An innovative project
A double paradigm shift for both pulmonary function testing and MRI
from forced to spontaneous breathing and from global to local measurements,
from standard to low- and very-low field MRI and from radiology to pulmonology departments.
Ventilation efficiency and respiratory diseases are clinically investigated mainly with spirometry as part of routine pulmonary function tests. From the shape of a flow-volume loop recorded over time at the mouth while the patient is performing forced breaths, the forced vital capacity (FVC) and the forced expiratory volume in one second (FEV) can be inferred and the pulmonologist is able to detect and characterize common respiratory diseases and to follow up treatments. This technique is non-invasive, simple, widely available, robust, and reproducible. Yet, spirometry outcomes rely on the operator’s training and the patient’s cooperation. Moreover, it cannot be performed in children younger than 5 years old and only provides global scalar information over the entire lung in extreme conditions. It is also well known that spirometry is less sensitive than imaging and provides very limited information about the nature and extent of physiological impairments or the resultant clinical consequences. The corollary is that exclusive reliance on spirometry for the evaluation of therapeutic efficacy can lead to significant misestimation of clinical benefit.
A new spirometry modality
A novel global strategy has recently been developed at UPSaclay to jointly analyse the ventilatory function and the mechanical behaviour of the lung. A 3D ultrashort echo time (UTE) sequence with 1.5 mm isotropic voxel size and 2 ms repetition time (TR) was implemented at 3 T to acquire images for 32 phases over a time-average human respiratory cycle by retrospective self-gating. Once displacement fields were computed from the 32 registered 3D images, tissue displacement loops could be inferred over the time-average respiratory cycle in every voxel and, from the local Jacobian and its time derivative, regional flow-volume loops were computed everywhere in the lung. The larger dimensionality advantageously allows the extraction of original metrics that characterize, beyond the flow-volume loops, the anisotropic and hysteretic regional mechanical behaviour of the lung. Maps of the Green-Lagrange strain tensor and of the related fractional anisotropy were inferred in this line as fully described in UPSaclay pioneering work. The outcomes set the grounds of 3D MR spirometry.
Tidal volume 3D maps
Local tidal volume derived from the gas flow-volume loops in every lung parenchyma voxel throughout the lung as averaged over a hundred dataset for 25 freely-breathing healthy volunteers after normalization (coronal view). The voxel is (1.5×1.5×1.5) mm3. The mean local tidal volume is close to 0.5 µL and the overall tidal volume – as it would be measured at the mouth – sums up to 0.5 L.
Spontaneous peak expiratory flow 3D maps
Local peak expiratory flow derived from the gas flow-volume loops in every lung parenchyma voxel throughout the lung as averaged over a hundred dataset for 25 freely-breathing healthy volunteers after normalization (sagittal view). The voxel is (1.5×1.5×1.5) mm3. The mean local peak expiratory flow is close to 0.5 µL·s-1 and it goes up to 1.5 µL·s-1
Anisotropic deformation index 3D maps
Local anisotropic deformation index derived from the Green-Lagrange tensor in every lung parenchyma voxel throughout the lung as averaged over a hundred dataset for 25 freely-breathing healthy volunteers after normalization (axial view). The voxel is (1.5×1.5×1.5) mm3. The mean anisotropic deformation index is close to 0.13 and it goes up to 0.54