A Constant-Volume Ventilator and Gas Recapture System for Hyperpolarized Gas MRI of Mouse and Rat Lungs

Assessment of Global and Regional Lung Compliance in Pulmonary Fibrosis With Hyperpolarized Gas MRI

BACKGROUND

Lung compliance, a biomarker of pulmonary fibrosis, is generally measured globally. Hyperpolarized 129Xe gas MRI offers the potential to evaluate lung compliance regionally, allowing for visualization of changes in lung compliance associated with fibrosis.

PURPOSE

To assess global and regional lung compliance in a rat model of pulmonary fibrosis using hyperpolarized 129Xe gas MRI.

STUDY TYPE

Prospective.

ANIMAL MODEL

Twenty Sprague-Dawley male rats with bleomycin-induced fibrosis model (N = 10) and saline-treated controls (N = 10).

FIELD STRENGTH/SEQUENCE

7-T, fast low-angle shot (FLASH) sequence.

ASSESSMENT

Lung compliance was determined by fitting lung volumes derived from segmented 129Xe MRI with an iterative selection method, to corresponding airway pressures. Similarly, lung compliance was obtained with computed tomography for cross-validation. Direction-dependencies of lung compliance were characterized by regional lung compliance ratios (R) in different directions. Pulmonary function tests (PFTs) and histological analysis were used to validate the pulmonary fibrosis model and assess its correlation with 129Xe lung compliance.

STATISTICAL TESTS

Shapiro-Wilk tests, unpaired and paired t-tests, Mann-Whitney U and Wilcoxon signed-rank tests, and Pearson correlation coefficients. P < 0.05 was considered statistically significant.

RESULTS

For the entire lung, the global and regional lung compliance measured with 129Xe gas MRI showed significant differences between the groups, and correlated with the global lung compliance measured using PFTs (global: r = 0.891; regional: r = 0.873). Additionally, for the control group, significant difference was found in mean regional compliance between areas, eg, 0.37 (0.32, 0.39) × 10-4 mL/cm H2O and 0.47 (0.41, 0.56) × 10-4 mL/cm H2O for apical and basal lung, respectively. The apical-basal direction R was 1.12 ± 0.09 and 1.35 ± 0.13 for fibrosis and control groups, respectively, indicating a significant difference.

DATA CONCLUSION

Our findings demonstrate the feasibility of using hyperpolarized gas MRI to assess regional lung compliance.

EVIDENCE LEVEL

2 TECHNICAL EFFICACY: Stage 1.

Low-boiling Point Perfluorocarbon Nanodroplets as Dual-Phase Dual-Modality MR/US Contrast Agent

Detection of bare gas microbubbles by magnetic resonance (MR) at low concentrations typically used in clinical contrast-ultrasound studies was recently demonstrated using hyperCEST. Despite the enhanced sensitivity achieved with hyperCEST, in vivo translation is challenging as on-resonance saturation of the gas-phase core of microbubbles consequently results in saturation of the gas-phase hyperpolarized 129 Xe within the lungs. Alternatively, microbubbles can be condensed into the liquid phase to form perfluorocarbon nanodroplets, where 129 Xe resonates at a chemical shift that is separated from the gas-phase signal in the lungs. For ultrasound applications, nanodroplets can be acoustically reverted back into their microbubble form to act as a phase-change contrast agent. Here, we show that low-boiling point perfluorocarbons, both in their liquid and gas form, generate phase-dependent hyperCEST contrast. Magnetic resonance detection of ultrasound-mediated phase transition demonstrates that these perfluorocarbons could be used as a dual-phase dual-modality MR/US contrast agent.

Quantitative assessment of regional lung ventilation in emphysematous mice using hyperpolarized 129Xe MRI with a continuous flow hyperpolarizing system

BACKGROUND

Lung ventilation function in small animals can be assessed by using hyperpolarized gas MRI. For these experiments a free breathing protocol is generally preferred to mechanical ventilation as mechanical ventilation can often lead to ventilation lung injury, while the need to maintain a gas reservoir may lead to a partial reduction of the polarization.

PURPOSE

To evaluate regional lung ventilation of mice by a simple but fast method under free breathing and give evidence for effectiveness with an elastase instilled emphysematous mice.

ANIMAL MODEL

Emphysematous mice.

MATERIALS AND METHODS

A Look-Locker based saturation recovery sequence was developed for continuous flow hyperpolarized (CF-HP) ¹²⁹Xe gas experiments, and the apparent gas-exchange rate, k', was measured by the analysis of the saturation recovery curve.

RESULTS

In mice with elastase-induced mild emphysema, reductions of 15-30% in k' values were observed as the results of lesion-induced changes in the lung.

DATA CONCLUSION

The proposed method was applied to an emphysematous model mice and ventilation dysfunctions have been approved as a definite decrease in k' values, supporting the usefulness for a non-invasive assessment of the lung functions in preclinical study by the CF-HP ¹²⁹Xe experiments.

In vivo hyperCEST imaging: Experimental considerations for a reliable contrast

PURPOSE

HyperCEST contrast relies on the reduction of the solvent signal after selective saturation of the solute magnetization. The scope of this work is to outline the experimental conditions needed to obtain a reliable hyperCEST contrast in vivo, where the "solvent" signal (ie, the dissolved-phase signal) may change over time due to the increase in xenon (Xe) accumulation into tissue.

METHODS

Hyperpolarized ¹²⁹ Xe was delivered to mice at a constant volume and rate using a mechanical ventilator, which triggered the saturation, excitation, and acquisition of the MR signal during the exhale phase of the breath cycle-either every breath or every 2, 3, or 4 breaths. Serial Z-spectra and hyperCEST images were acquired before and after a bolus injection of cucurbit[6]uril to assess possible signal fluctuations and instabilities.

RESULTS

The intensity of the dissolved-phase Xe signal was observed to first increase immediately after the beginning of the hyperpolarized gas inhalation and NMR acquisition, and then decrease before reaching a steady-state condition. Once a steady-state dissolved-phase magnetization was established, a reliable hyperCEST contrast, exceeding 40% signal reduction, was observed.

CONCLUSION

A reliable hyperCEST contrast can only be obtained after establishing a steady-state dissolved phase ¹²⁹ Xe magnetization. Under stable physiological conditions, a steady-state dissolved-phase Xe magnetization is only achieved after a series of Xe inhalations and RF excitations, and it requires synchronization of the breathing rate with the MR acquisition.

An open-source, low-cost NMR spectrometer operating in the mT field regime

In recent years, low field and ultra-low field NMR spectrometers have gained interest due to their portability, lower cost, and reduced subject-induced magnetic field inhomogeneities. Here, we describe the design of a low-cost multinuclear NMR spectrometer operating in the ultra-low field regime (ULF), which possesses high spectral resolution and enables arbitrary pulse programming. An inexpensive multifunction input/output (I/O) device is used to handle waveform generation and digitization in the kHz operating range. A home-built radio frequency (RF) mixing circuit is used to down-mix the NMR signals, allowing for the slower sampling rates and lower memory requirements needed to enable minute-long acquisitions using a standard Windows PC. The LabVIEW code, along with a bill of materials for all components used in the spectrometer, is included. As proof of concept, ¹H relaxation measurements and the simultaneous detection of ¹H with gas phase and dissolved ¹²⁹Xe frequencies using the described low field NMR spectrometer are demonstrated.

Tiny golden angle ultrashort echo-time lung imaging in mice

Imaging the lung parenchyma with MRI is particularly difficult in small animals due to the high respiratory and heart rates, and ultrashort T2* at high magnetic field strength caused by the high susceptibilities induced by the air-tissue interfaces. In this study, a 2D ultrashort echo-time (UTE) technique was combined with tiny golden angle (tyGA) ordering. Data were acquired continuously at 11.7 T and retrospective center-of-k-space gating was applied to reconstruct respiratory multistage images. Lung (proton) density (f_P ), T2*, signal-to-noise ratio (SNR), fractional ventilation (FV) and perfusion (f) were quantified, and the application to dynamic contrast agent (CA)-enhanced (DCE) qualitative perfusion assessment tested. The interobserver and intraobserver and interstudy reproducibility of the quantitative parameters were investigated. High-quality images of the lung parenchyma could be acquired in all animals. Over all lung regions a mean T2* of 0.20 ± 0.05 ms was observed. FV resulted as 0.31 ± 0.13, and a trend towards lower SNR values during inspiration (EX: SNR = 12.48 ± 6.68, IN: SNR = 11.79 ± 5.86) and a significant (P < 0.001) decrease in lung density (EX: f_P  = 0.69 ± 0.13, IN: f_P  = 0.62 ± 0.13) were observed. Quantitative perfusion results as 34.63 ± 9.05 mL/cm³ /min (systole) and 32.77 ± 8.55 mL/cm³ /min (diastole) on average. The CA dynamics could be assessed and, because of the continuous nature of the data acquisition, reconstructed at different temporal resolutions. Where a good to excellent interobserver reproducibility and an excellent intraobserver reproducibility resulted, the interstudy reproducibility was only fair to good. In conclusion, the combination of tiny golden angles with UTE (2D tyGA UTE) resulted in a reliable imaging technique for lung morphology and function in mice, providing uniform k-space coverage and thus low-artefact images of the lung parenchyma after gating.

Magnetic Resonance Detection of Gas Microbubbles via HyperCEST: A Path Toward Dual Modality Contrast Agent

Gas microbubbles are an established clinical ultrasound contrast agent. They could also become a powerful magnetic resonance (MR) intravascular contrast agent, but their low susceptibility-induced contrast requires high circulating concentrations or the addition of exogenous paramagnetic nanoparticles for MR detection. In order to detect clinical in vivo concentrations of raw microbubbles via MR, an alternative detection scheme must be used. HyperCEST is an NMR technique capable of indirectly detecting signals from very dilute molecules (concentrations well below the NMR detection threshold) that exchange hyperpolarized 129 Xe. Here, we use quantitative hyperCEST to show that microbubbles are very efficient hyperCEST agents. They can accommodate and saturate millions of 129 Xe atoms at a time, allowing for their indirect detection at concentrations as low as 10 femtomolar. The increased MR sensitivity to microbubbles achieved via hyperCEST can bridge the gap for microbubbles to become a dual modality contrast agent.

Validating in vivo hyperpolarized 129 Xe diffusion MRI and diffusion morphometry in the mouse lung

PURPOSE

Diffusion and lung morphometry imaging using hyperpolarized gases are promising tools to quantify pulmonary microstructure noninvasively in humans and in animal models. These techniques assume the motion encoded is exclusively diffusive gas displacement, but the impact of cardiac motion on measurements has never been explored. Furthermore, although diffusion morphometry has been validated against histology in humans and mice using 3 He, it has never been validated in mice for 129 Xe. Here, we examine the effect of cardiac motion on diffusion imaging and validate 129 Xe diffusion morphometry in mice.

THEORY AND METHODS

Mice were imaged using gradient-echo-based diffusion imaging, and apparent diffusion-coefficient (ADC) maps were generated with and without cardiac gating. Diffusion-weighted images were fit to a previously developed theoretical model using Bayesian probability theory, producing morphometric parameters that were compared with conventional histology.

RESULTS

Cardiac gating had no significant impact on ADC measurements (dual-gating: ADC = 0.020 cm2 /s, single-gating: ADC = 0.020 cm2 /s; P = .38). Diffusion-morphometry-generated maps of ADC (mean, 0.0165 ± 0.0001 cm2 /s) and acinar dimensions (alveolar sleeve depth [h] = 44 µm, acinar duct radii [R] = 99 µm, mean linear intercept [Lm ] = 74 µm) that agreed well with conventional histology (h = 45 µm, R = 108 µm, Lm = 63 µm).

CONCLUSION

Cardiac motion has negligible impact on 129 Xe ADC measurements in mice, arguing its impact will be similarly minimal in humans, where relative cardiac motion is reduced. Hyperpolarized 129 Xe diffusion morphometry accurately and noninvasively maps the dimensions of lung microstructure, suggesting it can quantify the pulmonary microstructure in mouse models of lung disease.

Improved preclinical hyperpolarized 129 Xe ventilation imaging with constant flip angle 3D radial golden means acquisition and keyhole reconstruction

Hyperpolarized (HP) 129 Xe MRI is increasingly used to noninvasively probe regional lung structure and function in the preclinical setting. As in human imaging, the primary barrier to quantitative imaging with HP gases is nonequilibrium magnetization, which is depleted by T1 relaxation and radio frequency excitation. Preclinical HP gas imaging commonly involves mechanically ventilating small animals and encoding k-space over tens or hundreds of breaths, with small subsets of k-space data collected within each breath. Breath-to-breath magnetization renewal enables the use of large flip angles, but the resulting magnetization decay generates large view-to-view differences in within-breath signal intensity, leading to artifacts and degraded image quality. This deleterious signal decay has motivated the use of variable flip angle (VFA) sampling schemes, in which the flip angle is progressively increased to maintain constant view-to-view signal intensity. However, VFA imaging complicates data acquisition and provides only a global correction that fails to compensate for regional differences in signal dynamics. When constant flip angle (CFA) imaging is used alongside 3D radial golden means acquisition, the center of k-space is sampled with every excitation, thereby encoding signal dynamics alongside imaging data. Here, keyhole reconstruction is used to generate multiple images to capture in-breath HP 129 Xe signal dynamics in mice and thus provide flip angle maps to quantitatively correct images without extra data collection. These CFA images display SNR that is not significantly different from VFA images, and further, high frequency k-space scaling can be used to mitigate decay-induced image artifacts. Results are supported by point spread function calculations and simulations of radial imaging with preclinical signal dynamics. Together, these results show that CFA 3D radial golden means ventilation imaging provides comparable image quality with VFA in small animals and allows for keyhole reconstruction, which can be used to generate flip angle maps and correct images for signal depletion.

Preclinical hyperpolarized 129 Xe MRI: ventilation and T2 * mapping in mouse lungs at 7 T using multi-echo flyback UTE

Fast apparent transverse relaxation (short T₂ *) is a common obstacle when attempting to perform quantitative ¹ H MRI of the lungs. While T₂ * times are longer for pulmonary hyperpolarized (HP) gas functional imaging (in particular for gaseous ¹²⁹ Xe), T₂ * can still lead to quantitative inaccuracies for sequences requiring longer echo times (such as diffusion weighted images) or longer readout duration (such as spiral sequences). This is especially true in preclinical studies, where high magnetic fields lead to shorter relaxation times than are typically seen in human studies. However, the T₂ * of HP ¹²⁹ Xe in the most common animal model of human disease (mice) has not been reported. Herein, we present a multi-echo radial flyback imaging sequence and use it to measure HP ¹²⁹ Xe T₂ * at 7 T under a variety of respiratory conditions. This sequence mitigates the impact of T₁ relaxation outside the animal by using multiple gradient-refocused echoes to acquire images at a number of effective echo times for each RF excitation. After validating the sequence using a phantom containing water doped with superparamagnetic iron oxide nanoparticles, we measured the ¹²⁹ Xe T₂ * in vivo for 10 healthy C57Bl/6 J mice and found T₂ * ~ 5 ms in the lung airspaces. Interestingly, T₂ * was relatively constant over all experimental conditions, and varied significantly with sex, but not age, mass, or the O₂ content of the inhaled gas mixture. These results are discussed in the context of T₂ * relaxation within porous media.

Direct detection of brown adipose tissue thermogenesis in UCP1-/- mice by hyperpolarized 129Xe MR thermometry

Brown adipose tissue (BAT) is a type of fat specialized in non-shivering thermogenesis. While non-shivering thermogenesis is mediated primarily by uncoupling protein 1 (UCP1), the development of the UCP1 knockout mouse has enabled the study of possible UCP1-independent non-shivering thermogenic mechanisms, whose existence has been shown so far only indirectly in white adipose tissue and still continues to be a matter of debate in BAT. In this study, by using magnetic resonance thermometry with hyperpolarized xenon, we produce the first direct evidence of UCP1-independent BAT thermogenesis in knockout mice. We found that, following adrenergic stimulation, the BAT temperature of knockout mice increases more and faster than rectal temperature. While with this study we cannot exclude or separate the physiological effect of norepinephrine on core body temperature, the fast increase of iBAT temperature seems to suggest the existence of a possible UCP1-independent thermogenic mechanism responsible for this temperature increase.

A portable ventilator with integrated physiologic monitoring for hyperpolarized 129Xe MRI in rodents

Hyperpolarized (HP) ¹²⁹Xe MRI is emerging as a powerful, non-invasive method to image lung function and is beginning to find clinical application across a range of conditions. As clinical implementation progresses, it becomes important to translate back to well-defined animal models, where novel disease signatures can be characterized longitudinally and validated against histology. To date, preclinical ¹²⁹Xe MRI has been limited to only a few sites worldwide with 2D imaging that is not generally sufficient to fully capture the heterogeneity of lung disease. To address these limitations and facilitate broader dissemination, we report on a compact and portable HP gas ventilator that integrates all the gas-delivery and physiologic monitoring capabilities required for high-resolution 3D hyperpolarized ¹²⁹Xe imaging. This ventilator is MR- and HP-gas compatible, driven by inexpensive microcontrollers and open source code, and allows for precise control of the tidal volume and breathing cycle in perorally intubated mice and rats. We use the system to demonstrate data acquisition over multiple breath-holds, during which lung motion is suspended to enable high-resolution 3D imaging of gas-phase and dissolved-phase ¹²⁹Xe in the lungs. We demonstrate the portability and versatility of the ventilator by imaging a mouse model of lung cancer longitudinally at 2 Tesla, and a healthy rat at 7 Tesla. We also report the detection of subtle spectroscopic fluctuations in phase with the heart rate, superimposed onto larger variations stemming from the respiratory cycle. This ventilator was developed to facilitate duplication and gain broad adoption to accelerate preclinical ¹²⁹Xe MRI research.

19 F MRI of the Lungs Using Inert Fluorinated Gases: Challenges and New Developments

Fluorine-19 (¹⁹ F) MRI using inhaled inert fluorinated gases is an emerging technique that can provide functional images of the lungs. Inert fluorinated gases are nontoxic, abundant, relatively inexpensive, and the technique can be performed on any MRI scanner with broadband multinuclear imaging capabilities. Pulmonary ¹⁹ F MRI has been performed in animals, healthy human volunteers, and in patients with lung disease. In this review, the technical requirements of ¹⁹ F MRI are discussed, along with various imaging approaches used to optimize the image quality. Lung imaging is typically performed in humans using a gas mixture containing 79% perfluoropropane (PFP) or sulphur hexafluoride (SF₆ ) and 21% oxygen. In lung diseases, such as asthma, chronic obstructive pulmonary disease (COPD), and cystic fibrosis (CF), ventilation defects are apparent in regions that the inhaled gas cannot access. ¹⁹ F lung images are typically acquired in a single breath-hold, or in a time-resolved, multiple breath fashion. The former provides measurements of the ventilation defect percent (VDP), while the latter provides measurements of gas replacement (ie, fractional ventilation). Finally, preliminary comparisons with other functional lung imaging techniques are discussed, such as Fourier decomposition MRI and hyperpolarized gas MRI. Overall, functional ¹⁹ F lung MRI is expected to complement existing proton-based structural imaging techniques, and the combination of structural and functional lung MRI will provide useful outcome measures in the future management of pulmonary diseases in the clinic. Level of Evidence: 3 Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2019;49:343-354.

Quantitative evaluation of pulmonary gas-exchange function using hyperpolarized 129 Xe CEST MRS and MRI

Hyperpolarized ¹²⁹ Xe gas MR has been a powerful tool for evaluating pulmonary structure and function due to the extremely high enhancement in spin polarization, the good solubility in the pulmonary parenchyma, and the excellent chemical sensitivity to its surrounding environment. Generally, the quantitative structural and functional information of the lung are evaluated using hyperpolarized ¹²⁹ Xe by employing the techniques of chemical shift saturation recovery (CSSR) and xenon polarization transfer contrast (XTC). Hyperpolarized ¹²⁹ Xe chemical exchange saturation transfer (Hyper-CEST) is another method for quantifying the exchange information of hyperpolarized ¹²⁹ Xe by using the exchange of xenon signals according to its different chemical shifts, and it has been widely used in biosensor studies in vitro. However, the feasibility of using hyperpolarized ¹²⁹ Xe CEST to quantify the pulmonary gas exchange function in vivo is still unclear. In this study, the technique of CEST was used to quantitatively evaluate the gas exchange in the lung globally and regionally via hyperpolarized ¹²⁹ Xe MRS and MRI, respectively. A new parameter, the pulmonary apparent gas exchange time constant (T_app ), was defined, and it increased from 0.63 s to 0.95 s in chronic obstructive pulmonary disease (COPD) rats (induced by cigarette smoke and lipopolysaccharide exposure) versus the controls with a significant difference (P = 0.001). Additionally, the spatial distribution maps of T_app in COPD rats' pulmonary parenchyma showed a regionally obvious increase compared with healthy rats. These results indicated that hyperpolarized ¹²⁹ Xe CEST MR was an effective method for globally and regionally quantifying the pulmonary gas exchange function, which would be helpful in diagnosing lung diseases that are related to gas exchange, such as COPD.

Simple and robust referencing system enables identification of dissolved-phase xenon spectral frequencies

PURPOSE

To assess the effect of macroscopic susceptibility gradients on the gas-phase referenced dissolved-phase 129 Xe (DPXe) chemical shift (CS) and to establish the robustness of a water-based referencing system for in vivo DPXe spectra.

METHODS

Frequency shifts induced by spatially varying magnetic susceptibility are calculated by finite-element analysis for the human head and chest. Their effect on traditional gas-phase referenced DPXe CS is then assessed theoretically and experimentally. A water-based referencing system for the DPXe resonances that uses the local water protons as reference is proposed and demonstrated in vivo in rats.

RESULTS

Across the human brain, macroscopic susceptibility gradients can induce an apparent variation in the DPXe CS of up to 2.5 ppm. An additional frequency shift as large as 6.5 ppm can exist between DPXe and gas-phase resonances. By using nearby water protons as reference for the DPXe CS, the effect of macroscopic susceptibility gradients is eliminated and consistent CS values are obtained in vivo, regardless of shimming conditions, region of interest analyzed, animal orientation, or lung inflation. Combining in vitro and in vivo spectroscopic measurements finally enables confident assignment of some of the DPXe peaks observed in vivo.

CONCLUSION

To use hyperpolarized xenon as a biological probe in tissues, the DPXe CS in specific organs/tissues must be reliably measured. When the gas-phase is used as reference, variable CS values are obtained for DPXe resonances. Reliable peak assignments in DPXe spectra can be obtained by using local water protons as reference. Magn Reson Med 80:431-441, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Brain Imaging Using Hyperpolarized 129Xe Magnetic Resonance Imaging

Hyperpolarized (HP) ¹²⁹Xe magnetic resonance imaging (MRI) is a novel iteration of traditional MRI that relies on detecting the spins of ¹H. Since ¹²⁹Xe is a gaseous signal source, it can be used for lung imaging. Additionally, ¹²⁹Xe dissolves in the blood stream and can therefore be detectable in the brain parenchyma and vasculature. In this work, we provide detailed information on the protocols that we have developed to image ¹²⁹Xe within the brains of both rodents and human subjects.

Optically polarized 3He

This article reviews the physics and technology of producing large quantities of highly spin-polarized 3He nuclei using spin-exchange (SEOP) and metastability-exchange (MEOP) optical pumping. Both technical developments and deeper understanding of the physical processes involved have led to substantial improvements in the capabilities of both methods. For SEOP, the use of spectrally narrowed lasers and K-Rb mixtures has substantially increased the achievable polarization and polarizing rate. For MEOP nearly lossless compression allows for rapid production of polarized 3He and operation in high magnetic fields has likewise significantly increased the pressure at which this method can be performed, and revealed new phenomena. Both methods have benefitted from development of storage methods that allow for spin-relaxation times of hundreds of hours, and specialized precision methods for polarimetry. SEOP and MEOP are now widely applied for spin-polarized targets, neutron spin filters, magnetic resonance imaging, and precision measurements.

Diffusion lung imaging with hyperpolarized gas MRI

Lung imaging using conventional 1 H MRI presents great challenges because of the 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 because of the very strong signal originating from inhaled gas residing in the lung airspaces and relatively slow gas T2 * relaxation (typical T2 * is about 20-30 ms). However, in vivo human experiments should be performed very rapidly - 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 the results of modeling of gas diffusion in lung airspaces and diffusion measurements with variable diffusion-sensitizing gradients allows the extraction of quantitative information on the lung microstructure at the alveolar level. From an MRI scan of less than 15 s, this approach, called in vivo lung morphometry, allows the provision of quantitative values and spatial distributions of the same physiological parameters as measured by means of 'standard' invasive stereology (mean linear intercept, surface-to-volume ratio, density of alveoli, etc.). In addition, 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 experiment based on the in vivo lung morphometry technique combined with quantitative computed tomography measurements, as well as with 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 and length of the acinar airways, and allow the evaluation of lung parenchymal and non-parenchymal tissue. Copyright © 2015 John Wiley & Sons, Ltd.

Vagal innervation is required for pulmonary function phenotype in Htr4-/- mice

Human genome-wide association studies have identified over 50 loci associated with pulmonary function and related phenotypes, yet follow-up studies to determine causal genes or variants are rare. Single nucleotide polymorphisms in serotonin receptor 4 (HTR4) are associated with human pulmonary function in genome-wide association studies and follow-up animal work has demonstrated that Htr4 is causally associated with pulmonary function in mice, although the precise mechanisms were not identified. We sought to elucidate the role of neural innervation and pulmonary architecture in the lung phenotype of Htr4^-/- animals. We report here that the Htr4^-/- phenotype in mouse is dependent on vagal innervation to the lung. Both ex vivo tracheal ring reactivity and in vivo flexiVent pulmonary functional analyses demonstrate that vagotomy abrogates the Htr4^-/- airway hyperresponsiveness phenotype. Hyperpolarized ³He gas magnetic resonance imaging and stereological assessment of wild-type and Htr4^-/- mice reveal no observable differences in lung volume, inflation characteristics, or pulmonary microarchitecture. Finally, control of breathing experiments reveal substantive differences in baseline breathing characteristics between mice with/without functional HTR4 in breathing frequency, relaxation time, flow rate, minute volume, time of inspiration and expiration and breathing pauses. These results suggest that HTR4's role in pulmonary function likely relates to neural innervation and control of breathing.

In vivo detection of cucurbit[6]uril, a hyperpolarized xenon contrast agent for a xenon magnetic resonance imaging biosensor

The Hyperpolarized gas Chemical Exchange Saturation Transfer (HyperCEST) Magnetic Resonance (MR) technique has the potential to increase the sensitivity of a hyperpolarized xenon-129 MRI contrast agent. Signal enhancement is accomplished by selectively depolarizing the xenon within a cage molecule which, upon exchange, reduces the signal in the dissolved phase pool. Herein we demonstrate the in vivo detection of the cucurbit[6]uril (CB6) contrast agent within the vasculature of a living rat. Our work may be used as a stepping stone towards using the HyperCEST technique as a molecular imaging modality.

Accurate MR thermometry by hyperpolarized 129 Xe

PURPOSE

To investigate the temperature dependence of the resonance frequency of lipid-dissolved xenon (LDX) and to assess the accuracy of LDX-based MR thermometry.

METHODS

The chemical shift temperature dependence of water protons, methylene protons, and LDX was measured from samples containing tissues with varying fat contents using a high-resolution NMR spectrometer. LDX results were then used to acquire relative and absolute temperature maps in vivo and the results were compared with PRF-based MR thermometry.

RESULTS

The temperature dependence of proton resonance frequency (PRF) is strongly affected by the specific distribution of water and fat. A redistribution of water and fat compartments can reduce the apparent temperature dependence of the water chemical shift from -0.01 ppm/°C to -0.006 ppm, whereas the LDX chemical shift shows a consistent temperature dependence of -0.21 ppm/°C. The use of the methylene protons resonance frequency as internal reference improves the accuracy of LDX-based MR thermometry, but degrades that of PRF-based MR thermometry, as microscopic susceptibility gradients affected lipid and water spins differently.

CONCLUSION

The LDX resonance frequency, with its higher temperature dependence, provides more accurate and precise temperature measurements, both in vitro and in vivo. More importantly, the resonance frequency of nearby methylene protons can be used to extract absolute temperature information. Magn Reson Med 78:1070-1079, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Fractional ventilation mapping using inert fluorinated gas MRI in rat models of inflammation and fibrosis

The purpose of this study was to extend established methods for fractional ventilation mapping using (19) F MRI of inert fluorinated gases to rat models of pulmonary inflammation and fibrosis. In this study, five rats were instilled with lipopolysaccharide (LPS) in the lungs two days prior to imaging, six rats were instilled with bleomycin in the lungs two weeks prior to imaging and an additional four rats were used as controls. (19) F MR lung imaging was performed at 3 T with rats continuously breathing a mixture of sulfur hexafluoride and O2 . Fractional ventilation maps were obtained using a wash-out approach, by switching the breathing mixture to pure O2 , and acquiring images following each successive wash-out breath. The mean fractional ventilation (r) was 0.29 ± 0.05 for control rats, 0.23 ± 0.10 for LPS-instilled rats and 0.19 ± 0.03 for bleomycin-instilled rats. Bleomycin-instilled rats had a significantly decreased mean r value compared with controls (P = 0.010). Although LPS-instilled rats had a slightly reduced mean r value, this trend was not statistically significant (P = 0.556). Fractional ventilation gradients were calculated in the anterior/posterior (A/P) direction, and the mean A/P gradient was -0.005 ± 0.008 cm(-1) for control rats, 0.013 ± 0.005 cm(-1) for LPS-instilled rats and 0.009 ± 0.018 cm(-1) for bleomycin-instilled rats. Fractional ventilation gradients were significantly different for control rats compared with LPS-instilled rats only (P = 0.016). The ventilation gradients calculated from control rats showed the expected gravitational relationship, while ventilation gradients calculated from LPS- and bleomycin-instilled rats showed the opposite trend. Histology confirmed that LPS-instilled rats had a significantly elevated alveolar wall thickness, while bleomycin-instilled rats showed signs of substantial fibrosis. Overall, (19)F MRI may be able to detect the effects of pulmonary inflammation and fibrosis using a simple and inexpensive imaging approach that can potentially be translated to humans.

Establishing an accurate gas phase reference frequency to quantify 129 Xe chemical shifts in vivo

PURPOSE

¹²⁹ Xe interacts with biological media to exhibit chemical shifts exceeding 200 ppm that report on physiology and pathology. Extracting this functional information requires shifts to be measured precisely. Historically, shifts have been reported relative to the gas-phase resonance originating from pulmonary airspaces. However, this frequency is not fixed-it is affected by bulk magnetic susceptibility, as well as Xe-N₂ , Xe-Xe, and Xe-O₂ interactions. In this study, we addressed this by introducing a robust method to determine the 0 ppm ¹²⁹ Xe reference from in vivo data.

METHODS

Respiratory-gated hyperpolarized ¹²⁹ Xe spectra from the gas- and dissolved-phases were acquired in four mice at 2T from multiple axial slices within the thoracic cavity. Complex spectra were then fitted in the time domain to identify peaks.

RESULTS

Gas-phase ¹²⁹ Xe exhibited two distinct resonances corresponding to ¹²⁹ Xe in conducting airways (varying from -0.6 ± 0.2 to 1.3 ± 0.3 ppm) and alveoli (relatively stable, at -2.2 ± 0.1 ppm). Dissolved-phase ¹²⁹ Xe exhibited five reproducible resonances in the thorax at 198.4 ± 0.4, 195.5 ± 0.4, 193.9 ± 0.2, 191.3 ± 0.2, and 190.7 ± 0.3 ppm.

CONCLUSION

The alveolar ¹²⁹ Xe resonance exhibits a stable frequency across all mice. Therefore, it can provide a reliable in vivo reference frequency by which to characterize other spectroscopic shifts. Magn Reson Med 77:1438-1445, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Probing lung microstructure with hyperpolarized noble gas diffusion MRI: theoretical models and experimental results

The introduction of hyperpolarized gases ((3)He and (129)Xe) has opened the door to applications for which gaseous agents are uniquely suited-lung MRI. One of the pulmonary applications, diffusion MRI, relies on measuring Brownian motion of inhaled hyperpolarized gas atoms diffusing in lung airspaces. In this article we provide an overview of the theoretical ideas behind hyperpolarized gas diffusion MRI and the results obtained over the decade-long research. We describe a simple technique based on measuring gas apparent diffusion coefficient (ADC) and an advanced technique, in vivo lung morphometry, that quantifies lung microstructure both in terms of Weibel parameters (acinar airways radii and alveolar depth) and standard metrics (mean linear intercept, surface-to-volume ratio, and alveolar density) that are widely used by lung researchers but were previously available only from invasive lung biopsy. This technique has the ability to provide unique three-dimensional tomographic information on lung microstructure from a less than 15 s MRI scan with results that are in good agreement with direct histological measurements. These safe and sensitive diffusion measurements improve our understanding of lung structure and functioning in health and disease, providing a platform for monitoring the efficacy of therapeutic interventions in clinical trials.

Quantitative evaluation of radiation-induced lung injury with hyperpolarized xenon magnetic resonance

PURPOSE

To demonstrate the feasibility of quantitative and comprehensive global evaluation of pulmonary function and microstructural changes in rats with radiation-induced lung injury (RILI) using hyperpolarized xenon MR.

METHODS

Dissolved xenon spectra were dynamically acquired using a modified chemical shift saturation recovery pulse sequence in five rats with RILI (bilaterally exposed by 6-MV x-ray with a dose of 14 Gy 3 mo. prior to MR experiments) and five healthy rats. The dissolved xenon signals were quantitatively analyzed, and the pulmonary physiological parameters were extracted with the model of xenon exchange.

RESULTS

The obtained pulmonary physiological parameters and the ratio of (129) Xe signal in red blood cells (RBCs) versus barrier showed a significant difference between the groups. In RILI rats versus controls, the exchange time increased from 44.5 to 112 ms, the pulmonary capillary transit time increased from 0.51 to 1.48 s, and the ratio of (129) Xe spectroscopic signal in RBCs versus barrier increased from 0.294 to 0.484.

CONCLUSION

Hyperpolarized xenon MR is effective for quantitative and comprehensive global evaluation of pulmonary function and structural changes without the use of radiation. This may open the door for its use in the diagnosis of lung diseases that are related to gas exchange. Magn Reson Med 76:408-416, 2016. © 2015 Wiley Periodicals, Inc.

Regional fractional ventilation mapping in spontaneously breathing mice using hyperpolarized ¹²⁹Xe MRI

The feasibility of ventilation imaging with hyperpolarized (HP) (129) Xe MRI has been investigated for quantitative and regional assessment of ventilation in spontaneously breathing mice. The multiple breath ventilation imaging technique was modified to the protocol of spontaneous inhalation of HP (129) Xe delivered continuously from a (129) Xe polarizer. A series of (129) Xe ventilation images was obtained by varying the number of breaths before the (129) Xe lung imaging. The fractional ventilation, r, was successfully evaluated for spontaneously breathing mice. An attempt was made to detect ventilation dysfunction in the emphysematous mouse lung induced by intratracheal administration of porcine pancreatic elastase (PPE). As a result, the distribution of fractional ventilation could be visualized by the r map. Significant dysfunction of ventilation was quantitatively identified in the PPE-treated group. The whole-lung r value of 0.34 ± 0.01 for control mice (N = 4) was significantly reduced, to 0.25 ± 0.07, in PPE-treated mice (N = 4) (p = 0.038). This study is the first application of multiple breath ventilation imaging to spontaneously breathing mice, and shows that this methodology is sensitive to differences in the pulmonary ventilation. This methodology is expected to improve simplicity as well as noninvasiveness when assessing regional ventilation in small rodents.

3D MRI of impaired hyperpolarized 129Xe uptake in a rat model of pulmonary fibrosis

A variety of pulmonary pathologies, in particular interstitial lung diseases, are characterized by thickening of the pulmonary blood-gas barrier, and this thickening results in reduced gas exchange. Such diffusive impairment is challenging to quantify spatially, because the distributions of the metabolically relevant gases (CO2 and O2) cannot be detected directly within the lungs. Hyperpolarized (HP) (129)Xe is a promising surrogate for these metabolic gases, because MR spectroscopy and imaging allow gaseous alveolar (129)Xe to be detected separately from (129)Xe dissolved in the red blood cells (RBCs) and the adjacent tissues, which comprise blood plasma and lung interstitium. Because (129)Xe reaches the RBCs by diffusing across the same barrier tissues (blood plasma and interstitium) as O2, barrier thickening will delay (129)Xe transit and, thus, reduce RBC-specific (129)Xe MR signal. Here we have exploited these properties to generate 3D, MR images of (129)Xe uptake by the RBCs in two groups of rats. In the experimental group, unilateral fibrotic injury was generated prior to imaging by instilling bleomycin into one lung. In the control group, a unilateral sham instillation of saline was performed. Uptake of (129)Xe by the RBCs, quantified as the fraction of RBC signal relative to total dissolved (129)Xe signal, was significantly reduced (P = 0.03) in the injured lungs of bleomycin-treated animals. In contrast, no significant difference (P = 0.56) was observed between the saline-treated and untreated lungs of control animals. Together, these results indicate that 3D MRI of HP (129)Xe dissolved in the pulmonary tissues can provide useful biomarkers of impaired diffusive gas exchange resulting from fibrotic thickening.

Very-low-field MRI of laser polarized xenon-129

We describe a homebuilt MRI system for imaging laser-polarized xenon-129 at a very low holding field of 2.2mT. A unique feature of this system was the use of Maxwell coils oriented at so-called "magic angles" to generate the transverse magnetic field gradients, which provided a simple alternative to Golay coils. We used this system to image a laser-polarized xenon-129 phantom with both a conventional gradient-echo and a fully phase-encoded pulse sequence. In other contexts, a fully phase-encoded acquisition, also known as single-point or constant-time imaging, has been used to enable distortion-free imaging of short-T₂^∗ species. Here we used this technique to overcome imperfections associated with our homebuilt MRI system while also taking full advantage of the long T₂^∗ available at very low field. Our results demonstrate that xenon-129 image quality can be dramatically improved at low field by combining a fully phase-encoded k-space acquisition with auxiliary measurements of system imperfections including B₀ field drift and gradient infidelity.

Enabling hyperpolarized (129) Xe MR spectroscopy and imaging of pulmonary gas transfer to the red blood cells in transgenic mice expressing human hemoglobin

PURPOSE

Hyperpolarized (HP) (129) Xe gas in the alveoli can be detected separately from (129) Xe dissolved in pulmonary barrier tissues (blood plasma and parenchyma) and red blood cells (RBCs) of humans, allowing this isotope to probe impaired gas uptake. Unfortunately, mice, which are favored as lung disease models, do not display a unique RBC resonance, thus limiting the preclinical utility of (129) Xe MR. Here we overcome this limitation using a commercially available strain of transgenic mice that exclusively expresses human hemoglobin.

METHODS

Dynamic HP (129) Xe MR spectroscopy, and three-dimensional radial MRI of gaseous and dissolved (129) Xe were performed in both wild-type (C57BL/6) and transgenic mice.

RESULTS

Unlike wild-type animals, transgenic mice displayed two dissolved (129) Xe NMR peaks at 198 and 217 ppm, corresponding to (129) Xe dissolved in barrier tissues and RBCs, respectively. Moreover, signal from these resonances could be imaged separately, using a 1-point variant of the Dixon technique.

CONCLUSION

It is now possible to examine the dynamics and spatial distribution of pulmonary gas uptake by the RBCs of mice using HP (129) Xe MR spectroscopy and imaging. When combined with ventilation imaging, this ability will enable translational "mouse-to-human" studies of impaired gas exchange in a variety of pulmonary diseases.

Imaging regional PAO2 and gas exchange

Several methods allow regional gas exchange to be inferred from imaging of regional ventilation and perfusion (V/Q) ratios. Each method measures slightly different aspects of gas exchange and has inherent advantages and drawbacks that are reviewed. Single photon emission computed tomography can provide regional measure of ventilation and perfusion from which regional V/Q ratios can be derived. PET methods using inhaled or intravenously administered nitrogen-13 provide imaging of both regional blood flow, shunt, and ventilation. Electric impedance tomography has recently been refined to allow simultaneous measurements of both regional ventilation and blood flow. MRI methods utilizing hyperpolarized helium-3 or xenon-129 are currently being refined and have been used to estimate local PaO(2) in both humans and animals. Microsphere methods are included in this review as they provide measurements of regional ventilation and perfusion in animals. One of their advantages is their greater spatial resolution than most imaging methods and the ability to use them as gold standards against which new imaging methods can be tested. In general, the reviewed methods differ in characteristics such as spatial resolution, possibility of repeated measurements, radiation exposure, availability, expensiveness, and their current stage of development.

Effects of corticosteroid treatment on airway inflammation, mechanics, and hyperpolarized ³He magnetic resonance imaging in an allergic mouse model

The purpose of this study was to assess the effects of corticosteroid therapy on a murine model of allergic asthma using hyperpolarized (3)He magnetic resonance imaging (MRI) and respiratory mechanics measurements before, during, and after methacholine (MCh) challenge. Three groups of mice were prepared, consisting of ovalbumin sensitized/ovalbumin challenged (Ova/Ova, n = 5), Ova/Ova challenged but treated with the corticosteroid dexamethasone (Ova/Ova+Dex, n = 3), and ovalbumin-sensitized/saline-challenged (Ova/PBS, n = 4) control animals. All mice underwent baseline 3D (3)He MRI, then received a MCh challenge while 10 2D (3)He MR images were acquired for 2 min, followed by post-MCh 3D (3)He MRI. Identically treated groups underwent respiratory mechanics evaluation (n = 4/group) and inflammatory cell counts (n = 4/group). Ova/Ova animals exhibited predominantly large whole lobar defects at baseline, with significantly higher ventilation defect percentage (VDP = 19 ± 4%) than Ova/PBS (+2 ± 1%, P = 0.01) animals. Such baseline defects were suppressed by dexamethasone (0%, P = 0.009). In the Ova/Ova group, MCh challenge increased VDP on both 2D (+30 ± 8%) and 3D MRI scans (+14 ± 2%). MCh-induced VDP changes were diminished in Ova/Ova+Dex animals on both 2D (+21 ± 9%, P = 0.63) and 3D scans (+7 ± 2%, P = 0.11) and also in Ova/PBS animals on 2D (+6 ± 3%, P = 0.07) and 3D (+4 ± 1%, P = 0.01) scans. Because MCh challenge caused near complete cessation of ventilation in four of five Ova/Ova animals, even as large airways remained patent, this implies that small airway (<188 μm) obstruction predominates in this model. This corresponds with respiratory mechanics observations that MCh challenge significantly increases elastance and tissue damping but only modestly affects Newtonian airway resistance.

In vivo MR imaging of pulmonary perfusion and gas exchange in rats via continuous extracorporeal infusion of hyperpolarized 129Xe

BACKGROUND

Hyperpolarized (HP) (129)Xe magnetic resonance imaging (MRI) permits high resolution, regional visualization of pulmonary ventilation. Additionally, its reasonably high solubility (>10%) and large chemical shift range (>200 ppm) in tissues allow HP (129)Xe to serve as a regional probe of pulmonary perfusion and gas transport, when introduced directly into the vasculature. In earlier work, vascular delivery was accomplished in rats by first dissolving HP (129)Xe in a biologically compatible carrier solution, injecting the solution into the vasculature, and then detecting HP (129)Xe as it emerged into the alveolar airspaces. Although easily implemented, this approach was constrained by the tolerable injection volume and the duration of the HP (129)Xe signal.

METHODS AND PRINCIPAL FINDINGS

Here, we overcome the volume and temporal constraints imposed by injection, by using hydrophobic, microporous, gas-exchange membranes to directly and continuously infuse (129)Xe into the arterial blood of live rats with an extracorporeal (EC) circuit. The resulting gas-phase (129)Xe signal is sufficient to generate diffusive gas exchange- and pulmonary perfusion-dependent, 3D MR images with a nominal resolution of 2×2×2 mm(3). We also show that the (129)Xe signal dynamics during EC infusion are well described by an analytical model that incorporates both mass transport into the blood and longitudinal relaxation.

CONCLUSIONS

Extracorporeal infusion of HP (129)Xe enables rapid, 3D MR imaging of rat lungs and, when combined with ventilation imaging, will permit spatially resolved studies of the ventilation-perfusion ratio in small animals. Moreover, EC infusion should allow (129)Xe to be delivered elsewhere in the body and make possible functional and molecular imaging approaches that are currently not feasible using inhaled HP (129)Xe.