In-vivo lung flooding was invented for thoracoscopic sonographic guidance¹ and lately we could show its use for lung cancer ablation using focused ultrasound (HIFU)². MR guidance is a valid method for non-invasive ablation and allows thermal dose control using PRFS thermometry³. The limitation of MR imaging in ventilated lung regarding nodule detection and thermometry might be overcome by replacing air with water (saline). The invasivity of lung flooding is justified for therapeutic pulmonary interventions. The purpose of the study was to investigate the feasibility of in-vivo lung flooding in MRI environment as well as to use the high sensitivity of MR imaging to susceptibility artifacts especially at tissue-air and liquid-air interfaces to qualitatively monitor complete flooding without trapped air.Unilateral lung flooding was performed on six female pigs “Deutsches Landschwein” bred, female 35-60 kg, anesthetized with propofol (10 mg/kg/h), fentanyl (0.08 μg/kg/min) and pancuronium (2.5 μg/kg/min). Mechanical ventilation was performed with an ICU respirator (Servo 900, Siemens AG, Munich, Germany) on a pressure-controlled setting trough a left-sided double-lumen tube (39 Ch, Mallinckrodt Medical, Dublin, Ireland). The MR examinations were performed with a 3 T MRI (Prisma Fit, Siemens Medical Solutions, Erlangen, Germany) and spine and body array coils. The positioning of the animal was lateral left, feet first. Vital parameters were monitored during examination using pulse-oximeter, respiration belt and MR compatible ECG sensors. Before unilateral flooding, lungs were ventilated with FIO₂ = 1.0 for about 30 minutes. Flooding was performed with 0.7 -1 L tempered (35°C) saline of the left lung wing. Flooding preassure was maintained stable at 20 cm H₂O column for 60-90 min, followed by re-ventilation for 30 min. The animal was euthanized with KCl in deep narcosis. MR imaging was performed using HASTE, segmented EPI and T1 weighted 2D flash sequences. Imaging with breath hold (inspiration or expiration end point), respiration triggered as well free breathing was performed.MRI examination during unilateral lung flooding was successfully performed in six of six animals. As expected, the flooded lung parenchyma appears hyperintense in T2 weighted images (Fig. 1) and hypointense in T1 (Fig. 2). Bronchial and vascular structures in the flooded lung lobe clearly appear as hypointense structures in T2 weighted image at high level of detail. GRE based phase images (Fig. 3) showed a homogenous phase in bronchial or alveolar tissue of the flooded lung wing. The absence of small local phase wraps and thus strong susceptibility changes in the flooded lung tissue suggests a complete flooding without trapped air. No streaming or outflow effects occurred in flooded lung parenchyma and bronchial structure in T1w and T2w images. This indicate that the water is statical trapped in flooded lung.To the best of our knowledge, this is the first study, which demonstrates the feasibility of in-vivo unilateral lung flooding during MRI examination. MR phase imaging proved qualitatively gas free flooding of the left (in one case of the right) lung lobe. We demonstrated gas free filling, which enables new imaging approaches for detection and classification of pulmonary nodules. It is very likely that the current detection limit for nodule in ventilated lung (> 4 mm)⁴ can be improved using the presented approach. The invasive nature of this method prevents general clinical diagnostics. However, the homogeneity of the phase over the entire lung lobe is further essential for PRFS based MR thermometry⁵ as found in flooded condition. This could enable monitoring of thermal interventions (LITT or preferably HIFU) in lung and justify its invasivity. Further research investigating PRFS based thermometry in flooded lung using invasive and temperature controlled lesions and finally the application in cancer models is required.