TECHNICAL ASPECT OF HRCT; NORMAL LUNG ANATOMY & HRCT FINDINGS OF LUNG DISEASE

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Presenter : Dr. Sarbesh Tiwari PGT Radiodiagnosis Assam Medical College Dibrugarh

HRCT ------ MEANING

o It is often used for anything and everything to do with “high resolution”.

o Resolution : Means ability to resolve small object that are close together ,as separate form.

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)

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INTRODUCTION

• 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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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

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TECHNICAL ASPECT

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

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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

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Reconstruction Algorithm

• Denotes the frequency at which the acquired scan data are recorded when creating the image.

• 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 better depicts normal and abnormal parenchymal interface.

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Standard algorithmHigh resolution algorithm9

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.10

• For routine technique – Kvp -- 120-140 mAs -- 200- 300

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

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

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WINDOW SETTINGS

Lung window• Window level setting ranging from – 600 to – 700 HU and

window widths of 1000 to 1500 HU are appropriate for a routine lung window.

Soft tissue window• Window level/width setting of 40-50/ 350-450 HU are best for

evaluation of the mediastinum, hila, and pleura.

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LOW DOSE HRCT

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

• Equivalent to conventional HRCT in 97 % of cases

• Disadvantage : Fails to identify GGO in few cases and have more prominent streak artifact.

• Not recommended for initial evaluation of patients with lung disease.

• Indicated in following up patients with a known lung abnormality or in screening large populations at risk for lung ds.

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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. (commonly 35 to 40)

• 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.

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• 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

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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

• Prone scan is useful in differentiating dependent

lung atelectasis from early lung fibrosis

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Axial CT image shows opacity in the posterior part of the lung which could represent dependent opacity or pulmonary inflammation. The prone images shows complete resolution of the opacity suggesting dependent atelectasis.

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

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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

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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. It delivers 3-5 times greater radiation. 21

Multidetector Helical HRCT

• Multidetector CT is equipped with a multiple row detector array

• Multiple images are acquired due to presence of multiple detectors

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

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

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Which is better HRCT or MD- HRCT

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

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

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Radiation dose

• Annual background radiation ----- --- 2.5 mSv

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

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

• 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.

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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. 25

Motion-related artifacts

• Pulsation / Star artefacts

• Doubling artefacts.

• Stair-step artefacts in sag/coro

reconstruction.

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MODIFICATION OF SCAN PROTOCOL

Scan protocol can be modified in relation to disease or patients comfort.

If a disease has basal predominance, it may be wise to begin scanning near the diaphragm and proceed cephalic .

Caudad for disease with an upper-lobe predominance (e.g., sarcoidosis)

An alternative approach - cephalad in all patients.

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REVIEW OF ANATOMY

PART 2

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LUNG ANATOMY

• Right lung is divided by major and minor fissure into 3 lobes and 10 broncho-pulmonary segments

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

1.1 kg

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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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TRACHEAL ANATOMY

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

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

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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

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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. left bronchial or right 3 rd 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)

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BRONCHOARTERIAL RATIO (B/A)

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

• 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.

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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

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Anatomy of the Secondary Lobule and Its Components

1. Interlobular septa and contiguous subpleural interstitium,

2. Centrilobular structures, and

3. Lobular parenchyma and acini.

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Interlobular septa and contiguous subpleural interstitium

The secondary pulmonary lobule is marginated by septa which extends from the pleural surface.

They measure 0.1 mm in thickness.

They are less well defined in central lung

Lobular core :

The secondary lobule is supplied by arteries and bronchioles that measures approximately 1 mm in diameter.

It consists of functioning lung parenchyma namely the alveoli, alveolar duct and vessels. The parenchyma is supported by network of central and peripheral fibers of interstitium.

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PULMONARY ACINUS

Portion of lung parenchyma supplied by a single respiratory Bronchiole.

Size is 7 to 8 mm in adults

3 to 24 acini = Sec Pul. Lobule

Primary Lobule: Lung parenchyma associated with a single Alveolar duct.

4-5 Primary Lobules Acinus40

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A group of terminal bronchioles

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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 Stroma

LUNG INTERSTITUM

Lung interstitium

Axiel fiber system

Peribronchovascular

interstitium

Centrilobular

interstitium

Peripheral fiber sysem

Subpleural interstitium

Interlobular septa

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• 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

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

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Anatomy of pleural surfaces and chest wall.

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NORMAL LUNG

ATTENUATION

• Normal lung attenuation : –700 to – 900 HU

• Attenuation gradient : densest at dependent region of lung as a result of 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 is greater than adults.

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NORMAL EXPIRATORY 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, normal variant.

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PATTERN OF LUNG DISEASE IN HRCT

PART 3

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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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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 SCANS56

LINEAR AND RETICULAR OPACITIES

• Represents thickening of interstitial fibers of lung by

- fluid or

- fibrous tissue or

- infiltration by cells

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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.

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Peribronchovascular Interstitial Thickening

PBIT

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

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sarcoidosis

UNILATERAL LYMPHANGITIC SPREAD OF CARCINOMA

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INTERLOBULAR SEPTAL THICKENING

• Normally, only a few septa seen

• On HRCT, if numerous interlobular septas are seen, it almost always indicate abnormality.

• Septal thickening d/t -interstitial fluid, cellular infiltration or fibrosis.

• The thickened interstitium outline the secondary pulmonary lobules and are perpendicular to the pleura.

• D/D are similar to that of PBIT. 61

Smooth Septal thickening

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

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Nodular Septal thickening

Focal septal thickening in lymphangitic carcinomatosis

Lymphangitic carcinomatosis : show diffuse smooth and nodular septal thickening. Sarcoidosis :

right lung base shows interlobular septal thickening associated with several septal nodules giving beaded appearance

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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.

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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.

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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.

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HONEYCOMBING

• Defined 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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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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Size, Distribution, Appearance

Nodules and Nodular Opacities

Size

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 69

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 structures

PERILYMPHATIC: corresponds to distribution of lymphatics

CENTRILOBULAR: related to centrilobular structuresDistribution

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Perilymphatic distribution

Nodules in relation to pulmonary lymphatics at

# perihilar peribronchovascular interstitium,

# interlobular septa,

# subpleural regions, and

# centrilobular interstitium.

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Perilymphatic nodules: D/D

Sarcoidosis

Lymphangitic carcinomatosis

Lymphocytic interstitial pneumonia (LIP)

Lymphoproliferative disorders

Amyloidosis

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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.

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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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Centrilobular nodules with or without tree-in-bud opacity: D/D :

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

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Random nodules Random nodules – No definable distribution

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.

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Random nodules: D/D

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

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Parenchymal Opacification

Ground-glass opacity

Consolidation

Lung calcification & high attenuation opacities.

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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 82

IMPORTANCE OF GGO

• Can represent - microscopic interstitial disease

(alveolar interstitium)

- microscopic alveolar space disease

- combination of both

In the absence of fibrosis, mostly indicates the presence of an ongoing, active, potentially treatable process

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

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DIFFERNTIAL DIAGNOSIS : GGO

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

• Upper zone predominance:

Respiratory bronchiolitis

PCP.

• Lower zone predominance: UIP, NSIP, DIP.

• Centrilobular distribution:

Hypersensitivity pneumonitis,

Respiratory bronchiolitis

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

Combination of GGO with fibrosis and tractional bronchiectasis-- NSIP

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

superimposed interlobular septal thickening and intralobular lines.

• Causes:

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

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CONSOLIDATION

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

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

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

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

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Patchy ground-glass opacity, consolidation, and nodule mainly with peribronchovascular distribution with reversed halo signs (central ground-glass opacity and surrounding air-space consolidation)

Peripheral consolidations with upper lobe predominance (photo negative of pulmonary edema)

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

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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

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HRCT findings manifesting as decreased lung opacity

Lung Cysts,

Emphysema,

and

Bronchiectasis

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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 interstitial pneumonia

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

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

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

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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.

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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

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Coned axial HRCT image shows bronchial dilation with lack of tapering . Bronchial morphology is consistent with varicose bronchiectasis. 100

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 lobe101

Types

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.

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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

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Differential diagnosis

1. Infective causes : specially childhood pneumonia,

pertusis, measles, tuberculosis

2. Non- infective causes : Bronchopulmonary aspergillosis, 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.

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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 (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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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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Paraseptal (distal acinar) emphysema

• Affects the peripheral parts of the secondary pulmonary lobule

• Produces subpleural lucencies.

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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 109

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 emphysema

Absent

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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

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Pneumatocele

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

• Associated with acute pneumonia or hydrocarbon aspiration.

• 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.

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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 113

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 vessels

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

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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

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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

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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.

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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

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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.

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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

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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.

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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 122

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

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Additional findings

Pleural effusion is seen in:

• Pulmonary edema

• Lymphangitic spread of carcinoma - often unilateral

• Tuberculosis

• Lymphangiomyomatosis (LAM)

• Asbestosis

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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 .

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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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