Regional Lung Inflammation and Expansion in Mechanically Ventilated Patients - a PET/CT Study

Inclusion Criteria: Group 1 * Mechanically ventilated patients scheduled to undergo a CT scanning for their routine clinical care * More than 24 hours of mechanical ventilation * Less than 15 days of mechanical ventilation if patient presents ALI/ARDS (in order to exclude the late, fibrotic, stage of ARDS) Group 2 * Mechanically ventilated patients with a diagnosis of sepsis * Less than 96 hours of mechanical ventilation Exclusion Criteria: * Age less than 18 years * Hemodynamic instability, defined as: systolic blood pressure (SBP) \< 90 mmHg that is not adequately stabilized by vasopressors or inotropic agents. For these purposes, SBP will not be considered "adequately stabilized" if the dose of the vasopressor/inotrope has not been stable for at least one hour * Hypoxemia, defined as: PaO2 \< 70 mmHg on an inspired oxygen fraction (FiO2) greater than or equal to 0.9; FiO2 greater than or equal to 0.8 at baseline * Hemodynamic and/or respiratory instability (as defined, respectively, in 2 and 3), that develop when the patient is mobilized during routine nursing care such as repositioning/washing the patient or changing their bed linens * Hemodynamic and/or respiratory instability (as defined, respectively, in 2 and 3), that develop when a 15-25 second respiratory pause is introduced as per the study protocol. This will be tested by inducing such a pause prior to transporting the patient * Patients must be deemed by the treating clinical staff to be able to tolerate leaving the ICU for 90 minutes (40-60 minute imaging time + 30 minute safety margin). Treating MD, RT, and RN must all agree that the patient is stable for transport * Any acute or chronic condition which, in the opinion of the investigator, might confound the imaging measurements (such as, but not limited to, severe bronchospasm, pulmonary infection, and lung tumor) * "Air leaks" requiring tube thoracostomy (e.g., pneumothorax, bronchopleural fistula) * Pregnancy (since this is a study which would expose a fetus to radiation risk) * Patients who have neither an arterial nor a central venous line at the time of the enrollement