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
Here is 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.
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.
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.
Above 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.
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.
Bronchiectasis caused by primary airway disease should be differentiated from tracion bronchiectasis as a result of fibrosis.
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 above 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)
Yet 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.
Please the other posts on HRCT Thorax. To read click on the topics given below.
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