Microstructural Analysis of Peripheral Lung Tissue through CPMG Inter-Echo Time R2 Dispersion

Abstract

Since changes in lung microstructure are important indicators for (early stage) lung pathology, there is a need for quantifiable information of diagnostically challenging cases in a clinical setting, e.g. to evaluate early emphysematous changes in peripheral lung tissue. Considering alveoli as spherical air-spaces surrounded by a thin film of lung tissue allows deriving an expression for Carr-Purcell-Meiboom-Gill transverse relaxation rates R 2 with a dependence on inter-echo time, local air-tissue volume fraction, diffusion coefficient and alveolar diameter, within a weak field approximation. The model relaxation rate exhibits the same hyperbolic tangent dependency as seen in the Luz-Meiboom model and limiting cases agree with Brooks et al. and Jensen et al. In addition, the model is tested against experimental data for passively deflated rat lungs: the resulting mean alveolar radius of R A = 31.46 ± 13.15 μm is very close to the literature value (∼34 μm). Also, modeled radii obtained from relaxometer measurements of ageing hydrogel foam (that mimics peripheral lung tissue) are in good agreement with those obtained from μCT images of the same foam (mean relative error: 0.06 ± 0.01). The model’s ability to determine the alveolar radius and/or air volume fraction will be useful in quantifying peripheral lung microstructure.

In Vivo Measurements of T2 Relaxation Time of Mouse Lungs during Inspiration and Expiration

“…Theoretically, increasing inter-echo times should increase the sensitivity to alveolar size [23]. This theoretical relationship was recently examined in detail and confirmed experimentally in an ex-vivo model [26]. In this paper we report a non-significant increase in the difference in T2 between end-inspiration and end-expiration with a longer inter-echo time.…”

Generalized Moment Analysis of Magnetic Field Correlations for Accumulations of Spherical and Cylindrical Magnetic Perturbers

“…Therefore, knowledge of the correlation function allows obtaining microstructural parameters through CPMG measurements for accumulations of similarly shaped subvoxel structures, for example in the case of lung alveoli [60], microscopic blood products around or in malign cerebral tumors or for microvascular changes in skeletal muscle tissue [21]. Also, based on a recently developed measurement method, magnetic field correlation imaging has been shown promising in its potential to evaluate iron-associated neuropathologies [18,19].…”

“…The influence of magnetic susceptibility properties of peripheral lung tissue with near-spherical air-containing alveoli on MR signal decay was examined recently, see e.g. [60,63]. For a typical set of parameters with R i = 150 µm, δω = 1500 s −1 , η = 0.85, D = 1 · 10 −9 m 2 s −1 (τ = 22.5 s) [53,54], the correlation function K(t) is shown with mono-and bi-exponential approximations in Figure 6B.…”

Lineshape of Magnetic Resonance and its Effects on Free Induction Decay and Steady-State Free Precession Signal Formation

“…Typical examples are vessel networks in the brain [8][9][10][11], around axons [12], or in the myocardium, causing magnetic eld inhomogeneities due to the susceptibility di erence between blood and tissue [13][14][15]. Although imaging of lung tissue is typically performed with computed tomography, in recent years, enormous e orts have been put into the eld of lung MRI [16][17][18][19]. Transverse relaxation in lung tissue is dominated by susceptibility e ects between spherically shaped alveoli and surrounding tissues that cause nonexponential signal decay [20][21][22].…”