The Acute Respiratory Distress Syndrome (ARDS) is an acute, diffuse and progressive inflammatory lung injury characterized by an increased permeability of the alveolo-capillary membrane with accumulation of proteinaceous exudate in the alveolar and interstitial spaces.

The ARDS was first described in 1967 by Ashbough and colleagues based on their experience with 12 patients that developed dyspnoea and tachypnoea, cyanosis refractory to oxygen therapy, decreased lung compliance and diffuse infiltrates on chest radiograph.

In 1994, the European-American Consensus Conference Committee established a new definition that made the distinction between acute lung injury (PaFi 200-300) and ARDS (PaFi<200).

According to the Berlin criteria, which were developed by a panel of experts in 2012, the diagnosis of ARDS is based on the onset of hypoxemia and of bilateral chest opacities within 1 week of a known risk factor. This respiratory failure should not be fully explained by cardiac failure of fluid overload.

These new criteria indicate the severity of ARDS according to the value of PaO2/FiO2 ratio (mild, moderate and severe) and allow the early diagnosis of this syndrome ( Fig. 1 ).  

The clinical disorders associated with ARDS can be classified into those associated with direct injury to the lung and those that cause indirect lung injury.  Pulmonary aetiologies of ARDS include pulmonary contusion, fat embolism, pneumonia… and non-pulmonary causes include sepsis, intravascular coagulopathy, severe trauma… From all these causes, sepsis is associated with the highest risk of progression to ARDS (40%).

There are also several predisposing disorders which substantially increases the risk of developing ARDS. These factors include chronic alcohol abuse, chronic lung disease, and a low serum pH.

These categories defined the pulmonary and non-pulmonary ARDS, which usually have distinct radiological features. In addition to these "causes", there are secondary factors that also increase the risk of developing ARDS such as chronic alcohol abuse, chronic lung disease and acidemia.

This progressive respiratory failure presents typical and distinct radiographic manifestations for the different pathological stages of the disease (exudative, inflammatory and fibroproliferative phases) ( Fig. 2 ).  Besides, the respiratory mechanisms and imaging appearance of ARDS due to pulmonary and indirect lung injuries are not identical.

Imaging plays a key role in the diagnosis of ARDS and may also provide useful information for the detection of prognostic factors. In addition of its role in the detection of complications, CT increases the understanding of the pathophysiology.