Reproducibility of quantitative indices of lung function and microstructure from 129 Xe chemical shift saturation recovery (CSSR) MR spectroscopy

Regional variations in hyperpolarized 129Xe lung MRI: Insights from CSI-CSSR and CSSR in healthy and irradiated rat models

PURPOSE

To compare pulmonary function metrics obtained with hyperpolarized xenon-129 (HXe) MRS, using chemical shift saturation recovery (CSSR) and CSI-CSSR, in healthy rats and a rat model of radiation-induced lung injury.

METHODS

HXe-MR data were acquired in two healthy rats and one rat with radiation-induced lung injury using whole-lung spectroscopy and CSI-CSSR techniques. The CSI-CSSR acquisitions were performed with both fixed TE and variable TE. Apparent alveolar septal wall thickness, gas transfer dynamics, and regional lung function were quantified and compared across acquisition methods. Spectral analysis included alignment of dissolved-phase frequency spectra using the membrane resonance as reference, segmentation of gas-phase (GP) frequency distribution, and characterization of gas uptake in the vasculature.

RESULTS

Complex GP line shapes were observed in rat lungs, necessitating pixel-wise CSI analysis and membrane resonance alignment for improved quantification. Notable differences in alveolar septal wall thickness, dissolved-phase GP ratios, and GP and red blood cell frequencies were found between acquisition techniques and lung conditions. CSI-CSSR provided unique insights into regional lung function, including the identification of distinct GP frequency zones potentially corresponding to different airway structures, and the ability to map relative xenon gas transport. Metrics from fixed-TE and variable-TE acquisitions usually differed by less than 10%, but the latter yielded a 20% SNR gain.

CONCLUSION

HXe-MRS and CSI-CSSR techniques provide similar but not universally interchangeable insights into lung function, particularly in the presence of pathology.

A thermally polarized, dissolved-phase 129Xe phantom for quality-control and multisite comparisons of gas-exchange imaging

Harmonizing and validating 129Xe gas exchange imaging across multiple sites is hampered by a lack of a quantitative standard that 1) displays the unique spectral properties of 129Xe observed from human subjects in vivo and 2) has short enough T1 times to enable practical imaging. This work describes and demonstrates the development of two dissolved-phase, thermally polarized phantoms that mimic the in-vivo, red blood cell and membrane resonances of 129Xe dissolved in human lungs. Following optimization, combinations of two common organic solvents, acetone and dimethyl sulfoxide, resulted in two in-vivo-like dissolved-phase 129Xe phantoms yielding chemical shifts of 212.4 ppm and 193.9 ppm. By doping the solutions with iron(iii) acetylacetonate, the longitudinal relaxation time was reduced T1 = 1.2 s for both phantoms at 3 T and 7 T. There was minimal change in chemical shift (+1.58 ppm) and T1 (+1.2 %) over 1 year. In a 2D Dixon-type acquisition with 3 mm2 in-plane resolution, 129Xe dissolved-phase images yielded signal-to-noise ratios 6 and 12 for the RBC and membrane phantoms, respectively. A simple scaling of these phantoms to clinically relevant volumes of several liters would result in an SNR of 7 for the RBC phantom acquired in less than one minute. These findings demonstrate the ability to fabricate robust, quantitative, thermally polarized dissolved-phase phantoms, which will be needed to validate and harmonize gas exchange imaging in multi-site clinical trials.

Ultrasensitive 129Xe Magnetic Resonance Imaging: From Clinical Monitoring to Molecular Sensing

Magnetic resonance imaging (MRI) is a cornerstone technology in clinical diagnostics and in vivo research, offering unparalleled visualization capabilities. Despite significant advancements in the past century, traditional 1H MRI still faces sensitivity limitations that hinder its further development. To overcome this challenge, hyperpolarization methods have been introduced, disrupting the thermal equilibrium of nuclear spins and leading to an increased proportion of hyperpolarized spins, thereby enhancing sensitivity by hundreds to tens of thousands of times. Among these methods, hyperpolarized (HP) 129Xe MRI, also known as ultrasensitive 129Xe MRI, stands out for achieving the highest polarization enhancement and has recently received clinical approval. It effectively tackles the challenge of weak MRI signals from low proton density in the lungs. HP 129Xe MRI is valuable for assessing structural and functional changes in lung physiology during pulmonary disease progression, tracking cells, and detecting target molecules at pico-molar concentrations. This review summarizes recent developments in HP 129Xe MRI, including its physical principles, manufacturing methods, in vivo characteristics, and diverse applications in biomedical, chemical, and material sciences. In addition, it carefully discusses potential technical improvements and future prospects for enhancing its utility in these fields, further establishing HP 129Xe MRI's importance in advancing medical imaging and research.

Loss of pulmonary capillaries in idiopathic pulmonary arterial hypertension with low diffusion capacity is accompanied by early diffuse emphysema detected by 129Xe MRI

OBJECTIVES

Recent studies suggest the existence of an idiopathic pulmonary arterial hypertension (IPAH) phenotype affecting mostly patients with a smoking history, characterised by low diffusion capacity for carbon monoxide (DLCO) without clinically significant emphysema. This study's objective was to test the hypothesis of a loss of pulmonary capillaries as an underlying mechanism by comparison to other patient groups with and without pulmonary hypertension (PH).

MATERIALS AND METHODS

Between March 2019 and June 2023, patients of four groups were recruited for this observational study: IPAH with preserved (1) and low DLCO (2), combined pulmonary fibrosis and emphysema with PH (3), and emphysema without PH (4). Patients underwent clinical CT and 129Xe MRI including dissolved-phase imaging yielding the ratio of 129Xe in red blood cells and membrane tissues (RBC-M), chemical shift saturation recovery for determining RBC fraction η and diffusion-weighted imaging yielding surface-volume ratio. Kruskal-Wallis tests were used for statistical analysis.

RESULTS

Twenty-nine participants were recruited, of which 22 (age 64 ± 10, 11 male, 5/5/7/5 for the individual groups) could be included in the analysis. RBC-M and η were reduced in IPAH with low versus preserved DLCO and emphysema groups (p ≤ 0.01). CT low-attenuation area percentage was not increased in IPAH with low DLCO compared to any group. 129Xe MRI-derived surface-volume ratio was reduced in IPAH with low versus preserved DLCO (p = 0.04).

CONCLUSION

Results are consistent with a loss of pulmonary capillaries in patients with IPAH and low DLCO along with destruction of alveolar tissue, likely due to early diffuse emphysema.

KEY POINTS

Question A loss of pulmonary capillaries has been suggested in patients with IPAH and low diffusion capacity without clinically significant emphysema on CT. Findings 129Xe uptake in red blood cells and lung surface-volume ratio were reduced in IPAH patients with low compared to preserved diffusion capacity. Clinical relevance This study furthers the understanding of the underlying pathological mechanisms in IPAH with low diffusion capacity, providing evidence that loss of pulmonary capillaries is accompanied by alveolar tissue destruction despite near-normal CT.

Sensitivity analysis of models of gas exchange for lung hyperpolarised 129Xe MR

Sensitivity analysis enables the identification of influential parameters and the optimisation of model composition. Such methods have not previously been applied systematically to models describing hyperpolarised 129Xe gas exchange in the lung. Here, we evaluate the current 129Xe gas exchange models to assess their precision for identifying alterations in pulmonary vascular function and lung microstructure. We assess sensitivity using established univariate methods and scatter plots for parameter interactions. We apply them to the model described by Patz et al and the Model of Xenon Exchange (MOXE), examining their ability to measure: i) importance (rank), ii) temporal dependence and iii) interaction effects of each parameter across healthy and diseased ranges. The univariate methods and scatter plot analyses demonstrate consistently similar results for the importance of parameters common to both models evaluated. Alveolar surface area to volume ratio is identified as the parameter to which model signals are most sensitive. The alveolar-capillary barrier thickness is identified as a low-sensitivity parameter for the MOXE model. An acquisition window of at least 200 ms effectively demonstrates model sensitivity to most parameters. Scatter plots reveal interaction effects in both models, impacting output variability and sensitivity. Our sensitivity analysis ranks the parameters within the model described by Patz et al and within the MOXE model. The MOXE model shows low sensitivity to alveolar-capillary barrier thickness, highlighting the need for designing acquisition protocols optimised for the measurement of this parameter. The presence of parameter interaction effects highlights the requirement for care in interpreting model outputs.

Assessment of pulmonary physiological changes caused by aging, cigarette smoking, and COPD with hyperpolarized 129Xe magnetic resonance

OBJECTIVE

To comprehensively assess the impact of aging, cigarette smoking, and chronic obstructive pulmonary disease (COPD) on pulmonary physiology using 129Xe MR.

METHODS

A total of 90 subjects were categorized into four groups, including healthy young (HY, n = 20), age-matched control (AMC, n = 20), asymptomatic smokers (AS, n = 28), and COPD patients (n = 22). 129Xe MR was utilized to obtain pulmonary physiological parameters, including ventilation defect percent (VDP), alveolar sleeve depth (h), apparent diffusion coefficient (ADC), total septal wall thickness (d), and ratio of xenon signal from red blood cells and interstitial tissue/plasma (RBC/TP).

RESULTS

Significant differences were found in the measured VDP (p = 0.035), h (p = 0.003), and RBC/TP (p = 0.003) between the HY and AMC groups. Compared with the AMC group, higher VDP (p = 0.020) and d (p = 0.048) were found in the AS group; higher VDP (p < 0.001), d (p < 0.001) and ADC (p < 0.001), and lower h (p < 0.001) and RBC/TP (p < 0.001) were found in the COPD group. Moreover, significant differences were also found in the measured VDP (p < 0.001), h (p < 0.001), ADC (p < 0.001), d (p = 0.008), and RBC/TP (p = 0.032) between the AS and COPD groups.

CONCLUSION

Our findings indicate that pulmonary structure and functional changes caused by aging, cigarette smoking, and COPD are various, and show a progressive deterioration with the accumulation of these risk factors, including cigarette smoking and COPD.

CLINICAL RELEVANCE STATEMENT

Pathophysiological changes can be difficult to comprehensively understand due to limitations in common techniques and multifactorial etiologies. 129Xe MRI can demonstrate structural and functional changes caused by several common factors and can be used to better understand patients' underlying pathology.

KEY POINTS

Standard techniques for assessing pathophysiological lung function changes, spirometry, and chest CT come with limitations. 129Xe MR demonstrated progressive deterioration with accumulation of the investigated risk factors, without these limitations. 129Xe MR can assess lung changes related to these risk factors to stage and evaluate the etiology of the disease.

Establishing a hemoglobin adjustment for 129 Xe gas exchange MRI and MRS

PURPOSE

129 Xe MRI and MRS signals from airspaces, membrane tissues (M), and red blood cells (RBCs) provide measurements of pulmonary gas exchange. However, 129 Xe MRI/MRS studies have yet to account for hemoglobin concentration (Hb), which is expected to affect the uptake of 129 Xe in the membrane and RBC compartments. We propose a framework to adjust the membrane and RBC signals for Hb and use this to assess sex-specific differences in RBC/M and establish a Hb-adjusted healthy reference range for the RBC/M ratio.

METHODS

We combined the 1D model of xenon gas exchange (MOXE) with the principle of TR-flip angle equivalence to establish scaling factors that normalize the dissolved-phase signals with respect to a standardH b 0 $$ H{b}^0 $$ (14 g/dL). 129 Xe MRI/MRS data from a healthy, young cohort (n = 18, age = 25.0± $$ \pm $$ 3.4 years) were used to validate this model and assess the impact of Hb adjustment on M/gas and RBC/gas images and RBC/M.

RESULTS

Adjusting for Hb caused RBC/M to change by up to 20% in healthy individuals with normal Hb and had marked impacts on M/gas and RBC/gas distributions in 3D gas-exchange maps. RBC/M was higher in males than females both before and after Hb adjustment (p < 0.001). After Hb adjustment, the healthy reference value for RBC/M for a consortium-recommended acquisition of TR = 15 ms and flip = 20° was 0.589± $$ \pm $$ 0.083 (mean± $$ \pm $$ SD).

CONCLUSION

MOXE provides a useful framework for evaluating the Hb dependence of the membrane and RBC signals. This work indicates that adjusting for Hb is essential for accurately assessing 129 Xe gas-exchange MRI/MRS metrics.

Lung Volume Dependence and Repeatability of Hyperpolarized 129Xe MRI Gas Uptake Metrics in Healthy Volunteers and Participants with COPD

Purpose

To assess the effect of lung volume on measured values and repeatability of xenon 129 (129Xe) gas uptake metrics in healthy volunteers and participants with chronic obstructive pulmonary disease (COPD).

Materials and Methods

This Health Insurance Portability and Accountability Act-compliant prospective study included data (March 2014-December 2015) from 49 participants (19 with COPD [mean age, 67 years ± 9 (SD)]; nine women]; 25 older healthy volunteers [mean age, 59 years ± 10; 20 women]; and five young healthy women [mean age, 23 years ± 3]). Thirty-two participants underwent repeated 129Xe and same-breath-hold proton MRI at residual volume plus one-third forced vital capacity (RV+FVC/3), with 29 also undergoing one examination at total lung capacity (TLC). The remaining 17 participants underwent imaging at TLC, RV+FVC/3, and residual volume (RV). Signal ratios between membrane, red blood cell (RBC), and gas-phase compartments were calculated using hierarchical iterative decomposition of water and fat with echo asymmetry and least-squares estimation (ie, IDEAL). Repeatability was assessed using coefficient of variation and intraclass correlation coefficient, and volume relationships were assessed using Spearman correlation and Wilcoxon rank sum tests.

Results

Gas uptake metrics were repeatable at RV+FVC/3 (intraclass correlation coefficient = 0.88 for membrane/gas; 0.71 for RBC/gas, and 0.88 for RBC/membrane). Relative ratio changes were highly correlated with relative volume changes for membrane/gas (r = -0.97) and RBC/gas (r = -0.93). Membrane/gas and RBC/gas measured at RV+FVC/3 were significantly lower in the COPD group than the corresponding healthy group (P ≤ .001). However, these differences lessened upon correction for individual volume differences (P = .23 for membrane/gas; P = .09 for RBC/gas).

Conclusion

Dissolved-phase 129Xe MRI-derived gas uptake metrics were repeatable but highly dependent on lung volume during measurement.Keywords: Blood-Air Barrier, MRI, Chronic Obstructive Pulmonary Disease, Pulmonary Gas Exchange, Xenon Supplemental material is available for this article © RSNA, 2023.

Functional lung imaging using novel and emerging MRI techniques

Respiratory diseases are leading causes of death and disability in the world. While early diagnosis is key, this has proven difficult due to the lack of sensitive and non-invasive tools. Computed tomography is regarded as the gold standard for structural lung imaging but lacks functional information and involves significant radiation exposure. Lung magnetic resonance imaging (MRI) has historically been challenging due to its short T2 and low proton density. Hyperpolarised gas MRI is an emerging technique that is able to overcome these difficulties, permitting the functional and microstructural evaluation of the lung. Other novel imaging techniques such as fluorinated gas MRI, oxygen-enhanced MRI, Fourier decomposition MRI and phase-resolved functional lung imaging can also be used to interrogate lung function though they are currently at varying stages of development. This article provides a clinically focused review of these contrast and non-contrast MR imaging techniques and their current applications in lung disease.

Preclinical MRI Using Hyperpolarized 129Xe

Although critical for development of novel therapies, understanding altered lung function in disease models is challenging because the transport and diffusion of gases over short distances, on which proper function relies, is not readily visualized. In this review we summarize progress introducing hyperpolarized ¹²⁹Xe imaging as a method to follow these processes in vivo. The work is organized in sections highlighting methods to observe the gas replacement effects of breathing (Gas Dynamics during the Breathing Cycle) and gas diffusion throughout the parenchymal airspaces (3). We then describe the spectral signatures indicative of gas dissolution and uptake (4), and how these features can be used to follow the gas as it enters the tissue and capillary bed, is taken up by hemoglobin in the red blood cells (5), re-enters the gas phase prior to exhalation (6), or is carried via the vasculature to other organs and body structures (7). We conclude with a discussion of practical imaging and spectroscopy techniques that deliver quantifiable metrics despite the small size, rapid motion and decay of signal and coherence characteristic of the magnetically inhomogeneous lung in preclinical models (8).

Nichtinvasive funktionelle Lungenbildgebung mit hyperpolarisiertem Xenon

Hintergrund

Die Magnetresonanztomographie (MRT) ist ein nichtinvasives Verfahren mit hervorragendem Weichteilkontrast. Aufgrund der geringen Protonendichte und vielen Luft-Gewebe-Übergängen ist die Anwendung in der Lunge jedoch eingeschränkt, so dass hier häufig röntgenbasierte Methoden eingesetzt werden (mit den bekannten Nachteilen ionisierender Strahlung).

Fragestellung

In dieser Übersichtsarbeit wird die Lungen-MRT mit hyperpolarisiertem Xenon-129 (Xe-MRT) dargestellt. Die Xe-MRT erlaubt einzigartige wertvolle Einblicke in die Mikrostruktur und Funktion der Lunge, einschließlich des Gasaustauschs mit roten Blutkörperchen – Parameter, die mit klinischen Standardmethoden nicht zugänglich sind.

Material und Methoden

Durch die magnetische Markierung, die Hyperpolarisierung, wird das Signal von Xenon-129 um bis zu 100.000-fach verstärkt. Hierbei werden die Elektronen von Rubidium mittels Laserlicht zunächst auf 100 % polarisiert und dann durch Stöße auf Xenon übertragen. Danach wird das hyperpolarisierte Gas in einem Beutel zum Patienten gebracht und eingeatmet, kurz bevor die MRT-Aufnahmen beginnen.

Ergebnisse

Durch spezielle Programmierungen (Sequenzen) in der MRT kann die Ventilation, Mikrostruktur oder der Gasaustausch der Lunge in 3‑D dargestellt werden. Dies ermöglicht z. B. die quantitative Darstellung von Belüftungsdefekten, der Größe der Alveolen, der Gasaufnahme im Gewebe und des Gastransfers ins Blut.

Schlussfolgerung

Die Xe-MRT liefert einzigartige Informationen über den Zustand der Lunge – nichtinvasiv, in vivo und in weniger als einer Minute.

Hyperpolarized 129 Xe imaging of the brain: Achievements and future challenges

Hyperpolarized (HP) xenon-129 (¹²⁹ Xe) brain MRI is a promising imaging modality currently under extensive development. HP ¹²⁹ Xe is nontoxic, capable of dissolving in pulmonary blood, and is extremely sensitive to the local environment. After dissolution in the pulmonary blood, HP ¹²⁹ Xe travels with the blood flow to the brain and can be used for functional imaging such as perfusion imaging, hemodynamic response detection, and blood-brain barrier permeability assessment. HP ¹²⁹ Xe MRI imaging of the brain has been performed in animals, healthy human subjects, and in patients with Alzheimer's disease and stroke. In this review, the overall progress in the field of HP ¹²⁹ Xe brain imaging is discussed, along with various imaging approaches and pulse sequences used to optimize HP ¹²⁹ Xe brain MRI. In addition, current challenges and limitations of HP ¹²⁹ Xe brain imaging are discussed, as well as possible methods for their mitigation. Finally, potential pathways for further development are also discussed. HP ¹²⁹ Xe MRI of the brain has the potential to become a valuable novel perfusion imaging technique and has the potential to be used in the clinical setting in the future.

Protocols for multi-site trials using hyperpolarized 129 Xe MRI for imaging of ventilation, alveolar-airspace size, and gas exchange: A position paper from the 129 Xe MRI clinical trials consortium

Hyperpolarized (HP) ¹²⁹ Xe MRI uniquely images pulmonary ventilation, gas exchange, and terminal airway morphology rapidly and safely, providing novel information not possible using conventional imaging modalities or pulmonary function tests. As such, there is mounting interest in expanding the use of biomarkers derived from HP ¹²⁹ Xe MRI as outcome measures in multi-site clinical trials across a range of pulmonary disorders. Until recently, HP ¹²⁹ Xe MRI techniques have been developed largely independently at a limited number of academic centers, without harmonizing acquisition strategies. To promote uniformity and adoption of HP ¹²⁹ Xe MRI more widely in translational research, multi-site trials, and ultimately clinical practice, this position paper from the ¹²⁹ Xe MRI Clinical Trials Consortium (https://cpir.cchmc.org/XeMRICTC) recommends standard protocols to harmonize methods for image acquisition in HP ¹²⁹ Xe MRI. Recommendations are described for the most common HP gas MRI techniques-calibration, ventilation, alveolar-airspace size, and gas exchange-across MRI scanner manufacturers most used for this application. Moreover, recommendations are described for ¹²⁹ Xe dose volumes and breath-hold standardization to further foster consistency of imaging studies. The intention is that sites with HP ¹²⁹ Xe MRI capabilities can readily implement these methods to obtain consistent high-quality images that provide regional insight into lung structure and function. While this document represents consensus at a snapshot in time, a roadmap for technical developments is provided that will further increase image quality and efficiency. These standardized dosing and imaging protocols will facilitate the wider adoption of HP ¹²⁹ Xe MRI for multi-site pulmonary research.

Hyperpolarized 129 Xe multi-slice imaging of the human brain using a 3D gradient echo pulse sequence

PURPOSE

To demonstrate the possibility of performing multi-slice in-vivo human brain MRI using hyperpolarized (HP) xenon-129 (¹²⁹ Xe) in two different orientations and to calculate the signal-to-noise ratio (SNR).

METHODS

Two healthy female participants were imaged during a single breath-hold of HP ¹²⁹ Xe using a Philips Achieva 3.0T MRI scanner (Philips, Andover, MA). Each HP ¹²⁹ Xe multi-slice brain image was acquired during separate HP ¹²⁹ Xe breath-holds using 3D gradient echo (GRE) imaging. The acquisition started 10 s after the inhalation of 1 L of HP ¹²⁹ Xe. Overall, four sagittal and three axial images were acquired (seven imaging sessions per participant). The SNR was calculated for each slice in both orientations.

RESULTS

The first ever HP ¹²⁹ Xe multi-slice images of the brain were acquired in axial and sagittal orientations. The HP ¹²⁹ Xe signal distribution correlated well with the gray matter distribution. The highest SNR values were close in the axial and sagittal orientations (19.46 ± 3.25 and 18.76 ± 4.94, respectively). Additionally, anatomical features, such as the ventricles, were observed in both orientations.

CONCLUSION

The possibility of using multi-slice HP ¹²⁹ Xe human brain magnetic resonance imaging was demonstrated for the first time. HP ¹²⁹ Xe multi-slice MRI can be implemented for brain imaging to improve current diagnostic methods.

The effects of an initial depolarization pulse on dissolved phase hyperpolarized 129 Xe brain MRI

PURPOSE

To evaluate the effect of an initial 90° depolarization RF pulse on the dissolved-phase hyperpolarized (HP) xenon-129 (¹²⁹ Xe) brain imaging and to compare the SNR variability of HP ¹²⁹ Xe images acquired without an initial depolarization RF pulse to those following the initial depolarization pulse.

METHODS

Five cognitive normal healthy volunteers were imaged using a Philips Achieva 3.0T MRI scanner during a single breath-hold following inhalation of 1 L of HP ¹²⁹ Xe. Each participant underwent six HP ¹²⁹ Xe scans. Three scans were performed using conventional single-slice projection HP ¹²⁹ Xe brain imaging, and the other three scans were performed using the HP ¹²⁹ Xe time-of-flight imaging with an initial rectangular depolarization pulse.

RESULTS

Although the utilization of an initial depolarization results in the reduction of the mean image SNR, the presence of an initial depolarization RF pulse reduces the SNR variability of the HP ¹²⁹ Xe brain image by a factor of 2.26. The highest SNR variability was observed from the posterior brain region, where the anterior region possessed the lower level of signal variability.

CONCLUSION

An initial 90° depolarization RF pulse, applied prior to the HP ¹²⁹ Xe image acquisition, reduced the HP ¹²⁹ Xe signal variability more than two times between the different breath-hold images.

Pulmonary Functional Imaging: Part 1-State-of-the-Art Technical and Physiologic Underpinnings

Over the past few decades, pulmonary imaging technologies have advanced from chest radiography and nuclear medicine methods to high-spatial-resolution or low-dose chest CT and MRI. It is currently possible to identify and measure pulmonary pathologic changes before these are obvious even to patients or depicted on conventional morphologic images. Here, key technological advances are described, including multiparametric CT image processing methods, inhaled hyperpolarized and fluorinated gas MRI, and four-dimensional free-breathing CT and MRI methods to measure regional ventilation, perfusion, gas exchange, and biomechanics. The basic anatomic and physiologic underpinnings of these pulmonary functional imaging techniques are explained. In addition, advances in image analysis and computational and artificial intelligence (machine learning) methods pertinent to functional lung imaging are discussed. The clinical applications of pulmonary functional imaging, including both the opportunities and challenges for clinical translation and deployment, will be discussed in part 2 of this review. Given the technical advances in these sophisticated imaging methods and the wealth of information they can provide, it is anticipated that pulmonary functional imaging will be increasingly used in the care of patients with lung disease. © RSNA, 2021 Online supplemental material is available for this article.

In vivo methods and applications of xenon-129 magnetic resonance

Hyperpolarised gas lung MRI using xenon-129 can provide detailed 3D images of the ventilated lung airspaces, and can be applied to quantify lung microstructure and detailed aspects of lung function such as gas exchange. It is sensitive to functional and structural changes in early lung disease and can be used in longitudinal studies of disease progression and therapy response. The ability of 129Xe to dissolve into the blood stream and its chemical shift sensitivity to its local environment allow monitoring of gas exchange in the lungs, perfusion of the brain and kidneys, and blood oxygenation. This article reviews the methods and applications of in vivo129Xe MR in humans, with a focus on the physics of polarisation by optical pumping, radiofrequency coil and pulse sequence design, and the in vivo applications of 129Xe MRI and MRS to examine lung ventilation, microstructure and gas exchange, blood oxygenation, and perfusion of the brain and kidneys.

Effects of neonatal lung abnormalities on parenchymal R2 * estimates

Infants admitted to the neonatal intensive care unit (NICU) often suffer from multifaceted pulmonary morbidities that are not well understood. Ultrashort echo time (UTE) magnetic resonance imaging (MRI) is a promising technique for pulmonary imaging in this population without requiring exposure to ionizing radiation. The aims of this study were to investigate the effect of neonatal pulmonary disease on R₂ * and tissue density and to utilize numerical simulations to evaluate the effect of different alveolar structures on predicted R₂ *.This was a prospective study, in which 17 neonatal human subjects (five control, seven with bronchopulmonary dysplasia [BPD], five with congenital diaphragmatic hernia [CDH]) were enrolled. Twelve subjects were male and five were female, with postmenstrual age (PMA) at MRI of 39.7 ± 4.7 weeks. A 1.5T/multiecho three-dimensional UTE MRI was used. Pulmonary R₂ * and tissue density were compared across disease groups over the whole lung and regionally. A spherical shell alveolar model was used to predict the expected R₂ * over a range of tissue densities and tissue susceptibilities. Tests for significantly different mean R₂ * and tissue densities across disease groups were evaluated using analysis of variance, with subsequent pairwise group comparisons performed using t tests. Lung tissue density was lower in the ipsilateral lung in CDH compared to both controls and BPD patients (both p < 0.05), while only the contralateral lung in CDH (CDHc) had higher whole-lung R₂ * than both controls and BPD (both p < 0.05). R₂ * differences were significant between controls and CDHc within all tissue density ranges (all p < 0.05) with the exception of the 80%-90% range (p = 0.17). Simulations predicted an inverse relationship between alveolar tissue density and R₂ * that matches empirical human data. Alveolar wall thickness had no effect on R₂ * independent of density (p = 1). The inverse relationship between R₂ * and tissue density is influenced by the presence of disease globally and regionally in neonates with BPD and CDH in the NICU. LEVEL OF EVIDENCE: 2. TECHNICAL EFFICACY STAGE: 2.

Ventilation/Perfusion Relationships and Gas Exchange: Measurement Approaches

Ventilation-perfusion ( V ˙ A / Q ˙ ) matching, the regional matching of the flow of fresh gas to flow of deoxygenated capillary blood, is the most important mechanism affecting the efficiency of pulmonary gas exchange. This article discusses the measurement of V ˙ A / Q ˙ matching with three broad classes of techniques: (i) those based in gas exchange, such as the multiple inert gas elimination technique (MIGET); (ii) those derived from imaging techniques such as single-photon emission computed tomography (SPECT), positron emission tomography (PET), magnetic resonance imaging (MRI), computed tomography (CT), and electrical impedance tomography (EIT); and (iii) fluorescent and radiolabeled microspheres. The focus is on the physiological basis of these techniques that provide quantitative information for research purposes rather than qualitative measurements that are used clinically. The fundamental equations of pulmonary gas exchange are first reviewed to lay the foundation for the gas exchange techniques and some of the imaging applications. The physiological considerations for each of the techniques along with advantages and disadvantages are briefly discussed. © 2020 American Physiological Society. Compr Physiol 10:1155-1205, 2020.

Investigating biases in the measurement of apparent alveolar septal wall thickness with hyperpolarized 129Xe MRI

PURPOSE

To investigate biases in the measurement of apparent alveolar septal wall thickness (SWT) with hyperpolarized xenon-129 (HXe) as a function of acquisition parameters.

METHODS

The HXe MRI scans with simultaneous gas-phase and dissolved-phase excitation were performed using 1-dimensional projection scans in mechanically ventilated rabbits. The dissolved-phase magnetization was periodically saturated, and the dissolved-phase xenon uptake dynamics were measured at end inspiration and end expiration with temporal resolutions up to 10 ms using a Look-Locker-type acquisition. The apparent alveolar septal wall thickness was extracted by fitting the signal to a theoretical model, and the findings were compared with those from the more commonly use chemical shift saturation recovery MRI spectroscopy technique with several different delay time arrangements.

RESULTS

It was found that repeated application of RF saturation pulses in chemical shift saturation recovery acquisitions caused exchange-dependent gas-phase saturation that heavily biased the derived SWT value. When this bias was reduced by our proposed method, the SWT dependence on lung inflation disappeared due to an inherent insensitivity of HXe dissolved-phase MRI to thin alveolar structures with very short T 2 ∗ . Furthermore, perfusion-based macroscopic gas transport processes were demonstrated to cause increasing apparent SWTs with TE (2.5 μm/ms at end expiration) and a lung periphery-to-center SWT gradient.

CONCLUSION

The apparent SWT measured with HXe MRI was found to be heavily dependent on the acquisition parameters. A method is proposed that can minimize this measurement bias, add limited spatial resolution, and reduce measurement time to a degree that free-breathing studies are feasible.

Acquisition strategies for spatially resolved magnetic resonance detection of hyperpolarized nuclei

Hyperpolarization is an emerging method in magnetic resonance imaging that allows nuclear spin polarization of gases or liquids to be temporarily enhanced by up to five or six orders of magnitude at clinically relevant field strengths and administered at high concentration to a subject at the time of measurement. This transient gain in signal has enabled the non-invasive detection and imaging of gas ventilation and diffusion in the lungs, perfusion in blood vessels and tissues, and metabolic conversion in cells, animals, and patients. The rapid development of this method is based on advances in polarizer technology, the availability of suitable probe isotopes and molecules, improved MRI hardware and pulse sequence development. Acquisition strategies for hyperpolarized nuclei are not yet standardized and are set up individually at most sites depending on the specific requirements of the probe, the object of interest, and the MRI hardware. This review provides a detailed introduction to spatially resolved detection of hyperpolarized nuclei and summarizes novel and previously established acquisition strategies for different key areas of application.

Quantitative evaluation of lung injury caused by PM2.5 using hyperpolarized gas magnetic resonance

PURPOSE

To demonstrate the feasibility of ¹²⁹ Xe MR in evaluating the pulmonary physiological changes caused by PM_2.5 in animal models.

METHODS

Six rats were treated with PM_2.5 solution (16.2 mg/kg) by intratracheal instillation twice a week for 4 weeks, and another six rats treated with normal saline served as the control cohort. Pulmonary function tests, hyperpolarized ¹²⁹ Xe multi-b diffusion-weighted imaging, and chemical shift saturation recovery MR spectroscopy were performed on all rats, and the pulmonary structure and functional parameters were obtained from hyperpolarized ¹²⁹ Xe MR data. Additionally, histological analysis was performed on all rats to evaluate alveolar septal thickness. Statistical analysis of all the obtained parameters was performed using unpaired 2-tailed t tests.

RESULTS

Compared with the control group, the measured exchange time constant increased from 11.74 ± 2.39 to 14.00 ± 2.84 ms (P < .05), and the septal wall thickness increased from 6.17 ± 0.48 to 6.74 ± 0.52 μm (P < .05) in the PM_2.5 cohort by ¹²⁹ Xe MR spectroscopy, which correlated well with that obtained using quantitative histology (increased from 5.52 ± 0.32 to 6.20 ± 0.36 μm). Additionally, the mean TP/GAS ratio increased from 0.828 ± 0.115 to 1.019 ± 0.140 in the PM_2.5 cohort (P = .021).

CONCLUSIONS

Hyperpolarized ¹²⁹ Xe MR could quantify the changes in gas exchange physiology caused by PM_2.5 , indicating that the technique has the potential to be a useful tool for evaluation of pulmonary injury caused by air pollution in the future.

Noninvasive Monitoring of the Response of Human Lungs to Low-Dose Lipopolysaccharide Inhalation Challenge Using MRI: A Feasibility Study

BACKGROUND

Development of antiinflammatory drugs for lung diseases demands novel methods for noninvasive assessment of inflammatory processes in the lung.

PURPOSE

To investigate the feasibility of hyperpolarized ¹²⁹ Xe MRI, ¹ H T₁ time mapping, and dynamic contrast-enhanced (DCE) perfusion MRI for monitoring the response of human lungs to low-dose inhaled lipopolysaccharide (LPS) challenge compared to inflammatory cell counts from induced-sputum analysis.

STUDY TYPE

Prospective feasibility study.

POPULATION

Ten healthy volunteers underwent MRI before and 6 hours after inhaled LPS challenge with subsequent induced-sputum collection.

FIELD STRENGTH/SEQUENCES

1.5T/hyperpolarized ¹²⁹ Xe MRI: Interleaved multiecho imaging of dissolved and gas phase, ventilation imaging, dissolved-phase spectroscopy, and chemical shift saturation recovery spectroscopy. ¹ H MRI: Inversion recovery fast low-angle shot imaging for T₁ mapping, time-resolved angiography with stochastic trajectories for DCE MRI.

ASSESSMENT

Dissolved-phase ratios of ¹²⁹ Xe in red blood cells (RBC), tissue/plasma (TP) and gas phase (GP), ventilation defect percentage, septal wall thickness, surface-to-volume ratio, capillary transit time, lineshape parameters in dissolved-phase spectroscopy, ¹ H T₁ time, blood volume, flow, and mean transit time were determined and compared to cell counts.

STATISTICAL TESTS

Wilcoxon signed-rank test, Pearson correlation.

RESULTS

The percentage of neutrophils in sputum was markedly increased after LPS inhalation compared to baseline, P = 0.002. The group median RBC-TP ratio was significantly reduced from 0.40 to 0.31, P = 0.004, and ¹ H T₁ was significantly elevated from 1157.6 msec to 1187.8 msec after LPS challenge, P = 0.027. DCE MRI exhibited no significant changes in blood volume, P = 0.64, flow, P = 0.17, and mean transit time, P = 0.11.

DATA CONCLUSION

Hyperpolarized ¹²⁹ Xe dissolved-phase MRI and ¹ H T₁ mapping may provide biomarkers for noninvasive assessment of the response of human lungs to LPS inhalation. By its specificity to the alveolar region, hyperpolarized ¹²⁹ Xe MRI together with ¹ H T₁ mapping adds value to sputum analysis.

LEVEL OF EVIDENCE

1 Technical Efficacy Stage: 2 J. Magn. Reson. Imaging 2020;51:1669-1676.

Single breath-hold measurement of pulmonary gas exchange and diffusion in humans with hyperpolarized 129 Xe MR

Pulmonary diseases usually result in changes of the blood-gas exchange function in the early stages. Gas exchange across the respiratory membrane and gas diffusion in the alveoli can be quantified using hyperpolarized ¹²⁹ Xe MR via chemical shift saturation recovery (CSSR) and diffusion-weighted imaging (DWI), respectively. Generally, CSSR and DWI data have been collected in separate breaths in humans. Unfortunately, the lung inflation level cannot be the exactly same in different breaths, which causes fluctuations in blood-gas exchange and pulmonary microstructure. Here we combine CSSR and DWI obtained with compressed sensing, to evaluate the gas diffusion and exchange function within a single breath-hold in humans. A new parameter, namely the perfusion factor of the respiratory membrane (SVR_d/g ), is proposed to evaluate the gas exchange function. Hyperpolarized ¹²⁹ Xe MR data are compared with pulmonary function tests and computed tomography examinations in healthy young, age-matched control, and chronic obstructive pulmonary disease human cohorts. SVR_d/g decreases as the ventilation impairment and emphysema index increase. Our results indicate that the proposed method has the potential to detect the extent of lung parenchyma destruction caused by age and pulmonary diseases, and it would be useful in the early diagnosis of pulmonary diseases in clinical practice.

Accelerated interleaved spiral-IDEAL imaging of hyperpolarized 129 Xe for parametric gas exchange mapping in humans

PURPOSE

To demonstrate the feasibility of mapping gas exchange with single breath-hold hyperpolarized (HP) ¹²⁹ Xe in humans, acquiring parametric maps of lung physiology. The potential benefit of acceleration using parallel imaging for this application is also explored.

METHODS

Six healthy volunteers were scanned with a modified spiral-IDEAL sequence to acquire gas exchange-weighted images using a single dose of ¹²⁹ Xe. These images were fit with the model of xenon exchange (MOXE) on a voxel-wise basis calculating parametric maps of lung physiology, specifically: air-capillary barrier thickness (δ), alveolar septal thickness (d), capillary transit time (t_x ), pulmonary hematocrit (HCT), and alveolar surface area-to-volume ratio (SVR). An accelerated version of the sequence was also tested in subset of 4 volunteers and compared to the fully sampled (FS) results.

RESULTS

Mean image-wide values calculated from MOXE parametric maps derived from FS dissolved ¹²⁹ Xe spiral-IDEAL images were: δ = 0.89 ± 0.17 μm, d = 7.5 ± 0.5 μm, t_x = 1.1 ± 0.2s, HCT = 28.8 ± 2.3%, and SVR = 140 ± 16 cm^-1 , in good agreement with previously published values based on whole-lung spectroscopy of healthy human subjects. Parallel imaging sufficiently reduces artifacting in accelerated images, but increases disagreement with MOXE parameters derived from FS data with mean voxel-wise unsigned relative differences of: δ = 39 ± 9%, d = 22 ± 3%, t_x = 117 ± 43%, HCT = 11 ± 2%, and SVR = 31 ± 12%.

CONCLUSION

Dissolved HP ¹²⁹ Xe spiral-IDEAL imaging for gas exchange mapping is feasible in humans using a single breath-hold. Accelerated gas exchange mapping is also shown to be feasible but requires further improvements to increase quantitative accuracy.

Repeatability of regional pulmonary functional metrics of Hyperpolarized 129 Xe dissolved-phase MRI

BACKGROUND

MRI of hyperpolarized ¹²⁹ Xenon (HP ¹²⁹ Xe) is increasingly utilized for investigating pulmonary function. The solubility of HP ¹²⁹ Xe in lung tissue, blood plasma (Barrier), and red blood cells (RBC), with unique chemical shifts, enables spectroscopic imaging of potential imaging biomarkers of gas exchange and microstructural pulmonary physiology.

PURPOSE

To quantify global average and regional repeatability of Barrier:gas, RBC:gas, and RBC:Barrier ratios derived from dissolved-phase ¹²⁹ Xe imaging and their dependence on intervisit changes in lung inflation volume.

STUDY TYPE

Prospective.

POPULATION

Fourteen healthy volunteers. One subject was unable to complete the study resulting in 13 subjects for analysis (eight female, five male, ages 24-69, 53.8 ± 13.9).

FIELD STRENGTH

1.5T.

ASSESSMENT

Subjects were imaged using a 3D radial 1-point Dixon method to separate Barrier and RBC component signals, at two different timepoints, with ~1 month between visits. RBC:Gas, Barrier:Gas, and RBC:Barrier measures were compared across time and with pulmonary function tests (PFTs).

STATISTICAL TESTS

Repeatablilty was quantified using Bland-Altman plots, coefficient of repeatability, coefficient of variation (CV), and intraclass correlation coefficients (ICCs). Dependence of imaging measures on PFTs and lung volume was evaluated using Spearman and Pearson correlation coefficients, respectively. Statistical significance was determined by F-test for intraclass correlations, and t-test for Spearman correlations and regression.

RESULTS

Mean RBC:Gas, Barrier:Gas, and RBC:Barrier had CVs of 19.2%, 20.0%, and 11.5%, respectively, and had significant ICCs, equal to 0.78, 0.79, and 0.92, respectively. Intervisit differences in RBC:Barrier were significantly correlated with intervisit differences in DL_CO (r = 0.93, P = 0.007). Significant correlations with intervisit lung volume differences and intervisit differences in mean RBC:Gas (r = -0.73, P = 0.005) and Barrier:Gas (r = -0.69, P = 0.009) were found.

DATA CONCLUSION

Three commonly used ¹²⁹ Xe MRI-based measures of gas-exchange show good repeatability, particularly the Barrier:RBC ratio, which did not depend on lung inflation volume and was strongly associated with intervisit changes in DL_CO .

LEVEL OF EVIDENCE

1 Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2019;50:1182-1190.

New Developments in Imaging Idiopathic Pulmonary Fibrosis With Hyperpolarized Xenon Magnetic Resonance Imaging

Idiopathic pulmonary fibrosis (IPF) is a progressive pulmonary disease that is ultimately fatal. Although the diagnosis of IPF has been revolutionized by high-resolution computed tomography, this imaging modality still exhibits significant limitations, particularly in assessing disease progression and therapy response. The need for noninvasive regional assessment has become more acute in light of recently introduced novel therapies and numerous others in the pipeline. Thus, it will likely be valuable to complement 3-dimensional imaging of lung structure with 3-dimensional regional assessment of function. This challenge is well addressed by hyperpolarized (HP) Xe magnetic resonance imaging (MRI), exploiting the unique properties of this inert gas to image its distribution, not only in the airspaces, but also in the interstitial barrier tissues and red blood cells. This single-breath imaging exam could ultimately become the ideal, noninvasive tool to assess pulmonary gas-exchange impairment in IPF. This review article will detail the evolution of HP Xe MRI from its early development to its current state as a clinical research platform. It will detail the key imaging biomarkers that can be generated from the Xe MRI examination, as well as their potential in IPF for diagnosis, prognosis, and assessment of therapeutic response. We conclude by discussing the types of studies that must be performed for HP Xe MRI to be incorporated into the IPF clinical algorithm and begin to positively impact IPF disease diagnosis and management.

Regional investigation of lung function and microstructure parameters by localized 129 Xe chemical shift saturation recovery and dissolved-phase imaging: A reproducibility study

PURPOSE

To evaluate the reproducibility and regional variation of parameters obtained from localized ¹²⁹ Xe chemical shift saturation recovery (CSSR) MR spectroscopy in healthy volunteers and patients with chronic obstructive pulmonary disease (COPD) and to compare the results to ¹²⁹ Xe dissolved-phase MR imaging.

METHODS

Thirteen healthy volunteers and 10 COPD patients were scanned twice using ¹²⁹ Xe dissolved-phase imaging, CSSR, and ventilation imaging sequences. A 16-channel phased-array coil in combination with the regularized spectral localization achieved by sensitivity heterogeneity (SPLASH) method was used to perform a regional analysis of CSSR data. Lung function and microstructural parameters were obtained using Patz model functions and their reproducibility was assessed.

RESULTS

The Patz model alveolar wall thickness parameter shows good reproducibility on a regional basis with a median coefficient of variation of 6.5% in healthy volunteers and 12.4% in COPD patients. Significant regional differences of lung function parameters derived from localized CSSR were found in healthy volunteers and correlations with spirometric indices were found.

CONCLUSION

Localized ¹²⁹ Xe CSSR provides reproducible estimates of alveolar wall thickness and is able to detect regional differences of lung microstructure.

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.

Hyperpolarized gas MRI in pulmonology

Lung diseases have a high prevalence amongst the world population and their early diagnosis has been pointed out to be key for successful treatment. However, there is still a lack of non-invasive examination methods with sensitivity to early, local deterioration of lung function. Proton-based lung MRI is particularly challenging due to short T2* times and low proton density within the lung tissue. Hyperpolarized gas MRI is aan emerging technology providing a richness of methodologies which overcome the aforementioned problems. Unlike proton-based MRI, lung MRI of hyperpolarized gases may rely on imaging of spins in the lung's gas spaces or inside the lung tissue and thereby add substantial value and diagnostic potential to lung MRI. This review article gives an introduction to the MR physics of hyperpolarized media and presents the current state of hyperpolarized gas MRI of 3Headvasd and 129Xe in pulmonology. Key applications, ranging from static and dynamic ventilation imaging as well as oxygen-pressure mapping to 129Xe dissolved-phase imaging and spectroscopy are presented. Hyperpolarized gas MRI is compared to alternative examination methods based on MRI and future directions of hyperpolarized gas MRI are discussed.