Pattern Approach to Chest Radiography on Chest Radiography

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8 APPLIED RADIOLOGY www.appliedradiology.com December 2009

The excellent contrast affordedby the air-tissue interface of the

lungs lends itself to diagnostic

radiographic evaluation of multiple

pathologic processes. However, given

the confusion that often exists with

regard to diffuse lung diseases, the art

of interpreting the chest radiograph for

the detection and characterization of

interstitial disease has become less

appreciated. The inability of some

radiologists to properly recognize and

differentiate interstitial lung diseases

on chest radiography is unfortunate as

this is often the initial screening test for

patients with dyspnea. In many cases,

early, subtle pathological changes are

often overlooked or poorly character-

ized. Due to a confusing and often

changing classification system, it is

easy to become overwhelmed by the

vast array of diseases that affect the

lung interstitium, and developing a dif-

ferential diagnosis based on isolated

findings can be a challenging task. A

solution to this problem is to revisit the

basic pattern approach to interstitial

lung disease as first described by Fel-

son,1 and then review the major disease

entities that best fit into these recogniz-

able patterns. We will review the pat-

tern approach for the evaluation of

interstitial lung disease on chest radi-

ography, and we present the most com-

mon disease entities for each pattern.In addition, the HRCT correlation for

some patterns will be discussed to

enhance understanding.

Evaluating patterns of ILDThe first step to radiographic evalu-

ation of interstitial lung disease begins

with a fundamental working knowl-

edge of the pertinent anatomy of the

lung interstitium and the various pat-

terns of its possible derangement. The

anatomy of the lung interstitium as

encountered on the routine chest radi-

ograph is largely imperceptible unless

a pathologic process is overriding and

there is abnormal thickening. Concep-

tually, the lung interstitium can be

divided into 3 main parts: the axial

interstitium (or peribronchovascular

interstitium) which contains the bron-chovascular bundles; the centrilobular

interstitium, which contains the alveoli

and capillaries for gas exchange; and

the peripheral interstitium which con-

tains the pulmonary venules, lymphat-

ics and interlobular septae. The

peripheral interstitium interdigitates

with the centrilobular interstitium

through the interlobular septae to

Revisiting the pattern approach

to interstitial lung disease onchest radiography

Scott D. Perrin, MD, Adam Ulano, MD, and Todd R. Hazelton, MD

Dr. Perrin is a Diagnostic Radiology Resident, Department of Radiology,University of South Florida College of

Medicine, Tampa, FL;Dr. Ulano is an Intern, Lahey Clinic Medical Center,Burlington, MA, and Tufts UniversitySchool of Medcine, Boston, MA; andDr.

Hazelton is Chair and Associate Profes-sor, Department of Radiology, Univer-sity of South Florida College of

Medicine, Tampa, FL.


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www.appliedradiology.com APPLIED RADIOLOGY 9December 2009

INTERSTITIAL LUNG DISEASE

divide the centrilobular interstitium

into discrete units known as secondary

pulmonary lobules. Through this inter-

digitation, the venules and lymphatics

in the peripheral interstitium are able

to drain the secondary lobules of the

centrilobular interstitium.Although this conceptualization is

helpful, it is important to remember

that any disease that affects the lung

affects the interstitium at some level:

the intimate relationship of the lung

interstitium and the airways cannot be

overstated. The lung interstitium has a

limited response to injury and usually

exhibits thickening of some or all of its

components. It is from these basic

anatomic concepts of the lung intersti-

tium that the patterns of interstitial

lung disease emerge. Classically, thepatterns of interstitial disease encoun-

tered in conventional radiography rep-

resent abnormal thickening due to

pathologic infiltration of the intersti-

tium at some level, depending on the

nature of the disease process. The pat-

terns have been divided into the broad

categories of linear, nodular and reticu-

lar.2,3 (Table 1)

The linear patternThe linear pattern on chest radiog-

raphy consists of thin linear opacities

which are either 2 to 6 cm long within

the lungs oriented radially toward the

hila or 1 to 2 cm long at right angles to,

and in contact with, the lateral pleural

surfaces. These linear opacities have

been referred to as Kerley A and Ker-

ley B lines, respectively,4 although the

descriptors septal thickening or

septal lines are now preferred for the

latter.5 Histologically, this linear pat-

tern represents thickening of either the

bronchovascular/axial interstitium(Kerley A) or the peripheral intersti-

tium (Kerley B).6,7 The linear opacities

may be single or multiple, regional or

diffuse, and short or long, depending

on the etiology and severity of disease.

The most common cause of the linear

pattern is hydrostatic pulmonary

edema (Figure 1), but other etiologies

include lymphangitic carcinomatosis

Table 1. Pattern Approach to Interstitial Lung Disease.

Diagnosis Radiographic and clinical clues

Linear patternInterstitial pulmonary edema Changes rapidly

Lymphangitic carcinomatosis May be asymmetric

Nodular septal thickening

Interstital pneumonia May be asymmetric

Nodular patternMetastases History of cancer

Sarcoidosis Upper- and mid-lung zone distributionLymphadenopathty

Miliary tuberculosis or fungal infections Very tiny nodules

Hypersensitivity pneumonitis Indistinct nodules

Langerhans-cell histiocytosis Upper-lung zone distributionConcurrent cystsLung hyperinflation

Silicosis or coal workers Upper-lung zone distributionpneumoconiosis Egg-shell lymph node calcifications

Occupational history

Reticular PatternIdiopathic pulmonary fibrosis Small lung volumes

Peripheral reticular opacities

Collagen vascular diseases Small lung volumesPeripheral reticular opacitiesClavicular erosions

Dilated esophagusSkin calcifications

Asbestosis Small lung volumesPeripheral reticular opacitiesPleural plaques

Langerhans-cell histiocytosis NodulesDiffuse reticular (cystic) patternUpper-lung zone distributionPulmonary hyperinflation

Lymphangioleiomyomatosis Young femaleDiffuse reticular (cystic) patternPulmonary hyperinflation

Tuberous sclerosis

Emphysema with concurrent fibrotic Older patientinterstitial lung disease Peripheral reticular pattern

Hyperlucent lungsPulmonary hyperinflation

Bronchiectasis Ring shadows (central reticular pattern)Tram linesBronchi larger than adjacent artery


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(Figure 2), and atypical interstitial pneu-

monias such as those caused by myco-

plasma, chlamydia, cytomegalovirus

(CMV), and respiratory syncytial virus

(RSV).4 Interstitial pulmonary edema

tends to be symmetric in distribution

while atypical infections and lymphan-

gitic carcinomatosis may be asymmet-

rical. A linear pattern with nodular

interstitial thickening strongly suggests

a diagnosis of lymphangitic carcino-

matosis. In addition, clinical history is

often helpful in determining the etiol-

ogy as fever, cough and patient age

would suggest pneumonia while an

improvement in symptoms after a trial

of diuretics in a patient with known car-

diac disease would suggest congestiveheart failure (CHF). No improvement in

symptoms after treatment with diuretics

or antibiotics should raise suspicion for

lymphangitic carcinomatosis.

The linear pattern, as viewed with

high resolution computed tomography

(HRCT), is often referred to as inter-

lobular septal thickening. The normal

interlobular septum is approximately

0.1 mm in thickness and is occasion-

ally visible on normal scans. The inter-

lobular septae outline the secondary

pulmonary lobule and represent the

HRCT equivalent of Kerley B lines.7

Abnormal thickening can be described

as smooth, beaded or irregular.5 Causes

of smooth septal thickening include

pulmonary edema and atypical intersti-

tial pneumonias. Lymphangitic carci-

nomatosis may cause either beaded or

smooth septal thickening.7

The nodular patternThe nodular pattern on chest radiog-

raphy is characterized by multiple

small, discrete, rounded opacities that

range in diameter from 2 to 10 mm.

4,5,7

The differential diagnosis for the nodu-

lar pattern can be separated into 3 main

categories based on etiology: nodular

metastases, nodular pneumoconioses

and the granulomatous diseases. The

most common malignancies resulting

in this pattern are thyroid, breast and

renal-cell carcinoma, with the nodules

measuring up to 10 mm in diameter.4

The nodular pneumoconioses in-

clude silicosis (Figures 3) and coal

workers pneumoconiosis (CWP). In

these diseases, the nodules are small

and have sharp borders. When present,

peripheral egg shell calcification of

hilar and mediastinal lymph nodes is

virtually pathognomonic for these enti-

ties.4 Clinical history, particularly occu-

pational history, is helpful in diagnosis.

Miners, sandblasters, ceramic workers,

and manufacturers of paint and var-

nishes have significantly increased risk.

Granulomatous diseases include

pulmonary sarcoidosis (Figure 4)

hypersensitivity pneumonitis (HP),

Langerhans-cell histiocytosis (Fig-

ure 5, formerly known as pulmonaryhistiocytosis X or eosinophilic granu-

loma), and miliary infections caused

by tuberculosis, cryptococcosis, coc-

cidiomycosis, and histoplasmosis.

Pulmonary sarcoidosis is the most

common of the granulomatous diseases.4

Over 90% of patients have an abnormal

chest radiograph where the disease is

generally divided into 4 radiographic

FIGURE 1. Interstitial pulmonary edema.

Chest radiograph demonstrating interstitial

pulmonary edema with characteristic thick-

ening of the axial interstitium (Kerley A lines

denoted by the thin arrows) and peripheral

interstitium (Kerley B lines denoted by the

thick arrows).

FIGURE 2. Lymphangitic carcinomatosis. Chest radiograph (A) demonstrating beaded septal

thickening caused by lymphangitic spread of carcinoma. Correlated computed tomography (CT,

B) image demonstrating beaded septal thickening caused by lymphangitic spread of carcinoma.

FIGURE 3. Silicosis. Chest radiograph (A) demonstrating small discrete nodules discrete

nodules in the bilateral upper lobes. In addition there is egg-shell calcification of a left hilar

lymph node. CT image (B) demonstrating small discrete centrilobular nodules in the bilateral

upper lungs.

A B

A B


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stages. Stage I presents with lym-

phadenopathy. Stage II is characterized

by lymphadenopathy and nodular lung

disease. Stage III exhibits nodular lung

disease but little evidence of lym-

phadenopathy. Stage IV demonstrates

lung fibrosis that can resemble

advanced tuberculosis. As the radi-

ographic stage increases, the disease

prognosis worsens.6

It may be helpful to separate the dif-

ferential diagnoses of nodular lung dis-

ease by lung zone predominance. Both

silicosis and CWP generally appear in

the upper-lung zones. Sarcoidosis and

LCH typically have upper- and mid-

dle-lung zone predominance. HP,

metastases and miliary infections typi-

cally have a diffuse or disseminated

appearance.4

Nodules are typically easier to iden-tify and accurately diagnose on HRCT.

With HRCT, nodules can be further

described according to margins

(smooth vs. irregular), presence or

absence of cavitations, and distribu-

tion.7,9 While patterns of nodule distrib-

ution can be difficult to appreciate

radiographically, Raoof et al. describe

an algorithmic approach to diagnosis

FIGURE 4. (A) Stage II pulmonary sarcoidosis: Chest radiograph demonstrating sarcoidosis with medi-

astinal lymphadenopathy and associated parenchymal disease. (B) Stage III pulmonary sarcoidosis:

Chest radiograph demonstrating parenchymal involvement with lack of associated lymphadenopathy.

(C) Pulmonary sarcoidosis: Selected axial CT image demonstrating nperilymphtic nodules..

BA C

FIGURE 5. Langerhans-cell histiocytosis (eosinophilic granuloma). Chest radiograph (A)

demonstrates reticulonodular opacities with upper-lobe predominance. A selected CT image

(B) demonstrates centrilobular nodularity.

FIGURE 6. Idiopathic pulmonary fibrosis.

Chest radiograph demonstrates peripheraland basilar interstitial disease.

A B

FIGURE 7. Collagen vascular disease.Chest radiograph demonstrates diffuse

interstitial thickening of the periphery and

in the lung bases as well as a dilated

esophagus.

FIGURE 8. Drug-induced pneumonitis.Chest radiograph shows increased reticular

interstitial thickening of the lung periphery

as well as the bronchovascular (axial) inter-

stitium and cardiomegaly.


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on HRCT which can be used to better

characterize diffuse nodular lung dis-

ease.10 Based on their distribution with

respect to the secondary pulmonary

lobule on HRCT, nodules can be classi-

fied as affecting the central structures

(centrilobular nodules) or affectingperipheral structures (perilymphatic

nodules), and a more specific diagnosis

may be achievable.

The reticular patternThe reticular pattern as seen on

chest radiography and computed

tomography (CT or HRCT) is depicted

by numerous, small, linear opacities

which, by summation, have been

described as a lace-like or net-like in

appearance.4,5 The reticular pattern can

be divided into 3 distinct groups, eachof which suggests different diagnoses:

peripheral reticular pattern with small

lung volumes, diffuse reticular/cystic

pattern with normal or increased lung

volumes, and airway/central reticular

pattern.

Peripheral reticular patternwith small lung volumes

The peripheral reticular pattern

demonstrates lucent spaces, which are

typically


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most common of which is idiopathic

pulmonary fibrosis (IPF), as seen in

Figure 6. However, this entity often

remains a diagnosis of exclusion.11

Other common causes include collagen

vascular diseases (Figure 7) such as

rheumatoid disease and scleroderma.12

Clinical history, such as age and symp-

toms, and laboratory findings such as

elevated rheumatoid factor or antinu-

clear antibodies (ANA) may be help-

ful. Other radiologic findings such as

clavicular erosions in RA and

esophageal dilation in scleroderma

may also offer significant clues to the

diagnosis.4 Asbestosis, cryptogenic

organizing pneumonia (COP), and pul-

monary drug toxicity (Figure 8) can

also demonstrate this pattern.7 The

idiopathic pneumonias such as desqua-mative interstitial pneumonia (DIP),

acute interstitial pneumonia (AIP),

nonspecific interstitial pneumonia

(NSIP), and lymphoid interstitial pneu-

monia (LIP) may have features consis-

tent with the peripheral reticular

pattern. However, they will often have

other findings, such as consolidation in

AIP or nodules and cysts in LIP.13,14 The

peripheral reticular pattern is uncom-

mon in sarcoidosis.11

Reticular (cystic) patternwith normal or increasedlung volumes

The diffuse reticular pattern is char-

acterized by global involvement of the

lungs and is often referred to as a cystic

pattern. The disease processes are char-

acterized by normal and/or increased

lung volumes and include: emphysema

with concurrent pulmonary fibrosis

(Figure 9), Langerhans-cell histiocyto-

sis (Figure 10), and lymphangioleiomy-

omatosis (Figure 11), which is histo-logically identical to pulmonary in-

volvement in tuberous sclerosis (TS).7

Langerhans-cell histiocytosis is best

characterized by preservation of lung

volume with reticulonodular opacities

in the upper- and middle-lung zones of

a smoking patient with sparing of the

costophrenic angles. HRCT demonstrates

bizarre-shaped cysts with irregular cen-

trilobular nodules of the distal airways. In

comparison, lymphangiomyomatosis

(LAM) shows larger thin-walled cysts

that are diffusely distributed in the lungs

and eventually involve the entire lungparenchyma. Also, LAM can result in

hyperinflation of the lungs in the later

stages of the disease.

Airway (central reticular) patternThe airway pattern, or central reticu-

lar pattern, typically spares the periph-

ery of the lungs and may demonstrate

larger spaces (5 to 10 mm) than those

caused by the peripheral and diffuse

reticular patterns. The most common

cause of this pattern is bronchiectasis.4

On radiography, bronchiectasis appearsas nontapering bronchi, as well as

dilated bronchi, which are larger than

the adjacent pulmonary artery branch.

Bronchiectasis is often associated with

bronchial wall thickening. When

viewed face on, dilated bronchi appear

as thickened rings, while those viewed

in-plane appear as thickened parallel

lines. Based on etiology, diffuse bronchi-

ectasis can be divided into congenital

and acquired diseases. Congenital

causes include cystic fibrosis (CF,

Figure 12), immotile cilia syndrome

(Kartageners syndrome), hyper-IgE

syndrome, and common-variable

immunodeficiency. Some of the ac-

quired causes of diffuse bronchiectasis

include allergic bronchopulmonary

aspergillosis (Figure 13), fibrosis with

traction bronchiectasis, aspiration,

extrinsic bronchial obstruction, and

toxin inhalation.11

Conclusion

Conceptualization of the interstitialanatomy of the lung aids the radiolo-

gist in understanding of the basic pat-

terns of interstitial lung disease which

allows for more accurate recognition

and characterization of these disorders.

While the evaluation of a chest radi-

ograph with suspected interstitial lung

disease is often viewed as a formidable

task by many radiologists, a firm

understanding of the pattern-based

approach to characterization of diffuse

lung diseases is important. A summary

of this pattern approach for chest radi-

ography is provided in Table 1. Utiliz-ing this approach, it is often possible to

establish a relevant differential diagno-

sis while still recommending further

evaluation with HRCT to better char-

acterize distribution and extent of the

lung parenchymal abnormality.

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