HRCT Lung 19th May 2012

transcript

TECHNICAL ASPECT OF HRCT; NORMAL LUNG ANATOMY & HRCT FINDINGS OF LUNG DISEASE Presenter : Sarbesh Tiwari

Moderator

Dr (Prof) R.K GOGOI.

HRCT ------ MEANING

The problem with the terminologyo It is often used for anything and everything to do

with “high resolution”.

Actual meaning o A scan performed using high- spatial frequency

algorithm to accentuate the contrast between tissue of widely differing densities, eg.,

- air & vessels (lung)

- air & bone (temporal & paranasal sinus)

INTRODUCTION

• HRCT is the 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 in terms of improved detection of lung disease, provision for specific diagnosis and identification of reversible disease.

HISTORY

• 1982– The term HRCT was first used by TODO et. Al

• 1985 – Nakata et.al and Naidich et.al published first report on HRCT

Since then has been an important tool in pulmonary medicine

• Recent development of MDCT scanner capable of volumetric high resolution scanning has improved the investigation

TECHNICAL ASPECT

Parameterso Slice thicknesso Kvpo mAso Scan timeo FoVo Interslice gap (collimation)o Filming.

SLICE THICKNESS

• Thin sections 0.5 – 1.5 mm is essential for optimal spatial

resolution

• Thicker slices are prone for volume averaging and reduces

ability to resolve smaller structure

• Better for delineation of bronchi, wall thickness and

diameter

Reconstruction Algorithm

• Using a high-resolution algorithm is critical element in performing HRCT.

• High spatial frequency or sharp algorithm -- bone algorithm is used which reduces image smoothing and increases spatial resolution, making structure appear sharper.

Kilovolts (Peak), Milliamperes, and Scan Time

• In HRCT image, noise is more apparent than standard CT.

• Noise – 1/√ mAs X Kvp X scan time

• As increasing scan time is not feasible, mAs and Kvp are altered to reduce noise

• Noise decreases with increase in Kvp and mAs.

• For routine technique – Kvp -- 120-140 mAs -- 200- 340

• Increased patient and chest wall thickness are associated with increase image noise, may be reduced by increasing mA and Kvp

• Scan Time : As low as possible (1-2 sec) to minimize motion artifact.

LOW DOSE HRCT

• Low dose HRCT uses Kvp of 120- 140 and mA of 30-20 at 2 sec scan time.

• The low dose HRCT is equivalent to conventional HRCT in 97 % of cases

• The low dose HRCT may fail to identify ground glass opacity in few cases and have more prominent streak artifact.

Matrix size, Field of View, and Target reconstruction

• Matrix size : Largest available matrix s/b used – 512 x 512

• Field of view : smallest FOV that will encompass the patient is used as it will reduce the pixel size.

• Retrospectively targeting image reconstruction to a single lung instead of the entire thorax significantly reduces the FOV and image pixel size, and thus increases spatial resolution.

• Interslice gap – varies from examination to examination, but is usually 10- 20 mm

• INSPIRATORY LEVEL : Routine HRCT is obtained in suspended full inspiration, which

optimizes contrast between normal structures, various abnormalities and normal aerated lung parenchyma; and

reduces transient atelectasis, a finding that may mimic or obscure significant abnormalities.

• EXPIRATORY SCAN : valuable in obstructive lung disease or airway abnormality

Patient Position and the Use of Prone Scanning

• Supine adequate in most instances.

• Prone for diagnosing subtle lung abnormalities.

e.g., asbestosis, suspected early lung fibrosis

• Atelectasis is commonly seen in dependent lung both in

normal & abnormal subjects.

• Dependent density or Subpleural line.

Supine and prone scans with normal "dependent density.”

Persistent opacity in the posterior lung in a patient with pulmonary fibrosis.

TECHNIQUE OF SCAN ACQUISITION:

1. Spaced axial scans :

• Obtained at 1cm intervals from lung apices to bases. In this manner, HRCT is intended to “sample” lung anatomy

• It is assumed that the findings seen at the levels scanned will be representative of what is present throughout the lungs

• Results in low radiation dose as the individual scans are widely placed

2. Volumetric HRCT -

• MDCT scanner are capable of rapid scanning and thin slice acquisition.

Advantages :

1. viewing of contagious slice for better delineation of lung

abnormality

2. Complete imaging of lung and thorax

3. Reconstruction of scan data in any plane using MIPs or

MinIPs.

4. diagnosis of other lung abnormalities

Disadvantage : greater radiation dose

Multidetector Helical HRTC

• Multidetector CT is equipped with a multiple row detector array

• Multiple images are acquired due tompresence of multiple detectors

• Advantages : - shorter acquisition times and retrospective creation of both thinner and thicker sections from the same raw data

• Acquisition time of HRCT with multidetector CT is so short that whole-lung HRCT can be performed in one breath-hold.

Which is better HRCT or MD- HRCT

• Increased table speed may increase the volume-averaging artifact and may result in indistinctness of subtle pulmonary abnormalities.

• MDCT provides for better reconstruction in Z axis

• Various study shows the image quality of axial HRCT with multidetector CT is equal to that with conventional single-detector CT.

Radiation dose

• HRCT performed with spaced axial images results in low radiation dose as compared with MD-HRCT.

• PA CHEST Radiograph ----- ----- ----- 0.05 mSv

• Spaced axial HRCT (10mm space) ----- 0.7 mSv ( 14 X

• Spaced axial HRCT (20 mm space) ------ 0.35 mSv ( 7 X ray)

• Low Dose Spaced axial HRCT ------- 0.02 mSV

• MD-HRCT ------- 4 - 7 msv ( 60-80 x ray)

Combining HRCT scan at 20 mm interval with low mAs scan (40 mAs) would result in radiation comparable to conventional X ray.

HRCT ARTIFACT

• Streak Artefacts :

Fine, linear, or netlike opacities

Radiate from the edges of sharply

marginated , high-contrast structures

such as bronchial walls, ribs, or

vertebral bodies.

More evident on low mA

Mechanisms: beam hardening,

photon starvation, and aliasing.

Motion-related artifacts

• Pulsation / Star artifacts

• Doubling artifacts.

• Stair-step artifacts in sag/coro reconstruction.

• Scanners with rapid scan time

(100ms), ECG gating or Spirometry controlled

scanning.

REVIEW OF ANATOMY

PART 2

LUNG ANATOMY

• Right lung is divided by major and minor fissure into 3 lobes and 10 bronchopulmonary segments

• Left lung is divided by major fissure into 2 lobes with a lingular lobe and 8 bronchopulmonary segments

1.1 kg

TRACHEAL ANATOMY

• 10-12 cm in length, from C6 level to upper border of D5.

• Extrathoracic (2-4cm) and Intrathoracic(6-9cm beyond manubrium)

• In men, tracheal diameter – 25-27 mm

women – 21- 23 mm

• The posterior portion of the tracheal wall is a thin fibromuscular membrane----- allows for oesophageal expansion.

BRONCHIAL ANATOMY

• Approximately 23 generations of branches from the trachea to the alveoli.

• Bronchi with a wall thickness of less than 300 um is not visible on CT or HRCT.

• As a consequence, normal bronchi less than 2 mm in diameter or closer than 2 cm from pleural surfaces equivalent to seventh to ninth order airways are generally below the resolution even of high-resolution CT

BRONCHUS

BLOOD SUPPLY Bronchial Arteries—

2 on left side i.e. superior and inferior

1 on right side)

Left arises from thoracic aorta

Right from either thoracic aorta, sup. lt. bronchial or right 3rd intercostal artery

VENOUS DRAINAGE

on right- azygous vein

on left- left superior intercostal or accessory hemiazygous vein

• NERVE SUPPLY Pulmonary plexus at hilum (vagus and sympathetic)

BRONCHOARTERIAL RATIO (B/A)

• Internal diameter of both bronchus and accompanying arterial diameter calculated and ratio measured.

• Normally, the bronchi and adjacent pulmonary arteries similar in diameter.

• Normal ratio is 0.65-0.70

• B/A ratio >1 indicates bronchiectasis.

NB:: B/A ratio increases with age and may exceed 1 in normal patients > 40 years.

BRONCHIAL WALL THICKNESS (T/D)

• Wall thickness proportionately decreases as the airway divides further as according to the diameter of the airway.

SECONDARY PULMONARY LOBULE

• Smallest lung unit that is surrounded by connective tissue septa (Miller)

• The basic anatomic unit

• Irregular polyhedral in shape.

• Measures 1 to 2.5 cm

Anatomy of the Secondary Lobule and Its Components

1. Interlobular septa and contiguous subpleural interstitium,

2. Centrilobular structures, and

3. Lobular parenchyma and acini.

PULMONARY ACINUS

It is smaller than secondary lobule

Defined as portion of lung distal a terminal bronchiole and supplied by respiratory bronchiole

It is the largest lung unit in which all airways participate in gas exchange

Size is 7 to 8 mm in adults

Secondary pulmonary lobule comprises of a dozen or fewer acini (3 to 24)

A group of terminal bronchioles

Accompanying pulmonary arterioles

Surrounded by lymph vessels

Pulmonary veins

Pulmonary lymphatics

Connective Tissue Stroma

LUNG INTERSTITUM

Lung interstitium

Axiel fiber system

Peribronchovascular

interstitium

Centrilobular

interstitium

Peripheral fiber sysem

Subpleural interstitium

Interlobular septa

• The peribronchovascular interstitum invests the bronchi and pulmonary artery in the perihilar region.

• The centrilobular interstitium are associated with small centrilobular bronchioles and arteries

• The subpleural interstitium is located beneath the visceral pleura; envelops the lung into fibrous sac and sends connective tissue septa into lung parenchyma.

• Interlobular septa constitute the septas arising from the subpleural interstitium.

LUNG INTERSTITUM

The normal pulmonary vein branches are seen marginating pulmonary lobules. The centrilobular artery branches are visible as a rounded dot

Anatomy of pleural surfaces and chest wall.

NORMAL LUNG

ATTENUATION

• Normal lung attenuation : –700 to – 900 HU

• Attenuation gradient : densest at dependent region of lung due to regional difference in blood and gas density due to gravity

Difference in attenuation of anterior and posterior lung ranges from 50 – 100 HU

• In children, lung attenuation in children is greater than adults.

NORMAL EXCRETORY HRCT

• Performed to detect air trapping in small airway obstruction

• Attenuation increases with expiration (ranges from 100 to 130 HU)

• 60 % of normal individual shows air trapping in the superior segment of lower lobe and involving single lobule, needs consideration

PATTERN OF LUNG DISEASE IN HRCT

PART 3

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

INCREASED LUNG ATTENUATION

LINEAR AND RETICULAR OPACITIES

NODULES AND NODULAR OPACITIES

PARENCHYMAL OPACIFICATION

consolidation

Ground glass

DECREASED LUNG ATTENUATION

CYSTIC LESIONS, EMPHYSEMA, AND BRONCHIEACTASIS

MOSAIC ATTENUATION AND PERFUSION

AIR TRAPPING ON EXPIRATORY SCANS

LINEAR AND RETICULAR OPACITIES

• Represents thickening of interstitial fibers of lung by

- fluid or

- fibrous tissue or

- interstitial infiltration by cells

Interface sign

Irregular interfaces between the aerated lung parenchyma and bronchi, vessels, or visceral pleural surfaces.

Represent thickened interlobular septa, intralobular lines, or irregular scars.

Nonspecific.

Common in patients with an interstitial abnormality, fibrotic lung disease.

Peribronchovascular Interstitial Thickening

Smooth

Pulmonary edema/ hemorrhage

Lymphoma / leukemia

Lymphangitic spread of carcinoma

Nodular

Sarcoidosis

Lymphangitic spread of carcinoma

Irregular

Due to adjacent lung fibrosis

Sarcoidosis, silicosis, TB and

talcosis

Venous, lymphatic or infiltrative

diseaselymphatic or

infiltrative diseases

sarcoidosis

UNILATERAL LYMPHANGITIC SPREAD OF CARCINOMA

Smooth Septal thickening

Nodular Septal thickening

PARENCHYMAL BANDS

• Non tapering , reticular opacity usually 1 to 3 mm in thickness and from 2 to 5 cm in length.

• Is often peripheral and generally contracts the pleural surface

• D/D : 1. Asbestosis

2. Sarcoidosis

3. Silicosis/ coal worker pneumoconiosis

4. Tuberculosis with associated scarring.

Subpleural Interstitial Thickening

• Mimic thickening of fissure.• DD similar to that of interlobular septal

thickening.• more common than septal thickening in IPF or

UIP of any cause.

Intralobular interstitial thickening (Intralobular lines)

• Results in a fine reticular pattern on HRCT, with the visible lines separated by a few millimeters

• Fine lace- or netlike appearance

• Causes : Pulmonary fibrosis

Asbestosis

Chronic Eosinophilic pneumonitis.

HONEYCOMBING

• Defined as - small cystic spaces with irregularly thickened walls composed of fibrous tissue. They share wall.

• 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.

Causes

Lower lobe predominance : 1. UIP or interstitial fibrosis 2. collagen disease 3. hypersensitivity pneumonitis 4. Asbestosis 5. NSIP (rare)

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

Size, Distribution, Appearance

Nodules and Nodular Opacities

Small Nodules: <10 mm Miliary - <3 mm

Large Nodules: >10 mm Masses - >3 cms

Appearance

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

Interstial nodules Air space opacity

RANDOM: no consistent relationship to any structures

PERILYMPHATIC: corresponds to distribution of lymphatics

CENTRILOBULAR: related to centrilobular structures

Distribution

Perilymphatic distribution

Nodules in relation to pulmonary lymphatics at

# perihilar peribronchovascular interstitium,

# interlobular septa,

# subpleural regions, and

# centrilobular interstitium.

Perilymphatic nodules: D/D

Sarcoidosis

Lymphangitic carcinomatosis

Lymphocytic interstitial pneumonia (LIP)

Lymphoproliferative disorders

Amyloidosis

Centrilobular nodules

• Distributed primarily within the centre of the secondary pulmonary lobule

• Reflect the presence of either interstitial or airspace abnormalities

• Dense or ground-glass opacity

• Subpleural lung is typically spared- distinguishes from

diffuse random nodules.

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, associated with peribronchiolar inflammation .

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

This finding is almost always seen with pulmonary infections.

Box 1. Centrilobular nodules with or without tree-in-bud opacity: diagnostic considerations

With tree-in-bud opacity

Bacterial pneumonia

Typical and atypical mycobacteria infections

Bronchiolitis Diffuse

panbronchiolitis Aspiration Allergic

bronchopulmonary aspergillosis

Cystic fibrosis Endobronchial-

neoplasms (particularly

Bronchioloalveolar carcinoma)

Without tree-in-bud opacity

All causes of centrilobular nodules with tree-in-bud opacity

Hypersensitivity pneumonitis

Respiratory bronchiolitis

Cryptogenic organizing pneumonia

Pneumoconioses Langerhans’ cell

histiocytosis Pulmonary edema Vasculitis Pulmonary

hypertension

Random nodules Random nodules – No definable distribution

Random nodules, in contrast to perilymphatic nodules, usually do not show a patchy distribution in the lung parenchyma; rather, random nodules are usually distributed uniformly throughout the lung parenchyma in a bilaterally symmetric distribution.

Random nodules: Miliary tuberculosis.

Axial HRCT image shows multiple nodules scattered uniformly throughout the lung parenchyma.

Random nodules: diagnostic considerations

1. Haematogenous metastases

2. Miliary tuberculosis

3. Miliary fungal infection

4. Disseminated viral infection

5. Silicosis or coal-worker’s pneumoconiosis

6. Langerhans’ cell histiocytosis

Parenchymal Opacification

Ground-glass opacity

Consolidation

Lung calcification & high attenuation opacities.

GROUND GLASS OPACITIES• Hazy increased attenuation of lung, with

preservation of bronchial and vascular margins

• Pathology : it is caused by

# partial filling of air spaces,

# interstitial thickening,

# partial collapse of alveoli,

# normal expiration, or

# increased capillary blood volume

• D/T volume averaging of morphological abnormality too small to be resolved by HRCT

IMPORTANCE OF GGO

• Can represent - microscopic interstitial disease

(alveolar interstitium)

- microscopic alveolar space disease

- combination of both

Mostly indicates the presence of an ongoing, active, potentially treatable process (in the absence of fibrosis)

NB :: Ground Glass opacity should be diagnosed only on scans obtained with thin sections : with thicker sections volume averaging is more - leading to spurious GGO, regardless of the nature of abnormality

DIFFERNTIAL DIAGNOSIS : GGO

The location of the abnormalities in ground glass pattern can be helpful:

• Upper zone predominance:

• Lower zone predominance: UIP, NSIP, DIP.

• Centrilobular distribution:

Hypersensitivity pneumonitis,

GGO with few cystic lesion and reticular in HIV + ve patient -- PCP

Combination of GGO with fibrosis and tractional bronchiectasis-- NSIP

CRAZY PAVING PATTERN• It is scattered or diffuse ground-glass attenuation with

superimposed interlobular septal thickening and intralobular lines.

• Causes:

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

CONSOLIDATION

• Consolidation is defined as increased attenuation, which results in obscuration of the underlying vasculature, usually producing air bronchograms.

• The presence of consolidation implies that the air within affected alveoli has been replaced by another substance, such as blood, pus, edema, or cells.

• When consolidation is evident on a chest radiograph, HRCT does not usually provide additional diagnostically useful information.

CONSOLIDATION

D/D on the basis of presentation

Acute consolidation is seen in:

- Pneumonias (bacterial, mycoplasma, PCP)

- Pulmonary edema due to heart failure or ARDS

- Hemorrhage

- Acute eosinophilic pneumonia

Chronic consolidation is seen in:

- Organizing Pneumonia

- Chronic eosinophilic pneumonia

- Fibrosis in UIP and NSIP

- Bronchoalveolar carcinoma or lymphoma

Organizing pneumonia

Lung calcification & high attenuation opacities

Multifocal lung calcification

• Infectious granulomatous ds - TB, histoplasmosis, and varicella, pneumonia;

• Sarcoidosis , silicosis, Amyloidosis• Fat embolism associated with ARDS

Diffuse & dense lung calcification

• Metastatic calcification,

• Disseminated pulmonary ossification, or

• Alveolar microlithiasis

High attenuation opacity

• Talcosis asso with fibrotic mass,

• inhalation of metals (tin/barium)

Small focal areas of increased attenuation

• injection and embolized radiodense materials such as mercury or acrylic cement

Diffuse, increased lung attn in absence of calcification

• amiodarone lung toxicity or

• embolization of iodinated oil after chemoembolization

HRCT findings manifesting as decreased lung opacity

Lung Cysts,

Emphysema,

Bronchiectasis

Lung cysts

• Thin walled (less than 3mm) , well defined and circumscribed air containing lesions

• They are lined by cellular epithelium, usually fibrous or epithelial in nature.

• Common cause are : 1. Lymphangiomyomatosis

2. Langerhans Histiocytosis

3. Lymphoid interstial pneumonia

They need to be differentiated from emphysematous bullae, blebs and pneumatocele.

Axial HRCT image through the upper lobes shows multiple bilateral bizarre-shaped cysts (arrows) and small centrilobular nodules (arrowheads) in a smoker with Langerhans’ cell histiocytosis.

) Axial HRCT image through the upper lobes shows multiple bilateral uniform, thin-walled cysts.

BRONCHIEACTASIS

Bronchiectasis is defined as localized, irreversible dilation of the bronchial tree.

HRCT findings of the bronchiectasis include

# Bronchial dilatation

# Lack of bronchial tapering

# Visualization of peripheral airways.

BRONCHIAL DILATATION

# The broncho-arterial ratio (internal diameter of the bronchus /pulmonary artery) exceeds 1.

# In cross section it appears as “Signet Ring appearance”

LACK OF BRONCHIAL TAPERING

# The earliest sign of cylindrical bronchiectasis

# One indication is lack of change in the size of an airway over 2 cm after branching.

VISUALIZATION OF PERIPHERAL AIRWAYS

# Visualization of an airway within 1 cm of the costal pleura is abnormal and indicates potential bronchiectasis

Coned axial HRCT image shows bronchial dilation with lack of tapering (arrows). Bronchial morphology is consistent with varicose bronchiectasis.

Dilated bronchioles with arteries at periphery giving signet ring appearance

A NUMBER OF ANCILLARY FINDINGS ARE ALSO RECOGNIZED:

# bronchial wall thickening : normally wall of bronchus should be less than half the width of the accompanying pulmonary artery branch.

# mucoid impaction

# Air trapping and mosaic perfusion

Extensive, bilateral mucoid impaction Mosaic perfusion caused by large and small

airway obstruction.Small centrilobular nodules are visible in the

right lower lobe

1. CYLINDRICAL BRONCHIECTASIS

# mildest form of this disease,

# thick-walled bronchi that extend into the lung periphery and fail to show normal tapering

2. VARICOSE BRONCHIECTASIS

# beaded appearance of bronchial walls - dilated bronchi with areas of relative narrowing

# string of pearls.

# Traction bronchiectasis often appears varicose.

3. CYSTIC BRONCHIECTASIS :

# Group or cluster of air-filled cysts,

# cysts can also be fluid filled, giving the appearance of a cluster of grapes.

4.TRACTION BRONCHIECTASIS :

# Defined as dilatation of intralobular bronchioles because of surrounding fibrosis

# due to fibrotic lung diseases

Differential diagnosis

1. Infective causes : specially childhood pneumonia,

pertusis, measles, tuberculosis

2. Non- infective causes : Bronchopulmonary aspergilosis, inhalation of toxic fumes

3. Connective tissue disorder : Ehlers-Danlos Synd,

Marfan synd , tracheobronchomeglay

4. Ciliary diskinesia : Cystic fibrosis, Kartangener synd, agammaglobulinemia .

5. Tractional bronchiectasis in interstitial fibrosis.

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.

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.

PANLOBULAR EMPHYSEMA

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

• Characterized by complete destruction of the entire pulmonary lobule.

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

Paraseptal (distal acinar) emphysema

• Affects the peripheral parts of the secondary pulmonary lobule

• Produces subpleural lucencies.

Cicatricial Emphysema/ irregular air space enlargement

• previously known as irregular or cicatricial emphysema

• can be seen in association with fibrosis

• with silicosis and progressive massive fibrosis or

sarcoidosis

BULLOUS EMPHYSEMA :

• does not represent a specific histological abnormality

• Emphysema characterized by large bullae• Often associated with centrilobular and paraseptal

emphysema

Paraseptal Emphysema vs Honeycombing

Paraseptal emphysema Honeycomb cysts

occur in a single layer at the pleural surface

may occur in several layers in the subpleural lung

predominate in the upper lobes predominate at the lung bases

unassociated with significant fibrosis

Asso with other findings of fibrosis.

Associated with other findings of empysema

Absent

Bullae

A sharply demarcated area of emphysema ≥ 1 cm in diameter

a thin epithelialized wall ≤ 1 mm.

uncommon as isolated findings, except in the lung apices

Usually asso with evidence of extensive centrilobular or paraseptal emphysema

When emphysema is associated with predominant bullae, it may be termed bullous emphysema

Pneumatocele

• defined as a thin-walled, gas-filled space within the lung,

• usually occurring in asso. with acute pneumonia or hydrocarbon aspiration,

• is often transient.

• believed to arise from lung necrosis and bronchiolar obstruction.

• Mimics a lung cyst or bulla on HRCT and cannot be distinguished on the basis of HRCT findings.

• Clinical & microbiological correlation.

CAVITARY NODULE

• thicker and more irregular walls than lung cysts

• In diffuse lung diseases - LCH, TB, fungal infections, and sarcoidosis.

• Also seen in rheumatoid lung disease, septic embolism, pneumonia, metastatic tumor, tracheobronchial papillomatosis, and Wegener granulomatosis

Cavitary nodules or cysts in tracheobronchial papillomatosis.

fungal pneumonia

Mosaic attenuation & perfusion

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

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

• It is seen as inhomogeneous attenuation of lung parenchyma with focal region of lucency which show smaller size of vessles

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

Mosaic attenuation due to small airway disease

# Air trapping and bronchial dilatation commonly seen.

# Areas of increased attenuation have relatively large vessels, while areas of decreased attenuation have small vessels.

# Causes include: Bronchiectasis, cystic fibrosis and bronchiolitis obliterans.

Mosaic attenuation due to vascular disease

# common in patients with acute or chronic pulmonary

embolism (CPE), and

# decreased vessel size in less opaque regions is often

visible

MOSIAC PATTERN

DEPENDENT LUNG ONLY

PRONE POSITION

RESOLVE

PLATE ATELECTASIS

NOT RESOLVE

GROUND GLASS

NONDEPENDENT LUNG

EXPIRATION

NO AIR TRAPPING

VESSEL SIZE

DECREASED

VASCULAR

NORMAL

GROUND GLASS

AIR TRAPPING

AIRWAYS DISEASE

Inhomogeneous lung opacity: mosaic perfusion in a patient with bronchiectasis.

central bronchiectasis with multifocal, bilateral inhomogeneous lung opacity.

The vessels within the areas of abnormally low attenuation are smaller than their counterparts in areas of normal lung attenuation.

Air trapping on expiration• Most patients with air trapping seen on expiratory scans

have inspiratory scan abnormalities, such as bronchiectasis, mosaic perfusion, airway thickening, or nodules suggest the proper differential diagnosis.

• Occasionally, air trapping may be the sole abnormal finding on an HRCT study.

• The differential diagnosis include ---

bronchiolitis obliterans; asthma; chronic bronchitis; and hypersensitivity pneumonitis

Air trapping on expiratory imaging in the absence of inspiratory scan findings in a patient with bronchiolitis obliterans.

(A) Axial inspiratory image through the lower lobes shows no clear evidence of inhomogeneous lung opacity.

(B) Axial expiratory image shows abnormal low attenuation (arrows) caused by air trapping, representing failure of the expected increase in lung attenuation that should normally occur with expiratory imaging.

Head cheese sign

• It refers to mixed densities which includes presence of- # consolidation # ground glass opacities # normal lung # Mosaic perfusion

• Signifies mixed infiltrative and obstructive disease

• Common cause are : Hypersensitive pneumonitis Sarcoidosis DIP

Axial HRCT image in a patient with hypersensitivity pneumonitis shows a combination of ground-glass opacity, normal lung, and mosaic perfusion (arrow) on the same inspiratory image.

Distribution within the lung

Upper lung zone preference is seen in: 1.Inhaled particles: pneumoconiosis (silica or coal) 2.Smoking related diseases (centrilobular emphysema 3. Respiratory bronchiolitis (RB-ILD) 4.Langerhans cell histiocytosis 5.Hypersensitivity pneumonitis 6.Sarcoidosis

Lower zone preference is seen in: 1. UIP 2. Aspiration 3. Pulmonary edema

Central vs peripheral zone

• Central Zone Peripheral zone

1. Sarcoidosis 1.COP

2. Cardiogenic pulmonary 2. Ch Eosinophilic Pneumonia

edema 3. UIP

3. Bronchitis 4. Hematogenous mets

Additional findings

Pleural effusion is seen in:

• Pulmonary edema

• Lymphangitic spread of carcinoma - often unilateral

• Tuberculosis

• Lymphangiomyomatosis (LAM)

• Asbestosis

Hilar and mediastinal lymphadenopathy

# In sarcoidosis the common pattern is right paratracheal and bilateral hilar adenopathy ('1-2-3-sign').

# In lung carcinoma and lymphangitic carcinomatosis adenopathy is usually unilateral.

#'Eggshell calcification' in lymph nodes occurs in ----Silicosis and coal-worker's pneumoconiosis and is sometimes seen in sarcoidosis, post irradiation Hodgkin disease, blastomycosis and scleroderma .

Conclusion

• With the advent & availability of better techniques & more efficient CT machines, it has become more convenient for a

Radiologist to give a proper diagnosis

• 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.

• A thorough knowledge of the basic anatomy is of utmost importance.

HRCT Lung 19th May 2012

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