Lung imaging during acute respiratory distress syndrome : CT- and PET-scanning

In this article we review some of the main findings obtained by Computed Tomography (CT) and Positron Emission Tomography (PET) concerning the pathophysiology of Acute Respiratory Distress Syndrome (ARDS). CT is based on the detection of physical densities, allowing measurement of regional aeration. Beginning from the revolutionary concept of the " baby lung" , CT findings later led to a further evolution of the ARDS lung model, showing that the lungs behave as a wet " sponge" , with the dependent regions collapsing under the superimposed weight. More recently, CT scan has been used to quantify the potential for lung recruitment, showing that this is extremely heterogeneous among ARDS patient and tightly linked to mortality. PET (possibly combined with CT) is a functional imaging technique, based on the detection of a labeled molecule administered to a subject. Based on which molecule is used, different functions can be imaged. In the course of experimental ARDS injection of [13N]N2- labeled saline has been used to image regional gas exchange. Administration of [18F]FDG allows the image of cellular metabolic activity, reflecting neutrophils activation during inflammation. This technique has been applied in experimental ARDS and, more recently, in patients showing, for example, that inflammatory activity of the lungs is markedly increased also in " normally aerated" regions and, in some cases, even higher than in the non-aerated ones. We will present here some of the findings obtained by the two techniques in the clinical setting of ARDS, also discussing some of their possible future applications