Assessment of lung function in asthma and COPD using hyperpolarized 129Xe chemical shift saturation recovery spectroscopy and dissolved-phase MRI

Compressed sensing reconstruction for high-SNR, rapid dissolved 129Xe gas exchange MRI

PURPOSE

Three-dimensional hyperpolarized 129Xe gas exchange imaging suffers from low SNR and long breath-holds, which could be improved using compressed sensing (CS). The purpose of this work was to assess whether gas exchange ratio maps are quantitatively preserved in CS-accelerated dissolved-phase 129Xe imaging and to investigate the feasibility of CS-dissolved 129Xe imaging with reduced-cost natural abundance (NA) xenon.

METHODS

129Xe gas exchange imaging was performed at 1.5 T with a multi-echo spectroscopic imaging sequence. A CS reconstruction with an acceleration factor of 2 was compared retrospectively with conventional gridding reconstruction in a cohort of 16 healthy volunteers, 5 chronic obstructive pulmonary disease patients, and 23 patients who were hospitalized following COVID-19 infection. Metrics of comparison included normalized mean absolute error, mean gas exchange ratio, and red blood cell (RBC) image SNR. Dissolved 129Xe CS imaging with NA xenon was assessed in 4 healthy volunteers.

RESULTS

CS reconstruction enabled acquisition time to be halved, and it reduced background noise. Median RBC SNR increased from 6 (2-18) to 11 (2-100) with CS, and there was strong agreement between CS and gridding mean ratio map values (R2 = 0.99). Image fidelity was maintained for gridding RBC SNR > 5, but below this, normalized mean absolute error increased nonlinearly with decreasing SNR. CS increased the mean SNR of NA 129Xe images 3-fold.

CONCLUSION

CS reconstruction of dissolved 129Xe imaging improved image quality with decreased scan time, while preserving key gas exchange metrics. This will benefit patients with breathlessness and/or low gas transfer and shows promise for NA-dissolved 129Xe imaging.

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.

Advances in COPD imaging using CT and MRI: linkage with lung physiology and clinical outcomes

Recent years have witnessed major advances in lung imaging in patients with COPD. These include significant refinements in images obtained by computed tomography (CT) scans together with the introduction of new techniques and software that aim for obtaining the best image whilst using the lowest possible radiation dose. Magnetic resonance imaging (MRI) has also emerged as a useful radiation-free tool in assessing structural and more importantly functional derangements in patients with well-established COPD and smokers without COPD, even before the existence of overt changes in resting physiological lung function tests. Together, CT and MRI now allow objective quantification and assessment of structural changes within the airways, lung parenchyma and pulmonary vessels. Furthermore, CT and MRI can now provide objective assessments of regional lung ventilation and perfusion, and multinuclear MRI provides further insight into gas exchange; this can help in structured decisions regarding treatment plans. These advances in chest imaging techniques have brought new insights into our understanding of disease pathophysiology and characterising different disease phenotypes. The present review discusses, in detail, the advances in lung imaging in patients with COPD and how structural and functional imaging are linked with common resting physiological tests and important clinical outcomes.

Pulmonary MRI with hyperpolarized xenon-129 demonstrates novel alterations in gas transfer across the air-blood barrier in asthma

BACKGROUND

Individuals with asthma can vary widely in clinical presentation, severity, and pathobiology. Hyperpolarized xenon-129 (Xe129) MRI is a novel imaging method to provide 3-D mapping of both ventilation and gas exchange in the human lung.

PURPOSE

To evaluate the functional changes in adults with asthma as compared to healthy controls using Xe129 MRI.

METHODS

All subjects (20 controls and 20 asthmatics) underwent lung function measurements and Xe129 MRI on the same day. Outcome measures included the pulmonary ventilation defect and transfer of inspired Xe129 into two soluble compartments: tissue and blood. Ten asthmatics underwent Xe129 MRI before and after bronchodilator to test whether gas transfer measures change with bronchodilator effects.

RESULTS

Initial analysis of the results revealed striking differences in gas transfer measures based on age, hence we compared outcomes in younger (n = 24, ≤ 35 years) versus older (n = 16, > 45 years) asthmatics and controls. The younger asthmatics exhibited significantly lower Xe129 gas uptake by lung tissue (Asthmatic: 0.98% ± 0.24%, Control: 1.17% ± 0.12%, P = 0.035), and higher Xe129 gas transfer from tissue to the blood (Asthmatic: 0.40 ± 0.10, Control: 0.31% ± 0.03%, P = 0.035) than the younger controls. No significant difference in Xe129 gas transfer was observed in the older group between asthmatics and controls (P > 0.05). No significant change in Xe129 transfer was observed before and after bronchodilator treatment.

CONCLUSIONS

By using Xe129 MRI, we discovered heterogeneous alterations of gas transfer that have associations with age. This finding suggests a heretofore unrecognized physiological derangement in the gas/tissue/blood interface in young adults with asthma that deserves further study.

Hyperpolarized Gas Imaging in Lung Diseases: Functional and Artificial Intelligence Perspective

Pathophysiologic changes in lung diseases are often accompanied by changes in ventilation and gas exchange. Comprehensive evaluation of lung function cannot be obtained through chest X-ray and computed tomography. Proton-based lung MRI is particularly challenging due to low proton density within the lung tissue. In this review, we discuss an emerging technology--hyperpolarized gas MRI with inhaled 129Xe, which provides functional and microstructural information and has the potential as a clinical tool for detecting the early stage and progression of certain lung diseases. We review the hyperpolarized 129Xe MRI studies in patients with a range of pulmonary diseases, including chronic obstructive pulmonary disease, asthma, cystic fibrosis, pulmonary hypertension, radiation-induced lung injury and interstitial lung disease, and the applications of artificial intelligence were reviewed as well.

Current advances in pulmonary functional imaging

Recent advances in imaging analysis have enabled evaluation of ventilation and perfusion in specific regions by chest computed tomography (CT) and magnetic resonance imaging (MRI), in addition to modalities including dynamic chest radiography, scintigraphy, positron emission tomography (PET), ultrasound, and electrical impedance tomography (EIT). In this review, an overview of current functional imaging techniques is provided for each modality. Advances in chest CT have allowed for the analysis of local volume changes and small airway disease in addition to emphysema, using the Jacobian determinant and parametric response mapping with inspiratory and expiratory images. Airway analysis can reveal characteristics of airway lesions in chronic obstructive pulmonary disease (COPD) and bronchial asthma, and the contribution of dysanapsis to obstructive diseases. Chest CT is also employed to measure pulmonary blood vessels, interstitial lung abnormalities, and mediastinal and chest wall components including skeletal muscle and bone. Dynamic CT can visualize lung deformation in respective portions. Pulmonary MRI has been developed for the estimation of lung ventilation and perfusion, mainly using hyperpolarized 129Xe. Oxygen-enhanced and proton-based MRI, without a polarizer, has potential clinical applications. Dynamic chest radiography is gaining traction in Japan for ventilation and perfusion analysis. Single photon emission CT can be used to assess ventilation-perfusion (V˙/Q˙) mismatch in pulmonary vascular diseases and COPD. PET/CT V˙/Q˙ imaging has also been demonstrated using "Galligas". Both ultrasound and EIT can detect pulmonary edema caused by acute respiratory distress syndrome. Familiarity with these functional imaging techniques will enable clinicians to utilize these systems in clinical practice.

Simultaneous quantification of hyperpolarized xenon-129 ventilation and gas exchange with multi-breath xenon-polarization transfer contrast (XTC) MRI

PURPOSE

To demonstrate the feasibility of a multi-breath xenon-polarization transfer contrast (XTC) MR imaging approach for simultaneously evaluating regional ventilation and gas exchange parameters.

METHODS

Imaging was performed in five healthy volunteers and six chronic obstructive pulmonary disease (COPD) patients. The multi-breath XTC protocol consisted of three repeated schemes of six wash-in breaths of a xenon mixture and four normoxic wash-out breaths, with and without selective saturation of either the tissue membrane or red blood cell (RBC) resonances. Acquisitions were performed at end-exhalation while subjects maintained tidal breathing throughout the session. The no-saturation, membrane-saturation, and RBC-saturation images were fit to a per-breath gas replacement model for extracting voxelwise tidal volume (TV), functional residual capacity (FRC), and fractional ventilation (FV), as well as tissue- and RBC-gas exchange (fMem and fRBC , respectively). The sensitivity of the derived model was also evaluated via simulations.

RESULTS

With the exception of FRC, whole-lung averages for all metrics were decreased in the COPD subjects compared to the healthy cohort, significantly so for FV, fRBC , and fMem . Heterogeneity was higher overall in the COPD subjects, particularly for fRBC , fMem , and fRBC:Mem . The anterior-to-posterior gradient associated with the gravity-dependence of lung function in supine imaging was also evident for FV, fRBC , and fMem values in the healthy subjects, but noticeably absent in the COPD cohort.

CONCLUSION

Multi-breath XTC imaging generated high-resolution, co-registered maps of ventilation and gas exchange parameters acquired during tidal breathing and with low per-breath xenon doses. Clear differences between healthy and COPD subjects were apparent and consistent with spirometry.

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.

Hyperpolarized Xenon-129: A New Tool to Assess Pulmonary Physiology in Patients with Pulmonary Fibrosis

PURPOSE

The existing tools to quantify lung function in interstitial lung diseases have significant limitations. Lung MRI imaging using inhaled hyperpolarized xenon-129 gas (129Xe) as a contrast agent is a new technology for measuring regional lung physiology. We sought to assess the utility of the 129Xe MRI in detecting impaired lung physiology in usual interstitial pneumonia (UIP).

MATERIALS AND METHODS

After institutional review board approval and informed consent and in compliance with HIPAA regulations, we performed chest CT, pulmonary function tests (PFTs), and 129Xe MRI in 10 UIP subjects and 10 healthy controls.

RESULTS

The 129Xe MRI detected highly heterogeneous abnormalities within individual UIP subjects as compared to controls. Subjects with UIP had markedly impaired ventilation (ventilation defect fraction: UIP: 30 ± 9%; healthy: 21 ± 9%; p = 0.026), a greater amount of 129Xe dissolved in the lung interstitium (tissue-to-gas ratio: UIP: 1.45 ± 0.35%; healthy: 1.10 ± 0.17%; p = 0.014), and impaired 129Xe diffusion into the blood (RBC-to-tissue ratio: UIP: 0.20 ± 0.06; healthy: 0.28 ± 0.05; p = 0.004). Most MRI variables had no correlation with the CT and PFT measurements. The elevated level of 129Xe dissolved in the lung interstitium, in particular, was detectable even in subjects with normal or mildly impaired PFTs, suggesting that this measurement may represent a new method for detecting early fibrosis.

CONCLUSION

The hyperpolarized 129Xe MRI was highly sensitive to regional functional changes in subjects with UIP and may represent a new tool for understanding the pathophysiology, monitoring the progression, and assessing the effectiveness of treatment in UIP.

Childhood to adulthood: Accounting for age dependence in healthy-reference distributions in 129 Xe gas-exchange MRI

PURPOSE

Xenon-129 (129 Xe) gas-exchange MRI is a pulmonary-imaging technique that provides quantitative metrics for lung structure and function and is often compared to pulmonary-function tests. Unlike such tests, it does not normalize to predictive values based on demographic variables such as age. Many sites have alluded to an age dependence in gas-exchange metrics; however, a procedure for normalizing metrics has not yet been introduced.

THEORY

We model healthy reference values for 129 Xe gas-exchange MRI against age using generalized additive models for location, scale, and shape (GAMLSS). GAMLSS takes signal data from an aggregated heathy-reference cohort and fits a distribution with flexible median, variation, skewness, and kurtosis to predict age-dependent centiles. This approach mirrors methods by the Global Lung Function Initiative for modeling pulmonary-function test data and applies it to binning methods widely used by the 129 Xe MRI community to interpret and quantify gas-exchange data.

METHODS

Ventilation, membrane-uptake, red blood cell transfer, and red blood cell:membrane gas-exchange metrics were collected on 30 healthy subjects over an age range of 5 to 68 years. A GAMLSS model was fit against age and compared against widely used linear and generalized-linear binning 129 Xe MRI analysis schemes.

RESULTS

All 4 gas-exchange metrics had significant skewness, and membrane-uptake had significant kurtosis compared to a normal distribution. Age has significant impact on distribution parameters. GAMLSS-binning produced narrower bins compared to the linear and generalized-linear binning schemes and distributed signal data closer to a normal distribution.

CONCLUSION

The proposed "proof-of-concept" GAMLSS-binning approach can improve diagnostic accuracy of 129 Xe gas-exchange MRI by providing a means of modeling voxel distribution data against age.

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.

MR-Nucleomics: The study of pathological cellular processes with multinuclear magnetic resonance spectroscopy and imaging in vivo

Clinical medicine has experienced a rapid development in recent decades, during which therapies targeting specific cellular signaling pathways, or specific cell surface receptors, have been increasingly adopted. While these developments in clinical medicine call for improved precision in diagnosis and treatment monitoring, modern medical imaging methods are restricted mainly to anatomical imaging, lagging behind the requirements of precision medicine. Although positron emission tomography and single photon emission computed tomography have been used clinically for studies of metabolism, their applications have been limited by the exposure risk to ionizing radiation, the subsequent limitation in repeated and longitudinal studies, and the incapability in assessing downstream metabolism. Magnetic resonance spectroscopy (MRS) or spectroscopic imaging (MRSI) are, in theory, capable of assessing molecular activities in vivo, although they are often limited by sensitivity. Here, we review some recent developments in MRS and MRSI of multiple nuclei that have potential as molecular imaging tools in the clinic.

Lung function measurements in preclinical research: What has been done and where is it headed?

Due to the close interaction of lung morphology and functions, repeatable measurements of pulmonary function during longitudinal studies on lung pathophysiology and treatment efficacy have been a great area of interest for lung researchers. Spirometry, as a simple and quick procedure that depends on the maximal inspiration of the patient, is the most common lung function test in clinics that measures lung volumes against time. Similarly, in the preclinical area, plethysmography techniques offer lung functional parameters related to lung volumes. In the past few decades, many innovative techniques have been introduced for in vivo lung function measurements, while each one of these techniques has their own advantages and disadvantages. Before each experiment, depending on the sensitivity of the required pulmonary functional parameters, it should be decided whether an invasive or non-invasive approach is desired. On one hand, invasive techniques offer sensitive and specific readouts related to lung mechanics in anesthetized and tracheotomized animals at endpoints. On the other hand, non-invasive techniques allow repeatable lung function measurements in conscious, free-breathing animals with readouts related to the lung volumes. The biggest disadvantage of these standard techniques for lung function measurements is considering the lung as a single unit and providing only global readouts. However, recent advances in lung imaging modalities such as x-ray computed tomography and magnetic resonance imaging opened new doors toward obtaining both anatomical and functional information from the same scan session, without the requirement for any extra pulmonary functional measurements, in more regional and non-invasive manners. Consequently, a new field of study called pulmonary functional imaging was born which focuses on introducing new techniques for regional quantification of lung function non-invasively using imaging-based techniques. This narrative review provides first an overview of both invasive and non-invasive conventional methods for lung function measurements, mostly focused on small animals for preclinical research, including discussions about their advantages and disadvantages. Then, we focus on those newly developed, non-invasive, imaging-based techniques that can provide either global or regional lung functional readouts at multiple time-points.

Gas exchange and ventilation imaging of healthy and COPD subjects using hyperpolarized xenon-129 MRI and a 3D alveolar gas-exchange model

OBJECTIVES

To investigate the utility of hyperpolarized xenon-129 (HPX) gas-exchange magnetic resonance imaging (MRI) and modeling in a chronic obstructive pulmonary disease (COPD) cohort in comparison to a minimal CT-diagnosed emphysema (MCTE) cohort and a healthy cohort.

METHODS

A total of 25 subjects were involved in this study including COPD (n = 8), MCTE (n = 3), and healthy (n = 14) subjects. The COPD subjects were scanned using HPX ventilation, gas-exchange MRI, and volumetric CT. The healthy subjects were scanned using the same HPX gas-exchange MRI protocol with 9 of them scanned twice, 3 weeks apart. The coefficient of variation (CV) was used to quantify image heterogeneities. A three-dimensional computational fluid dynamic (CFD) model of gas exchange was used to derive functional volumes of pulmonary tissue, capillaries, and veins.

RESULTS

The CVs of gas distributions in the images showed that there was a statistically significant difference between the COPD and healthy subjects (p < 0.0001). The functional volumes of pulmonary tissue, capillaries, and veins were significantly lower in the subjects with COPD than in the healthy subjects (p < 0.001). The functional volume of pulmonary tissue was found to be (i) statistically different between the healthy and MCTE groups (p = 0.02) and (ii) dependent on the age of the subjects in the healthy group (p = 0.0008) while their CVs (p = 0.13) were not.

CONCLUSION

The novel HPX gas-exchange MRI and CFD model distinguished the healthy cohort from the MCTE and COPD cohorts. The proposed technique also showed that the functional volume of pulmonary tissue decreases with aging in the healthy group.

KEY POINTS

• The ventilation and gas-exchange imaging with hyperpolarized xenon-129 MRI has enabled the identification of gas-exchange variation between COPD and healthy groups. • This novel technique was promising to be sensitive to minimal CT-diagnosed emphysema and age-related changes in gas-exchange parameter in a small pilot cohort.

Investigating Rubidium Density and Temperature Distributions in a High-Throughput 129Xe-Rb Spin-Exchange Optical Pumping Polarizer

Accurate knowledge of the rubidium (Rb) vapor density, [Rb], is necessary to correctly model the spin dynamics of 129Xe-Rb spin-exchange optical pumping (SEOP). Here we present a systematic evaluation of [Rb] within a high-throughput 129Xe-Rb hyperpolarizer during continuous-flow SEOP. Near-infrared (52S1/2→52P1/2 (D1)/52P3/2 (D2)) and violet (52S1/2→62P1/2/62P3/2) atomic absorption spectroscopy was used to measure [Rb] within 3.5 L cylindrical SEOP cells containing different spatial distributions and amounts of Rb metal. We were able to quantify deviation from the Beer-Lambert law at high optical depth for D2 and 62P3/2 absorption by comparison with measurements of the D1 and 62P1/2 absorption lines, respectively. D2 absorption deviates from the Beer-Lambert law at [Rb]D2>4×1017 m−3 whilst 52S1/2→62P3/2 absorption deviates from the Beer-Lambert law at [Rb]6P3/2>(4.16±0.01)×1019 m−3. The measured [Rb] was used to estimate a 129Xe-Rb spin exchange cross section of γ′=(1.2±0.1)×10−21 m3 s−1, consistent with spin-exchange cross sections from the literature. Significant [Rb] heterogeneity was observed in a SEOP cell containing 1 g of Rb localized at the back of the cell. While [Rb] homogeneity was improved for a greater surface area of the Rb source distribution in the cell, or by using a Rb presaturator, the measured [Rb] was consistently lower than that predicted by saturation Rb vapor density curves. Efforts to optimize [Rb] and thermal management within spin polarizer systems are necessary to maximize potential future enhancements of this technology.

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.

Imaging in Asthma Management

Asthma is a heterogeneous disease characterized by chronic airway inflammation that affects more than 300 million people worldwide. Clinically, asthma has a widely variable presentation and is defined based on a history of respiratory symptoms alongside airflow limitation. Imaging is not needed to confirm a diagnosis of asthma, and thus the use of imaging in asthma has historically been limited to excluding alternative diagnoses. However, significant advances continue to be made in novel imaging methodologies, which have been increasingly used to better understand respiratory impairment in asthma. As a disease primarily impacting the airways, asthma is best understood by imaging methods with the ability to elucidate airway impairment. Techniques such as computed tomography, magnetic resonance imaging with gaseous contrast agents, and positron emission tomography enable assessment of the small airways. Others, such as optical coherence tomography and endobronchial ultrasound enable high-resolution imaging of the large airways accessible to bronchoscopy. These imaging techniques are providing new insights in the pathophysiology and treatments of asthma and are poised to impact the clinical management of asthma.

Pulmonary functional MRI: Detecting the structure-function pathologies that drive asthma symptoms and quality of life

Pulmonary functional MRI (PfMRI) using inhaled hyperpolarized, radiation-free gases (such as ³ He and ¹²⁹ Xe) provides a way to directly visualize inhaled gas distribution and ventilation defects (or ventilation heterogeneity) in real time with high spatial (~mm³ ) resolution. Both gases enable quantitative measurement of terminal airway morphology, while ¹²⁹ Xe uniquely enables imaging the transfer of inhaled gas across the alveolar-capillary tissue barrier to the red blood cells. In patients with asthma, PfMRI abnormalities have been shown to reflect airway smooth muscle dysfunction, airway inflammation and remodelling, luminal occlusions and airway pruning. The method is rapid (8-15 s), cost-effective (~$300/scan) and very well tolerated in patients, even in those who are very young or very ill, because unlike computed tomography (CT), positron emission tomography and single-photon emission CT, there is no ionizing radiation and the examination takes only a few seconds. However, PfMRI is not without limitations, which include the requirement of complex image analysis, specialized equipment and additional training and quality control. We provide an overview of the three main applications of hyperpolarized noble gas MRI in asthma research including: (1) inhaled gas distribution or ventilation imaging, (2) alveolar microstructure and finally (3) gas transfer into the alveolar-capillary tissue space and from the tissue barrier into red blood cells in the pulmonary microvasculature. We highlight the evidence that supports a deeper understanding of the mechanisms of asthma worsening over time and the pathologies responsible for symptoms and disease control. We conclude with a summary of approaches that have the potential for integration into clinical workflows and that may be used to guide personalized treatment planning.

Childhood Interstitial Lung Disease: Imaging Guidelines and Recommendations

Childhood interstitial lung disease (ChILD) is an umbrella term encompassing a diverse group of diffuse lung diseases affecting infants and children. Although the timely and accurate diagnosis of ChILD is often challenging, it is optimally achieved through the multidisciplinary integration of imaging findings with clinical data, genetics, and potentially lung biopsy. This article reviews the definition and classification of ChILD; the role of imaging, pathology, and genetics in ChILD diagnosis; treatment options; and future goals. In addition, a practical approach to ChILD imaging based on the latest available research and the characteristic imaging appearance of ChILD entities are presented.

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.

Hyperpolarized 129Xe magnetic resonance spectroscopy in a rat model of bronchopulmonary dysplasia

Premature infants often require mechanical ventilation and oxygen therapy, which can result in bronchopulmonary dysplasia (BPD), characterized by developmental arrest and impaired lung function. Conventional clinical methods for assessing the prenatal lung are not adequate for the detection and assessment of long-term health risks in infants with BPD, highlighting the need for a noninvasive tool for the characterization of lung microstructure and function. Theoretical diffusion models, like the model of xenon exchange (MOXE), interrogate alveolar gas exchange by predicting the uptake of inert hyperpolarized (HP) ¹²⁹Xe gas measured with HP ¹²⁹Xe magnetic resonance spectroscopy (MRS). To investigate HP ¹²⁹Xe MRS as a tool for noninvasive characterization of pulmonary microstructural and functional changes in vivo, HP ¹²⁹Xe gas exchange data were acquired in an oxygen exposure rat model of BPD that recapitulates the fewer and larger distal airways and pulmonary vascular stunting characteristics of BPD. Gas exchange parameters from MOXE, including airspace mean chord length (L_m), apparent hematocrit in the pulmonary capillaries (HCT), and pulmonary capillary transit time (t_x), were compared with airspace mean axis length and area density (MAL and ρ_A) and percentage area of tissue and air (PTA and PAA) from histology. L_m was significantly larger in the exposed rats (P = 0.003) and correlated with MAL, ρ_A, PTA, and PAA (0.59<|ρ|<0.66 and P < 0.05). Observed increase in HCT (P = 0.012) and changes in t_x are also discussed. These findings support the use of HP ¹²⁹Xe MRS for detecting fewer, enlarged distal airways in this rat model of BPD, and potentially in humans.

Pulmonary Functional Imaging: Part 2-State-of-the-Art Clinical Applications and Opportunities for Improved Patient Care

Pulmonary functional imaging may be defined as the regional quantification of lung function by using primarily CT, MRI, and nuclear medicine techniques. The distribution of pulmonary physiologic parameters, including ventilation, perfusion, gas exchange, and biomechanics, can be noninvasively mapped and measured throughout the lungs. This information is not accessible by using conventional pulmonary function tests, which measure total lung function without viewing the regional distribution. The latter is important because of the heterogeneous distribution of virtually all lung disorders. Moreover, techniques such as hyperpolarized xenon 129 and helium 3 MRI can probe lung physiologic structure and microstructure at the level of the alveolar-air and alveolar-red blood cell interface, which is well beyond the spatial resolution of other clinical methods. The opportunities, challenges, and current stage of clinical deployment of pulmonary functional imaging are reviewed, including applications to chronic obstructive pulmonary disease, asthma, interstitial lung disease, pulmonary embolism, and pulmonary hypertension. Among the challenges to the deployment of pulmonary functional imaging in routine clinical practice are the need for further validation, establishment of normal values, standardization of imaging acquisition and analysis, and evidence of patient outcomes benefit. When these challenges are addressed, it is anticipated that pulmonary functional imaging will have an expanding role in the evaluation and management of patients with lung disease.

Using hyperpolarized 129Xe gas-exchange MRI to model the regional airspace, membrane, and capillary contributions to diffusing capacity

Hyperpolarized ¹²⁹Xe MRI has emerged as a novel means to evaluate pulmonary function via 3D mapping of ventilation, interstitial barrier uptake, and RBC transfer. However, the physiological interpretation of these measurements has yet to be firmly established. Here, we propose a model that uses the three components of ¹²⁹Xe gas-exchange MRI to estimate accessible alveolar volume (V_A), membrane conductance, and capillary blood volume contributions to DL_CO. ¹²⁹Xe ventilated volume (VV) was related to V_A by a scaling factor k_V = 1.47 with 95% confidence interval [1.42, 1.52], relative ¹²⁹Xe barrier uptake (normalized by the healthy reference value) was used to estimate the membrane-specific conductance coefficient k_B = 10.6 [8.6, 13.6] mL/min/mmHg/L, whereas normalized RBC transfer was used to calculate the capillary blood volume-specific conductance coefficient k_R = 13.6 [11.4, 16.7] mL/min/mmHg/L. In this way, the barrier and RBC transfer per unit volume determined the transfer coefficient K_CO, which was then multiplied by image-estimated V_A to obtain DL_CO. The model was built on a cohort of 41 healthy subjects and 101 patients with pulmonary disorders. The resulting ¹²⁹Xe-derived DL_CO correlated strongly (R² = 0.75, P < 0.001) with the measured values, a finding that was preserved within each individual disease cohort. The ability to use ¹²⁹Xe MRI measures of ventilation, barrier uptake, and RBC transfer to estimate each of the underlying constituents of DL_CO clarifies the interpretation of these images while enabling their use to monitor these aspects of gas exchange independently and regionally.NEW & NOTEWORTHY The diffusing capacity for carbon monoxide (DL_CO) is perhaps one of the most comprehensive physiological measures used in pulmonary medicine. Here, we spatially resolve and estimate its key components-accessible alveolar volume, membrane, and capillary blood volume conductances-using hyperpolarized ¹²⁹Xe MRI of ventilation, interstitial barrier uptake, and red blood cell transfer. This image-derived DL_CO correlates strongly with measured values in 142 subjects with a broad range of pulmonary disorders.

Automated Low-Cost In Situ IR and NMR Spectroscopy Characterization of Clinical-Scale 129Xe Spin-Exchange Optical Pumping

We present on the utility of in situ nuclear magnetic resonance (NMR) and near-infrared (NIR) spectroscopic techniques for automated advanced analysis of the ¹²⁹Xe hyperpolarization process during spin-exchange optical pumping (SEOP). The developed software protocol, written in the MATLAB programming language, facilitates detailed characterization of hyperpolarized contrast agent production efficiency based on determination of key performance indicators, including the maximum achievable ¹²⁹Xe polarization, steady-state Rb-¹²⁹Xe spin-exchange and ¹²⁹Xe polarization build-up rates, ¹²⁹Xe spin-relaxation rates, and estimates of steady-state Rb electron polarization. Mapping the dynamics of ¹²⁹Xe polarization and relaxation as a function of SEOP temperature enables systematic optimization of the batch-mode SEOP process. The automated analysis of a typical experimental data set, encompassing ∼300 raw NMR and NIR spectra combined across six different SEOP temperatures, can be performed in under 5 min on a laptop computer. The protocol is designed to be robust in operation on any batch-mode SEOP hyperpolarizer device. In particular, we demonstrate the implementation of a combination of low-cost NIR and low-frequency NMR spectrometers (∼$1,100 and ∼$300 respectively, ca. 2020) for use in the described protocols. The demonstrated methodology will aid in the characterization of NMR hyperpolarization hardware in the context of SEOP and other hyperpolarization techniques for more robust and less expensive clinical production of HP ¹²⁹Xe and other contrast agents.

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.

Novel imaging techniques for cystic fibrosis lung disease

With an increasing number of patients with cystic fibrosis (CF) receiving highly effective CFTR (cystic fibrosis transmembrane regulator protein) modulator therapy, particularly at a young age, there is an increasing need to identify imaging tools that can detect and regionally visualize mild CF lung disease and subtle changes in disease state. In this review, we discuss the latest developments in imaging modalities for both structural and functional imaging of the lung available to CF clinicians and researchers, from the widely available, clinically utilized imaging methods for assessing CF lung disease-chest radiography and computed tomography-to newer techniques poised to become the next phase of clinical tools-structural/functional proton and hyperpolarized gas magnetic resonance imaging (MRI). Finally, we provide a brief discussion of several newer lung imaging techniques that are currently available only in selected research settings, including chest tomosynthesis, and fluorinated gas MRI. We provide an update on the clinical and/or research status of each technique, with a focus on sensitivity, early disease detection, and possibilities for monitoring treatment efficacy.

Characterisation of gas exchange in COPD with dissolved-phase hyperpolarised xenon-129 MRI

To investigate whether hyperpolarised xenon-129 MRI (HXeMRI) enables regional and physiological resolution of diffusing capacity limitations in chronic obstructive pulmonary disease (COPD), we evaluated 34 COPD subjects and 11 healthy volunteers. We report significant correlations between airflow abnormality quantified by HXeMRI and per cent predicted forced expiratory volume in 1 s; HXeMRI gas transfer capacity to red blood cells and carbon monoxide diffusion capacity (%DLCO); and HXeMRI gas transfer capacity to interstitium and per cent emphysema quantified by multidetector chest CT. We further demonstrate the capability of HXeMRI to distinguish varying pathology underlying COPD in subjects with low %DLCO and minimal emphysema.

Damaged lung gas exchange function of discharged COVID-19 patients detected by hyperpolarized 129Xe MRI

The recovery process of COVID-19 patients is unclear. Some recovered patients complain of continued shortness of breath. Vasculopathy has been reported in COVID-19, stressing the importance of probing pulmonary microstructure and function at the alveolar-capillary interface. While computed tomography (CT) detects structural abnormalities, little is known about the impact of disease on lung function. 129Xe magnetic resonance imaging (MRI) is a technique uniquely capable of assessing ventilation, microstructure, and gas exchange. Using 129Xe MRI, we found that COVID-19 patients show a higher rate of ventilation defects (5.9% versus 3.7%), unchanged microstructure, and longer gas-blood exchange time (43.5 ms versus 32.5 ms) compared with healthy individuals. These findings suggest that regional ventilation and alveolar airspace dimensions are relatively normal around the time of discharge, while gas-blood exchange function is diminished. This study establishes the feasibility of localized lung function measurements in COVID-19 patients and their potential usefulness as a supplement to structural imaging.

Imaging Inflammation - From Whole Body Imaging to Cellular Resolution

Imaging techniques have evolved impressively lately, allowing whole new concepts like multimodal imaging, personal medicine, theranostic therapies, and molecular imaging to increase general awareness of possiblities of imaging to medicine field. Here, we have collected the selected (3D) imaging modalities and evaluated the recent findings on preclinical and clinical inflammation imaging. The focus has been on the feasibility of imaging to aid in inflammation precision medicine, and the key challenges and opportunities of the imaging modalities are presented. Some examples of the current usage in clinics/close to clinics have been brought out as an example. This review evaluates the future prospects of the imaging technologies for clinical applications in precision medicine from the pre-clinical development point of view.

Measuring pulmonary gas exchange using compartment-selective xenon-polarization transfer contrast (XTC) MRI

PURPOSE

To demonstrate the feasibility of generating red blood cell (RBC) and tissue/plasma (TP)-specific gas-phase (GP) depolarization maps using xenon-polarization transfer contrast (XTC) MR imaging.

METHODS

Imaging was performed in three healthy subjects, an asymptomatic smoker, and a chronic obstructive pulmonary disease (COPD) patient. Single-breath XTC data were acquired through a series of three GP images using a 2D multi-slice GRE during a 12 s breath-hold. A series of 8 ms Gaussian inversion pulses spaced 30 ms apart were applied in-between the images to quantify the exchange between the GP and dissolved-phase (DP) compartments. Inversion pulses were either centered on-resonance to generate contrast, or off-resonance to correct for other sources of signal loss. For an alternative scheme, inversions of both RBC and TP resonances were inserted in lieu of off-resonance pulses. Finally, this technique was extended to a multi-breath protocol consistent with tidal breathing, involving 30 consecutive acquisitions.

RESULTS

Inversion pulses shifted off-resonance by 20 ppm to mimic the distance between the RBC and TP resonances demonstrated selectivity, and initial GP depolarization maps illustrated stark magnitude and distribution differences between healthy and diseased subjects that were consistent with traditional approaches.

CONCLUSION

The proposed DP-compartment selective XTC MRI technique provides information on gas exchange between all three detectable states of xenon in the lungs and is sufficiently sensitive to indicate differences in lung function between the study subjects. Investigated extensions of this approach to imaging schemes that either minimize breath-hold duration or the overall number of breath-holds open avenues for future research to improve measurement accuracy and patient comfort.

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.

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.

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.

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.

Xenon-129 MRI detects ventilation deficits in paediatric stem cell transplant patients unable to perform spirometry

BACKGROUND

Early detection of pulmonary morbidity following haematopoietic stem cell transplantation (HSCT) remains an important challenge for intervention, primarily due to the insensitivity of spirometry to early change, and in paediatrics, patient compliance provides additional challenges. Regional lung ventilation abnormalities in paediatric HSCT patients were quantified using hyperpolarised xenon-129 (129Xe) magnetic resonance imaging (MRI) and compared to spirometry.

METHODS

Medically stable, paediatric allogeneic HSCT patients (n=23, ages 6-16 years) underwent an outpatient MRI scan where regional ventilation was quantified with a breath-hold of hyperpolarised 129Xe gas. Ventilation deficits, regions of the lung that ventilate poorly due to obstruction, were quantified as a ventilation defect percentage (VDP) and compared to forced expiratory volume in 1 s (FEV1), FEV1/forced vital capacity (FVC) ratio, and forced expiratory flow at 25-75% of FVC (FEF25-75%) from spirometry using linear regression.

RESULTS

The mean±sd 129Xe VDP was 10.5±9.4% (range 2.6-41.4%). 129Xe VDP correlated with FEV1, FEV1/FVC ratio and FEF25-75% (p≤0.02 for all comparisons). Ventilation deficits were detected in patients with normal spirometry (i.e. FEV1 >80%), supporting the sensitivity of 129Xe MRI to early obstruction reported in other pulmonary conditions. Seven (30%) patients could not perform spirometry, yet ventilation deficits were observed in five of these patients, detecting abnormalities that otherwise may have gone undetected and untreated until advanced.

CONCLUSION

Lung ventilation deficits were detected using hyperpolarised 129Xe gas MRI in asymptomatic paediatric HSCT patients and in a subgroup who were unable to perform reliable spirometry. 129Xe MRI provides a reliable imaging-based assessment of pulmonary involvement in this potentially difficult to diagnose paediatric population.

Hyperpolarised xenon magnetic resonance spectroscopy for the longitudinal assessment of changes in gas diffusion in IPF

Prognosticating idiopathic pulmonary fibrosis (IPF) is challenging, in part due to a lack of sensitive biomarkers. A recent article in Thorax described how hyperpolarised xenon magnetic resonance spectroscopy may quantify regional gas exchange in IPF lungs. In a population of patients with IPF, we find that the xenon signal from red blood cells diminishes relative to the tissue/plasma signal over a 12-month time period, even when the diffusion factor for carbon monoxide is static over the same time period. We conclude that hyperpolarised 129Xe MR spectroscopy may be sensitive to short-term changes in interstitial gas diffusion in IPF.

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.

Probing Changes in Lung Physiology in COPD Using CT, Perfusion MRI, and Hyperpolarized Xenon-129 MRI

RATIONALE AND OBJECTIVES

Chronic obstructive pulmonary disease (COPD) is highly heterogeneous and not well understood. Hyperpolarized xenon-129 (Xe129) magnetic resonance imaging (MRI) provides a unique way to assess important lung functions such as gas uptake. In this pilot study, we exploited multiple imaging modalities, including computed tomography (CT), gadolinium-enhanced perfusion MRI, and Xe129 MRI, to perform a detailed investigation of changes in lung morphology and functions in COPD. Utility and strengths of Xe129 MRI in assessing COPD were also evaluated against the other imaging modalities.

MATERIALS AND METHODS

Four COPD patients and four age-matched normal subjects participated in this study. Lung tissue density measured by CT, perfusion measures from gadolinium-enhanced MRI, and ventilation and gas uptake measures from Xe129 MRI were calculated for individual lung lobes to assess regional changes in lung morphology and function, and to investigate correlations among the different imaging modalities.

RESULTS

No significant differences were found for all measures among the five lobes in either the COPD or age-matched normal group. Strong correlations (R > 0.5 or < -0.5, p < 0.001) were found between ventilation and perfusion measures. Also gas uptake by blood as measured by Xe129 MRI showed strong correlations with CT tissue density and ventilation measures (R > 0.5 or < -0.5, p < 0.001) and moderate to strong correlations with perfusion measures (R > 0.4 or < -0.5, p < 0.01). Four distinctive patterns of functional abnormalities were found in patients with COPD.

CONCLUSION

Xe129 MRI has high potential to uniquely identify multiple changes in lung physiology in COPD using a single breath-hold acquisition.

Using Hyperpolarized Xenon-129 MRI to Quantify Early-Stage Lung Disease in Smokers

RATIONALE AND OBJECTIVES

Hyperpolarized xenon-129 magnetic resonance (MR) provides sensitive tools that may detect early stages of lung disease in smokers before it has progressed to chronic obstructive pulmonary disease (COPD) apparent to conventional spirometric measures. We hypothesized that the functional alveolar wall thickness as assessed by hyperpolarized xenon-129 MR spectroscopy would be elevated in clinically healthy smokers before xenon MR diffusion measurements would indicate emphysematous tissue destruction.

MATERIALS AND METHODS

Using hyperpolarized xenon-129 MR we measured the functional septal wall thickness and apparent diffusion coefficient of the gas phase in 16 subjects with smoking-related COPD, 9 clinically healthy current or former smokers, and 10 healthy never smokers. All subjects were age-matched and characterized by conventional pulmonary function tests. A total of 11 data sets from younger healthy never smokers were added to determine the age dependence of the septal wall thickness measurements.

RESULTS

In healthy never smokers the septal wall thickness increased by 0.04 μm per year of age. The healthy smoker cohort exhibited normal pulmonary function test measures that did not significantly differ from the never-smoker cohort. The age-corrected septal wall thickness correlated well with diffusion capacity for carbon monoxide (R² = 0.56) and showed a highly significant difference between healthy subjects and COPD patients (8.8 μm vs 12.3 μm; p < 0.001), but was the only measure that actually discriminated healthy subjects from healthy smokers (8.8 μm vs 10.6 μm; p < 0.006).

CONCLUSION

Functional alveolar wall thickness assessed by hyperpolarized xenon-129 MR allows discrimination between healthy subjects and healthy smokers and could become a powerful new measure of early-stage lung disease.

Structural and Functional Pulmonary Magnetic Resonance Imaging in Pediatrics-From the Neonate to the Young Adult

The clinical imaging modalities available to investigate pediatric pulmonary conditions such as bronchopulmonary dysplasia, cystic fibrosis, and asthma are limited primarily to chest x-ray radiograph and computed tomography. As the challenges that historically limited the application of magnetic resonance imaging (MRI) to the lung have been overcome, its clinical potential has greatly expanded. In this review article, recent advances in pulmonary MRI including ultrashort echo time and hyperpolarized-gas MRI techniques are discussed with an emphasis on pediatric research and translational applications.

Measurement of Regional 2D Gas Transport Efficiency in Rabbit Lung Using Hyperpolarized 129Xe MRI

While hyperpolarized xenon-129 (HXe) MRI offers a wide array of tools for assessing functional aspects of the lung, existing techniques provide only limited quantitative information about the impact of an observed pathology on overall lung function. By selectively destroying the alveolar HXe gas phase magnetization in a volume of interest and monitoring the subsequent decrease in the signal from xenon dissolved in the blood inside the left ventricle of the heart, it is possible to directly measure the contribution of that saturated lung volume to the gas transport capacity of the entire lung. In mechanically ventilated rabbits, we found that both xenon gas transport and transport efficiency exhibited a gravitation-induced anterior-to-posterior gradient that disappeared or reversed direction, respectively, when the animal was turned from supine to prone position. Further, posterior ventilation defects secondary to acute lung injury could be re-inflated by applying positive end expiratory pressure, although at the expense of decreased gas transport efficiency in the anterior volumes. These findings suggest that our technique might prove highly valuable for evaluating lung transplants and lung resections, and could improve our understanding of optimal mechanical ventilator settings in acute lung injury.