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Mechanical ventilation plays a crucial role in managing critically ill patients. High stretch and overdistension of alveoli can cause parenchymal injury, worsening outcome. Prone posture and Positive End Expiratory Pressure (PEEP) are used as lung protective strategies during mechanical ventilation. We evaluated changes in inflation and stretch due to gravitational forces with PEEP and prone positioning in normal subjects. We hypothesized that prone posture redistributes mechanical forces, reducing stretch in regions of the lung that are already fully inflated and thus vulnerable. Six healthy subjects (4M/2F, age: 30±4yrs, FEV1: 91.8±9.2% Predicted) were imaged at functional residual capacity with a GE 1.5 T HDx TwinSpeed MRI scanner. A fast multi-echo gradient-echo sequence was used to measure density in 15mm sagittal slices, covering most of the right lung in supine and prone …