from nonvascular lesions without the aid of intravenously administered contrast material (9) and is often used to evaluate abnormalities

involving

the

spine

and

spinal

ca-

nal. Much of the available CT literature on the mediastinum has concentrated on the CT appearance of anterior and middle mediastinal

masses.

Less

attention

has

been

paid

Differential

True

pathologic

entities

as defined

representative

Vascular

to the

by means

(d)

cases,

discuss

U

NORMAL

.

Classification

Mediastinum

The mediastinum trathoracic region son

(10)

may be defined as the inoutside the pleural sac. Fel-

attempted

to divide

the

mediastinum

on the basis of a lateral radiograph and arbitrarily drew a line posterior to the pericardium, extending it cephalad anterior to the trachea, which divides the anterior from the middle mediastinum. The middle and postenor

mediastinum

line

connecting

are

separated

points

1 cm

anterior margin of the dorsal An attempt to compartmentalize

by a second posterior

to the

vertebrae. the

medi-

astinum with CT was introduced by Zylak (4), who favored the concept of the mediastinum being divided into three longitudinal cornpartments extending from the level of the thoracic inlet to that of the diaphragm. The middie mediastinal compartment is exclusively a vascular space that incorporates the pericardium and its contents, the great veins, and the anterior aorta and its major branches. The anterior mediastinal compartment, or the prevascular space, includes the thymus. The pos-

1046

U

RadioGraphics

U

Kawashinia

artery, sling

Azygos

et at

double

or hemiazygos

aortic

arch)

continuation

with

interruption of P/C Acquired abnormalities Aneurysm Dissection Obstruction

Foregut

of superior

vena

Bronchogenic Esophageal Neurenteric

cysts duplication cysts

goiters hernia

posterior areas

Adenopathy Paravertebral Neurogenic

Other

or NC

cysts

Mediastinal lipomatosis Fat-containing tumors (lipoma, coma, and teratoma) Mediastinal pancreatic pseudocyst

True

cava

cysts

Intrathoracic Fatty masses Bochdalek

Posterior

vanices

lesions

clavian Pulmonary

of CT

ANATOMY of the

mediastinum

Congenital abnormalities Aortic arch and subclavian artery anomalies (aberrant right subclavian artery, right aortic arch and aberrant left sub-

CT

techniques and the advantages of CT over other nadiologic modalities in the evaluation of pathologic entities, and (e) discuss problems in the differential diagnosis of the pathologic entities.

posterior

Esophageal lesions Neoplasms Esophageal dilatation Hiatal hernia Esophageal and paraesophageal

anatomy and abnormalities of the posterior mediastinum, as depicted at CT. In this articie, we (a) review the anatomy of the postenor mediastinum, (b) present a systematic review of the various posterior mediastinal masses, (C) illustrate how to recognize the with

of Posterior

Diagnoses Masses

Mediastinal

mediastinal

liposar-

and paravertebral

area tumors

paravertebral

lesions

Infectious spondylitis Spinal trauma Spinal neoplasms (primary

Extramedullary and sickle

and

hematopoiesis cell disease)

metastatic)

(thalassemia

tenor mediastinal compartment, or postvascular space, is bounded anteriorly by the pericardium and great vessels, posteriorly by the prevertebral fascia and anterior longitudinal ligaments, and laterally by the respective panetal pleura. It contains the esophagus, descending aorta, azygos and hemiazygos veins, thoracic duct, lymph nodes, and neural

Volume

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6

structures. By convention, the paravertebral areas are included in the posterior mediastinum. In Zylak’s definition, the posterior mediastinum includes the structures classified by Felson as middle and posterior mediastinum. The posterior mediastinum, according to Felson, is essentially the paravertebral (paraspinal) area. The pathologic conditions arising from the paravertebral area include neurogenic tumor, adenopathy, infectious spondylitis,

spinal

trauma,

spinal

tumor,

and

ex-

tramedullary hematopoiesis (Table). The pathologic conditions arising from the posterior mediastinum (true posterior mediastinum), excluding the panavertebral area, indude the lesions arising from the esophagus and the descending aorta, adenopathy, foregut cysts, intrathonacic goiters, and mediastinal pancreatic pseudocysts. Heitzman (1 1) emphasizes that the aortic and azygos arches can be used as physiologic and anatomic reference points to facilitate the understanding of anatomic relationships and the categorization of the pathologic processes on the basis of their relationships. The anatomy of the posterior mediastinum and the anatomic landmarks can easily be depicted with CT.

The middle layer (pretracheal fascia) lies anterior to the trachea and posterior to the strap muscles and extends mnferionly from the hyoid

with

bone

into

fascia

November

1991

thorax,

surrounds

where

the

it blends

aorta

and

peri-

cardium. The pretracheal fascia anteriorly, the prevertebral space posteriorly, and the carotid sheaths laterally define the visceral compartment, which is continuous with the mediastinum across the thonacic inlet. The visceral compartment contains the pharynx, larynx, trachea, esophagus, recurrent nerves, periesophageal lymph nodes, thyroid, and panathyroid glands. The posterior or retropharyngeal portion of this compartment continues inferiorly behind the esophagus, into the thorax, to the posterior mediastinum. The anterior layer of the deep cervical fascia anterior to the strap muscles arises from the hyoid bone and attaches the sternum and clavicle anteriorly, forming the suprasternal space

at the

thyroid.

The esophagus courses infeniorly along the posterior tracheal wall. The esophagus is on the right side of the proximal two-thirds of the descending aorta. The esophagus crosses the aorta anteriorly and, distal to this, is on the left side of the descending aorta. The esophagus

courses

along

esophageal . Anatomic Considerations The anatomic communication between the neck and the mediastinum is defined by the deep fascial layers of the region (Figs 1-3) (12). The understanding ofthe fascial cornpartments facilitates the space-specific differential diagnosis for the lesions on the basis of their space ofonigin (12,13). A fascial envelope, the deep cervical fascia, divides into three layers, posterior, middle, and anterior, that define the distinct compartments. The posterior layer (prevertebral fascia) delineates the prevertebral space posterior to the fascia that extends from the occipital bone to the thorax. The prevertebral space in the neck can extend to the thoracic inlet but is normally obliterated below T-i, at which point the pnevertebnal fascia merges with the anterior longitudinal ligament of the thoracic spine and endothoracic fascia.

the

that

into hiatus,

with

branch

the

abdomen which

the

of the through

is formed

vagus

left

nerve

gastric

the

and

vessels

esophageal

by decussation

of the

medial fiber of the right crus of the diaphragm at T-10 (14). In general, if the esophagus is distended, its wall is clearly visualized on CT scans

and

has

a sharp

outer

and

inner

con-

tour, a smooth surface, and a thickness of about i-i .5 mm (15, 16). If it is partly or totally collapsed, the wall thickness is greater than but does not exceed 3 mm. Its outer contour remains perfectly symmetric and sharp, whereas its inner contour is somewhat irregular, reflecting the more redundant mucosal surface. The descending aorta extends from the ligamentum arteniosum to the aortic hiatus in the diaphragm, which is an osseoaponeurotic

Kawashilna

et at

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1047

1.

2.

Figures

1-3.

(1) Diagram

anatomy projection,

of the mediastinum with the heart

of normal

in frontal and pericardium removed. a = artery. Ext = external, Inf = inferior, mt internal, v = vein. (2) Diagram of normal anatomy of the mediastinum in left anterior oblique projection. LPA = left pulmonary artery, LPV = left pulmonary vein, v = vein. (3) Diagram of normal mediastinal lymphatic chains in the posterior mediastinum in left posterior projection, with

the aortic removed.

arch SVC

and descending =

superior

vena

aorta cava.

opening located behind the diaphragm T-i2 and transmits the thoracic duct and gos and hemiazygos veins (14). The two phragmatic crura are connected in front the aorta, just above the celiac trunk, by fibrous median arcuate ligament. At CT, proximal, middle, and distal portions of descending aorta measure 2.6, 2.5, and

1048

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Kawashima

et at

at azydiaof the the the 2.4

cm in diameter, respectively (17). At no level is the normal descending aorta larger than the ascending aorta. The ratio of ascending aorta diameter to descending aorta diameter is as high as 2.2 in women younger than age 40 years,

but

for

women

older

than

age

the ratio is 1 .2: 1 .4 because of the tional increase in diameter of the aorta with respect to the ascending age (18).

Volume

11

60 years,

dispropordescending aorta with

Number

6

The azygos system is a paired paravertebral pathway in the posterior thorax (19). The azygos vein ascends along the right anterolatenal surface of the thoracic vertebrae and at T-4 arches anteriorly just cephalad to the night main

stem

bronchus

to join

the

superior

vena

cava. The hemiazygos vein ascends along the left anterolateral aspect of the thoracic vertebnae and crosses to the right, posterior to the aorta and esophagus, to join the azygos vein at T-8 or T-9. The accessory azygos vein runs cephalad in a left paravertebnal location and usually communicates with the azygos and left brachiocephalic veins. The accessory hemiazygos vein sometimes joins the hemiazygos vein so that both cross as one channel, rather than each having its own channel crossing toward the azygos vein. The hemiazygos and accessory hemiazygos veins vary in size relative to the azygos vein. The transverse connections (retroaortic anastomoses) between the azygos and the hemiazygos veins usually occur at T-8 to T-10 and are numbered one to five (19,20). The thoracic duct, continuous with the cisterna chyli, enters the thorax through the aortic side

hiatus

and

of the

courses

aorta

cephalad

(2 1). Approximately

to the

right at the

carina, the duct crosses the left main stem bronchus and runs cephalad along the left lateral wall of the trachea and esophagus, somewhat posteriorly. The duct leaves the thorax between the esophagus and left innominate vein and then joins with the left internal jugular vein or occasionally with the left subclavian, innominate, or external jugulan veins. The thoracic duct is rarely seen at CT, unless the duct is opacified because of lymphangiognaphic contrast media. The ganglionated sympathetic trunk, counsing vertically near the neck of the ribs, is connected with each intercostal nerve and passes through splanchnic the fifth medially

the

crus of the diaphragm (22). The nerves, formed by branches from to 12th sympathetic ganglia, course and mnferiorly and perforate the crus

of the diaphragm. The right vagus nerve enters on the right lateral wall of the trachea and falls back on the esophagus. After giving off the left recurrent nerve under the aortic arch posterior to the ligamentum arteriosum, the left vagus nerve courses behind the root of the lung and then along the esophagus into the abdomen. Groups of posterior mediastinal nodes indude those around the esophagus (penesophageal nodes) and those along the antenor and lateral aspect of the descending aorta (periaortic nodes) (Fig 3) (2 1). The posterior

November

1991

panietal (paraspinal) lymph nodes are located at the rib heads in the posterior intercostal spaces

(intercostal

nodes)

vertebrae

(juxtavertebral

municate

with

and

around

nodes),

periesophageal

the

which and

com-

peniaortic

lymph nodes. Each of the two diaphragmatic cruna is a musculotendinous pillar that attaches to the anterolateral surfaces of the lumbar vertebral bodies

and

disks

(Li-3

on

the

night,

Ll-2

on

the left) (14). The diaphragmatic crura form the anterior and lateral borders of the netnocrural spaces, which contain the aorta, azygos and hemiazygos veins, thoracic duct, and lymph

U

nodes

(retrocrural

nodes).

CT TECHNIQUE

Posterior

mediastinal

masses

are

routinely

studied with CT by using 8-mm-thick sections taken at i-cm intervals from the lung apex to the diaphragm. At our institution, typical scanning parameters for the Somatom Plus (Siemens

Medical

Systems,

Iselin,

NJ)

are

250

mM and 125 kVp, with i-second scan time; for the DRH scanner (Siemens Medical Systems), parameters are 3 10 mM and 125 kVp, with

4-second

scan

time.

viewed at a mediastinal dow width, 420; window window setting (width, and

a bone

center,

window

setting

when

appropriate.

150)

mediastinum,

fat

planes

great vessels, superior venous channels, and sels

to be

All images

well

seen

(width,

re-

CT

1,750;

In the

allow

vena central

on

are

window setting (wincenter, 36), a lung 1,650; center, -540),

the

normal

aorta

and

cava, major pulmonary scans,

even

yeswithout

intravenously administered contrast material. If the identification of the major vessels is important, the CT numbers of the vascular structunes can be increased well beyond the range of the other mediastinal structures after iodinated contrast media is intravenously administered.

Our

approximately contrast media Pharmaceuticals, 2.0

mL/sec

ton.

For

serial

current

with

an

a detailed

scanning

technique

is to inject

100-150 mL of6O% iodinated (Hypaque 60%; Winthrop New York) at a rate of 1.0automated

study after

power

oflocalized

a bolus

injec-

lesions,

injection

of

25-50 mL of 60% iodinated contrast media can be performed. If an esophageal or penesophageal lesion is suspected, use of a banium paste (Esopho-CAT; E-Z-Em, Westbury, NY) is particularly effective in coating the lumen of the esophagus.

Kawashima

et at

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1049

a.

b.

Figure

4.

Esophageal

cancer.

Sequential

CT scans

of the upper

ferential thickening of the wall of the middle of the esophagus, (black arrow). Esophagus above the obstruction is distended of the distal trachea and left main bronchus, with loss of the

U ESOPHAGEAL Fiberoptic endoscopy phy of the esophagus tion

about

gus, whereas normalities is currently disease and .

the

The

LESIONS

mucosal

lining

of the

Esophageal

ability

esopha-

CT

Cancer

Esophageal carcinoma at an early stage may appear as either an intraluminal mass or localized wall thickening of 3-5 mm and may become circumferential with or without intnaluminal narrowing as the tumor grows (15,23). CT is useful in staging carcinomas of the intrathoracic esophagus and can demonstrate the thickness of the lesion; length; and involvement of periesophageal tissues, nodes, and tracheobronchial tree (Fig 4) (23-25).

1050

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Kawashima

et at

thorax

esophagus

lacks

local invasion by the frequently encountered

and barium radiograprovide direct informa-

mural and extraluminal abare suggested only indirectly. used to evaluate extramucosal extension into the mediastinum.

and middle

demonstrate

a circum-

with intraluminal narrowing at the carina (a). Tumor slightly indents the posterior aspect intervening fat plane (white arrows in b).

of periesophageal

as a sign troversial with

a serosa

so that

direct

esophageal carcinoma at diagnosis. The fat

plane

is reli-

obliteration

of cancer, as defined with CT, is conin the literature, and further study

surgical

correlation

is necessary

(25).

Tracheobronchial invasion can be suspected when CT shows extension of an esophageal mass to the trachea or bronchus, both of which are displaced away from the spine, or indentation of the posterior wall of either the trachea or bronchus by an esophageal mass (25). One should be cautious in predicting aortic

invasion

with

CT

because

it is rare

(25).

The greater the contact between the tumor and aorta ( > 90#{176} of the aortic circumference), the more likely the possibility of aortic invasion (24). Prediction of pericardial invasion with CT should also be made with caution because fat planes between the pericardium and the esophagus may not be seen. Me-

Volume

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5.

6.

(5) Hematoma immediately after esophagectomy for esophageal cancer in a 72-year-old man. Unenhanced CT scan of the lower thorax reveals a large area of high attenuation (long arrow) in lower posterior mediastinum and a small right pleural effusion (short arrow) (6) Seroma 3 months after esophagogastrectomy and gastric pull-up for esophageal cancer in a 70-year-old man. Unenhanced CT scan of the lower thorax demonstrates posterior mediastinal loculated collection of fluid (long arrow) that compresses the contrast material-filled gastric pull-up (short arrow) Small bilateral pleural effusions are present. (Reprinted, with permission, from reference 27.) Figures

5, 6.

.

.

tastases to regional supraclavicular, celiac, and gastrohepatic ligament lymph nodes can be assessed. CT evaluation of the gastroesophageal junction requires caution because a “pseudomass” (thickening of the junction presumably representing the oblique insertion of the esophagus into the stomach) is often encountered. CT is useful in planning radiation therapy for esophageal cancer, although CT may not be useful for staging the disease after radiation therapy, since, within the field of the radiation port, mediastinal fat planes are obliterated and the normal esophagus is thickened (25). Complications occur in up to 60% of patients after esophagectomy (26). Early complications include anastomotic leaks, abscess, gastric outlet obstruction, gastric necrosis, and hematoma (Fig 5) (27) Late complications include benign strictures, tracheal aspiration, neflux esophagitis, bronchoesophageal fistula, and recurrent tumors (27,28). CT is useful in detecting recurrent tumor before barium radiography of the esophagus on endoscopy is performed because local recurrent disease is often located extramucosally (25,29). An enlarging extramucosal me.

diastinal

soft-tissue

mass

at the

level

of the

tomy,

since

it may

be

mistaken

for

recurrent

disease (25,29). Mediastinal fat planes may be obliterated in the early postoperative period. The differential diagnosis of fluid collection esophagogastnectomy includes anastoleak, abscess, lymphocele, and seroma (27). The anastomotic leaks are diagnosed with extravasation of contrast material at CT or barium examination and reportedly occur in 3.5%-25% of cases (19). Small self-contamed leaks can be treated conservatively unafter

motic

less

they

sepsis.

are

associated

Abscesses

cate with Lymphocele

may

an empyema can occur

with

an

contain

empyema air

or

or subphrenic as a sequela

or

communi-

abscess. of insult

to

thonacic duct during surgery in which chybus effusions are common. Negative CT attenuation values (-20 to 100 HU) in the the

-

mediastinal

collection

ing.

is a fluid

Seroma

is a characteristic collection

find-

in the

surgical

bed during esophagectomy and appears as a well-defined tubular collection of fluid with a thin on imperceptible wall (Fig 6) (27). Esophageal mucocele represents a closed thoracic segment filled with protemnaceous fluid, which results from surgical isolation of the thonacic esophagus, accompanied by esophageal bypass, usually a substernal gastric tube (26).

tumor is the most common initial CT finding of recurrence (29). One should be cautious in interpreting the undendistended, interposed stomach after esophagogastrecprimary

November

1991

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et at

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Figures 7, 8. (7) of the lower thorax esophagus (arrow), fluid

and

orally

Achalasia.

shows which

Unenhanced

marked is filled

administered

CT scan

dilatation of the with retained

contrast

material

and

has a thin wall. Dilated esophagus extends into a right paravertebral location and azygoesophageal recess. (Reprinted, with permission, from reference 31.) (8) Hiatal hernia in a 65-year-old patient with scan

a history of breast cancer. (a) of the upper abdomen reveals

ageal

or periesophageal

scan obtained tional contrast

tended

hiatal

mass

Enhanced a lower

(arrow).

after oral administration material demonstrates

sac with

gastric

folds

CT esoph-

(b) Another of addia better dis-

(arrow).

. Esophageal Dilatation The causes of diffuse esophageal dilatation can be divided into two major categories: secondary to esophageal motility disorders and to distal obstruction (30). The former group includes achalasia (Fig 7) (31), postvagotorny syndrome, Chagas disease, scleroderma, systemic lupus erythematosus, presbyesophagus, diabetic and alcoholic neuropathy, effects of anticholinergic drugs, and esophagitis. The latter group includes benign or malignant

stricture

and compression

trinsic mass. wall thickness

CT is useful secondary

caused by an exin demonstrating to tumor infiltration

(pseudoachalasia) or extrinsic compression. Although esophageal wall thickness with achalasia or other esophageal motility disorders would be expected to be normal, symmetric esophageal wall thickening at the gastroesophageal junction could be observed in achalasia

as a result

.

Hiatal

U

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et at

distention,

Hernia

Hiatal

hernia

mality

of the

The sliding periesophageal esophagus

1052

of inadequate

prior dilatation or myotomy, or secondary esophagitis and may be difficult to distinguish from tumor infiltration (31).

is a commonly lower

type lies

posterior

diagnosed

abnor-

mediastinum.

is seen more often than the type, in which the distal along the herniated stomach

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11

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Figure 9. Esophageal and periesophageal varices in a 40-year-old man with a history of upper gastrointestinal bleeding. (a) Topogram (scout view) oflower thorax and upper abdomen shows bilateral lower paravertebral widening (arrows). (b) CT scan of the lower chest obtained during a drip infusion

of contrast

material

shows

the

esophagus

is

compressed by extensive periesophageal varices and is not adequately visualized. Descending aorta (d) is also surrounded by the periesophageal varices and probably (c) Upper abdominal

abdominal and nodular larged,

enlarged CT scan

azygos shows

system. evidence

of

vanix (arrow). Liver (L) appears atrophic in contour and the spleen (S) is en-

indicating

underlying

portal hypertension, which ated on other sections.

liver

were

cirrhosis

better

and

appreci-

a.

b.

above

C.

the

(5). Air-fluid levels or hernimay be present in the hiatal sac. Hiatal can simulate a retnocrural or periesophageal mass, and this can be resolved by performing CT while the patient is sipping contrast material or swallowing air (Fig 8).

ated gastric

hiatus folds hernia

. Esophageal Varices Esophageal on periesophageal varices may appear as a retrocandiac mediastinal mass in patients with portal hypertension. CT can help promptly establish the nature and extent of the lesion (Fig 9). CT findings of esophageal varices include thickening of the esoph-

November

1991

ageal wall, slightly lobuJated outer contour, and scalloped esophageal luminal masses, which are better appreciated with contrast material-enhanced imaging (16). Periesophageal varices may be visualized only at CT. Associated CT findings include evidence of liver cirrhosis and portal hypertension, particularly with splenomegaly and abdominal varices.

As with

barium

radiography

of the

esoph-

agus, normal CT scans do not help rule out esophageal vanices because small vanices may escape detection, particularly in scans obtamed without a bolus injection of contrast material.

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et at

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Figures 10, 11. (10) Aberrant right subclavian artery in a 30-year-old man with a history ofdysphagia. (a) CT scan obtained at the level of the aortic arch shows an aberrant right subclavian

artery

tic arch scan

above

artery

(arrow)

(a). v the

(arrow)

oblique

aortic and

off the

distal

cava.

(b) CT

demonstrates

the

vena

arch

crossing

manner

aspect

coming

superior

=

the mediastinum indenting

of the esophagus

aor-

in an

the

(e) and

posterior trachea (t).

artery, s = left subclavian artery, v = right and left brachiocephalic veins joined together and forming the superior vena cava (v in a). (11) Pulmonary sling. Unenhanced CT scan obtained at the level of the lower trachea (t) and carmna reveals an anomaC

=

bus

left

left

common

main

ing from posteriorly tween the

carotid

pulmonary

the right and

main

crossing

trachea

artery

pulmonary the

(arrows)

artery

mediastinum

arms(r)

be-

11.

and esophagus.

U MASSES OF VASCULAR ORIGIN Approximately 10% of mediastinal masses in adults are ofvascular origin (32). Although most of these are aneurysms of the thoracic aorta and its branches, there are many vasculan entities that may simulate mediastinal abnormalities. CT, particularly with contrastenhanced scanning, minimizes the need for more invasive procedures to evaluate these abnormalities (1,33).

.

Congenital Abnormalities Arch and Subclavian Arte,y Anomalies.-An aberrant right subclavian artery associated with an otherwise normal aorta is the most common aortic arch anornAortic

aly, occurring in approximately 0.4%-2.3% of the population and 37% of children with Down syndrome who have congenital heart disease (34,35). The anomalous right subclavian artery is the last branch off the distal aortic arch

and

crosses

oblique manner (Fig 10) (35,36).

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et at

the

mediastinum

posterior Dilatation

in an

to the esophagus of the artery at its

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Figure 12. Azygos and hemiazygos continuation with interruption of NC. (a) Unenhanced CT scan of the upper abdomen shows no intrahepatic portion of the NC and enlargement of the hemiazygos vein (arrow), which crosses to the right to join the azygos vein above this section. The patient has abdominal situs solitus. (b ) CT scan obtained at the level of the canina reveals enlarged azygos vein (arrow), which joins the superior vena cava above this section.

origin

(diverticulum

of Kommerell, remnant arch) is common and occurs in up to 60% ofcases (37). A right aortic arch may be associated with an aberrant left subclavian artery, the most common of the right aortic variants. Congenitat heart disease is rarely associated with this anomaly. The left subclavian artery arises from

left, behind the trachea and in front of the esophagus, toward the hilum of the left lung. It may indent the trachea posteriorly and the esophagus anteriorly and is often symptom-

the descending

course

of the embryonic

right

aorta,

often

from a diverticu-

atic

in neonates

of the

relationship (Fig ii).

other anomalies of the aortic arch and great vessels. The two separate arches arise from a single ascending aorta. Each arch gives rise to subclavian and carotid arteries (“fourartery sign”) (36) and then joins to form a single descending aorta. The right aortic arch is usually somewhat larger and higher than

Congenital

which

is a remnant

of the

embryonic

arch (33,36,37). Double

aortic

arch

occurs

less

the left one.

Most patients experience toms because of tight vascular rings require surgery.

monary

artery

part

arises

and courses

artery of the

right

of the

November

main trachea

1991

from

over

causing

left pulmonary to and effect

artery and on the airway

its

and

sympmay

aberrant the right

left

pul-

pulmonary

the proximal

stem bronchus or the and then posteriorly

Interruption

oftbe Inferior or Hem iaxygos Continuation.-Congenital interruption of the inferior vena cava (NC) with azygos or hemiazygos continuation should be suspected when the CT study reveals enlargement of the azygos arch and continuous enlargement of the paravertebral and retrocrural portions of the azygos and hemiazygos veins without a definable mntrahepatic IVC (Fig 12) (19,39,

Cava

Vena

40).

Sling.-The

children,

commonly

than

Pulmonary

young

difficulties, as opposed to being usually asymptomatic in adults (38). CT allows delineation of the abnormal origin and

left

lum,

and

respiratory

It may

anomaly anomalies

with

Azygos

occur

as an

or in association of abdominal

isolated

asymptomatic

with congenital sims (polysplenia).

portion distal to the

Kawashima

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13. Figures 13-15. scan of the upper

(13) Aortic aneurysm. abdomen demonstrates

Enhanced a large

CT aneu-

rysm of the descending aorta with intramural thrombus and curvilinear calcification that erodes the anterior aspect ofthe vertebra (arrow). (14) Aortic dissection in a 68-year-old

man

obtained

with

at the level

hypertension.

Enhanced

of the right

main

CT scan

pulmonary

artery .

demonstrates a V-shaped, displaced intimal flap (arrowheads) separating true and false lumens of the descending aorta. (15) Enlarged azygos system in a 42-year-old

woman cally

with

Budd-Chiari

substantial

the NC. shows veins vessels

stenosis

Enhanced

syndrome

and hemodynami-

of an intrahepatic

CT scan

.

of the upper

portion

of

r

abdomen

enhancement of enlarged azygos and hemiazygos (curved arrows) and a few subcutaneous collateral (straight white arrows). Mesoatrial shunt placed

for portal arrow).

hypertension

had been

thrombosed

15.

(black

Abnormalities oftbe Aorta.-Thonacic aortic aneurysms are the most common vascular cause of a mediastinal mass in adults and may result from atherosclerosis, trauma, syphilis,

tunes, and occasionally erosion of the vertebral bodies (Fig 13) (17,41). Unenhanced CT may demonstrate high-attenuation fluid in the mediastinum, pericardium, or pleural sac, indicating rupture or impending rupture

cystic

medial

(17,41).

cosis.

They

.

Acquired

Aneu,ysm

necrosis, may

be

poststenosis, saccular

on fusiform

and

myin

configuration. Aneurysms of the descending aorta often project from the posterolateral aspect of the aorta. CT demonstrates local or diffuse aortic dilatation, intimal calcifications, enhancement of the patent lumen, intramural thrombus,

displacement

of mediastinal

struc-

Traumatic pseudoaneurysms are the second most common form of thoracic aortic aneurysm and typically occur in the proximal descending aorta just beyond the origin of the left subclavian artery at the aortic isthmus (17). Angiography has been the procedure of choice for evaluation of suspected acute aortic

injuries

can result Dynamic

1056

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Kawashima

et at

because

prompt

in a high rate contrast-enhanced

surgical

repairs

of patient survival. CT may be per-

Volume

11

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6

formed in clinically chest trauma, who

rior mediastinum

stable patients have widening

at plain chest

with blunt of the supe-

radiography.

CT can be used to image the entire aortic cumference, whereas focal false aneurysm may be seen at angiography when imaged in profile.

CT

findings

pseudoaneurysm timal

tear

of acute

include on flap,

periaortic

cir-

traumatic

false or

aneurysm,

in-

intraluminal

hematoma, and marginal irregularity of the opacified aortic lumen (17). Obliteration of mediastinal fat planes is not a reliable sign of aortic injury and may be due to contusion or venous hematoma. Dissection oftbe Aorta.-An aortic dissection is a separation of the intima and adventitia, which may involve a localized area of the aorta or the entire circumference. A distal aortic dissection (type B) is confined to the descending aorta and can be treated medically as opposed to acute proximal aortic dissection (type A), which has a worse prognosis and is treated surgically. Death is usually due to rupture of the false lumen into the pericardial or pleural sac. Other complications indude aortic insufficiency and obstruction of the coronary arteries and great vessels. Unenhanced CT may demonstrate the presence of a high-attenuation clot within the aortic wall that indicates acute dissection with a thrombosed false lumen (41,42). Dynamic CT can demonstrate true and false lumens with an intervening intimal flap (Fig 14) (41,42). CT rarely shows the sites of intimal tears and communications between the true and false channels (42). CT thus is a complementary study to angiography. In cases in which CT findings are indeterminate or a better definition of the true or false lumen is needed, angiography may be performed.

November

1991

Obstruction

Obstruction

oftbe of the

Venae superior

Cavae. vena cava -

or NC

can result from neoplastic, infectious, or iatrogenic causes. Contrast-enhanced CT may demonstrate the cause of mediastinal widening to be the presence of multiple venous collateral vessels, including the azygos and hemiazygos veins, with or without enlargement of superior intercostal veins; the vertebral venous system; the internal mammary veins; and lateral thoracic veins (Fig 15). U FOREGUT CYSTS Mediastinal cysts of fonegut origin constitute up to 9% of all primary mediastinal masses in surgical series (43,44). Mediastinal cysts result from embryologic aberrations and anomalous budding of the primitive foregut and early tracheobronchial tree. The spectrum of bronchopulmonary malformations includes bronchogenic cysts, esophageal duplications, and neurentenic cysts. Each type of foregut cyst has typical histologic features and characteristic

anatomic

locations

within

the

chest

(45).

Bronchogenic cysts are the most common intrathonacic foregut cysts, accounting for 54%-63% of cases in surgical series (43,45, 46), and most commonly are located in the carnal area (52%), followed by paratracheal area (19%), esophageal wall (14%), and retrocardiac area (9%) (47,48). Esophageal duplication cysts are usually located along the esophagus in the lower posterior mediastinum in up to 60% of cases. Neurenteric cysts, the least common type, communicate with meninges and are associated with spinal dysraphism, butterfly vertebrae on hemivertebnae,

and

scoliosis.

Kawashima

et at

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16.

17.

18.

19.

Figures 16-19. (16) Bronchogenic cyst in an asymptomatic 57-year-old man. Enhanced CT scan obtained below the carmna demonstrates a cystic subcarinal mass (arrow) with attenuation similar to that of water, with a thin wall protruding into the azygoesophageal recess. (17) Esophageal duplication cyst in an asymptomatic 70-year-old

man.

level of the left atrium reveals a cystic periesophageal was confirmed at surgery. (18) High-attenuation bronchogenic cyst in an asymptomatic 16-year-old boy. Unenhanced CT scan of the upper abdomen demonstrates a periesophageal mass (arrow) with attenuation of 54.9 HU. At surgery, the mass was cystic and contamed viscid creamy fluid with respiratory epithelial lining, findings consistent with bronchogenic cyst.

mass

(arrow)

(19)

Intrathoracic

strates branches

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Enhanced

goiter

Continuity

U

CT scan

attenuation

a well-defined of the aortic

the mass.

1058

with

obtained

in a 50-year-old

mass arch

with

Kawashima

at the

of 12.3 HU. Diagnosis

woman.

Enhanced

with enhancement that and right brachiocephalic

cervical

thyroid

et at

tissue

displaces vein.

was seen

CT scan the There

obtained

esophagus are areas

on other

above the aortic (e, arrow), trachea of cystic

degeneration

arch (t),

demonand major

(arrow)

in

images.

Volume

11

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6

Distinction among the various types of congenital cysts is difficult, however, when they share overlying anatomic locations and similar histologic features. In addition, when inflammation and hemorrhage occur in the foregut cyst, the type-specific lining may be replaced by nonspecific granulation tissue. Such nonspecific cysts account for i7%-20% of all foregut cysts (44,46). Although patients, especially adults, with bronchogenic and esophageal duplication cysts usually do not have symptoms, they may be produced by compression of adjacent structures, such as the esophagus (vomiting and dysphagia) or the tracheobronchial tree (persistent cough on dyspnea). Bleeding on infection may cause the cyst to grow, exacerbating symptoms (45,46,48). CT demonstrates a single, clearly defined, rounded mediastinal cystic mass of homogeneous attenuation, commonly located in the carnal, paratracheal, or paraesophageal regions (Figs 16, 17). The cystic mass shows a thin on imperceptible wall. The attenuation values of foregut cysts are expected to be the same as those ofwater. Up to half of bronchogenic cysts, however, may have a higher attenuation than that ofwater as a result of the presence of calcium or milk of calcium, proteinaceous fluid, mucus, or blood debni within the cysts (Fig 18) (49-52). The complete absence of enhancement after the intravenous administration of contrast material may be a clue to the diagnosis. Aspiration (transbronchial, tnansesophageal, or pencutaneous) or surgery may sometimes be necessary for diagnosis (53).

November

1991

U INTRATHORACIC GOITERS An intrathoracic goiter is usually an extension of cervical thyroid tissue and one of the most common causes of an apparent mediastinal mass.

It represents

masses

resected

up

to

5%

of mediastinal

at thoracotomy,

most

of

which represent a multinodular goiter (43,54-56). A primary intrathoracic goiter is rare (54,55). One-fourth of intrathoracic goiters are posterior (54), arise from the postenor

and

lateral

and descend trathoracic night

side,

aspects

into goiter since

of the

the thorax. exclusively the

left

thyroid

gland,

Posterior inoccurs on the

bnachiocephalic

vein

and aortic arch prevent the goiter from descending on the left. Most patients are asymptomatic, but when symptoms are present, they usually relate to tracheal and esophageal compression and recurrent laryngeal nerve involvement. The CT findings of intrathoracic goiter

include

anatomic

continuity

with

the

cervical thyroid, well-defined borders and frequent focal calcifications, relatively high CT attenuation numbers, increased CT attenuation numbers after the administration of an intravenous bolus injection of contrast material with prolonged enhancement, and inhomogeneity with discrete unenhanced lowattenuation areas corresponding to degenerative cystic areas (Fig 19) (55,56). A lack of continuity between the cervical thyroid and the intrathoracic mass does not exclude a goiter, since the connection may be narrow, fibrous, or a vascular pedicle (56).

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20. Mediastinal pancreatic pseudocyst in a 34-year-old CT scan of the upper abdomen demonstrates a periaortic

Figure

hanced esophagus

tion

U

.

.;

&-

b.

(short

arising

arrow)

from

FATTY

anteriorly.

the lesser

(b)

sac; the crus

CT scan

obtained

. Bochdalek Hernia Bochdalek hernia is a congenital eral defect resulting in a persistent penitoneal canal; it occurs in one one of 12,500 live births (5,14,57). on the left side, with a left-to-right

posterolatpleuroof 2,200 to Most are ratio of 9:1 as a result of earlier closure of the right pleuroperitoneal canal and relative protection by the liver (58). Visceral herniation may involve the stomach, spleen, large or small bowel, omentum, or kidney. The remainder of the diaphragm and the diaphragmatic crura remain normal. A small, asymptomatic Bochdalek hernia containing fat has been reported to occur

with

a frequency

below

of the left diaphragm

MASSES

of 6% at CT (58).

man with acute fluid collection the

level

is slightly

pancreatitis. (a) Unen(long arrow) displacing

of a demonstrates

splayed

the

laterally

fluid

the

collec-

(arrow).

. Fat-containing Tumors Lipomas and teratomas most commonly arise in the anterior mediastinum, whereas liposarcomas have a predilection for the posterior mediastinum. Characteristic calcifications may be demonstrated in cases of teratomas (52). Only 3%-8% of teratomas occur in the postenor mediastinum. CT attenuation values of lipomas are -30 to 100 HU. Malignancy should be suspected when the CT number exceeds -30 HU (59,60). -

U PANCREATIC PSEUDOCYSTS Extension of a pancreatic pseudocyst into the mediastinum is uncommon. A cystic posterior mediastinal mass developing over a short time in a patient

with

evidence

of pancreatitis

is

to be a pseudocyst, which represents an encapsulated collection of pancreatic secretions, often with blood and necrotic material. Mediastinal extension results from migration of pancreatic secretions and inflammatory products through the esophageal hiatus, aortic hiatus, or defect of the diaphragm, appearing in the lower posterior mediastinum (57). CT is superior to ultrasonography in the evaluation of mediastinal extension of abdominal disease. CT helps confirm the cystic nature of the mass; define the boundaries of the cyst; and demonstrate marked splaying of the dialikely

.

Abnormal

Infiltration Lipomatosis)

Fatty

(Mediastinal

Widening of the anterior mediastinum and the supra-azygos and supra-aortic areas by fatty infiltration secondary to Cushing disease, high-dose known

erable of the pleural

steroid (1,5,57,59).

therapy, There

or also

obesity may

is well be

fat deposition in the paravertebral posterior mediastinum and in the area adjacent to the ribs.

consid-

area extra-

phragmatic

1060

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RadioGrapbics

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Kawashima

et at

of the

poste-

nor stomach and esophagus, indicating the mass crosses the aortic on esophageal tus, respectively (Fig 20) (61). Attenuation may be higher than that of water because superimposed hemorrhage on infection.

crura

or compression

that hia-

Volume

11

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of

6

Figures 21, 22. (21) Metastatic colon cancer in a 56-year-old man. (a) Enhanced CT scan of the lower chest shows bilateral periaortic adenopathy (arrows) (b) Enhanced CT scan of the upper abdomen reveals retrocrural adenopathy (short arrow) and metastases (long arrow) in the right hepatic lobe. (22) Chronic lymphocytic leukemia in a 55.

year-old

man.

Unenhanced

CT scan

ofthe

abdomen reveals extensive coalesced periesophageal, and posterior parietal adenopathy.

descending

Splenomegaly

(S) is also

upper

periaortic, (paraspinal) noted. d

=

aorta.

22.

U

ADENOPATHY

Posterior Mediastinal Adenopathy CT scans of the lower thorax are particularly useful in outlining posterior mediastinal adenopathy (Figs 2 1, 22) (2 1,62). In the retrocrural space, lymph nodes greater than 5-6 mm are considered abnormally enlarged (63). Enlargement of posterior mediastinal lymph nodes may be due to granulomatous disease, lymphoma, or metastasis. CT is highly sensitive for detection of adenopathy but cannot help distinguish among a variety of causes. CT S

November

1991

can help guide needle aspiration for histologic examination or culture. Posterior mediastinal lymph node involvement has been seen in 10% ofpatients with non-Hodgkin lymphoma and 5% of patients with Hodgkin lymphoma (64,65). Especially in cases of lymphoma, pear as a paravertebral brae, and extend into

Kawashima

adenopathy may mass, erode the the spinal canal.

et at

U

apverte-

RadioGraphics

U

1061

Figures old

man.

23, 24.

(23)

CT scan

Low-attenuation

ofthe

upper

carnal adenopathy from metastatic the carina demonstrates subcarinal

(