HRCT CHEST

Dr. Thambidurai

HRCT Technique

Thinnest collimation (1.0-1.5mm) High spatial frequency/sharp algorithm/bone algorithm kvp:120-140;mA:240 Matrix size: Largest (512 * 512) Scan time =<1sec Windows:-700/1000 HU or -600/1500 HU, soft tissues :

50/350

LOW DOSE HRCT Study by Zwirewich et al collimation - 1.5 mm scan time - 2 sec kV – 120 mA – 20 ( low dose) and 200(high dose)RESULTS: 1. Two techniques are equally diagnostic in

97% 2. low dose failed to demonstrate ground

glass in 2 and emphysema in 1 patient

LOW DOSE HRCT

Used only for,

1.follow up of patients with known lung abnormalty

2.screening large populations for lung dis

TARGETED RECONSTRUCTION Optional Retrospectively targeting image image

reconstruction to a single lung , using a smaller FOV increases the spatial resolution by decreasing the pixel size

For 512 *512 matrix, if FOV is 40 cms,pixel size wil be 0.78 mm, for a FOV of 15cms, pixel size would be only 0.29mm thus improving the spatial resolution.

Disad- requires addition time,filming and raw data needs to be saved

TECHNICAL MODIFICATIONS PATIENT POSITION –PRONE-early lung

fibrosis EXPIRATORY- obstrutive lung diseases SCAN SPACING

-1 cm intervals

- 3 0r 4 cm intervals

- LIMITED HRCT-3 preselected levels( aortic arch, carina,2cm above Rt hemidiaphragm)

GANTRY ANGULATION- 20 degrees caudally in bronchiectasis

EXPIRATORY HRCT

Post expiratory Dynamic expiratory(forced expiration) Spirometrically

triggered(specific,reproducible user selected lung volumes)

3D expiratory(spiral CT+8mm collimation+3D recon)

RECOMMENDED PROTOCOLS

SUSPECTED EMPHYSEMA,AIRWAY DISEASE OR OBSTRUCTIVE LUNG DIS

-full inspiration supine scans with 1 cm interval from lung apices to base

-expiratory scans at 3 or more levels

PULMONARY VASCULAR DIS Full inspiration supine scans

with 1cm spacing

Exp scans at 3 or more levels

Contrast enhanced spiral CT

HEMOPTYSIS

Full inspiration supine scan with 5mm collimation thro the hila

HRCT with 1 cm spacing at other levels

Exp scans at 3 or more levels

COMBINED DIAGNOSIS OF DIFFUSE LUNG DIS AND FOCAL ABNORMALTIES

Full ins HRCT with 1cm spacing Prone scan if necessary Exp scans at 3 or more levels Spiral CT with or without contrast

RESTRICTIVE OR FIBROTIC OR UNKNOWN DIFFUSE LUNG DIS CHEST RADIOGRAPH ABNORMAL

-full ins supine scans with 1 cm spacing from apices to base

-exp scans at 3 or more levels

CHEST RADIOGRAPH ABNORMAL

-option 1

-option 2

OPTION 1

-full ins scans with 2 cm spacing in both prone and supine from apices to base

-exp scans at 3 or more levels

OPTION 2

-full ins supine scans with 1 cm spacing from apices to bases

- prone scans if dependant densities present

- exp scans at 3 or more levels

RADIATION DOSE

CONVENTIONAL CT - 20 mGy HRCT – 120 Kv,200mA, 2 sec

-4.4 mGy for 1.5 mm at 10mm intervals(12%)

-2.1 mGy for 20 mm intervals(6%)

-36.3 mGy for conventional 10mm scans at 10 mm intervals

Low dose HRCT at 20 mm interval=chest x ray

ARTIFACTS

STREAK

- due to aliasing or correlated noise

- fine linear netlike opacities that radiate from the edges of sharply marginated high contrast structures such as ribs, vertebral bodies,bronchial wall most commonly found paralleling pleura or posterior chest wall

MOTION ARTIFACTS PULSATION OR STAR ARTIFACT

-thin streaks radiating from vessels at left lung base adjacent to heart

DOUBLING ARTIFACT

-major fissure, bronchi or vessels may be seen as double because of cardiac pulsation or respiration and can mimic bronchiectasis

Motion artifacts can be reduced by ECG gating of scan acquistion or spirometrically controlled respiration

HRCT-BASIC INTERPRETATION

SECONDARY LOBULE

• Basic anatomic unit• Smallest lung unit surrounded by connective tissue

septa• Measures 1-2 cm • Made up of 5-15 pulmonary acini, that contain the

alveoli• 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

• Two lymphatic systems: central network- bronchovascular bundle towards the centre of the lobule ; peripheral network- within the interlobular septa and along the pleural linings

Centrilobular area Central part of the secundary lobule Site of diseases, that enter the lung through the airways Eg: Hypersensitivity pneumonitis Respiratory bronchiolitis Centrilobular emphysema

Perilymphatic area Peripheral part of the secundary lobule Site of diseases, that are located in the lymphatics of in the interlobular septa Eg: Sarcoid Lymphangitic carcinomatosis Pulmonary edema

What is the dominant HR-pattern: Reticular Nodular High attenuation (ground-glass, consolidation) Low attenuation (emphysema, cystic)

Where is it located within the secondary lobule (centrilobular, perilymphatic or random)

Is there an upper versus lower zone or a central versus peripheral predominance

Are there additional findings (pleural fluid, lymphadenopathy, traction bronchiectasis)

BASIC INTERPRETATION

RETICULAR PATTERN

Thickening of the interlobular septa / fibrosis as in honeycombing

SEPTAL THICKENING

Thickening of lung interstitium by fluid, fibrous tissue, or infiltration by cells results in a pattern of reticular opacities due to thickening of the interlobular septa

Focal septal thickening in lymphangitic carcinomatosis

NODULAR PATTERN

SARCOIDOSIS

Centrilobular nodules of ground glass density-Hypersensitivity pneumonitis

TREE-IN-BUD APPEARANCE:

Irregular and nodular branching structure, most easily identified in the lung periphery

Represents dilated and impacted (mucus or pus-filled) centrilobular bronchioles

Tree-in-bud indicates the presence of:

Endobronchial spread of infection (TB, MAC, any bacterial bronchopneumonia)Airway disease associated with infection (cystic fibrosis, bronchiectasis)Airway disease associated primarily with mucus retention (allergic bronchopulmonary aspergillosis, asthma)

ACTIVE TB-TREE IN BUD

RANDOM NODULES

Result of the hematogenous spread of the infection

Small random nodules are seen in: Hematogenous metastasesMiliary tuberculosisMiliary fungal infectionsSarcoidosis may mimick this pattern, when very extensiveLangerhans cell histiocytosis (early nodular stage)

Ground-glass-opacity = hazy increase in lung opacity without obscuration of underlying vessels

Consolidation = increase in lung opacity which obscures the vessels

HIGH ATTENUATION PATTERN

Ground-glass opacity →Filling of the alveolar spaces with pus, edema, hemorrhage, inflammation or tumor cells→Thickening of the interstitium or alveolar walls below the spatial resolution of the HRCT as seen in fibrosis

Upper zone predominance: Respiratory bronchiolitis, PCP

Lower zone predominance: UIP, NSIP, DIP Centrilobular distribution: Hypersensitivity pneumonitis,

Respiratory bronchiolitis

Broncho-alveolar cell carcinoma

'Mosaic attenuation' = density differences between affected and non-affected lung areas

When ground glass opacity presents as mosaic attenuation to consider: Infiltrative process adjacent to normal lung Normal lung appearing relatively dense adjacent to lung with air-trapping Hyperperfused lung adjacent to oligemic lung due to chronic thromboembolic disease

MOSAIC ATTENUATION

Combination of ground glass opacity with superimposed septal thickening

• 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

ALVEOLAR PROTEINOSIS

Consolidation → Airspace disease Pus, edema, blood ,tumor cells or fibrosis=Replace

air

CONSOLIDATION

Chronic eosinophilic granuloma

LOW ATTENUATION PATTERN

Decreased lung attenuation or air-filled lesions.

These include:

Emphysema

Lung cysts (LAM, LIP, Langerhans cell histiocytosis)

Bronchiectasis

Honeycombing

Areas of low attenuation without visible walls as a result of parenchymal destruction

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

EMPHYSEMA

Panlobular emphysema

Affects the whole secondary lobule

Lower lobe predominance

In alpha-1-antitrypsin deficiency, but also seen in

smokers with advanced emphysema

Paraseptal emphysema

Adjacent to the pleura and interlobar fissures

Can be isolated phenomenon in young adults, or in

older patients with centrilobular emphysema

In young adults = spontaneous pneumothorax

Lung cysts: Radiolucent areas with wall thickness

of less than 4mm

Cavities -Radiolucent areas with wall thickness of

more than 4mm and are seen in infection (TB,

Staph, fungal, hydatid), septic emboli, squamous

cell carcinoma and Wegener's disease

CYSTIC LUNG DISEASE

Langerhans cell histiocytosis

Bronchiectasis is defined as localized bronchial dilatation

Bronchial dilatation (signet-ring sign)Bronchial wall thickeningLack of normal tapering with visibility of airways in the peripheral lungMucus retention in the bronchial lumenAssociated atelectasis and sometimes air trapping

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

BRONCHIECTASIS

Honeycombing is defined by the presence of small

cystic spaces with irregularly thickened walls

composed of fibrous tissue

Predominate in the peripheral and subpleural lung

regions

Subpleural honeycomb cysts occur in several

contiguous layers

HONEYCOMBING

DISTRIBUTION WITHIN THE LUNG

ADDITIONAL FINDINGS