Alveolar Gas Diffusion MRI as a Function of Pulmonary Pressure

INTRODUCTION: Acute Respiratory Distress Syndrome (ARDS) is a critical lung disorder characterized by impairment of respiratory mechanics and gas exchange. ARDS can lead to inflammation in the alveoli, and eventually to atelectasis (alveolar collapse) and gas exchange deficiency. Alveolar recruitment maneuvers with varying and fixed positive end-expiratory pressure (PEEP) values have been devised to recruit collapsed alveoli and keep them open with the aid of mechanical ventilation. However the optimal strategy for the alveolar recruitment strategy is still uncertain. Various combinations of high/low tidal volume (TV) and higher-than-normal PEEP maneuvers have been proposed with different and sometimes contradictory results in terms of survival rate. Hyperpolarized (HP) gas MRI has the potential to noninvasively and quantitatively measure the dynamics of alveolar expansion and recruitment and assess the efficacy of the PEEP maneuvers to gain better understanding in this area of lung research. METHODS: Healthy male Sprague Dawley rats were intubated and connected to a customdesigned small animal ventilator (SAV). The SAV is capable of delivering the breathing gas with an accuracy of ±100μL/breath and real-time monitoring of peak inspiration pressure (PIP). The rats breathed a mixture of He:O2 (4:1) at 60 BPM with I:E=1:2 at a TV=3⁄4FRC as measured with a rodent plethysmography spirometry system. The PEEP was applied by submerging the SAV exhale line in a graduated water column. For imaging, rats were ventilated with five identical breaths of HP He:O2 (4:1) followed by a 3-s breath-hold – either at end-inhale (EI) or end-exhale (EE) – during which ADC He images (Figure 2) were acquired using a double acquisition diffusion-weighted gradient echo imaging pulse sequence in a 50-cm bore 4.7-T MRI scanner (Varian, Inc.) equipped with 12-cm, 25-G/cm gradients and a 2-3/4”-ID quadrature 8-leg birdcage body coil (Stark Contrast). Imaging parameters were as following: diffusion time Δ=1ms along phase encoding direction, b-values = 0.00, 3.73, 2.18, 1.00 and 0.00 s/cm, FOV=6cm, ST=6mm, MS=64×64 and α=5°. The PEEP value was varied between 0 (black) and 12 cm H2O (red) as shown in Figure 3.