HRCT

DR Shahid Pervaiz

Post Graduate trainee

Dept. Of Pumonology

Nishtar Hospital Multan

INTRODUCTIONINTRODUCTION

• HRCT -- Use of thin section CT images (0.625 to 2 mm slice thickness) often with a high-spatial-frequency reconstruction algorithm to detect and characterize disease affecting the pulmonary parenchyma and airways.

• Superior to chest radiography for detection of lung disease, points a specific diagnosis and helps in identification of reversible disease.

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Thin section produces better contrast between lung parenchyma and bronchus and pulmonary vessel. A scan obtained with increased slice thickness, produces volume averaging with blurring of pathological details.

The division of trachea gives rise to the left and right mainstream bronchi, which further divides into lobar and segmental bronchi. Segmental bronchi divides after 6 to 20 division they no longer contain cartilage in their walls and are referred to as bronchioles.

There are approximately 23 generation of dichotomous branchingFrom trachea to the alveolar sac

HRCT can identify upto 8th order central bronchioles

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SUBJECTS

Anatomy of the secondary lobule

Basic HRCT patterns

Distribution of abnormalities

Differential diagnosis of interstitial lung diseases

Secondary lobule

• The secondary lobule is the basic anatomic unit of pulmonary structure and function.Interpretation of interstitial lung diseases is based on the type of involvement of the secondary lobule.It is the smallest lung unit that is surrounded by connective tissue septa.It measures about 1-2 cm and is made up of 5-15 pulmonary acini, that contain the alveoli for gas exchange.

Secondary lobuleBasic anatomic unit of pulmonary structure and function.

1-2 cm and is made up of 5-15 pulmonary acini

Supplied by a small bronchiole (terminal bronchiole) in the center, that is parallelled by the centrilobular artery.

Pulmonary veins and lymphatics run in the periphery

Two lymphatic systems: central network peripheral network

• The secondary lobule is supplied by a small bronchiole (terminal bronchiole) in the center, that is parallelled by the centrilobular artery. Pulmonary veins and lymphatics run in the periphery of the lobule within the interlobular septa.Under normal conditions only a few of these very thin septa will be seen.

There are two lymphatic systems: a central network, that runs along the bronchovascular bundle towards the centre of the lobule and a peripheral network, that is located within the interlobular septa and along the pleural linings.

The terminal bronchiole in the center divides into respiratory bronchioli with acini that contain alveoli. Lymphatics and veins run within the interlobular septa

Centrilobular area

It is the central part of the secondary lobule.It is usually the site of diseases, that enter the lung through the airways ( i.e. hypersensitivity pneumonitis, respiratory bronchiolitis, centrilobular emphysema ).

Centrilobular area in blue perilymphatic area in yellow

Perilymphatic area

Perilymphatic areais the peripheral part of the secundary lobule.It is usually the site of diseases, that are located in the lymphatics of in the interlobular septa ( i.e. sarcoid, lymphangitic carcinomatosis, pulmonary edema). These diseases are usually also located in the central network of lymphatics that surround the bronchovascular bundle.

Raoof, S. , CHEST 2006; 129:805

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A group of terminal bronchioles

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Unit of lung (0.5-3 cm)Irregularly polyhedral Supplied by a group of terminal bronchioles and accompanying pulmonary arterioles surrounded by lymph vesselsDemarcated by “interlobular septa”

pulmonary veinspulmonary lymphaticsconnective tissue stroma

Accompanying pulmonary arterioles

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Surrounded by lymph vessels

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Pulmonary veins

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Pulmonary lymphatics

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Connective Tissue StromaConnective Tissue Stroma

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Perilymphatic distribution

Centrilobular distribution

Random distribution

DOTSss.....

TO SUM UP..

• Random – touch pleura – scattered in lung

• Centrilobular –away from pleura

• Perilymphatic – around vessels, bronchi – touch pleura or fissure

Size, Distribution, Appearance

Nodules and Nodular Opacities

SizeSize

Small Nodules: <10 mm Miliary - <3 mmSmall Nodules: <10 mm Miliary - <3 mm

Large Nodules: >10 mm Masses - >3 cmsLarge Nodules: >10 mm Masses - >3 cms

AppearanceAppearance

Interstitial opacity: Well-defined, homogenous,Soft-tissue densityObscures the edges of vessels or adjacent structure

Interstitial opacity: Well-defined, homogenous,Soft-tissue densityObscures the edges of vessels or adjacent structure

Air space: Ill-defined, inhomogeneous.Less dense than adjacent vessel – GGOsmall nodule is difficult to identify

Air space: Ill-defined, inhomogeneous.Less dense than adjacent vessel – GGOsmall nodule is difficult to identify

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Interstitial nodules

Air space opacity

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Miliary tuberculosis

sarcoidosis

in a lung transplant patient with bronchopneumonia

RANDOM: no consistent relationship to any structuresRANDOM: no consistent relationship to any structures

PERILYMPHATIC: corresponds to distribution of lymphaticsPERILYMPHATIC: corresponds to distribution of lymphatics

CENTRILOBULAR: related to centrilobular structuresCENTRILOBULAR: related to centrilobular structuresDistributionDistribution

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Reticular patternIn the reticular pattern there are too many lines, either as a result of thickening of the interlobular septa or as a result of fibrosis as in honeycombing.

Focal septal thickening in lymphangitic carcinomatosis

Septal thickening and ground-glass opacity with a gravitational distribution in a patient with cardiogenic pulmonary edema.

Notice the nodules along the fissures indicating a perilymphatic distribution (red arrows).

The majority of nodules located along the bronchovascular bundle (yellow arrow).

Sarcoidosis

The majority of nodules located along the bronchovascular bundle (yellow arrow).

PERILYMPHATIC NODULES

Perilymphatic and Random

distribution of nodules , seen in

sarcoidosis.

Centrilobular distribution

Hypersensitivity pneumonitis Respiratory bronchiolitis in smokers infectious airways diseases (endobronchial spread of tuberculosis or nontuberculous mycobacteria, bronchopneumonia) Uncommon in bronchioloalveolar carcinoma, pulmonary edema, vasculitis

Tree-in-bud Centrilobular nodules m/b further characterized by presence or

absence of ‘‘tree-in-bud.’’

Tree-in-bud -- Impaction of centrilobular bronchus with mucous, pus, or fluid, resulting in dilation of the bronchus, with associated peribronchiolar inflammation .

Dilated, impacted bronchi produce Y- or V-shaped structures

This finding is almost always seen with pulmonary infections.

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Tree-in-budTree-in-bud describes the appearance of an irregular and often nodular branching structure, most easily identified in the lung periphery.

Typical Tree-in-bud appearance in a patient with active TB.

Random distribution

Small random nodules are seen in: Hematogenous metastases

Miliary tuberculosis

Miliary fungal infections

Sarcoidosis may mimick this pattern, when very extensive

Langerhans cell histiocytosis (early nodular stage)

Langerhans cell histiocytosis: early nodular stage before the typical cysts appear.

Attenuation pattern

High Attenuation pattern

GROUND GLASS CONSOLIDATION

Low Attenuation pattern

Emphysema Lung cysts (LAM, LIP, Langerhans cell histiocytosis) Bronchiectasis Honeycombing

Dark bronchus sign in ground glass opacity. Complete obscuration of vessels in consolidation.

Ground-glass opacity

Broncho-alveolar cell carcinoma with ground-glass opacity and consolidation

Consolidation

Two patients with chronic consolidations as a result of COP (cryptogenic organizing pneumonia)

Mosaic attenuationThe term 'mosaic attenuation' is used to describe density differences between affected and non-affected lung areas.

Mosaic attenuation

Lung density and attenuation depends partially on amount of blood in lung tissue.

May be due to vascular obstruction, abnormal ventilation or airway disease

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Mosaic pattern in a patient with hypersensitivity pneumonitis

Mosaic pattern in a patient with chronic thromboemboli

Crazy Paving PatternCrazy Paving is a combination of ground glass opacity with

superimposed septal thickening

Crazy Paving can be seen in: Alveolar proteinosis Sarcoid NSIP Organizing pneumonia (COP/BOOP) Infection (PCP, viral, Mycoplasma, bacterial) Neoplasm (Bronchoalveolarca (BAC) Pulmonary hemorrhage Edema (heart failure, ARDS, AIP)

CRAZY PAVING PATTERNIt is scattered or diffuse ground-glass attenuation with superimposed interlobular septal thickening and intralobular lines.

Causes:

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Crazy Paving in a patient with Alveolar proteinosis.

Crazy Paving

Combination of ground glass opacity and septal thickening : Alveolar proteinosis.

Combination of ground glass opacity and septal thickening : Alveolar proteinosis

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Head cheese signIt refers to mixed densities which includes

# consolidation # ground glass

opacities # normal lung # Mosaic perfusion

• Signifies mixed infiltrative and obstructive disease

Head cheese signCommon cause are :

1. Hypersensitive pneumonitis

2. Sarcoidosis

3. DIP

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Headcheese sign

Headcheese sign in hypersensitivity pneumonitis.

HRCT scan shows lung with a geographic appearance, which represents a combination of patchy or lobular ground-glass opacity (small arrows) and mosaic perfusion (large arrows).

Low Attenuation pattern

Emphysema

Lung cysts (LAM, LIP, Langerhans cell histiocytosis)

Bronchiectasis

Honeycombing

Emphysema

Emphysema typically presents as areas of low attenuation without visible walls as a result of parenchymal destruction.

EMPHYSEMA Permanent, abnormal enlargement of air spaces distal to the terminal bronchiole and accompanied by the destruction of the walls of the involved air spaces.

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Centrilobular emphysema Most common type Irreversible destruction of alveolar walls

in the centrilobular portion of the lobule Upper lobe predominance and uneven

distribution Strongly associated with smoking.

Centrilobular (proximal or centriacinar) emphysema

Found most commonly in the upper lobes

Manifests as multiple small areas of low attenuation without a

perceptible wall, producing a punched-out appearance.

Often the centrilobular artery is visible within the centre of these lucencies.

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Centrilobular emphysema due to smoking. The periphery of the lung is spared (blue arrows). Centrilobular artery (yellow arrows) is seen in the center of the hypodense area.

Panlobular emphysema Affects the whole secondary lobule Lower lobe predominance In alpha-1-antitrypsin deficiency, but

also seen in smokers with advanced emphysema

PANLOBULAR EMPHYSEMA Affects the entire secondary pulmonary lobule and is more pronounced in the lower zones

Complete destruction of the entire pulmonary lobule.

Results in an overall decrease in lung attenuation and a reduction in size of pulmonary vessels

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PANLOBULAR EMPHYSEMA

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Panlobular emphysema

Paraseptal (distal acinar) emphysema

Affects the peripheral parts of the secondary pulmonary lobule

Produces subpleural lucencies.

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Paraseptal emphysema

Cystic lung disease

Lung cysts are defined as radiolucent areas with a wall thickness of less than 4mm.

Langerhans cell histiocytosis

Lymphangiomyomatosis complicated by pneumothorax

Bronchiectasis

Bronchiectasis is defined as localized bronchial dilatation. (signet-ring sign)

bronchial wall thickening

lack of normal tapering with visibility of airways in the peripheral lung

mucus retention in the broncial lumen

associated atelectasis and sometimes air trapping

ABPA: glove-finger shadow due to mucoid impaction in central bronchiectasis in a patient with asthma.

Signet-Ring Sign

A signet-ring sign represents an axial cut of a dilated bronchus (ring) with its accompanying small artery (signet).

Tram Tracks

Bronchial dilation with lack of tapering .

HONEYCOMBINGDefined as - small cystic spaces with irregularly thickened walls composed of fibrous tissue.

Predominate in the peripheral and subpleural lung regions

Subpleural honeycomb cysts typically occur in several contiguous layers. D/D- paraseptal emphysema in which subpleural cysts usually occur in a single layer.

Indicates the presence of “END stage” disease regardless of the cause.

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HoneycombingHoneycombing is defined by the presence of small cystic spaces with irregularly thickened walls composed of fibrous tissue.

Causes

Lower lobe predominance : 1. UIP or interstitial fibrosis 2. Connective tissue disorders 3. Hypersensitivity pneumonitis 4. Asbestosis 5. NSIP (rare)

Upper lobe predominance : 1. End stage sarcodosis 2. Radiation 3. Hypersensitivity Pneumonitis 4. End stage ARDS

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Honeycombing

HRCT showing subpleural broncheolectasis

Honeycombing and traction bronchiectasis in UIP.

Typical UIP with honeycombing and traction bronchiectasis in a patient with idiopathic pulmonary fibrosis (IPF)

Distribution within the lung

Additional findings

Differential diagnosis of interstitial lung diseases

Reticular pattern

Nodular pattern

High Attenuation pattern

Low Attenuation pattern

Lymphangitic carcinomatosis: irregular septal thickening, usually focal or unilateral 50% adenopathy', known carcinoma.

Cardiogenic pulmonary edema: incidental finding in HRCT, smooth septal thickening with basal predominance (Kerley B lines), ground-glass opacity with a gravitational and perihilar distribution, (peribronchial cuffing)

Cardiogenic pulmonary edema

Lymphangitic carcinomatosis

Lymphangitic carcinomatosis with hilar adenopathy

Nodular pattern

1.Hypersensitivity pneumonitis:2.Miliary TB: random nodules 3.Sarcoidosis4.Hypersensitivity pneumonitis

Nodular pattern

Hypersensitivity pneumonitis Miliary TB

Sarcoidosis Hypersensitivity pneumonitis

Low Attenuation pattern

Lymphangiomyomatosis (LAM) LCH

Honeycombing Centrilobular emphysema

Low Attenuation pattern (2)

Centrilobular emphysema: Langerhans cell histiocytosis (LCH)

Honeycombing. Lymphangiomyomatosis (LAM)

Q.1. What is the dominant HR-pattern ?

Q.2. Where is it located within the secondary lobule (centrilobular, Perilymphatic or random) ?

Q.3. Is there an upper versus lower zone or a central versus peripheral predominance ?

Q.4. Are there additional findings (pleural fluid, lymphadenopathy, traction bronchiectasis) ?

STRUCTURED APPROACH

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Conclusion • A thorough knowledge of the basic anatomy is of

utmost importance.

When attempting to reach a diagnosis or differential diagnosis of lung disease using HRCT, the overall distribution of pulmonary abnormalities should be considered along with their morphology, HRCT appearance, and distribution relative to lobular structures.

Correlation of the radiological findings with patients clinical and laboratory findings to reach a likely diagnosis

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