A protocol for quantifying cardiogenic oscillations in dynamic 129 Xe gas exchange spectroscopy: The effects of idiopathic pulmonary fibrosis

Age, sex, and lung volume dependence of dissolved xenon-129 MRI gas exchange metrics

PURPOSE

To characterize the dependence of Xe-MRI gas transfer metrics upon age, sex, and lung volume in a group of healthy volunteers.

METHODS

Sixty-five subjects with no history of chronic lung disease were assessed with 129Xe-MRI using a four-echo 3D radial spectroscopic imaging sequence and a dose of xenon titrated according to subject height that was inhaled from a lung volume of functional residual capacity (FRC). Imaging was repeated in 34 subjects at total lung capacity (TLC). Regional maps of the fractions of dissolved xenon in red blood cells (RBC), membrane (M), and airspace (Gas) were acquired at an isotropic resolution of 2 cm, from which global averages of the ratios RBC:M, RBC:Gas, and M:Gas were computed.

RESULTS

Data from 26 males and 36 females with a median age of 43 y (range: 20-69 y) were of sufficient quality to analyze. Age (p = 0.0006) and sex (p < 0.0001) were significant predictors for RBC:M, and a linear regression showed higher values and steeper decline in males: RBC:M(Males) = -0.00362 × Age + 0.60 (p = 0.01, R2 = 0.25); RBC:M(Females) = -0.00170 × Age + 0.44 (p = 0.02, R2 = 0.15). Similarly, age and sex were significant predictors for RBC:Gas but not for M:Gas. RBC:M, M:Gas and RBC:Gas were significantly lower at TLC than at FRC (plus inhaled volume), with an average 9%, 30% and 35% decrease, respectively.

CONCLUSION

Expected age and sex dependence of pulmonary function concurs with 129Xe RBC:M imaging results, demonstrating that these variables must be considered when reporting Xe-MRI metrics. Xenon doses and breathing maneuvers should be controlled due to the strong dependence of Xe-MRI metrics upon lung volume.

Emerging multimodality imaging techniques for the pulmonary circulation

Pulmonary hypertension (PH) remains a challenging condition to diagnose, classify and treat. Current approaches to the assessment of PH include echocardiography, ventilation/perfusion scintigraphy, cross-sectional imaging using computed tomography and magnetic resonance imaging, and right heart catheterisation. However, these approaches only provide an indirect readout of the primary pathology of the disease: abnormal vascular remodelling in the pulmonary circulation. With the advent of newer imaging techniques, there is a shift toward increased utilisation of noninvasive high-resolution modalities that offer a more comprehensive cardiopulmonary assessment and improved visualisation of the different components of the pulmonary circulation. In this review, we explore advances in imaging of the pulmonary vasculature and their potential clinical translation. These include advances in diagnosis and assessing treatment response, as well as strategies that allow reduced radiation exposure and implementation of artificial intelligence technology. These emerging modalities hold the promise of developing a deeper understanding of pulmonary vascular disease and the impact of comorbidities. They also have the potential to improve patient outcomes by reducing time to diagnosis, refining classification, monitoring treatment response and improving our understanding of disease mechanisms.

Functional gas exchange measures on 129Xe MRI and spectroscopy are associated with age, sex, and BMI in healthy subjects

Introduction

Hyperpolarized 129Xe MRI and spectroscopy is a rapidly growing technique for assessing lung function, with applications in a wide range of obstructive, restrictive, and pulmonary vascular disease. However, normal variations in 129Xe measures of gas exchange across healthy subjects are not well characterized, presenting an obstacle to differentiating disease processes from the consequences of expected physiological heterogeneity. Here, we use multivariate models to evaluate the role of age, sex, and BMI in a range of commonly used 129Xe measures of gas exchange.

Materials and methods

Healthy subjects (N = 40, 16F, age 44.3 ± 17.8 yrs., min-max 22-87 years) with no history of cardiopulmonary disease underwent 129Xe gas exchange MRI and spectroscopy. We used multivariate linear models to assess the associations of age, sex, and body mass index (BMI) with the RBC:Membrane (RBC:M), membrane to gas (Mem:Gas), and red blood cell to gas (RBC:Gas) ratios, as well as measurements of RBC oscillation amplitude and RBC chemical shift.

Results

Age, sex, and BMI were all significant covariates in the RBC:M model. Each additional 10 years of age was associated with a 0.05 decrease in RBC:M (p < 0.001), each additional 10 points of BMI was associated with a decrease of 0.07 (p = 0.02), and males were associated with a 0.17 higher RBC:M than females (p < 0.001). For Mem:Gas, male sex was associated with a decrease and BMI was associated with an increase. For RBC:Gas, age was associated with a decrease and male sex was associated with an increase. RBC oscillation amplitude increased with age and RBC chemical shift was not associated with any of the three covariates.

Discussion

129Xe MRI and spectroscopy measurements in healthy subjects, particularly the widely used RBC:M measurement, exhibit heterogeneity associated in part with variations in subject age, sex, and BMI. Elucidating the contributions of these and other factors to 129Xe gas exchange measurements is a critical component for differentiating disease processes from expected variation in healthy subjects. Notably, the Mem:Gas and RBC chemical shift appear to be stable with aging, suggesting that unexplained deviations in these metrics may be signs of underlying abnormalities.

Review of Hyperpolarized Pulmonary Functional 129 Xe MR for Long-COVID

The respiratory consequences of acute COVID-19 infection and related symptoms tend to resolve 4 weeks post-infection. However, for some patients, new, recurrent, or persisting symptoms remain beyond the acute phase and persist for months, post-infection. The symptoms that remain have been referred to as long-COVID. A number of research sites employed 129 Xe magnetic resonance imaging (MRI) during the pandemic and evaluated patients post-infection, months after hospitalization or home-based care as a way to better understand the consequences of infection on 129 Xe MR gas-exchange and ventilation imaging. A systematic review and comprehensive search were employed using MEDLINE via PubMed (April 2023) using the National Library of Medicine's Medical Subject Headings and key words: post-COVID-19, MRI, 129 Xe, long-COVID, COVID pneumonia, and post-acute COVID-19 syndrome. Fifteen peer-reviewed manuscripts were identified including four editorials, a single letter to the editor, one review article, and nine original research manuscripts (2020-2023). MRI and MR spectroscopy results are summarized from these prospective, controlled studies, which involved small sample sizes ranging from 9 to 76 participants. Key findings included: 1) 129 Xe MRI gas-exchange and ventilation abnormalities, 3 months post-COVID-19 infection, and 2) a combination of MRI gas-exchange and ventilation abnormalities alongside persistent symptoms in patients hospitalized and not hospitalized for COVID-19, 1-year post-infection. The persistence of respiratory symptoms and 129 Xe MRI abnormalities in the context of normal or nearly normal pulmonary function test results and chest computed tomography (CT) was consistent. Longitudinal improvements were observed in long-term follow-up of long-COVID patients but mean 129 Xe gas-exchange, ventilation heterogeneity values and symptoms remained abnormal, 1-year post-infection. Pulmonary functional MRI using inhaled hyperpolarized 129 Xe gas has played a role in detecting gas-exchange and ventilation abnormalities providing complementary information that may help develop our understanding of the root causes of long-COVID. LEVEL OF EVIDENCE: 1 TECHNICAL EFFICACY: Stage 5.

Optimized quantitative mapping of cardiopulmonary oscillations using hyperpolarized 129 Xe gas exchange MRI: Digital phantoms and clinical evaluation in CTEPH

PURPOSE

The interaction between 129 Xe atoms and pulmonary capillary red blood cells provides cardiogenic signal oscillations that display sensitivity to precapillary and postcapillary pulmonary hypertension. Recently, such oscillations have been spatially mapped, but little is known about optimal reconstruction or sensitivity to artifacts. In this study, we use digital phantom simulations to specifically optimize keyhole reconstruction for oscillation imaging. We then use this optimized method to re-establish healthy reference values and quantitatively evaluate microvascular flow changes in patients with chronic thromboembolic pulmonary hypertension (CTEPH) before and after pulmonary thromboendarterectomy (PTE).

METHODS

A six-zone digital lung phantom was designed to investigate the effects of radial views, key radius, and SNR. One-point Dixon 129 Xe gas exchange MRI images were acquired in a healthy cohort (n = 17) to generate a reference distribution and thresholds for mapping red blood cell oscillations. These thresholds were applied to 10 CTEPH participants, with 6 rescanned following PTE.

RESULTS

For undersampled acquisitions, a key radius of0.14 k max $$ 0.14{k}_{\mathrm{max}} $$ was found to optimally resolve oscillation defects while minimizing excessive heterogeneity. CTEPH participants at baseline showed higher oscillation defect + low (32 ± 14%) compared with healthy volunteers (18 ± 12%, p < 0.001). For those scanned both before and after PTE, oscillation defect + low decreased from 37 ± 13% to 23 ± 14% (p = 0.03).

CONCLUSIONS

Digital phantom simulations have informed an optimized keyhole reconstruction technique for gas exchange images acquired with standard 1-point Dixon parameters. Our proposed methodology enables more robust quantitative mapping of cardiogenic oscillations, potentially facilitating effective regional quantification of microvascular flow impairment in patients with pulmonary vascular diseases such as CTEPH.

Initial feasibility and challenges of hyperpolarized 129 Xe MRI in neonates with bronchopulmonary dysplasia

PURPOSE

The underlying functional and microstructural lung disease in neonates who are born preterm (bronchopulmonary dysplasia, BPD) remains poorly characterized. Moreover, there is a lack of suitable techniques to reliably assess lung function in this population. Here, we report our preliminary experience with hyperpolarized 129 Xe MRI in neonates with BPD.

METHODS

Neonatal intensive care patients with established BPD were recruited (N = 9) and imaged at a corrected gestational age of median:40.7 (range:37.1, 44.4) wk using a 1.5T neonatal scanner. 2D 129 Xe ventilation and diffusion-weighted images and dissolved phase spectroscopy were acquired, alongside 1 H 3D radial UTE. 129 Xe images were acquired during a series of short apneic breath-holds (˜3 s). 1 H UTE images were acquired during tidal breathing. Ventilation defects were manually identified and qualitatively compared to lung structures on UTE. ADCs were calculated on a voxel-wise basis. The signal ratio of the 129 Xe red blood cell (RBC) and tissue membrane (M) resonances from spectroscopy was determined.

RESULTS

Spiral-based 129 Xe ventilation imaging showed good image quality and sufficient sensitivity to detect mild ventilation abnormalities in patients with BPD. 129 Xe ADC values were elevated above that expected given healthy data in older children and adults (median:0.046 [range:0.041, 0.064] cm2 s-1 ); the highest value obtained from an extremely pre-term patient. 129 Xe spectroscopy revealed a low RBC/M ratio (0.14 [0.06, 0.21]).

CONCLUSION

We have demonstrated initial feasibility of 129 Xe lung MRI in neonates. With further data, the technique may help guide management of infant lung diseases in the neonatal period and beyond.

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.

Hyperpolarised xenon-129 diffusion-weighted magnetic resonance imaging for assessing lung microstructure in idiopathic pulmonary fibrosis

Background

Hyperpolarised 129-xenon (129Xe) magnetic resonance imaging (MRI) shows promise in monitoring the progression of idiopathic pulmonary fibrosis (IPF) due to the lack of ionising radiation and the ability to quantify functional impairment. Diffusion-weighted (DW)-MRI with hyperpolarised gases can provide information about lung microstructure. The aims were to compare 129Xe DW-MRI measurements with pulmonary function tests (PFTs), and to assess whether they can detect early signs of disease progression in patients with newly diagnosed IPF.

Methods

This is a prospective, single-centre, observational imaging study of patients presenting with IPF to Northern General Hospital (Sheffield, UK). Hyperpolarised 129Xe DW-MRI was performed at 1.5 T on a whole-body General Electric HDx scanner and PFTs were performed on the same day as the MRI scan.

Results

There was an increase in global 129Xe apparent diffusion coefficient (ADC) between the baseline and 12-month visits (mean 0.043 cm2·s-1, 95% CI 0.040-0.047 cm2·s-1 versus mean 0.045 cm2·s-1, 95% CI 0.040-0.049 cm2·s-1; p=0.044; n=20), with no significant change in PFTs over the same time period. There was also an increase in 129Xe ADC in the lower zone (p=0.027), and an increase in 129Xe mean acinar dimension in the lower zone (p=0.033) between the baseline and 12-month visits. 129Xe DW-MRI measurements correlated strongly with diffusing capacity of the lung for carbon monoxide (% predicted), transfer coefficient of the lung for carbon monoxide (KCO) and KCO (% predicted).

Conclusions

129Xe DW-MRI measurements appear to be sensitive to early changes of microstructural disease that are consistent with progression in IPF at 12 months. As new drug treatments are developed, the ability to quantify subtle changes using 129Xe DW-MRI could be particularly valuable.

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.

Imaging gas-exchange lung function and brain tissue uptake of hyperpolarized 129 Xe using sampling density-weighted MRSI

PURPOSE

Imaging of the different resonances of hyperpolarized ¹²⁹ Xe in the brain and lungs was performed using a 3D sampling density-weighted MRSI technique in healthy volunteers.

METHODS

Four volunteers underwent dissolved-phase hyperpolarized ¹²⁹ Xe imaging in the lung with the MRSI technique, which was designed to improve the point-spread function while preserving SNR (1799 phase-encoding steps, 14-s breath hold, 2.1-cm isotropic resolution). A frequency-tailored RF excitation pulse was implemented to reliably excite both the ¹²⁹ Xe gas and dissolved phase (tissue/blood signal) with 0.1° and 10° flip angles, respectively. Images of xenon gas in the lung airspaces and xenon dissolved in lung tissue/blood were used to generate quantitative signal ratio maps. The method was also optimized and used for imaging dissolved resonances of ¹²⁹ Xe in the brain in 2 additional volunteers.

RESULTS

High-quality regional spectra of hyperpolarized ¹²⁹ Xe were achieved in both the lung and the brain. Ratio maps of the different xenon resonances were obtained in the lung with sufficient SNR (> 10) at both 1.5 T and 3 T, making a triple Lorentzian fit possible and enabling the measurement of relaxation times and xenon frequency shifts on a voxel-wise basis. The imaging technique was successfully adapted for brain imaging, resulting in the first demonstration of 3D xenon brain images with a 2-cm isotropic resolution.

CONCLUSION

Density-weighted MRSI is an SNR and encoding-efficient way to image ¹²⁹ Xe resonances in the lung and the brain, providing a valuable tool to quantify regional spectroscopic information.

Assessment of pulmonary morphometry using hyperpolarized 129 Xe diffusion-weighted MRI with variable-sampling-ratio compressed sensing patterns

BACKGROUND

Hyperpolarized (HP) ¹²⁹ Xe multiple b-values diffusion-weighted magnetic resonance imaging (DW-MRI) has been widely used for quantifying pulmonary microstructural morphometry. However, the technique requires long acquisition times, making it hard to apply in patients with severe pulmonary diseases, who cannot sustain long breath holds.

PURPOSE

To develop and evaluate the technique of variable-sampling-ratio compressed sensing (VCS) patterns for accelerating HP ¹²⁹ Xe multiple b-values DW-MRI in humans.

METHODS

Optimal variable sampling ratios and corresponding k-space undersampling patterns for each b-value were obtained by retrospective simulations based on the fully sampled (FS) DW-MRI dataset acquired from six young healthy volunteers. Then, the FS datasets were retrospectively undersampled using both VCS patterns and conventional compressed sensing (CS) pattern with a similar average acceleration factor. The quality of reconstructed images with retrospective VCS (rVCS) and CS (rCS) datasets were quantified using mean absolute error (MAE) and structural similarity (SSIM). Pulmonary morphometric parameters were also evaluated between rVCS and FS datasets. In addition, prospective VCS multiple b-values ¹²⁹ Xe DW-MRI datasets were acquired from 14 cigarette smokers and 13 age-matched healthy volunteers. The differences of lung morphological parameters obtained with the proposed method were compared between the groups using independent samples t-test. Pearson correlation coefficient was also utilized for evaluating the correlation of the pulmonary physiological parameters obtained with VCS DW-MRI and pulmonary function tests.

RESULTS

Lower MAE and higher SSIM values were found in the reconstructed images with rVCS measurement when compared to those using conventional rCS measurement. The details and quality of the images obtained with rVCS and FS measurements were found to be comparable. The mean values of the morphological parameters derived from rVCS and FS datasets showed no significant differences (p > 0.05), and the mean differences of measured acinar duct radius, mean linear intercept, surface-to-volume ratio, and apparent diffusion coefficient with cylinder model were -0.87%, -2.42%, 2.04%, and -0.50%, respectively. By using the VCS technique, significant differences were delineated between the pulmonary morphometric parameters of healthy volunteers and cigarette smokers (p < 0.001), while the acquisition time was reduced by four times.

CONCLUSION

A fourfold reduction in acquisition time was achieved using the proposed VCS method while preserving good image quality. Our preliminary results demonstrated that the proposed method can be used for evaluating pulmonary injuries caused by cigarette smoking and may prove to be helpful in diagnosing lung diseases in clinical practice.

Quantitative Imaging Metrics for the Assessment of Pulmonary Pathophysiology: An Official American Thoracic Society and Fleischner Society Joint Workshop Report

Multiple thoracic imaging modalities have been developed to link structure to function in the diagnosis and monitoring of lung disease. Volumetric computed tomography (CT) renders three-dimensional maps of lung structures and may be combined with positron emission tomography (PET) to obtain dynamic physiological data. Magnetic resonance imaging (MRI) using ultrashort-echo time (UTE) sequences has improved signal detection from lung parenchyma; contrast agents are used to deduce airway function, ventilation-perfusion-diffusion, and mechanics. Proton MRI can measure regional ventilation-perfusion ratio. Quantitative imaging (QI)-derived endpoints have been developed to identify structure-function phenotypes, including air-blood-tissue volume partition, bronchovascular remodeling, emphysema, fibrosis, and textural patterns indicating architectural alteration. Coregistered landmarks on paired images obtained at different lung volumes are used to infer airway caliber, air trapping, gas and blood transport, compliance, and deformation. This document summarizes fundamental "good practice" stereological principles in QI study design and analysis; evaluates technical capabilities and limitations of common imaging modalities; and assesses major QI endpoints regarding underlying assumptions and limitations, ability to detect and stratify heterogeneous, overlapping pathophysiology, and monitor disease progression and therapeutic response, correlated with and complementary to, functional indices. The goal is to promote unbiased quantification and interpretation of in vivo imaging data, compare metrics obtained using different QI modalities to ensure accurate and reproducible metric derivation, and avoid misrepresentation of inferred physiological processes. The role of imaging-based computational modeling in advancing these goals is emphasized. Fundamental principles outlined herein are critical for all forms of QI irrespective of acquisition modality or disease entity.

Developing a Method to Estimate the Downstream Metabolite Signals from Hyperpolarized [1-13C]Pyruvate

Hyperpolarized carbon-13 MRI has the advantage of allowing the study of glycolytic flow in vivo or in vitro dynamically in real-time. The apparent exchange rate constant of a metabolite dynamic signal reflects the metabolite changes of a disease. Downstream metabolites can have a low signal-to-noise ratio (SNR), causing apparent exchange rate constant inconsistencies. Thus, we developed a method that estimates a more accurate metabolite signal. This method utilizes a kinetic model and background noise to estimate metabolite signals. Simulations and in vitro studies with photon-irradiated and control groups were used to evaluate the procedure. Simulated and in vitro exchange rate constants estimated using our method were compared with the raw signal values. In vitro data were also compared to the Area-Under-Curve (AUC) of the cell medium in ¹³C Nuclear Magnetic Resonance (NMR). In the simulations and in vitro experiments, our technique minimized metabolite signal fluctuations and maintained reliable apparent exchange rate constants. In addition, the apparent exchange rate constants of the metabolites showed differences between the irradiation and control groups after using our method. Comparing the in vitro results obtained using our method and NMR, both solutions showed consistency when uncertainty was considered, demonstrating that our method can accurately measure metabolite signals and show how glycolytic flow changes. The method enhanced the signals of the metabolites and clarified the metabolic phenotyping of tumor cells, which could benefit personalized health care and patient stratification in the future.

Noninvasive diagnosis of pulmonary hypertension with hyperpolarised 129Xe magnetic resonance imaging and spectroscopy

Background

The diagnosis of pulmonary hypertension (PH) remains challenging. Pre- and post-capillary PH have different signatures on noninvasive ¹²⁹Xe gas-exchange magnetic resonance imaging (MRI) and dynamic MR spectroscopy (MRS). We tested the accuracy of ¹²⁹Xe MRI/MRS to diagnose PH status compared to right heart catheterisation (RHC).

Methods

¹²⁹Xe MRI/MRS from 93 subjects was used to develop a diagnostic algorithm, which was tested in 32 patients undergoing RHC on the same day (n=20) or within 5 months (42±40 days) (n=12). Three expert readers, blinded to RHC, used ¹²⁹Xe MRI/MRS to classify subjects as pre-capillary PH, post-capillary PH, no PH and no interstitial lung disease (ILD), or ILD.

Results

For pre-capillary PH, ¹²⁹Xe MRI/MRS diagnostic accuracy was 75% (95% CI 66–84) with a sensitivity of 67% (95% CI 54–79) and a specificity of 86% (95% CI 75–96); for post-capillary PH accuracy was 69% (95% CI 59–78) with sensitivity of 54% (95% CI 34–74) and specificity of 74% (95% CI 63–84). The model performed well in straightforward cases of pre-capillary PH but was less accurate in its diagnosis in the presence of mixed disease, particularly in the presence of ILD or combined post- and pre-capillary PH.

Conclusion

This study demonstrates the potential to develop ¹²⁹Xe MRI/MRS into a modality with good accuracy in detecting pre- and post-capillary PH. Furthermore, the combination of ¹²⁹Xe dynamic MRS and gas-exchange MRI uniquely provide concurrent, noninvasive assessment of both haemodynamics and gas-exchange impairment that may aid in the detection of PH.

Diagnostic models using ¹²⁹Xe MRI/MRS metrics can noninvasively detect pre-capillary PH, post-capillary PH and ILD. The combination of ¹²⁹Xe MRI/MRS provides a comprehensive assessment of haemodynamics and gas-exchange impairment in individual patients.https://bit.ly/3tDJw5P

Novel Approaches to Imaging the Pulmonary Vasculature and Right Heart

There is an increased appreciation for the importance of the right heart and pulmonary circulation in several disease states across the spectrum of pulmonary hypertension and left heart failure. However, assessment of the structure and function of the right heart and pulmonary circulation can be challenging, due to the complex geometry of the right ventricle, comorbid pulmonary airways and parenchymal disease, and the overlap of hemodynamic abnormalities with left heart failure. Several new and evolving imaging modalities interrogate the right heart and pulmonary circulation with greater diagnostic precision. Echocardiographic approaches such as speckle-tracking and 3-dimensional imaging provide detailed assessments of regional systolic and diastolic function and volumetric assessments. Magnetic resonance approaches can provide high-resolution views of cardiac structure/function, tissue characterization, and perfusion through the pulmonary vasculature. Molecular imaging with positron emission tomography allows an assessment of specific pathobiologically relevant targets in the right heart and pulmonary circulation. Machine learning analysis of high-resolution computed tomographic lung scans permits quantitative morphometry of the lung circulation without intravenous contrast. Inhaled magnetic resonance imaging probes, such as hyperpolarized 129Xe magnetic resonance imaging, report on pulmonary gas exchange and pulmonary capillary hemodynamics. These approaches provide important information on right ventricular structure and function along with perfusion through the pulmonary circulation. At this time, the majority of these developing technologies have yet to be clinically validated, with few studies demonstrating the utility of these imaging biomarkers for diagnosis or monitoring disease. These technologies hold promise for earlier diagnosis and noninvasive monitoring of right heart failure and pulmonary hypertension that will aid in preclinical studies, enhance patient selection and provide surrogate end points in clinical trials, and ultimately improve bedside care.

Lung MRI with hyperpolarised gases: current & future clinical perspectives

The use of pulmonary MRI in a clinical setting has historically been limited. Whilst CT remains the gold-standard for structural lung imaging in many clinical indications, technical developments in ultrashort and zero echo time MRI techniques are beginning to help realise non-ionising structural imaging in certain lung disorders. In this invited review, we discuss a complementary technique - hyperpolarised (HP) gas MRI with inhaled ³He and ¹²⁹Xe - a method for functional and microstructural imaging of the lung that has great potential as a clinical tool for early detection and improved understanding of pathophysiology in many lung diseases. HP gas MRI now has the potential to make an impact on clinical management by enabling safe, sensitive monitoring of disease progression and response to therapy. With reference to the significant evidence base gathered over the last two decades, we review HP gas MRI studies in patients with a range of pulmonary disorders, including COPD/emphysema, asthma, cystic fibrosis, and interstitial lung disease. We provide several examples of our experience in Sheffield of using these techniques in a diagnostic clinical setting in challenging adult and paediatric lung diseases.

Enabling Clinical Technologies for Hyperpolarized 129 Xenon Magnetic Resonance Imaging and Spectroscopy

Hyperpolarization is a technique that can increase nuclear spin polarization with the corresponding gains in nuclear magnetic resonance (NMR) signals by 4-8 orders of magnitude. When this process is applied to biologically relevant samples, the hyperpolarized molecules can be used as exogenous magnetic resonance imaging (MRI) contrast agents. A technique called spin-exchange optical pumping (SEOP) can be applied to hyperpolarize noble gases such as 129 Xe. Techniques based on hyperpolarized 129 Xe are poised to revolutionize clinical lung imaging, offering a non-ionizing, high-contrast alternative to computed tomography (CT) imaging and conventional proton MRI. Moreover, CT and conventional proton MRI report on lung tissue structure but provide little functional information. On the other hand, when a subject breathes hyperpolarized 129 Xe gas, functional lung images reporting on lung ventilation, perfusion and diffusion with 3D readout can be obtained in seconds. In this Review, the physics of SEOP is discussed and the different production modalities are explained in the context of their clinical application. We also briefly compare SEOP to other hyperpolarization methods and conclude this paper with the outlook for biomedical applications of hyperpolarized 129 Xe to lung imaging and beyond.

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 129Xe MRI and Spectroscopy of Gas-Exchange Abnormalities in Nonspecific Interstitial Pneumonia

Background Recent studies demonstrate that antifibrotic drugs previously reserved for idiopathic pulmonary fibrosis (IPF) may slow progression in other interstitial lung diseases (ILDs), creating an urgent need for tools that can sensitively assess disease activity, progression, and therapy response across ILDs. Hyperpolarized xenon 129 (¹²⁹Xe) MRI and spectroscopy have provided noninvasive measurements of regional gas-exchange abnormalities in IPF. Purpose To assess gas exchange function using ¹²⁹Xe MRI in a group of study participants with nonspecific interstitial pneumonia (NSIP) compared with healthy control participants. Materials and Methods In this prospective study, participants with NSIP and healthy control participants were enrolled between November 2017 and February 2020 and underwent ¹²⁹Xe MRI and spectroscopy. Quantitative imaging provided three-dimensional maps of ventilation, interstitial barrier uptake, and transfer into the red blood cell (RBC) compartment. Spectroscopy provided parameters of the static RBC and barrier uptake compartments, as well as cardiogenic oscillations in RBC signal amplitude and chemical shift. Differences between NSIP and healthy control participants were assessed using the Wilcoxon rank-sum test. Results Thirty-six participants with NSIP (mean age, 57 years ± 11 [standard deviation]; 27 women) and 15 healthy control participants (mean age, 39 years ± 18; two women) were evaluated. Participants with NSIP had no difference in ventilation compared with healthy control participants (median, 4.4% [first quartile, 1.5%; third quartile, 8.7%] vs 6.0% [first quartile, 2.8%; third quartile, 6.9%]; P = .91), but they had a higher barrier uptake (median, 6.2% [first quartile, 1.8%; third quartile, 23.9%] vs 0.53% [first quartile, 0.33%; third quartile, 2.9%]; P = .003) and an increased RBC transfer defect (median, 20.6% [first quartile, 11.6%; third quartile, 27.8%] vs 2.8% [first quartile, 2.3%; third quartile, 4.9%]; P < .001). NSIP participants also had a reduced ratio of RBC-to-barrier peaks (median, 0.24 [first quartile, 0.19; third quartile, 0.31] vs 0.57 [first quartile, 0.52; third quartile, 0.67]; P < .001) and a reduced RBC chemical shift (median, 217.5 ppm [first quartile, 217.0 ppm; third quartile, 218.0 ppm] vs 218.2 ppm [first quartile, 217.9 ppm; third quartile, 218.6 ppm]; P = .001). Conclusion Participants with nonspecific interstitial pneumonia had increased barrier uptake and decreased red blood cell (RBC) transfer compared with healthy controls measured using xenon 129 gas-exchange MRI and reduced RBC-to-barrier ratio and RBC chemical shift measured using spectroscopy. © RSNA, 2021 Online supplemental material is available for this article. See also the editorial by Wild in this issue.

Regional Gas Exchange Measured by 129 Xe Magnetic Resonance Imaging Before and After Combination Bronchodilators Treatment in Chronic Obstructive Pulmonary Disease

Background

Hyperpolarized ¹²⁹Xe magnetic resonance imaging (MRI) provides a non‐invasive assessment of regional pulmonary gas exchange function. This technique has demonstrated that chronic obstructive pulmonary disease (COPD) patients exhibit ventilation defects, reduced interstitial barrier tissue uptake, and poor transfer to capillary red blood cells (RBCs). However, the behavior of these measurements following therapeutic intervention is unknown.

Purpose

To characterize changes in ¹²⁹Xe gas transfer function following administration of an inhaled long‐acting beta‐agonist/long‐acting muscarinic receptor antagonist (LABA/LAMA) bronchodilator.

Study Type

Prospective.

Population

Seventeen COPD subjects (GOLD II/III classification per Global Initiative for Chronic Obstructive Lung Disease criteria) were imaged before and after 2 weeks of LABA/LAMA therapy.

Field Strength/Sequences

Dedicated ventilation imaging used a multi‐slice 2D gradient echo sequence. Three‐dimensional images of ventilation, barrier uptake, and RBC transfer used an interleaved, radial, 1‐point Dixon sequence. Imaging was acquired at 3 T.

Assessment

¹²⁹Xe measurements were quantified before and after LABA/LAMA treatment by ventilation defect + low percent (ven_{def + low}) and by barrier uptake and RBC transfer relative to a healthy reference population (bar_%ref and RBC_%ref). Pulmonary function tests, including diffusing capacity of the lung for carbon monoxide (DL_CO), were also performed before and after treatment.

Statistical Tests

Paired t‐test, Pearson correlation coefficient (r).

Results

Baseline ven_{def + low} was 57.8 ± 8.4%, bar_%ref was 73.2 ± 19.6%, and RBC_%ref was 36.5 ± 13.6%. Following treatment, ven_{def + low} decreased to 52.5 ± 10.6% (P < 0.05), and improved in 14/17 (82.4%) of subjects. However, RBC_%ref decreased in 10/17 (58.8%) of subjects. Baseline measurements of bar_%ref and DL_CO were correlated with the degree of post‐treatment change in ven_{def + low} (r = −0.49, P < 0.05 and r = −0.52, P < 0.05, respectively).

Conclusion

LABA/LAMA therapy tended to preferentially improve ventilation in subjects whose ¹²⁹Xe barrier uptake and DL_CO were relatively preserved. However, newly ventilated regions often revealed RBC transfer defects, an aspect of lung function opaque to spirometry. These microvasculature abnormalities must be accounted for when assessing the effects of LABA/LAMA therapy.

Level of Evidence

1

Technical Efficacy Stage

4

Effect of inhaled oxygen concentration on 129 Xe chemical shift of red blood cells in rat lungs

PURPOSE

To investigate the dependence of dissolved ¹²⁹ Xe chemical shift on the fraction of inhaled oxygen, F_i O₂ , in the lungs of healthy rats.

METHODS

The chemical shifts of ¹²⁹ Xe dissolved in red blood cells, δ_RBC , and blood plasma and/or tissue, δ_Plasma , were measured using MRS in 12 Sprague Dawley rats mechanically ventilated at F_i O₂ values of 0.14, 0.19, and 0.22. Regional effects on the chemical shifts were controlled using a chemical shift saturation recovery sequence with a fixed delay time. MRS was also performed at an F_i CO₂ value of 0.085 to investigate the potential effect of the vascular response on δ_RBC and δ_Plasma .

RESULTS

δ_RBC increased with decreasing F_i O₂ (P = .0002), and δ_Plasma showed no dependence on F_i O₂ (P = .23). δ_RBC at F_i CO₂ = 0 (210.7 ppm ± 0.1) and at F_i CO₂ = 0.085 (210.6 ppm ± 0.2) were not significantly different (P = .67). δ_Plasma at F_i CO₂ = 0 (196.9 ppm ± 0.3) and at F_i CO₂ = 0.085 (197.0 ppm ± 0.1) were also not significantly different (P = .81).

CONCLUSION

Rat lung δ_RBC showed an inverse relationship to F_i O₂ , opposite to the relationship previously demonstrated for in vitro human blood. Rat lung δ_RBC did not depend on F_i CO₂ .

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.

Mapping cardiopulmonary dynamics within the microvasculature of the lungs using dissolved 129Xe MRI

Magnetic resonance (MR) imaging and spectroscopy using dissolved hyperpolarized (HP) 129Xe have expanded the ability to probe lung function regionally and noninvasively. In particular, HP 129Xe imaging has been used to quantify impaired gas uptake by the pulmonary tissues. Whole-lung spectroscopy has also been used to assess global cardiogenic oscillations in the MR signal intensity originating from 129Xe dissolved in the red blood cells of pulmonary capillaries. Herein, we show that the magnitude of these cardiogenic dynamics can be mapped three dimensionally using radial MRI, because dissolved 129Xe dynamics are encoded directly in the raw imaging data. Specifically, 1-point Dixon imaging is combined with postacquisition keyhole image reconstruction to assess regional blood volume fluctuations within the pulmonary microvasculature throughout the cardiac cycle. This "oscillation mapping" was applied in healthy subjects (mean amplitude 9% of total RBC signal) and patients with pulmonary arterial hypertension (PAH; mean 4%) and idiopathic pulmonary fibrosis (IPF; mean 14%). Whole-lung mean values from these oscillation maps correlated strongly with spectroscopy and clinical pulmonary function testing, but exhibited significant regional heterogeneity, including gravitationally dependent gradients in healthy subjects. Moreover, regional oscillations were found to be sensitive to disease state. Greater percentages of the lungs exhibit low-amplitude oscillations in PAH patients, and longitudinal imaging shows high-amplitude oscillations increase significantly over time (4-14 mo, P = 0.02) in IPF patients. This technique enables regional dynamics within the pulmonary capillary bed to be measured, and in doing so, provides insight into the origin and progression of pathophysiology within the lung microvasculature.NEW & NOTEWORTHY Spatially heterogeneous abnormalities within the lung microvasculature contribute to pathology in various cardiopulmonary diseases but are difficult to assess noninvasively. Hyperpolarized 129Xe MRI is a noninvasive method to probe lung function, including regional gas exchange between pulmonary air spaces and capillaries. We show that cardiogenic oscillations in the raw dissolved 129Xe MRI signal from pulmonary capillary red blood cells can be imaged using a postacquisition reconstruction technique, providing a new means of assessing regional lung microvasculature function and disease state.

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.

Transverse relaxation rates of pulmonary dissolved-phase Hyperpolarized 129 Xe as a biomarker of lung injury in idiopathic pulmonary fibrosis

PURPOSE

The MR properties (chemical shifts and R 2 ∗ decay rates) of dissolved-phase hyperpolarized (HP) ¹²⁹ Xe are confounded by the large magnetic field inhomogeneity present in the lung. This work improves measurements of these properties using a model-based image reconstruction to characterize the R 2 ∗ decay rates of dissolved-phase HP ¹²⁹ Xe in healthy subjects and patients with idiopathic pulmonary fibrosis (IPF).

METHODS

Whole-lung MRS and 3D radial MRI with four gradient echoes were performed after inhalation of HP ¹²⁹ Xe in healthy subjects and patients with IPF. A model-based image reconstruction formulated as a regularized optimization problem was solved iteratively to measure regional signal intensity in the gas, barrier, and red blood cell (RBC) compartments, while simultaneously measuring their chemical shifts and R 2 ∗ decay rates.

RESULTS

The estimation of spectral properties reduced artifacts in images of HP ¹²⁹ Xe in the gas, barrier, and RBC compartments and improved image SNR by over 20%. R 2 ∗ decay rates of the RBC and barrier compartments were lower in patients with IPF compared to healthy subjects (P < 0.001 and P = 0.005, respectively) and correlated to DL_CO (R = 0.71 and 0.64, respectively). Chemical shift of the RBC component measured with whole-lung spectroscopy was significantly different between IPF and normal subjects (P = 0.022).

CONCLUSION

Estimates for R 2 ∗ in both barrier and RBC dissolved-phase HP ¹²⁹ Xe compartments using a regional signal model improved image quality for dissolved-phase images and provided additional biomarkers of lung injury in IPF.

Diverse cardiopulmonary diseases are associated with distinct xenon magnetic resonance imaging signatures

BACKGROUND

As an increasing number of patients exhibit concomitant cardiac and pulmonary disease, limitations of standard diagnostic criteria are more frequently encountered. Here, we apply noninvasive ¹²⁹Xe magnetic resonance imaging (MRI) and spectroscopy to identify patterns of regional gas transfer impairment and haemodynamics that are uniquely associated with chronic obstructive pulmonary disease (COPD), idiopathic pulmonary fibrosis (IPF), left heart failure (LHF) and pulmonary arterial hypertension (PAH).

METHODS

Healthy volunteers (n=23) and patients with COPD (n=8), IPF (n=12), LHF (n=6) and PAH (n=10) underwent ¹²⁹Xe gas transfer imaging and dynamic spectroscopy. For each patient, three-dimensional maps were generated to depict ventilation, barrier uptake (¹²⁹Xe dissolved in interstitial tissue) and red blood cell (RBC) transfer (¹²⁹Xe dissolved in RBCs). Dynamic ¹²⁹Xe spectroscopy was used to quantify cardiogenic oscillations in the RBC signal amplitude and frequency shift.

RESULTS

Compared with healthy volunteers, all patient groups exhibited decreased ventilation and RBC transfer (both p≤0.01). Patients with COPD demonstrated more ventilation and barrier defects compared with all other groups (both p≤0.02). In contrast, IPF patients demonstrated elevated barrier uptake compared with all other groups (p≤0.007), and increased RBC amplitude and shift oscillations compared with healthy volunteers (p=0.007 and p≤0.01, respectively). Patients with COPD and PAH both exhibited decreased RBC amplitude oscillations (p=0.02 and p=0.005, respectively) compared with healthy volunteers. LHF was distinguishable from PAH by enhanced RBC amplitude oscillations (p=0.01).

CONCLUSION

COPD, IPF, LHF and PAH each exhibit unique ¹²⁹Xe MRI and dynamic spectroscopy signatures. These metrics may help with diagnostic challenges in cardiopulmonary disease and increase understanding of regional lung function and haemodynamics at the alveolar-capillary level.

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.