Ventilation Defects Observed with Hyperpolarized 3He Magnetic Resonance Imaging in a Mouse Model of Acute Lung Injury

Regions of diminished ventilation are often evident during functional pulmonary imaging studies, including hyperpolarized gas magnetic resonance imaging (MRI), positron emission tomography, and computed tomography (CT). The objective of this study was to characterize the hypointense regions observed via He-3 MRI in a murine model of acute lung injury. LPS at doses ranging from 1550 mu g was intratracheally administered to C57BL/6 mice under anesthesia. Four hours after exposure to either LPS or saline vehicle, mice were imaged via hyperpolarized He-3 MRI. All images were evaluated to identify regions of hypointense signals. Lungs were then characterized by conventional histology, or used to obtain tissue samples from regions of normal and hypointense He-3 signals and analyzed for cytokine content. The characterization of He-3 MRI images identified three distinct types of hypointense patterns: persistent defects, atelectatic defects, and dorsal lucencies. Persistent defects were associated with the administration of LPS. The number of persistent defects depended on the dose of LPS, with a significant increase in mean number of defects in 30-50-mu g LPS-dosed mice versus saline-treated control mice. Atelectatic defects predominated in LPS-dosed mice under conditions of low-volume ventilation, and could be reversed with deep inspiration. Dorsal lucencies were present in nearly all mice studied, regardless of the experimental conditions, including control animals that did not receive LPS. A comparison of He-3 MRI with histopathology did not identify tissue abnormalities in regions of low He-3 signal, with the exception of a single region of atelectasis in one mouse. Furthermore, no statistically significant differences were evident in concentrations of IL-1 beta, IL-6, macrophage inflammatory protein (MIP)-1 alpha MIP-2, chemokine (C-X-C motif) ligand 1 (KC), TNF alpha, and monocyte chemotactic protein (MCP)-1 between hypointense and normally ventilated lung regions in LPS-dosed mice. Thus, this study defines the anatomic, functional, and biochemical characteristics of ventilation defects associated with the administration of LPS in a murine model of acute lung injury.