1. High Resolution Computed Tomography (HRCT)

2. Content HRCT – Methods Basic lung anatomy HRCT interpretation
HRCT features in ILDs
Reticular abnormalities
Nodules
High attenuation
Low attenuation
Mosaic attenuation
IIP patterns on HRCT
Diagnosis of UIP
Diffentiating between UIP and NSIP

3. HRCT offers two different techniques:5
HRCT - Methods
In HRCT, thin sections of <1.5 mm width are acquired and reconstructed with a high spatial frequency algorithm, which allows the operator to visualise submillimetre structures and detect discrete abnormalities as small as 0.3 mm at the level of the secondary pulmonary lobule.2,5
HRCT offers two different techniques:5
Spaced axial HRCT
Thin sections, spaced at 1-2 cm intervals, are acquired from the lung apices to the bases
Low radiation dose compared to volumetric imaging
Sufficient to detect abnormalities in diffuse lung diseases
Volumetric HRCT (multidetector row CT)
Detection of all abnormalities
Easier and better interpretation owing to ability to perform coronal and sagittal reformations
Greater radiation dose compared to axial imaging (3-5 times)

4. basic lung anatomy
The secondary lobule is the basic functional unit of the lung. Understanding its anatomy is key to interpreting the imaging results of interstitial lung diseases (ILDs) and is based on the type of secondary-lobule involvement.
Secondary pulmonary lobules measure between 1 and 2.5 cm in diameter. Each lobule is made up of up to a dozen acini and primary pulmonary lobules. Each secondary lobule is supplied by a terminal bronchiole and a pulmonary artery branch and surrounded by a lobular septum within which pulmonary veins and lymphatics are located. These surrounding septa are very thin and only a few of them will be seen under healthy conditions.6
The lymphatic system is comprised of two parts:6
A central network (centrilobular area): runs along the bronchovascular bundle towards the centre of the lobule
A peripheral network (perilymphatic area): located within the interlobular septa and along the pleural linings.

5. basic lung anatomy

6. Distribution within lung
HRCT interpretation
For a structured approach to the interpretation of HRCT (high resolution computed tomography) images, the following aspects should be looked at carefully:7
Dominant HRCT features
Feature location within the secondary lung lobule
Predominant distribution within the lung
For any diagnosis, all morphological findings from HRCT must always be combined with the patient’s history and additional clinical findings (e.g. pleural fluid, traction bronchiectasis).
Dominant pattern
Reticular, nodular high density, low density ▼
Distribution in
second lobule
Centrilobular, perilymphatic, random
Distribution within lung
Upper zones, lower zones central or peripheral

7. HRCT features in ILDs
HRCT scans show diverse features which, individually or in combination, can indicate the presence of a certain type of disease.
In diagnosing ILDs, there are 4 general features to be considered that indicate the presence of pathological abnormalities on HRCT scans and are important to the interpretation of HRCT scans:2
Reticular abnormalities
Nodules
Increased lung opacity (high attenuation)
Decreased lung opacity (low attenuation)
Further consideration of potential co-occurrence/overlap of features, additional findings and the distribution of abnormalities in the axial and coronal planes can help narrow the differential diagnosis.8

8. Reticular abnormalities
Interlobular septal thickening
Reticular opacities seen in HRCT images are a result of the thickening of the interlobular septa or fibrosis (honeycombing).2 Interlobular septal thickening is defined as the thickening of the interstitium that surrounds the secondary pulmonary lobule by fluid, fibrous tissue, or infiltration by cells. It can have a smooth, nodular or irregular appearance.
Visualisation of numerous septa, which are normally about 0.1 mm thick, indicates an abnormal condition.2,6,9
Clinical and histopathological correlations:
Pulmonary oedema
Lymphangitic spread of tumour
Sarcoidosis
Erdheim-Chester disease
Lymphoid pulmonary lesions

9. Reticular abnormalities
Honeycombing
The term ‘honeycombing’ is used to describe the end stage of diseases that cause diffuse fibrotic destruction, are irreversible and have a poor prognosis. If present, HRCT shows thick-walled clustered cystic air spaces ( cm in diameter), which are distributed subpleurally and basally. These cysts resemble a honeycomb in cross-section. This feature is typically accompanied by other signs of fibrosis, e.g. traction bronchiectasis.2,6

10. Reticular abnormalities
Honeycombing
Clinical and histopathological correlations:
Collagen vascular diseases
Hypersensitivity pneumonitis
Fibrosing sarcoidosis (stage IV)
Nonspecific interstitial pneumonia
Drug-induced lung disease
Asbestosis
For a differential diagnosis it is also important to consider pulmonary emphysema, which can mimic honeycombing.

11. Reticular abnormalities
Traction bronchiectasis
An abnormal dilatation of the bronchial tree occurring as a consequence of interstitial fibrosis is referred to as traction bronchiectasis. The bronchus is pulled apart by the traction of surrounding parenchymal fibrosis. It can arise from a number of underlying causes which result in lung fibrosis.2,6
Clinical and histopathological correlations:
Specific for fibrosis, especially in the presence of honeycombing.

12. Reticular Abnormalities
Traction bronchiectasis

13. Nodules
Nodules can be categorised in several ways with the most important one being the distribution within the secondary lobule. This distribution is essential for the accurate diagnosis of the nodular pattern. Nodules can be distributed in three different ways:
Perilymphatic
Centrilobular
Random

14. Nodules Perilymphatic2,6,7
Present in the interlobular septa, pleural surfaces, and the bronchovascular sheath
Most commonly seen in sarcoidosis or neoplasms
Centrilobular2,6,7
Present only in the centrilobular region, spaced 5-10 mm from the pleura and up to 1 cm in size, although not necessarily in the centres of the secondary lobules, the pleural surfaces are spared
Can appear in rosettes or be diffuse
Not necessarily disease-related
Random2
Randomly distributed relative to structures of the secondary lobule throughout the lung parenchyma
Can be associated with haematogenous metastases, miliary tuberculosis, miliary fungal infections, or Langerhans cell histiocytosis (early nodular stage)

15. Centrilobular nodules – “tree-in-bud” sign
Centrilobular nodules can be accompanied by structures called "tree- in-bud" opacities, which resemble a budding tree. Showing a branching structure, it represents an impacted centrilobular bronchus dilated by pus, mucus or fluid. This occurrence is almost always associated with inflammation due to pulmonary infections. The identification of Y- and V-shaped structures on HRCT images is of value for narrowing the differential diagnosis.7,9
Tree-in-bud structures are seen in:6,7
Hypersensitivity pneumonitis
Respiratory bronchiolitis in smokers
Uncommon in bronchioloalveolar carcinoma, pulmonary edema, vasculitis

16. High attenuation
Increased lung attenuation refers to an opacification on HRCT images that can either obscure underlying vessels in the case of consolidations or present without obscuration as in so-called ground-glass opacities.2
Both findings can be associated with active or reversible lung disease, although in cases with predominant fibrosis, ground-glass opacity can be seen more often.2
There is substantial overlap between the diagnoses of ground-glass opacity and consolidation, and a differential diagnosis should be based on symptom duration or acute vs chronic disease, respectively.2

17. Ground-glass opacities
High attenuation
Ground-glass opacities
Ground-glass opacification does not obscure underlying vasculature and can be caused by various abnormalities:2,9
Air space disease: filling of the alveolar spaces with pus, oedema, hemorrhage, inflammation or tumour cells
Interstitial lung disease: thickening of the interstitium or alveolar walls below the spatial resolution of HRCT as seen in fibrosis.
A combination of the above
These opacities are non-specific and can also be due to the limitations of HRCT resolution.9

18. High attenuation Consolidation
The term consolidation refers to the presence of opacification that obscures the underlying vasculature. It is synonymous with airspace disease and implies that the air is replaced by blood, pus, oedema, or cells. It can also be caused by fibrotic changes.7 Special consideration of the distribution of consolidation is important for a successful differential diagnosis. Underlying causes for a peripheral or subpleural distribution can be: cryptogenic organising pneumonia, chronic eosinophilic pneumonia, atypical pulmonary oedema, Churg-Strauss syndrome, drug reactions, pulmonary contusion, pulmonary infarct, or sarcoidosis.9

19. High attenuation
Consolidation

20. Low attenuation Abnormally low attenuation on HRCT can be caused by:2
Emphysema
Cystic diseases (Lymphangioleiomyomatosis, lymphoid Interstitial pneumonia, Langerhans cell histiocytosis)
Bronchiectasis, honeycombing

21. Low attenuation Emphysema
Emphysema describes an abnormal, irreversible enlargement of the distal airspaces which is accompanied by destruction of their parenchymal walls resulting in areas of low attenuation on HRCT images.7,9,2
Classification of lung emphysema:
Centrilobular (upper lobe, centrilobular origin portion of the lobule)
Panlobular (lower lobe, diffuse distribution, affects the whole secondary lobule)
Paraseptal (upper lobe, subpleural and interlobar distribution, can resemble honeycomb changes)

22. Low attenuation Cystic diseases
Pulmonary cysts present as circumscribed and radiolucent lesions with a thin wall (usually <3 mm thick). They are often caused by fibrotic changes resulting in honeycombing. These cysts can have a diameter ranging from several millimetres to several centimetres. Adjacent cysts share the wall, a finding not seen in other cystic lung diseases.2
Cystic lung diseases include lymphangioleiomyomatosis, Langerhans cell histiocytosis, lymphocytic interstitial pneumonia, pneumatoceles.9

23. Low attenuation Bronchiectasis
Bronchiectasis is defined as an abnormal, irreversible bronchial dilatation. It is often a result of prior infections, chronic bronchitis, COPD, asthma and cystic fibrosis.7,10
To distinguish the various causes of bronchiectasis during diagnosis, a combination of the following additional findings should be considered:
“Signet-ring sign” reflecting bronchial dilatation
Bronchial wall thickening
Lack of tapering
Mucus retention in the bronchial lumen
Air trapping
NB: Bronchiectasis must be discriminated from traction bronchiectasis, which is a result of fibrosis.

24. Mosaic attenuation
Mosaic attenuation is the presence of regions of differing attenuation appearing as patchy areas of black and white lung on HRCT. These correspond to affected and non-affected lung areas, respectively. Different attenuation is a result of alterations in lung parenchymal perfusion that creates hyperperfused as well as hypoperfused lung areas.2,9,11
HRCT at the level of the upper lobes exhibits a “mosaic attenuation pattern” with patchy areas of increased attenuation (Courtesy of A. Oikonomou12)

25. Mosaic attenuation Conditions underlying:9,11 Low attenuation
Obstructive small airways disease (e.g. bronchiectasis, cystic fibrosis, constrictive bronchiolitis)
Occlusive vascular disease (e.g. chronic pulmonary embolism)
High attenuation
Parenchymal disease: represents ground-glass opacity
HRCT at the level of the upper lobes exhibits a “mosaic attenuation pattern” with patchy areas of increased attenuation (Courtesy of A. Oikonomou12)

26. Recognition of IIP patterns on HRCT
Idiopathic interstitial pneumonias are defined as an entity of interstitial lung diseases of unknown cause. They can be classified based on characteristic HRCT features that are associated with certain histological patterns.13
These HRCT patterns are unique for each of the entities included in the group of IIPs classified by the ATS/ERS according to certain morphologic patterns.13–15 (see table)
Morphological Pattern
HRCT Features
Distribution on CT
UIP
Reticular opacities, honeycombing, traction bronchiectasis, focal ground-glass opacity
Peripheral, Subpleural
Basal, Lower lung zones
NSIP
Ground-glass opacities, irregular linear or reticular opacities, micronodules, consolidation, microcystic honeycombing
Peripheral
COP
Airspace consolidation, mild bronchial dilatation, ground-glass opacity, large nodules (rare)
Peribronchial
RP-ILD
Centrilobular nodules, patchy ground-glass opacities, bronchial wall thickening
Diffuse or upper lung predominance
DIP
Ground-glass opacities, irregular linear or reticular opacities, occasionally cysts
Lower lung zones
Peripheral predominance
LIP
Ground-glass opacities, perivascular cysts, septal thickening, centrilobular nodules
Basilar predominance or diffuse
AIP
Exudative phase shows ground-glass opacities, airspace consolidation, organising phase shows bronchial dilatation, architectural distortion 
Diffuse
(adapted from Mueller-Mang )

27. Diagnosis of ILDs of unknown cause: UIP
UIP – Usual Interstitial Pneumonia – describes the typical histological and CT patterns occurring in IPF (Idiopathic Pulmonary Fibrosis).
This pattern is required for the diagnosis of IPF, when other known causes or features inconsistent with UIP can be ruled out.1,16
The most predominant UIP features to be found on HRCT in cases of IPF are reticular changes like honeycombing and traction bronchiectasis with the disease being most extensive in the lower lobes of the lung and having a peripheral distribution.16
Although honeycombing on HRCT is paramount for diagnosis, identification of this feature can be challenged through frequent disagreement among radiologists on their findings mainly due to other conditions that can mimic honeycombing, e.g. emphysema or traction bronchiectasis.16

28. Distribution and CT pattern of UIP
CT shows honeycombing (green), reticular opacities (white), traction bronchiectasis (yellow), and ground-glass opacity (gray).
The distribution is subpleural with an apicobasal gradient.
(adapted from Mueller-Mang )

29. HRCT criteria for UIP pattern1
(All 4 features)
Possible UIP pattern
(All 3 features)
Inconsistent with UIP pattern
(Any of the 7 features)
Subpleural, basal predominance
Upper or mid-lung predominance
Reticular abnormality
Peribronchovascular predominance
Honeycombing with or without traction bronchiectasis
Absence of features listed as inconsistent with UIP pattern
Extensive ground glass abnormality (extent. reticular abnormality)
Profuse micronodules (bilateral, predominantly upper lobes)
Discrete cysts (multiple, bilateral, away from areas of honeycombing)
Diffuse mosaic attenuation/air-trapping (bilateral, in three or more lobes)
Consolidation in bronchopulmonary segment(s)/lobe(s)

30. Diffentiating between UIP and nsip
Although less common than UIP, non-specific idiopathic pneumonia (NSIP) is still the most frequent differential diagnosis of UIP. On high-resolution CT the patterns of both conditions overlap considerably, complicating the diagnosis.13,16
Some distinct imaging features favour the diagnosis of NSIP over UIP. These include homogeneous lung involvement without an obvious apico-basal gradient, extensive ground- glass abnormalities that do not progress to areas of honeycombing, a finer reticular pattern, and micronodules.13,16
Although the different distribution and CT patterns may point to either UIP or NSIP, distinguishing these conditions remains challenging, and a lung biopsy is a necessary measure for clarification.13

31. Diffentiating between UIP and nsip
No obvious gradient
Obvious apico-basal gradient
Homogeneous
Heterogeneous
Ground-glass opacities
Honeycombing
Micronodules
Traction bronchiectasis
(adapted from Mueller-Mang )

32. Literature
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Knipe H, Gaillard F, et al. Bronchiectasis | Radiopaedia.org. Available at: Accessed July 7,
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Weerakkody Y, Henson N, et al. Mosaic attenuation pattern in lung | Radiopaedia.org. Available at: Accessed April 20, 2015.
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