HRCT is used to detect small lung abnormalities not seen on regular CT. It is indicated for diffuse lung disease with normal or equivocal chest x-rays. HRCT allows visualization of lung anatomy down to the secondary pulmonary lobule. Patterns seen on HRCT include reticular opacities from septal thickening or honeycombing, nodular opacities, ground glass opacity, consolidation, and low attenuation areas like emphysema. Specific findings provide clues to diagnoses like lymphangitic carcinomatosis, hypersensitivity pneumonitis, and idiopathic interstitial pneumonias. IV contrast is not needed for lung parenchyma evaluation on HRCT.

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HRCT: PATTERNS OF HRCT
*LUNG ANATOMY
*PATTERNS OF HRCT
Dr. Asma Javed Qureshi

2.  When we need to see smaller objects which are closely spaced
together
 High resolution CT is a scanning protocol in which thin sections
(usually 0.625 to 1.25 mm) are acquired and reconstructed using a
sharp algorithm (e.g. bone algorithm).
 It has been used for:
 Lung imaging
 Temporal bone imaging
Why do we need to do HRCT?

3.  for detection in patients with normal or equivocal
plain CXR appearances who have symptoms or
pulmonary function tests suggestive of diffuse lung
disease.
 where the symptoms and/or plain CXR findings are
non-specific, to attempt a specific diagnosis.
 to assess activity of the disease.
 to select an optimal biopsy site.
 for investigation of hemoptysis in selected patients
 to select an optimal biopsy site.
Main indications

5.  Use of intravenous (IV) iodinated contrast
should not be used when performing an HRCT
to evaluate the lung parenchyma and small
airways primarily, as subtle pulmonary
findings may be obscured by intrapulmonary
contrast. In addition, IV contrast adds little
value to the interpretation of diffuse lung
disease while exposing patients to the risks
associated with the administration of iodinated
contrast.
Do we use I/V contrast?

6. Lung Anatomy
Lung anatomy

7.  The basic pulmonary unit visible by HRCT
represents the secondary pulmonary lobule:
 Polyhedral 1.5cm structure surrounded by
connective tissue (interlobular septa)
 Central artery and bronchiole
 Peripheral pulmonary veins and lymphatics in
septum
HRCT Anatomy

8. Secondary pulmonary lobule
Secondary lobules. The centrilobular artery (in blue: oxygen-poor blood)
and the terminal bronchiole run in the center. Lymphatics and veins (in
red: oxygen-rich blood) run within the interlobular septa

9. Centrilobular area in blue (left) and perilymphatic area in yellow (right)

10.  Centrilobular area 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 ).
 Perilymphatic area is the peripheral part of the secondary
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).

11. Lung interstitium

12.  Normal pulmonary
vein branches are
seen marginating
pulmonary lobule
 Centrilobular artery
branches are seen as
a rounded dot

13. Basic interpretation of HRCT

14.  In 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.
 Septal thickening
 Honeycombing
Reticular pattern

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

17. focal irregular septal thickening in the
right upper lobe in a patient with a known
malignancy.
This finding is typical for lymphangitic
carcinomatosis.
A patient with both septal thickening and
ground glass opacity in a patchy
distribution.
Some lobules are affected and others are
not.
This combination of findings is called
'crazy paving'.

18. Honeycombing represents the second
reticular pattern recognizable on
HRCT.
Pathologically, honeycombing is
defined by the presence of small cystic
spaces lined by bronchiolar epithelium
with thickened walls composed of
dense fibrous tissue.
Honeycombing is the typical feature of
usual interstitial pneumonia (UIP).

20. Nodular pattern
 Random distribution
Usually seen in pleural surfaces and
along the fissures but lack the
subpleural distribution.
 Centrilobular distribution
In certain diseases, nodules are limited
to the centrilobular region & spare the
pleural surfaces. The most peripheral
nodules are centered 5-10mm from
fissures or the pleural surface.
 Perilymphatic distribution
Nodules are seen in relation to pleural
surfaces, interlobular septa and the
peribronchovascular interstitium.
Nodules are almost always visible in a
subpleural location, particularly in
relation to the fissures.

21.  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

24. *nodules along the fissures indicating a
perilymphatic distribution (red arrows).
*majority of nodules located along the
bronchovascular bundle (yellow arrow).
In addition to the perilymphatic nodules,
there are multiple enlarged lymph nodes,
which is also typical for sarcoidosis.
In end stage sarcoidosis we will see
fibrosis, which is also predominantly
located in the upper lobes and perihilar.

25. Tree in bud
Tree-in-bud describes the appearance of an irregular and often nodular branching
structure, most easily identified in the lung periphery.
It represents dilated and impacted (mucus or pus-filled) centrilobular
bronchioles.

34. High attenuation: GGO
 An area of increased attenuation in the lung on computed
tomography (CT) with preserved bronchial and vascular
markings.
Ground-glass opacity (GGO) represents:
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.
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

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

38. PCP with ground glass opacity

41. Ground-glass opacity is a non-specific
term that refers to the presence of
increased hazy opacity within the lungs
that is not associated with obscured
underlying vessels (obscured underlying
vessels is known as consolidation). It can
reflect minimal thickening of the septal
or alveolar interstitium, thickening of
alveolar walls, or the presence of cells or
fluid filling the alveolar spaces. In an
acute setting, it can represent active
disease such as pulmonary edema,
pneumonia, or diffuse alveolar damage.

44.  Air bronchogram refers to the phenomenon of air-filled
bronchi (dark) being made visible by the opacification of
surrounding alveoli (grey/white).

45.  The term 'mosaic attenuation' is used to describe density differences
between affected and non-affected lung areas.
There are patchy areas of black and white lung.
 Can be seen in vascular obstruction, airway disease or abnormal
ventilation
Mosaic attenuation

46. Mosaic pattern in a patient with
hypersensitivity pneumonitis

49. Crazy paving

52.  Consolidation
 Consolidation is synonymous with airspace disease.
When you think of the causes of consolidation, think of
'what is replacing the air in the alveoli'?
Is it pus, edema, blood or tumor cells.
Even fibrosis as in UIP, NSIP and long standing
sarcoidosis can replace the air in the alveoli and cause
consolidation.
Consolidation

53. There are patchy non-segmental consolidations in a subpleural and
peripheral distribution.

54.  Emphysema
 Lung cysts (LAM, LIP, Langerhans cell
histiocytosis)
 Bronchiectasis
 Honeycombing
Low attenuation pattern

55. Paraseptal emphysema with small bullae
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.

57. Idiopathic indicates unknown cause and interstitial pneumonia refers
to involvement of the lung parenchyma by varying combinations of
fibrosis and inflammation.

59. The diagnosis of NSIP requires histological proof.
In all patients with a NSIP pattern, the clinician should be advised to look for
connective tissue diseases, hypersensitivity pneumonitis or drugs .
Note the varying
combination of GGO and
fibrosis (traction
bronchiectasis), but the
lack of honeycombing.

62. HRCT findings in UIP
Honeycombing consisting of multilayered thick-walled cysts.
Architectural distortion with traction bronchiectasis due to fibrosis.
Predominance in basal and subpleural region.
Mild mediastinal lymphadenopathy

63. HRCT Gallery

64. A case of AML with ANC of 200/µl, presented with high grade
fever and dyspnea. HRCT chest reveals bilateral diffuse GGO
with air space consolidation and subpleural sparing and a few air
cysts classical of Pneumocystis jiroveci pneumonia

65. A case of myelofibrosis with ANC of 200/µl. HRCT chest shows multiple
small randomly distributed nodules (2-3 mm) in both lungs with tree-in-bud
appearance at places suggestive of miliary tuberculosis. Patient's sputum
was positive for AFB

66. Figure 3: (a, b) A patient of AML (postchemotherapy), with fever and an
ANC of 120/µl. HRCT chest shows B/L multifocal consolidation (a) with
small nodules in both upper lobes (b) suggestive of pyogenic infection.
Sputum was positive for group A streptococcus

67. A renal transplant recipient with fever and cough with expectoration (ANC of 120/µl).
HRCT (a) lung and (b) mediastinal window shows consolidation with areas of
cavitation in right upper lobe. HRCT diagnosis was necrotizing pneumonia.
However, BAL yielded aspergillus

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