Abstract
Background: Computed tomography is the gold standard for lung aeration assessment, but exposure to ionizing radiation limits its application. We assessed the ability of magnetic resonance imaging (MRI) to detect changes in lung aeration in ex vivo isolated swine lung and the potential of translation of the findings to human MRI scans.Methods: We performed MRI scans in 11 isolated non-injured and injured swine lungs, as well as 6 patients both pre- and post-operatively. Images were obtained using a 1.5 T MRI scanner, with T1 – weighted volumetric interpolated breath-hold examination (VIBE) and T2 – weighted half-Fourier acquisition single-shot turbo spin-echo (HASTE) sequences. We scanned swine lungs, with reference samples of water and muscle, at different airway pressure levels: 0, 40, 10, 2 cmH2O. We investigated the relations between MRI signal intensity and both lung density and gas content fraction. We analyzed patients’ images according to the findings of the ex vivo model.Results: In the ex vivo samples, the lung T1 – VIBE signal intensity normalized to water or muscle reference signal correlated with lung density (r2 = 0.98). Thresholds for poorly and non-aerated lung tissue, expressed as MRI intensity attenuation factor compared to the deflated lung, were estimated as 0.70 [95% CI: 0.65–0.74] and 0.28 [95% CI: 0.27–0.30], respectively. In patients, dorsal versus ventral regions had a higher MRI signal intensity both pre- and post-operatively (p = 0.031). Comparing post- versus pre-operative scans, lung volume decreased (p = 0.028), while the following increased: MRI signal intensity in ventral (p = 0.043) and dorsal (p < 0.0001) regions, and percentages of non-aerated (p = 0.028) and poorly aerated tissue volumes (p = 0.028).Conclusion: Magnetic resonance imaging signal intensity is a function of lung density, decreasing linearly with increasing gas content. Lung MRI might be useful for estimating lung aeration. Compared to CT, this technique is radiation-free but requires a longer acquisition time and has a lower spatial resolution.
Highlights
The quantitative analysis of lung aeration, usually performed with computed tomography (CT), has deeply changed the understanding and clinical management of the acute respiratory distress syndrome (ARDS) (Puybasset et al, 1998; Gattinoni et al, 2001)
Despite the lower SNR achieved with higher continuous positive airway pressure (CPAP) levels, the manual segmentation could be manually performed in all scans, as the interface between lung and air could be identified by visual inspection
Lung density could be described as a function of magnetic resonance imaging (MRI) signal; as shown in Table 2, the lowest AICc values were obtained with the T1 – VIBE sequences, MRI normalization to either water or muscle signal, and a quadratic model
Summary
The quantitative analysis of lung aeration, usually performed with computed tomography (CT), has deeply changed the understanding and clinical management of the acute respiratory distress syndrome (ARDS) (Puybasset et al, 1998; Gattinoni et al, 2001). Safety concerns limit the clinical application of quantitative lung CT to critically ill patients, where the high mortality and the clinical value of this technique possibly justify the biological hazards of radiation exposure. From CT, MRI signal intensity is not calibrated to a standard measurement unit, making quantitative analysis of images more difficult. Computed tomography is the gold standard for lung aeration assessment, but exposure to ionizing radiation limits its application. We assessed the ability of magnetic resonance imaging (MRI) to detect changes in lung aeration in ex vivo isolated swine lung and the potential of translation of the findings to human MRI scans
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