The goal of this study is to investigate acute respiratory distress syndrome (ARDS) and septic lung with positron emission tomography (PET) imaging and to examine the distribution of inflammation, as measured by neutrophil metabolic activity.

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

Adult intensive care patients requiring mechanical ventilation.