Characterization of and tissue density in the human lung: Application to neonatal imaging in the intensive care unit

Abstract

Purpose Novel demonstration of R2∗ and tissue density estimation in infant lungs using 3D ultrashort echo time MRI. Differences between adult and neonates with no clinical indication of lung pathology is explored, as well as relationships between parameter estimates and gravitationally dependent position and lung inflation state. This provides a tool for probing physiologic processes that may be relevant to pulmonary disease and progression in newborns. Methods R2∗ and tissue density were estimated in a phantom consisting of standards allowing for ground truth comparisons and in human subjects (N = 5 infants, N = 4 adults, no clinical indication of lung dysfunction) using a 3D radial multiecho ultrashort echo time MRI sequence. Whole lung averages were compared between infants and adults. Dependence of the metrics on anterior‐posterior position as well as between end‐tidal inspiration and expiration were explored, in addition to the general relationship between R2∗ and tissue density. Results Estimates in the phantom did not differ significantly from ground truth. Neonates had significantly lower mean R2∗ (P = .006) and higher mean tissue density (P = 1.5e‐5) than adults. Tissue density and R2∗ were both significantly dependent on anterior‐posterior position and lung inflation state (P < .005). An overall inverse relationship was found between R2∗ and tissue density, which was similar in both neonates and adults. Conclusion Estimation of tissue density and R2∗ in free breathing, nonsedated, neonatal patients is feasible using multiecho ultrashort echo time MRI. R2∗ was no different between infants and adults when matched for tissue density, although density of lung parenchyma was, on average, lower in adults than neonates.

Effects of neonatal lung abnormalities on parenchymal R2* estimates

“…Finally, simulations were performed to investigate whether simple variations in alveolar geometry, specifically alveolar septal wall thickness (differences in which might be associated with disease), influence the R 2 *. The results of our model and measurements support the inverse relationship between tissue density and R 2 * predicted by normal lung inflation, consistent with similar simulated findings from previous work 29 and empirical results both in this work and elsewhere 12 …”

“…The R 2 * was estimated by sampling S ( t ) at the echo times used for imaging and fitting a monoexponential decay to these data. The echo times used were 0.200 ms, 0.95 ms, 1.700 ms, and 2.45 ms for the simulations of neonatal alveoli and 0.09 ms, 0.59 ms, 1.09 ms, and 1.59 ms for simulations of adult alveoli 12 …”

Quantification of lung water density with UTE Yarnball MRI

“…Dualecho acquisitions also enable detection of larger T * 2 of lung tissue associated with increased water content. 27,28 Dualecho trajectories were designed with matched parameters to the single-echo Yarnball sequence, with a 500-kHz readout bandwidth and fully sampled k-space for FOV of 300 mm, 2.5-mm isotropic resolution, ρ = 0.30, T RO = 1.3 ms per echo, and with TE 1 = 0.07 ms, TE 2 = 2.79 ms, for which TR = 3.54 ms. The dual-echo trajectories were slightly less efficient than the single echo, requiring 7381 projections.…”

“…For application in the lungs, dual‐echo Yarnball facilitates simultaneous UTE and dark lung imaging, where the latter is proposed to enhance automated selection of the short lung parenchyma. Dual‐echo acquisitions also enable detection of larger of lung tissue associated with increased water content 27,28 . Dual‐echo trajectories were designed with matched parameters to the single‐echo Yarnball sequence, with a 500‐kHz readout bandwidth and fully sampled k‐space for FOV of 300 mm, 2.5‐mm isotropic resolution, ρ = 0.30, T RO = 1.3 ms per echo, and with TE 1 = 0.07 ms, TE 2 = 2.79 ms, for which TR = 3.54 ms.…”

Modern pulmonary imaging of bronchopulmonary dysplasia

“…Regional measurement of MR relaxation parameters (e.g. T 1 , T 2 , and T 2 *) is also feasible at various inflation states and may have promise in identifying patients with BPD-related lung disease (69,70).…”