Three‐dimensional oxygen‐enhanced MRI of the human lung at 1.5T with ultra‐fast balanced steady‐state free precession

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With the development of dedicated pulse sequences and optimized acquisition schemes, MRI has become an attractive radiation‐free modality for morphological and functional imaging of the lung 1. For the assessment of pulmonary perfusion, dynamic contrast‐enhanced (DCE) MRI is a well‐established technique that provides high spatial and temporal resolution, but requires the intravenous injection of contrast agents. For ventilation imaging, non‐proton‐based MRI with inhaled hyperpolarized 3He or 129Xe gases yields a direct measure of pulmonary ventilation and has demonstrated compelling results 3. However, its broad application is hindered by the requirement of dedicated equipment.
An interesting alternative to hyperpolarized gas imaging may be found in oxygen‐enhanced (OE) proton‐based MRI 5 since it does not require additional equipment and is based on the paramagnetic properties of oxygen itself. When breathed, oxygen acts as a weak contrast agent in the lung, shortening the longitudinal relaxation time (T1) and the apparent transverse relaxation time (T2*) 6. The observed decrease in T1 under hyperoxic condition is attributed to an increased concentration of dissolved O2 in the parenchyma, blood vessels, and lung tissue 7. OE‐MRI is usually considered an indirect measure for ventilation, diffusion, and perfusion on T1‐weighted images 8. If only one of the three functions is regionally hampered, a lower signal intensity (SI) enhancement is detected locally. In contrast, the T2* shortening effect in the lungs while breathing oxygen can be attributed to the mesoscopic magnetic susceptibility changes at the tissue/gas interfaces in the alveoli and reflects a measurement for pulmonary ventilation 7.
Oxygen‐related signal enhancement in the lung is commonly derived voxel‐wise based on the relative difference of images acquired in hyperoxic (100% O2) and normoxic (room air, 21% O2) conditions and has shown promising results for patients with diseases such as cystic fibrosis, pulmonary embolism, and emphysema 9. Moreover, the technique was recently applied to monitoring the severity of asthma, evaluating pharmacological treatment in chronic obstructive pulmonary disease and defining candidates for lung volume reduction surgery 11. However, the overall magnitude of the signal enhancement from oxygen generally is weak, that is, 5% to 10% 6. As a result, the oxygen enhancement typically is inferior to the natural SI variations of the lung parenchyma induced by different but similar inspiratory volumes (e.g. in expiration), and OE‐MRI can be easily flawed 14. Consequently, a strategy to account for the SI modulations of the lung at diverse inspiratory phases appears mandatory. Moreover, most of the OE‐MRI methods are confined to 2D acquisitions and thus are limited either in speed and/or chest coverage.
Here, we investigate the feasibility of 3D oxygen‐enhanced MRI using ultra‐fast balanced steady‐state free precession (ufSSFP) imaging, recently introduced for improved morphological and functional lung imaging 15. Based on multi‐volumetric ufSSFP lung scans (acquired at different inspiratory phases), we propose a new framework for robust volumetric OE‐MRI that accounts for signal intensity variations of the lung parenchyma using an adapted sponge model 16.

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