HRCT - Basic Interpretation

Anatomy of Secondary lobule

Knowledge of the lung anatomy is essential for understanding HRCT.
The interpretation of interstitial lung diseases is based on the type of involvement of the secondary lobule.
The secondary lobule is the basic anatomic unit of pulmonary structure and function.
It is the smallest lung unit that is surrounded by connective tissue septa.
It measures about 1-2 cm and is made up of 5-15 pulmonary acini, that contain the alveoli for gas exchange.
The secondary lobule is supplied by a small bronchiole or terminal bronchiole in the center, which is parallelled by the centrilobular artery.
The pulmonary veins and lymphatics run in the periphery of the lobule within the interlobular septa.
Under normal conditions only a few of these very thin septa can be seen.
There are two lymphatic systems: a central network, that runs along the bronchovascular bundle towards the centre of the lobule and a peripheral network, that is located within the interlobular septa and along the pleural linings.

Centrilobular area is the central part of the secundary 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 secundary 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).
These diseases are usually also located in the central network of lymphatics that surround the bronchovascular bundle.

Basic Interpretation

A structured approach to interpretation of HRCT involves the following questions:

• 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 HR-pattern:
  • centrilobular
  • perilymphatic
  • random
• Is there an upper versus lower zone or a central versus peripheral predominance
• Are there additional findings:
  • pleural fluid
  • lymphadenopathy
  • traction bronchiectasis

These morphologic findings have to be combined with the history of the patient and important clinical findings.
When we study patients with HRCT, we have to realize that we are looking at a selected group of patients.
Common diseases like pneumonias, pulmonary emboli, cardiogenic edema and lungcarcinoma are already ruled out.

This means that uncommon diseases like Sarcoidosis, Hypersensitivity pneumonitis, Langerhans cell histiocytosis, Lymphangitic carcinomatosis, Usual Interstitial Pneumonitis (UIP) and many others become regular HRCT diagnoses and can be real Aunt Minnies.

Image
Aunt Minnie of a typical UIP.

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

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.
Although thickening of the interlobular septa is relatively common in patients with interstitial lung disease, it is uncommon as a predominant finding and has a limited differential diagnosis (Table).

Smooth septal thickening is usually seen in interstitial pulmonary edema (Kerley B lines on chest film); lymphangitic spread of carcinoma or lymphoma and alveolar proteinosis.

Nodular or irregular septal thickening occurs in lymphangitic spread of carcinoma or lymphoma; sarcoidosis and silicosis.

On the left we see focal irregular septal thickening in the right upper lobe in a patient with a known malignancy.
This finding is typical for lymphangitic carcinomatosis.
There are also additional findings, that support this diagnosis like mediastinal lymph nodes and a nodular lesion in the left lung, that probably represents a metastasis.

Pulmonary lymphangitic carcinomatosis (PLC)
In 50% of patients the septal thickening is focal or unilateral.
This finding is helpful in distinguishing PLC from other causes of interlobular septal thickening like Sarcoidosis or cardiogenic pulmonary edema.
Hilar lymphadenopathy is visible in 50% and usually there is a history of malignancy.
Identical findings can be seen in patients with lymphoma and in children with HIV infection, who develop Lymphocytic interstitial pneumonitis (LIP), a rare benign infiltrative lymphocytic disease.

On the left a patient who had a CT to rule out pulmonary embolism.
There is a combination of smooth septal thickening and ground-glass opacity with a gravitational distribution.
The diagnosis based on this CT was cardiogenic pulmonary edema.

Cardiogenic pulmonary edema generally results in a combination of septal thickening and ground-glass opacity.
There is a tendency for hydrostatic edema to show a perihilar and gravitational distribution.
Thickening of the peribronchovascular interstitium, which is called peribronchial cuffing, and fissural thickening are also common.
Common additional findings are an enlarged heart and pleural fluid.
Usually these patient are not imaged with HRCT as the diagnosis is readily made based on clinical and radiographic findings, but sometimes unsuspected hydrostatic pulmonary edema is found.

On the left 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'.
Crazy paving was thought to be specific for alveolar proteinosis, but is also seen in many other diseases such as pneumocystis carinii pneumonia, bronchoalveolar carcinoma, sarcoidosis, nonspecific interstitial pneumonia (NSIP), organizing pneumonia (COP), adult respiratory distress syndrome and pulmonary hemorrhage.

Alveolar proteinosis is a rare diffuse lung disease of unknown etiology characterized by alveolar and interstitial accumulation of a periodic acid-Schiff (PAS) stain-positive phospholipoprotein derived from surfactant.

Honeycombing

Honeycombing represents the second reticular pattern recognizable on HRCT.
Because of the cystic appearance, honeycombing is also discussed in the chapter on the low attenuation pattern.

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).

The distribution of nodules shown on HRCT is the most important factor in making an accurate diagnosis in the nodular pattern.
In most cases small nodules can be placed into one of three categories: perilymphatic, centrilobular or random distribution.
Random refers to no preference for a specific location in the secondary lobule.

Perilymphatic distribution
In patients with a 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.

Centrilobular distribution
In certain diseases, nodules are limited to the centrilobular region.
Unlike perilymphatic and random nodules, centrilobular nodules spare the pleural surfaces.
The most peripheral nodules are centered 5-10mm from fissures or the pleural surface.

Random distribution
Nodules are randomly distributed relative to structures of the lung and secondary lobule.
Nodules can usually be seen to involve the pleural surfaces and fissures, but lack the subpleural predominance often seen in patients with a perilymphatic distribution.

Algorithm for nodular pattern

The algorithm to distinguish perilymphatic, random and centrilobular nodules is the following:

• Look for the presence of pleural nodules.
  These are often easiest to see along the fissures.
  If pleural nodules are absent or few in number, the distribution is likely centrilobular.
• If pleural nodules are visible, the pattern is either random (miliary) or perilymphatic.
• If there are pleural nodules and also nodules along the central bronchovascular interstitium and along interlobular septa, you are dealing with a periplymphatic distribution.
• If the nodules are diffuse and uniformly distributed, it is likely a random distribution.

Perilymphatic distribution

Perilymphatic nodules are most commonly seen in sarcoidosis.
They also occur in silicosis, coal-worker's pneumoconiosis and lymphangitic spread of carcinoma.
Notice the overlap in differential diagnosis of perilymphatic nodules and the nodular septal thickening in the reticular pattern.
Sometimes the term reticulonodular is used.

On the left a typical case of perilymphatic distribution of nodules in a patient with sarcoidosis.
Notice the nodules along the fissures indicating a perilymphatic distribution (red arrows).
Always look carefully for these nodules in the subpleural region and along the fissures, because this finding is very specific for sarcoidosis.
Typically in sarcoidosis there is an upper lobe and perihilar predominance and in this case we see the majority of nodules located along the bronchovascular bundle (yellow arrow).

On the left another typical case of sarcoidosis.
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.

Centrilobular distribution

Centrilobular nodules are seen in diseases, that enter the lung through the airways.
The pathogens enter the central area of the secondary lobule via the terminal bronchiole:

• Hypersensitivity pneumonitis
• Respiratory bronchiolitis in smokers
• Infectious airways diseases (endobronchial spread of tuberculosis or nontuberculous mycobacteria, bronchopneumonia)
• Uncommon in bronchioloalveolar carcinoma, pulmonary edema, vasculitis

In many cases centrilobular nodules are of ground glass density and ill defined (figure).
They are sometimes called acinair nodules.

Tree-in-bud

In centrilobular nodules the recognition of 'tree-in-bud' is of value for narrowing the differential diagnosis.
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.

Tree-in-bud almost always indicates the presence of:

• Endobronchial spread of infection: TB, MAC or any bacterial bronchopneumonia.
• Airway disease associated with infection: cystic fibrosis, bronchiectasis.
• less often, an airway disease associated primarily with mucus retention like allergic bronchopulmonary aspergillosis and asthma.
• Aspiration.

On the left a tree-in-bud is seen.
In the proper clinical setting suspect active endobronchial spread of TB.

In most patients with active tuberculosis, the HRCT shows evidence of bronchogenic spread of disease even before bacteriologic results are available (6).

Random distribution

On the left a patient with random nodules as a result of miliary TB.
The random distribution is a result of the hematogenous spread of the infection.
Small random nodules are seen in:

• Hematogenous metastases
• Miliary tuberculosis
• Miliary fungal infections
• Sarcoidosis
• Langerhans cell histiocytosis (early nodular stage)

Sarcoidosis usually has a perilymphatic distribution.
However, when it is very extensive, it spreads along the lymphatics in the bronchovascular bundle to the periphery of the lung and may reach the centrilobular area.
This may result in a combined perilymphatic-centrilobular pattern which can simulate the random pattern.

Here a typical random nodular pattern in a patient with Langerhans cell histiocytosis (LCH).
LCH is an uncommon disease characterised by multiple irregular cysts in patients with nicotine abuse.

LCH in the early phase is a nodular disease (figure).
These nodules eventually cavitate and become cysts.
As in all smoking related diseases, there is an upper lobe predominance.

Increased lung attenuation is called ground-glass-opacity (GGO) if there is a hazy increase in lung opacity without obscuration of underlying vessels and is called consolidation if the increase in lung opacity obscures the vessels.

In both ground glass and consolidation the increase in lung density is the result of replacement of air in the alveoli by fluid, cells or fibrosis.
In GGO the density of the intrabronchial air appears darker as the air in the surrounding alveoli.
This is called the dark bronchussign
In consolidation, there is exclusively air left in the bronchi.
This is called the air bronchogram.

Ground-glass opacity

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.

So ground-glass opacification may either be the result of air space disease (filling of the alveoli) or interstitial lung disease (i.e. fibrosis).

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

• Upper zone predominance: Respiratory bronchiolitis, Pneumocystis pneumonia.
• Lower zone predominance: UIP, NSIP, DIP.
• Centrilobular distribution: Hypersensitivity pneumonitis, Respiratory bronchiolitis

The ground glass pattern itself is rather unspecific.
Not suprisingly, there is a big overlap in the causes of ground-glass opacity and consolidation and some diseases may present with both areas of ground-glass and consolidation.
On the left we see consolidation and ground-glass opacity in a patient with persistent chest abnormalities and weight loss without signs of infection.
This suggested a chronic disease.
There is no honeycombing or traction bronchiectasis, so we can rule out fibrosis.
The weight loss is suggestive of a malignant disease.
Histology revealed broncho-alveolar cell carcinoma

Broncho-alveolar cell carcinoma (BAC) may present as:

1. solitary nodule or mass (40% of patients)
2. focal or diffuse consolidation (30%) as in this case.
3. diffuse ill-defined centrilobular nodules (30%) due to endobronchial spread.

Treatable or not treatable?
Ground-glass opacity is nonspecific, but a highly significant finding since 60-80% of patients with ground-glass opacity on HRCT have an active and potentially treatable lung disease.
In the other 20-40% of the cases the lung disease is not treatable and the ground-glass pattern is the result of fibrosis.
In those cases there are usually associated HRCT findings of fibrosis, such as traction bronchiectasis and honeycombing.
The images show two cases with GGO, one without fibrosis and potentially treatable and the other with traction bronchiectasis indicating fibrosis.

Here two images af a patient with GGO as the dominant pattern.
In addition there is traction bronchiectasis indicating the presence of fibrosis.
This case is one of the possible patterns of nonspecific interstitial pneumonia (NSIP).

NSIP is characterized histologically by a relatively uniform pattern of cellular interstitial inflammation associated with variable degrees of fibrosis.
As in UIP (usual interstitial pneumonia) it mainly involves the dependent regions of the lower lobes, but NSIP lacks the extensive fibrosis with honeycombing.
NSIP may be idiopathic or associated with collagen vascular diseases or exposure to drugs or chemicals.
NSIP has a relative good prognosis and the majority of patients respond to treatment with corticosteroids.
This outcome is quite different from that seen in UIP, which has a poor prognosis.

Mosaic attenuation

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.
The role of the radiologist is to determine which part is abnormal: the black or the white lung.
When ground glass opacity presents as mosaic attenuation consider:

• Infiltrative process adjacent to normal lung.
• Normal lung appearing relatively dense adjacent to lung with air-trapping.
• Hyperperfused lung adjacent to hypoperfused lung due to chronic thromboembolic disease.

It can be difficult to distinguish these three entities.

There are two diagnostic hints for further differentiation:

• Look at expiratory scans for air trapping
• Look at the vessels

If the vesses are difficult to see in the 'black' lung as compared to the 'white' lung, than it is likely that the 'black' lung is abnormal.
Then there are two possibilities: obstructive bronchiolitis or chronic pulmonary embolism.
Sometimes these can be differentiated with an expiratory scan.

If the vessels are the same in the 'black' lung and 'white' lung, then you are looking at a patient with infiltrative lung disease, like the one on the right with the pulmonary hemmorrhage.

Temporary bronchiolitis with air trapping is seen in:

• (post) infection
• Inhalation of toxin
• Rheumatoid arthritis, Sjogren
• Post transplant
• Drug reaction (penicillamine)

On the left a patient with ground glass pattern in a mosaic distribution.
Some lobules are involved and others are not.
The differential diagnosis is hypersensitivity pneumonitis, bronchiolitis or thromboembolic disease.
The history was typical for hypersensitivity pneumonitis.
Hypersensitivity pneumonitis usually presents with centrilobular nodules of ground glass density (acinar nodules).
When they are confluent, HRCT shows diffuse ground glass.

Hypersensitivity pneumonitis (HP) is an allergic lung disease caused by the inhalation of antigens contained in a variety of organic dusts.
Farmer's lung is the best-known HP syndrome and results from the inhalation of fungal organisms that grow in moist hay or exposure to birds as pets (1).
HP usually presents in two forms either as ground glass in a mosaic distribution as in this case or as centrilobular nodules of ground glass density (acinar nodules).

On the left a patient with ground glass pattern in a mosaic distribution.
The clue here is the enlargement of pulmonary arteries (arrow) in the areas of ground glass.
The ground glass appearance is the result of hyperperfused lung adjacent to oligemic lung with reduced vessel caliber due to chronic thromboembolic disease.

On the left another patient with ground glass pattern in a mosaic distribution.
Again the ground glass appearance is the result of hyperperfused lung with large vessels adjacent to oligemic lung with small vessels due to chronic thromboembolic disease.
Emboli adherent to the wall and intravascular septa are typical for chronic thromboemboli in which partial recanalization took place.

Crazy Paving

Crazy Paving is a combination of ground glass opacity with superimposed septal thickening (5).
It was first thought to be specific for alveolar proteinosis, but later was also seen in other diseases.

Crazy Pavin can also be seen in:

• Alveolar proteinosis
• Sarcoid
• NSIP
• Organizing pneumonia (COP/BOOP)
• Infection (PCP, viral, Mycoplasma, bacterial)
• Neoplasm (Bronchoalveolarca (BAC)
• Pulmonary hemorrhage
• Edema (heart failure, ARDS, AIP)

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 (Table on the left).
Even fibrosis as in UIP, NSIP and long standing sarcoidosis can replace the air in the alveoli and cause consolidation.

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

Most patients who are evaluated with HRCT, will have chronic consolidation, which limits the differential diagnosis.

On the left two cases with chronic consolidation.
There are patchy non-segmental consolidations in a subpleural and peripheral distribution.

The differential diagnosis is the same as the list above.
The final diagnosis was cryptogenic organizing pneumonia (COP).
In chronic eosinophilic pneumonia the HRCT findings will be the same, but there will be eosinophilia.
In fibrosis there will be other signs of fibrosis like honeycombing or traction bronchiectasis.
Bronchoalveolar carcinoma can also look like this.

Organizing pneumonia (OP)
Organizing pneumonia represents an inflammatory process in which the healing process is characterized by organization and cicatrization of the exudate rather than by resolution and resorption.
It is also described as 'unresolved pneumonia'.
If no cause can be identified it is called cryptogenic organizing pneumonia (COP).
It was described in earlier years as Bronchiolitis-obliterans-organizing pneumonia (BOOP).
Patients with COP typically present with a several-month history of nonproductive cough.
Many cases are idiopathic, but OP may also be seen in patients with pulmonary infection, drug reactions, collagen vascular disease, Wegener's granulomatosis and after toxic-fume inhalation.

On the left a case of chronic eosinophilic pneumonia.
It was a patient with low-grade fever, progressive shortness of breath and an abnormal chest radiograph.
There was a marked eosinophilia in the peripheral blood.
Like in COP we see patchy non-segmental consolidations in a subpleural distribution.

Chronic eosinophilic pneumonia is an idiopathic condition characterized by extensive filling of alveoli by an infiltrate consisting primarily of eosinophils.
Chronic eosinophilic pneumonia is usually associated with an increased number of eosinophils in the peripheral blood and patients respond promptly to treatment with steroids.

The fourth pattern includes abnormalities that result in decreased lung attenuation or air-filled lesions.
These include:

• Emphysema
• Lung cysts (LAM, LIP, Langerhans cell histiocytosis)
• Bronchiectasis
• Honeycombing

Most diseases with a low attenuation pattern can be readily distinguished on the basis of HRCT findings.

Emphysema

Emphysema typically presents as 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.
• 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 often associated with spontaneous pneumothorax

Paraseptal emphysema
Paraseptal emphysema is localized near fissures and pleura and is frequently associated with bullae formation (area of emphysema larger than 1 cm in diameter).
Apical bullae may lead to spontaneous pneumothorax.
Giant bullae occasionally cause severe compression of adjacent lung tissue.

Panlobular emphysema
On the left a typical case of panlobular emphysema.
There is uniform destruction of the underlying architecture of the secondary pulmonary lobules, leading to widespread areas of abnormally low attenuation.
Pulmonary vessels in the affected lung appear fewer and smaller than normal.
Panlobular emphysema is diffuse and is most severe in the lower lobes.
In severe panlobular emphysema, the characteristic appearance of extensive lung destruction and the associated paucity of vascular markings are easily distinguishable from normal lung parenchyma.
On the other hand, mild and even moderately severe panlobular emphysema can be very subtle and difficult to detect on HRCT(1).

Cystic lung disease

Lung cysts are defined as radiolucent areas with a wall thickness of less than 4mm.
Cystic lung diseases as listed in the table on the left.

Cavities are defined as radiolucent areas with a wall thickness of more than 4mm and are seen in infection (TB, Staph, fungal, hydatid), septic emboli, squamous cell carcinoma and Wegener's disease.

On the left a case with multiple round and bizarre shaped cysts.
There was an upper lobe predominance.
The patient had a long history of smoking.
This combination of findings is typical for Langerhans cell histiocytosis.

Langerhans cell histiocytosis (LCH) is an idiopathic disease characterized in its early stages by granulomatous nodules containing Langerhans histiocytes and eosinophils.
In its later stages, the granulomas are replaced by fibrosis and the formation of cysts.
It is an uncommon condition.
The majority of patients are young or middle-aged adults presenting with nonspecific symptoms of cough and dyspnea. Up to 20% of patients present with pneumothorax and over 90% of patients are smokers.
Most cysts appear round, but can also have bizarre shapes (bilobed or clover-leaf shaped).
An upper lobe predominance in the size and number of cysts is common.

On the left a case with multiple cysts that are evenly distributed througout the lung ( in contrast to LCH).
Notice the pneumothorax.
There was no history of smoking and this was a 40 year old female.
This combination of findings is typical for Lymphangiomyomatosis (LAM).

Lymphangiomyomatosis is a rare disease characterized by progressive proliferation of spindle cells, resembling smooth muscle.
Proliferation of these cells along the bronchioles leads to air trapping and the development of thin-walled lung cysts.
Rupture of these cysts can result in pneumothorax.
Other features of LAM include adenopathy and pleural effusion.

Lymphangiomyomatosis occurs only in women, usually of child-bearing age, between 17 and 50 years. Identical clinical, radiologic, and pathologic pulmonary changes are seen in about 1% of patients with tuberous sclerosis.
Most patients die within 10 years of the onset of symptoms.

Bronchiectasis
Bronchiectasis is defined as localized bronchial dilatation.
The diagnosis of bronchiectasis is usually based on a combination of the following findings:

• bronchial dilatation (signet-ring sign)
• bronchial wall thickening
• lack of normal tapering with visibility of airways in the peripheral lung
• mucus retention in the broncial lumen
• associated atelectasis and sometimes air trapping

A signet-ring sign represents an axial cut of a dilated bronchus (ring) with its accompanying small artery (signet).
The most common cause of bronchiectasis is prior infection, usually viral, at an early age.
It also occurs in patients with chronic bronchitis, COPD and cystic fibrosis.

Bronchiectasis may mimic cystic lung disease and bullous emphysema.
Bronchiectasis caused by primary airway disease should be differentiated from tracion bronchiectasis as a result of fibrosis.

On the left we see a chest film with a typical finger-in-glove shadow.
The HRCT shows focal bronchiectasis with extensive mucoid impaction, which is in the appropriate clinical setting (asthma and serum eosinophilia) typical for Allergic bronchopulmonary aspergillosis (ABPA).

Allergic bronchopulmonary aspergillosis is a lung disease occurring in patients with asthma or cystic fibrosis, triggered by a hypersensitivity reaction to the presence of Aspergillus fumigatus in the airways.
It characteristically presents with the findings of central bronchiectasis, mucoid impaction and atelectasis.

Honeycombing

Honeycombing is defined by the presence of small cystic spaces with irregularly thickened walls composed of fibrous tissue.
Honeycomb cysts often predominate in the peripheral and subpleural lung regions regardless of their cause.
Subpleural honeycomb cysts typically occur in several contiguous layers.
This finding can allow honeycombing to be distinguished from paraseptal emphysema in which subpleural cysts usually occur in a single layer.

The case on the left shows subpleural honeycomb cysts in several contiguous layers.
There is also a lower lobe predominance and widespread traction bronchiectasis.
These findings are typical for Usual Interstitial Pneumonia (UIP).

UIP or 'end-stage lung' is a pathology diagnosis and usually shown at lungbiopsy, when honeycombing is visible.
Idiopathic pulmonary fibrosis (IPF), accounts for more than 60% of the cases of UIP.
UIP with lung fibrosis is also a common pattern of auto-immune disease and drug-related lung injury.
A long list of drugs have been implicated, but this pattern is most commonly the result of cytotoxic chemotherapeutic agents such as bleomycin, busulfan, vincristine, methotrexate, adriamycin, and carmustine (BCNU).

On the left another case of UIP.
The lower zone predominance is demonstrated when you scroll through the images.
Notice the ground glass opacity in the left lower lobe as a result of fibrous tissue replacing the air in the alveoli.

Upper versus lower zone distribution

Upper lung zone preference is mostly seen in inhalation diseases:

• Inhaled particles: pneumoconiosis (silica or coal)
• Smoking related diseases (centrilobular emphysema)
• Respiratory bronchiolitis (RB-ILD)
• Langerhans cell histiocytosis
• Hypersensitivity pneumonitis
• Sarcoidosis

Lower zone preference is seen in:

• UIP
• Aspiration
• Pulmonary edema

Central versus peripheral distribution

Central distribution is seen in sarcoidosis, bronchitis and cardiogenic pulmonary edema.

Peripheral distribution is mainly seen in cryptogenic organizing pneumonia (COP), chronic eosinophilic pneumonia and UIP.

Pleural effusion

Diseases that manifest with pleural effusion are listed in the table.

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
This is commonly seen in lymph nodes in patients with silicosis and coal-worker's pneumoconiosis and is sometimes seen in sarcoidosis, postirradiation Hodgkin disease, blastomycosis and scleroderma.

Examples of reticular pattern:

1. Lymphangitic carcinomatosis: irregular septal thickening, usually focal or unilateral 50% adenopathy', known carcinoma.
2. Cardiogenic pulmonary edema: incidental finding in HRCT, smooth septal thickening with basal predominance (Kerley B lines), ground-glass opacity with a gravitational and perihilar distribution, thickening of the peribronchovascular interstitium (peribronchial cuffing)
3. Lymphangitic carcinomatosis
4. Lymphangitic carcinomatosis with hilar adenopathy.
5. Alveolar proteinosis: ground glass attenuation with septal thickening (crazy paving).
6. Cardiogenic pulmonary edema.

Examples of nodular pattern

1. Hypersensitivity pneumonitis: ill defined centrilobular nodules.
2. Miliary TB: random nodules
3. Sarcoidosis: nodules with perilymphatic distribution, along fissures, adenopathy.
   
4. Hypersensitivity pneumonitis: centrilobular nodules, notice sparing of the area next to pleura and fissure.

More nodular pattern

1. Sarcoidosis: nodules with perilymphatic distribution, along fissures, adenopathy.
2. TB: Tree-in-bud appearance in a patient with active TB.
3. Langerhans cell histiocytosis: early nodular stage before the typical cysts appear.
4. Respiratory bronchiolitis in infection.

Examples of High Attenuation pattern

1. Chronic eosinophilic pneumonia with peripheral areas of ground glass opacity.
2. Sarcoid end-stage with massive fibrosis in upper lobes presenting as areas of consolidation. Notice lymphadenopathy.
3. Chronic eosinophilic pneumonia with peripheral areas of consolidation.
4. Broncho-alveolar cell carcinoma with both areas of ground glass opacity and consolidation

High Attenuation pattern (2)

1. Non specific interstitial pneumonitis (NSIP): ground glass with traction bronchiectasis, no honeycombing.
2. Cryptogenic organizing pneumonia (COP).
3. Sarcoidosis end-stage: consolidation as a result of massive fibrosis perihilar and in upper lobes.
4. COP.

Low Attenuation pattern

1. Lymphangiomyomatosis (LAM): uniform cysts in woman of child-bearing age; no history of smoking; adenopathy and pleural effusion; sometimes pneumothorax.
2. LCH: multiple round and bizarre shaped cysts; smoking history.
3. Honeycombing
4. Centrilobular emphysema: low attenuation areas without walls.

Low Attenuation pattern (2)

1. Centrilobular emphysema: low attenuation areas without walls. Notice the centrilobular artery in the center.
2. Langerhans cell histiocytosis (LCH): multiple thick walled cysts; smoking history.
3. Honeycombing.
4. Lymphangiomyomatosis (LAM): regular cysts in woman of child-bearing age.