Diffusion lung imaging with hyperpolarized gas MRI

Abstract

Lung imaging using conventional 1 H MRI presents great challenges due to low density of lung tissue, lung motion and very fast lung tissue transverse relaxation (typical T2* is about 1-2 ms).MRI with hyperpolarized gases ( 3 He and 129 Xe) provides a valuable alternative due to a very strong signal originated from inhaled gas residing in the lung airspaces and relatively slow gas T2* relaxation (typical T2* is about 20-30 ms). Though in vivo human experiments should be done very fast -usually during a single breath-hold. In this review we describe the recent developments in diffusion lung MRI with hyperpolarized gases. We show that a combination of modeling results of gas diffusion in lung airspaces and diffusion measurements with variable diffusion-sensitizing gradients allows extracting quantitative information on the lung microstructure at the alveolar level. This approach, called in vivo lung morphometry, allows from a less than 15-second MRI scan, providing quantitative values and spatial distributions of the same physiological parameters as are measured by means of the "standard" invasive stereology (mean linear intercept, surface-tovolume ratio, density of alveoli, etc.). Besides, the approach makes it possible to evaluate some advanced Weibel parameters characterizing lung microstructure -average radii of alveolar sacs and ducts, as well as the depth of their alveolar sleeves. Such measurements, providing in vivo information on the integrity of pulmonary acinar airways and their changes in different diseases, are of great importance and interest to a broad range of physiologists and clinicians. We also discuss a new type of experiments that are based on the in vivo lung morphometry technique combined with quantitative CT measurements as well as with the Gradient Echo MRI measurements of hyperpolarized gas transverse relaxation in the lung airspaces. Such experiments provide additional information on the blood vessel volume fraction, specific gas volume, the length of acinar airways, and allows evaluation of lung parenchymal and non-parenchymal tissue.

Feasibility, tolerability and safety of pediatric hyperpolarized 129Xe magnetic resonance imaging in healthy volunteers and children with cystic fibrosis

“…Additional imaging parameters included flip angle of 9–12°, repetition time/echo time [TR/TE] 8/4 ms, matrix size 52–96 × 96–144, voxel size 3 × 3 × 15 mm 3 , 9–14 slices, with a total scan time of less than 16 s. For eight children, 129 Xe diffusion images were acquired using multiple b values (b = 6.25, 12.5, 18.75, 25 s/cm 2 ) bi-polar diffusion-sensitizing gradient-echo sequence [63] (flip angle 5–10°, diffusion time [Δ] 3.5 ms, lobe duration [δ] 3.1 ms, TR/TE 15/10 ms, matrix size 25–96 × 48–96, voxel size 3–7 × 3–7 × 15–30 mm 3 , 4–10 slices), using a maximum scan duration of 16 s. 129 Xe apparent diffusion coefficient (ADC) maps were generated from the diffusion images using code written in MATLAB (MathWorks, Natick, MA) and the R language. Hyperpolarized 129 Xe chemical shift saturation recovery (CSSR) MR spectroscopy [48, 49] was performed using a variable delay time ranging from 3.5 ms to 900 ms and a 2-ms Gaussian radiofrequency excitation pulse to generate a free induction decay, which was repeated 32 times during a single breath-hold.…”

How high resolution 3-dimensional imaging changes our understanding of postnatal lung development

“…The combination of diffusion measurements with variable diffusion times, diffusion-sensitizing gradients, and modeling of gas diffusion in lung airspaces permits the estimation of quantitative structural information at alveolar level. This approach was named in vivo lung morphometry and allows a structural characterization of a living lung (Yablonskiy et al 2017). It was utilized to show that juvenile humans still form new alveoli, even if the alveolar septa are believed to be mature at an age 2–3 years (Herring et al 2014; Narayanan et al 2012).…”

T1, T1 contrast, and Ernst‐angle images of four rat‐lung pathologies

“…There have been substantial advances in functional pulmonary MRI in the last few decades from imaging gases dominated by polarized noble gases and from tissue MRI, notably from the effect of breathing pure oxygen on the tissue T 1 . The present study is anatomical in comparison.…”

“…To be certain we are not missing signal, we have used free induction decay (FID)-projection imaging (as in 11,[20][21][22] dubbed ZTE, for "zero echo time" 23 ) with acquisition times appropriate for the T * 2 calculated from the 8 ppm line width. 21 There have been substantial advances in functional pulmonary MRI in the last few decades from imaging gases 24 dominated by polarized noble gases [25][26][27][28][29][30][31] and from tissue MRI, notably from the effect of breathing pure oxygen on the tissue T 1 . [32][33][34][35][36][37][38][39][40] The present study is anatomical in comparison.…”