MR imaging of apparent 3He gas transport in narrow pipes and rodent airways

Abstract

High sensitivity makes hyperpolarized 3He an attractive signal source for visualizing gas flow with magnetic resonance (MR) imaging. Its rapid Brownian motion, however, can blur observed flow lamina and alter measured diffusion rates when excited nuclei traverse shear-induced velocity gradients during data acquisition. Here, both effects are described analytically, and predicted values for measured transport during laminar flow through a straight, 3.2-mm-diameter pipe are validated using two-dimensional (2D) constant-time images of different binary gas mixtures. Results show explicitly how measured transport in narrow conduits is characterized by apparent values that depend on underlying gas dynamics and imaging time. In ventilated rats, this is found to obscure acquired airflow images. Nevertheless, flow splitting at airway branches is still evident and use of 3D vector flow mapping is shown to reveal surprising detail that highlights the correlation between gas dynamics and lung structure.

Phase-contrast MRI and CFD modeling of apparent 3He gas flow in rat pulmonary airways

“…Predicted transport, however, is so detailed that new experimental methods are needed to assess simulation results [5]. Phase-contrast (PC) magnetic resonance imaging (MRI) with hyperpolarized 3 He is particularly promising since it has already been successfully applied for visualizing respiratory airflows in rats [6] and humans [7]. This is especially important for inhalation toxicology given the rat’s widespread use as a human surrogate for safety testing, and the established role of CFD for relating airborne exposure from one species to another [8].…”

“…This is particularly challenging when comparing PC-MRI to CFD since rapid gas diffusion blurs laminar flow structure in measured results [6,10]. Physically, this occurs when gas diffuses across shear-induced velocity gradients and excited nuclei traverse different laminar flow steams during raw data collection.…”

“…Physically, this occurs when gas diffuses across shear-induced velocity gradients and excited nuclei traverse different laminar flow steams during raw data collection. In straight pipes this process has been described analytically, and predicted patterns of measured airflow have been experimentally validated [6]. Results show explicitly how 3 He PC-MRI provides apparent values of local gas velocity that not only depend on actual airflow but also on the rate of gas diffusion and underlying imaging parameters [6].…”

Resolution enhancement in MRI of laser polarized 3He by control of diffusion

“…Hence it can be envisioned that the presented idea of admixing heavier buffer gases can become useful in clinical applications to increase spatial and/or temporal resolution without diminishing the diagnostic contrast as recently demonstrated in 3 He transport in rodent airways [32]. Finally d 32 was replaced by d.…”

Spectral separation of gaseous fluorocarbon mixtures and measurement of diffusion constants by 19F gas phase DOSY NMR

“…In gaseous systems, there has been significant interest in the determination of diffusion rates, both as surface probes in materials science [7][8][9][10] and as an experimental datum in pulmonary MRI studies. This last application most often uses hyperpolarized 3 He [11][12][13] and 129 Xe [7,11] (and even 83 Kr [14]), though the use of fluorinated gases has been reported as well [12,15].…”