Human pulmonary imaging and spectroscopy with hyperpolarized 129Xe at 0.2T

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.

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.

Lung functional imaging

Pulmonary functional imaging modalities such as computed tomography, magnetic resonance imaging and nuclear imaging can quantitatively assess regional lung functional parameters and their distributions. These include ventilation, perfusion, gas exchange at the microvascular level and biomechanical properties, among other variables. This review describes the rationale, strengths and limitations of the various imaging modalities employed for lung functional imaging. It also aims to explain some of the most commonly measured parameters of regional lung function. A brief review of evidence on the role and utility of lung functional imaging in early diagnosis, accurate lung functional characterisation, disease phenotyping and advancing the understanding of disease mechanisms in major respiratory disorders is provided.

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.

Hyperpolarized 129Xe MRI at low field: Current status and future directions

Magnetic Resonance Imaging (MRI) is dictated by the magnetization of the sample, and is thus a low-sensitivity imaging method. Inhalation of hyperpolarized (HP) noble gases, such as helium-3 and xenon-129, is a non-invasive, radiation-risk free imaging technique permitting high resolution imaging of the lungs and pulmonary functions, such as the lung microstructure, diffusion, perfusion, gas exchange, and dynamic ventilation. Instead of increasing the magnetic field strength, the higher spin polarization achievable from this method results in significantly higher net MR signal independent of tissue/water concentration. Moreover, the significantly longer apparent transverse relaxation time T₂* of these HP gases at low magnetic field strengths results in fewer necessary radiofrequency (RF) pulses, permitting larger flip angles; this allows for high-sensitivity imaging of in vivo animal and human lungs at conventionally low (<0.5 T) field strengths and suggests that the low field regime is optimal for pulmonary MRI using hyperpolarized gases. In this review, theory on the common spin-exchange optical-pumping method of hyperpolarization and the field dependence of the MR signal of HP gases are presented, in the context of human lung imaging. The current state-of-the-art is explored, with emphasis on both MRI hardware (low field scanners, RF coils, and polarizers) and image acquisition techniques (pulse sequences) advancements. Common challenges surrounding imaging of HP gases and possible solutions are discussed, and the future of low field hyperpolarized gas MRI is posed as being a clinically-accessible and versatile imaging method, circumventing the siting restrictions of conventional high field scanners and bringing point-of-care pulmonary imaging to global facilities.

Preclinical MRI Using Hyperpolarized 129Xe

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

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.

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.

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.

Helium-rich mixtures for improved batch-mode clinical-scale spin-exchange optical pumping of Xenon-129

We present studies of spin-exchange optical pumping (SEOP) using ternary xenon-nitrogen-helium gas mixtures at high xenon partial pressures (up to 1330 Torr partial pressure at loading, out of 2660 Torr total pressure) in a 500-mL volume SEOP cell, using two automated batch-mode clinical-scale ¹²⁹Xe hyperpolarizers operating under continuous high-power (~170 W) pump laser irradiation. In this pilot study, we explore SEOP in gas mixtures with up to 45% ⁴He content under a wide range of experimental conditions. When an aluminum jacket cooling/heating design was employed (GEN-3 hyperpolarizer), ¹²⁹Xe polarization (%P_Xe) of 55.9 ± 0.9% was observed with mono-exponential build-up rate γ_SEOP of 0.049 ± 0.001 min^-1 for the ⁴He-rich mixture (1000 Torr Xe/900 Torr He, 100 Torr N₂), compared to %P_Xe of 49.3 ± 3.3% at γ_SEOP of 0.035 ± 0.004 min^-1 for the N₂-rich gas mixture (1000 Torr Xe/100 Torr He, 900 Torr N₂). When forced-air cooling/heating was used (GEN-2 hyperpolarizer), %P_Xe of 83.9 ± 2.7% was observed at γ_SEOP of 0.045 ± 0.005 min^-1 for the ⁴He-rich mixture (1000 Torr Xe/900 Torr He, 100 Torr N₂), compared to %P_Xe of 73.5 ± 1.3% at γ_SEOP of 0.028 ± 0.001 min^-1 for the N₂-rich gas mixture (1000 Torr Xe and 1000 Torr N₂). Additionally, %P_Xe of 72.6 ± 1.4% was observed at a build-up rate γ_SEOP of 0.041 ± 0.003 min^-1 for a super-high-density ⁴He-rich mixture (1330 Torr Xe/1200 Torr ⁴He/130 Torr N₂), compared to %P_Xe = 56.6 ± 1.3% at a build-up rate of γ_SEOP of 0.034 ± 0.002 min^-1 for an N₂-rich mixture (1330 Torr Xe/1330 Torr N₂) using forced air cooling/heating. The observed SEOP hyperpolarization performance under these conditions corresponds to %P_Xe improvement by a factor of 1.14 ± 0.04 at 1000 Torr Xe density and by up to a factor of 1.28 ± 0.04 at 1330 Torr Xe density at improved SEOP build-up rates by factors of 1.61 ± 0.18 and 1.21 ± 0.11 respectively. Record %P_Xe levels have been obtained here: 83.9 ± 2.7% at 1000 Torr Xe partial pressure and 72.6 ± 1.4% at 1330 Torr Xe partial pressure. In addition to improved thermal stability for SEOP, the use of ⁴He-rich gas mixtures also reduces the overall density of produced inhalable HP contrast agents; this property may be desirable for HP ¹²⁹Xe inhalation by human subjects in clinical settings-especially in populations with heavily impaired lung function. The described approach should enjoy ready application in the production of inhalable ¹²⁹Xe contrast agent with near-unity ¹²⁹Xe nuclear spin polarization.

Batch-Mode Clinical-Scale Optical Hyperpolarization of Xenon-129 Using an Aluminum Jacket with Rapid Temperature Ramping

We present spin-exchange optical pumping (SEOP) using a third-generation (GEN-3) automated batch-mode clinical-scale ¹²⁹Xe hyperpolarizer utilizing continuous high-power (∼170 W) pump laser irradiation and a novel aluminum jacket design for rapid temperature ramping of xenon-rich gas mixtures (up to 2 atm partial pressure). The aluminum jacket design is capable of heating SEOP cells from ambient temperature (typically 25 °C) to 70 °C (temperature of the SEOP process) in 4 min, and perform cooling of the cell to the temperature at which the hyperpolarized gas mixture can be released from the hyperpolarizer (with negligible amounts of Rb metal leaving the cell) in approximately 4 min, substantially faster (by a factor of 6) than previous hyperpolarizer designs relying on air heat exchange. These reductions in temperature cycling time will likely be highly advantageous for the overall increase of production rates of batch-mode (i.e., stopped-flow) ¹²⁹Xe hyperpolarizers, which is particularly beneficial for clinical applications. The additional advantage of the presented design is significantly improved thermal management of the SEOP cell. Accompanying the heating jacket design and performance, we also evaluate the repeatability of SEOP experiments conducted using this new architecture, and present typically achievable hyperpolarization levels exceeding 40% at exponential build-up rates on the order of 0.1 min^-1.

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.

Alveolar dynamics during mechanical ventilation in the healthy and injured lung

Mechanical ventilation is a life-saving therapy in patients with acute respiratory distress syndrome (ARDS). However, mechanical ventilation itself causes severe co-morbidities in that it can trigger ventilator-associated lung injury (VALI) in humans or ventilator-induced lung injury (VILI) in experimental animal models. Therefore, optimization of ventilation strategies is paramount for the effective therapy of critical care patients. A major problem in the stratification of critical care patients for personalized ventilation settings, but even more so for our overall understanding of VILI, lies in our limited insight into the effects of mechanical ventilation at the actual site of injury, i.e., the alveolar unit. Unfortunately, global lung mechanics provide for a poor surrogate of alveolar dynamics and methods for the in-depth analysis of alveolar dynamics on the level of individual alveoli are sparse and afflicted by important limitations. With alveolar dynamics in the intact lung remaining largely a "black box," our insight into the mechanisms of VALI and VILI and the effectiveness of optimized ventilation strategies is confined to indirect parameters and endpoints of lung injury and mortality.In the present review, we discuss emerging concepts of alveolar dynamics including alveolar expansion/contraction, stability/instability, and opening/collapse. Many of these concepts remain still controversial, in part due to limitations of the different methodologies applied. We therefore preface our review with an overview of existing technologies and approaches for the analysis of alveolar dynamics, highlighting their individual strengths and limitations which may provide for a better appreciation of the sometimes diverging findings and interpretations. Joint efforts combining key technologies in identical models to overcome the limitations inherent to individual methodologies are needed not only to provide conclusive insights into lung physiology and alveolar dynamics, but ultimately to guide critical care patient therapy.

Magnetic resonance imaging of interstitial lung diseases: A state-of-the-art review

Magnetic resonance imaging (MRI) has been emerging as an imaging modality to assess interstitial lung diseases (ILD). An optimal chest MRI protocol for ILDs should include non-contrast breath-holding sequences, steady-state free-precession sequences, and contrast-enhanced sequences. One of the main MRI applications in ILDs is the differentiation between areas of active inflammation (i.e. reversible stage) and fibrosis. Alveolitis presents high signal intensity on T2-weighted sequences (WS) and early-enhancement on contrast-enhanced MR sequences, while fibrotic-predominant lesions present low signal and late-enhancement in these sequences, respectively. MRI can be useful in connective tissue diseases, idiopathic pulmonary fibrosis, and sarcoidosis. The aim of this state-of-the-art review was to perform a state-of-the-art review on the use of MRI in ILDs, and propose the optimal MRI protocols for imaging ILDs.

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.

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

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

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.

Magnetic resonance imaging of the time course of hyperpolarized 129Xe gas exchange in the human lungs and heart

Purpose

To perform magnetic resonance imaging (MRI), human lung imaging, and quantification of the gas-transfer dynamics of hyperpolarized xenon-129 (HPX) from the alveoli into the blood plasma.

Materials and methods

HPX MRI with iterative decomposition of water and fat with echo asymmetry and least-square estimation (IDEAL) approach were used with multi-interleaved spiral k-space sampling to obtain HPX gas and dissolved phase images. IDEAL time-series images were then obtained from ten subjects including six normal subjects and four patients with pulmonary emphysema to test the feasibility of the proposed technique for capturing xenon-129 gas-transfer dynamics (XGTD). The dynamics of xenon gas diffusion over the entire lung was also investigated by measuring the signal intensity variations between three regions of interest, including the left and right lungs and the heart using Welch’s t test.

Results

The technique enabled the acquisition of HPX gas and dissolved phase compartment images in a single breath-hold interval of 8 s. The y-intersect of the XGTD curves were also found to be statistically lower in the patients with lung emphysema than in the healthy group (p < 0.05).

Conclusion

This time-series IDEAL technique enables the visualization and quantification of inhaled xenon from the alveoli to the left ventricle with a clinical gradient strength magnet during a single breath-hold, in healthy and diseased lungs.

Key Points

• The proposed hyperpolarized xenon-129 gas and dissolved magnetic resonance imaging technique can provide regional and temporal measurements of xenon-129 gas-transfer dynamics.

• Quantitative measurement of xenon-129 gas-transfer dynamics from the alveolar to the heart was demonstrated in normal subjects and pulmonary emphysema.

• Comparison of gas-transfer dynamics in normal subjects and pulmonary emphysema showed that the proposed technique appears sensitive to changes affecting the alveoli, pulmonary interstitium, and capillaries.

Electronic supplementary material

The online version of this article (10.1007/s00330-018-5853-9) contains supplementary material, which is available to authorized users.

Assessment of Pulmonary Gas Transport in Rabbits Using Hyperpolarized Xenon-129 Magnetic Resonance Imaging

Many forms of lung disease manifest themselves as pathological changes in the transport of gas to the circulatory system, yet the difficulty of imaging this process remains a central obstacle to the comprehensive diagnosis of lung disorders. Using hyperpolarized xenon-129 as a surrogate marker for oxygen, we derived the temporal dynamics of gas transport from the ratio of two lung images obtained with different timing parameters. Additionally, by monitoring changes in the total hyperpolarized xenon signal intensity in the left side of the heart induced by depletion of xenon signal in the alveolar airspaces of interest, we quantified the contributions of selected lung volumes to the total pulmonary gas transport. In a rabbit model, we found that it takes at least 200 ms for xenon gas to enter the lung tissue and travel the distance from the airspaces to the heart. Additionally, our method shows that both lungs contribute fairly equally to the gas transport in healthy rabbits, but that this ratio changes in a rabbit model of acid aspiration. These results suggest that hyperpolarized xenon-129 MRI may improve our ability to measure pulmonary gas transport and detect associated pathological changes.

Rapid assessment of pulmonary gas transport with hyperpolarized 129Xe MRI using a 3D radial double golden-means acquisition with variable flip angles

PURPOSE

To demonstrate the feasibility of using a 3D radial double golden-means acquisition with variable flip angles to monitor pulmonary gas transport in a single breath hold with hyperpolarized xenon-129 MRI.

METHODS

Hyperpolarized xenon-129 MRI scans with interleaved gas-phase and dissolved-phase excitations were performed using a 3D radial double golden-means acquisition in mechanically ventilated rabbits. The flip angle was either held fixed at 15 ° or 5 °, or it was varied linearly in ascending or descending order between 5 ° and 15 ° over a sampling interval of 1000 spokes. Dissolved-phase and gas-phase images were reconstructed at high resolution (32 × 32 × 32 matrix size) using all 1000 spokes, or at low resolution (22 × 22 × 22 matrix size) using 400 spokes at a time in a sliding-window fashion. Based on these sliding-window images, relative change maps were obtained using the highest mean flip angle as the reference, and aggregated pixel-based changes were tracked.

RESULTS

Although the signal intensities in the dissolve-phase maps were mostly constant in the fixed flip-angle acquisitions, they varied significantly as a function of average flip angle in the variable flip-angle acquisitions. The latter trend reflects the underlying changes in observed dissolve-phase magnetization distribution due to pulmonary gas uptake and transport.

CONCLUSION

3D radial double golden-means acquisitions with variable flip angles provide a robust means for rapidly assessing lung function during a single breath hold, thereby constituting a particularly valuable tool for imaging uncooperative or pediatric patient populations.

Using hyperpolarized 129Xe MRI to quantify regional gas transfer in idiopathic pulmonary fibrosis

BACKGROUND

Assessing functional impairment, therapeutic response and disease progression in patients with idiopathic pulmonary fibrosis (IPF) continues to be challenging. Hyperpolarized 129Xe MRI can address this gap through its unique capability to image gas transfer three-dimensionally from airspaces to interstitial barrier tissues to red blood cells (RBCs). This must be validated by testing the degree to which it correlates with pulmonary function tests (PFTs) and CT scores, and its spatial distribution reflects known physiology and patterns of disease.

METHODS

13 healthy individuals (33.6±15.7 years) and 12 patients with IPF (66.0±6.4 years) underwent 129Xe MRI to generate three-dimensional quantitative maps depicting the 129Xe ventilation distribution, its uptake in interstitial barrier tissues and its transfer to RBCs. For each map, mean values were correlated with PFTs and CT fibrosis scores, and their patterns were tested for the ability to depict functional gravitational gradients in healthy lung and to detect the known basal and peripheral predominance of disease in IPF.

RESULTS

129Xe MRI depicted functional impairment in patients with IPF, whose mean barrier uptake increased by 188% compared with the healthy reference population. 129Xe MRI metrics correlated poorly and insignificantly with CT fibrosis scores but strongly with PFTs. Barrier uptake and RBC transfer both correlated significantly with diffusing capacity of the lungs for carbon monoxide (r=-0.75, p<0.01 and r=0.72, p<0.01), while their ratio (RBC/barrier) correlated most strongly (r=0.94, p<0.01). RBC transfer exhibited significant anterior-posterior gravitational gradients in healthy volunteers, but not in IPF, where it was significantly impaired in the basal (p=0.02) and subpleural (p<0.01) lung.

CONCLUSIONS

Hyperpolarized129Xe MRI is a rapid and well-tolerated exam that provides region-specific quantification of interstitial barrier thickness and RBC transfer efficiency. With further development, it could become a robust tool for measuring disease progression and therapeutic response in patients with IPF, sensitively and non-invasively.

High-resolution hyperpolarized in vivo metabolic 13C spectroscopy at low magnetic field (48.7mT) following murine tail-vein injection

High-resolution ¹³C NMR spectroscopy of hyperpolarized succinate-1-¹³C-2,3-d₂ is reported in vitro and in vivo using a clinical-scale, biplanar (80cm-gap) 48.7mT permanent magnet with a high homogeneity magnetic field. Non-localized ¹³C NMR spectra were recorded at 0.52MHz resonance frequency over the torso of a tumor-bearing mouse every 2s. Hyperpolarized ¹³C NMR signals with linewidths of ∼3Hz (corresponding to ∼6ppm) were recorded in vitro (2mL in a syringe) and in vivo (over a mouse torso). Comparison of the full width at half maximum (FWHM) for ¹³C NMR spectra acquired at 48.7mT and at 4.7T in a small-animal MRI scanner demonstrates a factor of ∼12 improvement for the ¹³C resonance linewidth attainable at 48.7mT compared to that at 4.7T in vitro. ¹³C hyperpolarized succinate-1-¹³C resonance linewidths in vivo are at least one order of magnitude narrower at 48.7mT compared to those observed in high-field (≥3T) studies employing HP contrast agents. The demonstrated high-resolution ¹³C in vivo spectroscopy could be useful for high-sensitivity spectroscopic studies involving monitoring HP agent uptake or detecting metabolism using HP contrast agents with sufficiently large ¹³C chemical shift differences.

Hyperpolarized Gas Magnetic Resonance Lung Imaging in Children and Young Adults

The assessment of early pulmonary disease and its severity can be difficult in young children, as procedures such as spirometry cannot be performed on them. Computed tomography provides detailed structural images of the pulmonary parenchyma, but its major drawback is that the patient is exposed to ionizing radiation. In this context, magnetic resonance imaging (MRI) is a promising technique for the evaluation of pediatric lung disease, especially when serial imaging is needed. Traditionally, MRI played a small role in evaluating the pulmonary parenchyma. Because of its low proton density, the lungs display low signal intensity on conventional proton-based MRI. Hyperpolarized (HP) gases are inhaled contrast agents with an excellent safety profile and provide high signal within the lung, allowing for high temporal and spatial resolution imaging of the lung airspaces. Besides morphologic information, HP MR images also offer valuable information about pulmonary physiology. HP gas MRI has already made new contributions to the understanding of pediatric lung diseases and may become a clinically useful tool. In this article, we discuss the HP gas MRI technique, special considerations that need to be made when imaging children, and the role of MRI in 2 of the most common chronic pediatric lung diseases, asthma and cystic fibrosis. We also will discuss how HP gas MRI may be used to evaluate normal lung growth and development and the alterations occurring in chronic lung disease of prematurity and in patients with a congenital diaphragmatic hernia.

Magnetic resonance imaging with hyperpolarized agents: methods and applications

In the past decade, hyperpolarized (HP) contrast agents have been under active development for MRI applications to address the twin challenges of functional and quantitative imaging. Both HP helium (³He) and xenon (¹²⁹Xe) gases have reached the stage where they are under study in clinical research. HP ¹²⁹Xe, in particular, is poised for larger scale clinical research to investigate asthma, chronic obstructive pulmonary disease, and fibrotic lung diseases. With advances in polarizer technology and unique capabilities for imaging of ¹²⁹Xe gas exchange into lung tissue and blood, HP ¹²⁹Xe MRI is attracting new attention. In parallel, HP ¹³C and ¹⁵N MRI methods have steadily advanced in a wide range of pre-clinical research applications for imaging metabolism in various cancers and cardiac disease. The HP [1-¹³C] pyruvate MRI technique, in particular, has undergone phase I trials in prostate cancer and is poised for investigational new drug trials at multiple institutions in cancer and cardiac applications. This review treats the methodology behind both HP gases and HP ¹³C and ¹⁵N liquid state agents. Gas and liquid phase HP agents share similar technologies for achieving non-equilibrium polarization outside the field of the MRI scanner, strategies for image data acquisition, and translational challenges in moving from pre-clinical to clinical research. To cover the wide array of methods and applications, this review is organized by numerical section into (1) a brief introduction, (2) the physical and biological properties of the most common polarized agents with a brief summary of applications and methods of polarization, (3) methods for image acquisition and reconstruction specific to improving data acquisition efficiency for HP MRI, (4) the main physical properties that enable unique measures of physiology or metabolic pathways, followed by a more detailed review of the literature describing the use of HP agents to study: (5) metabolic pathways in cancer and cardiac disease and (6) lung function in both pre-clinical and clinical research studies, concluding with (7) some future directions and challenges, and (8) an overall summary.

129 Xe chemical shift in human blood and pulmonary blood oxygenation measurement in humans using hyperpolarized 129 Xe NMR

PURPOSE

To evaluate the dependency of the 129 Xe-red blood cell (RBC) chemical shift on blood oxygenation, and to use this relation for noninvasive measurement of pulmonary blood oxygenation in vivo with hyperpolarized 129 Xe NMR.

METHODS

Hyperpolarized 129 Xe was equilibrated with blood samples of varying oxygenation in vitro, and NMR was performed at 1.5 T and 3 T. Dynamic in vivo NMR during breath hold apnea was performed at 3 T on two healthy volunteers following inhalation of hyperpolarized 129 Xe.

RESULTS

The 129 Xe chemical shift in RBCs was found to increase nonlinearly with blood oxygenation at 1.5 T and 3 T. During breath hold apnea, the 129 Xe chemical shift in RBCs exhibited a periodic time modulation and showed a net decrease in chemical shift of ∼1 ppm over a 35 s breath hold, corresponding to a decrease of 7-10 % in RBC oxygenation. The 129 Xe-RBC signal amplitude showed a modulation with the same frequency as the 129 Xe-RBC chemical shift.

CONCLUSION

The feasibility of using the 129 Xe-RBC chemical shift to measure pulmonary blood oxygenation in vivo has been demonstrated. Correlation between 129 Xe-RBC signal and 129 Xe-RBC chemical shift modulations in the lung warrants further investigation, with the aim to better quantify temporal blood oxygenation changes in the cardiopulmonary vascular circuit. Magn Reson Med 77:1399-1408, 2017. © 2016 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

NMR Spin-Lock Induced Crossing (SLIC) dispersion and long-lived spin states of gaseous propane at low magnetic field (0.05T)

When parahydrogen reacts with propylene in low magnetic fields (e.g., 0.05T), the reaction product propane develops an overpopulation of pseudo-singlet nuclear spin states. We studied how the Spin-Lock Induced Crossing (SLIC) technique can be used to convert these pseudo-singlet spin states of hyperpolarized gaseous propane into observable magnetization and to detect ¹H NMR signal directly at 0.05T. The theoretical simulation and experimental study of the NMR signal dependence on B₁ power (SLIC amplitude) exhibits a well-resolved dispersion, which is induced by the spin-spin couplings in the eight-proton spin system of propane. We also measured the exponential decay time constants (T_LLSS or T_S) of these pseudo-singlet long-lived spin states (LLSS) by varying the time between hyperpolarized propane production and SLIC detection. We have found that, on average, T_S is approximately 3 times longer than the corresponding T₁ value under the same conditions in the range of pressures studied (up to 7.6atm). Moreover, T_S may exceed 13s at pressures above 7atm in the gas phase. These results are in agreement with the previous reports, and they corroborate a great potential of long-lived hyperpolarized propane as an inhalable gaseous contrast agent for lung imaging and as a molecular tracer to study porous media using low-field NMR and MRI.

Quantification of regional early stage gas exchange changes using hyperpolarized (129)Xe MRI in a rat model of radiation-induced lung injury

PURPOSE

To assess the feasibility of hyperpolarized (HP) (129)Xe MRI for detection of early stage radiation-induced lung injury (RILI) in a rat model involving unilateral irradiation by assessing differences in gas exchange dynamics between irradiated and unirradiated lungs.

METHODS

The dynamics of gas exchange between alveolar air space and pulmonary tissue (PT), PT and red blood cells (RBCs) was measured using single-shot spiral iterative decomposition of water and fat with echo asymmetry and least-squares estimation images of the right and left lungs of two age-matched cohorts of Sprague Dawley rats. The first cohort (n = 5) received 18 Gy irradiation to the right lung using a (60)Co source and the second cohort (n = 5) was not irradiated and served as the healthy control. Both groups were imaged two weeks following irradiation when radiation pneumonitis (RP) was expected to be present. The gas exchange data were fit to a theoretical gas exchange model to extract measurements of pulmonary tissue thickness (LPT) and relative blood volume (VRBC) from each of the right and left lungs of both cohorts. Following imaging, lung specimens were retrieved and percent tissue area (PTA) was assessed histologically to confirm RP and correlate with MRI measurements.

RESULTS

Statistically significant differences in LPT and VRBC were observed between the irradiated and non-irradiated cohorts. In particular, LPT of the right and left lungs was increased approximately 8.2% and 5.0% respectively in the irradiated cohort. Additionally, VRBC of the right and left lungs was decreased approximately 36.1% and 11.7% respectively for the irradiated cohort compared to the non-irradiated cohort. PTA measurements in both right and left lungs were increased in the irradiated group compared to the non-irradiated cohort for both the left (P < 0.05) and right lungs (P < 0.01) confirming the presence of RP. PTA measurements also correlated with the MRI measurements for both the non-irradiated (r = 0.79, P < 0.01) and irradiated groups (r = 0.91, P < 0.01).

CONCLUSIONS

Regional RILI can be detected two weeks post-irradiation using HP (129)Xe MRI and analysis of gas exchange curves. This approach correlates well with histology and can potentially be used clinically to assess radiation pneumonitis associated with early RILI to improve radiation therapy outcomes.

Uncovering a third dissolved-phase 129 Xe resonance in the human lung: Quantifying spectroscopic features in healthy subjects and patients with idiopathic pulmonary fibrosis

PURPOSE

The purpose of this work was to accurately characterize the spectral properties of hyperpolarized ¹²⁹ Xe in patients with idiopathic pulmonary fibrosis (IPF) compared to healthy volunteers.

METHODS

Subjects underwent hyperpolarized ¹²⁹ Xe breath-hold spectroscopy, during which 38 dissolved-phase free induction decays (FIDs) were acquired after reaching steady state (echo time/repetition time = 0.875/50 ms; bandwidth = 8.06 kHz; flip angle≈22 °). FIDs were averaged and then decomposed into multiple spectral components using time-domain curve fitting. The resulting amplitudes, frequencies, line widths, and starting phases of each component were compared among groups using a Mann-Whitney-Wilcoxon U test.

RESULTS

Three dissolved-phase resonances, consisting of red blood cells (RBCs) and two barrier compartments, were consistently identified in all subjects. In subjects with IPF relative to healthy volunteers, the RBC frequency was 0.70 parts per million (ppm) more negative (P = 0.05), the chemical shift of barrier 2 was 0.6 ppm more negative (P = 0.009), the line widths of both barrier peaks were ∼2 ppm narrower (P < 0.001), and the starting phase of barrier 1 was 20.3 ° higher (P =  0.01). Moreover, the ratio RBC:barriers was reduced by 52.9% in IPF (P < 0.001).

CONCLUSIONS

The accurate decomposition of ¹²⁹ Xe spectra not only has merit for developing a global metric of pulmonary function, but also provides necessary insights to optimize phase-sensitive methods for imaging ¹²⁹ Xe gas transfer. Magn Reson Med 78:1306-1315, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Signal-to-noise ratio, T2 , and T2* for hyperpolarized helium-3 MRI of the human lung at three magnetic field strengths

PURPOSE

To evaluate T₂ , T2*, and signal-to-noise ratio (SNR) for hyperpolarized helium-3 (³ He) MRI of the human lung at three magnetic field strengths ranging from 0.43T to 1.5T.

METHODS

Sixteen healthy volunteers were imaged using a commercial whole body scanner at 0.43T, 0.79T, and 1.5T. Whole-lung T₂ values were calculated from a Carr-Purcell-Meiboom-Gill spin-echo-train acquisition. T2* maps and SNR were determined from dual-echo and single-echo gradient-echo images, respectively. Mean whole-lung SNR values were normalized by ventilated lung volume and administered ³ He dose.

RESULTS

As expected, T₂ and T2* values demonstrated a significant inverse relationship to field strength. Hyperpolarized ³ He images acquired at all three field strengths had comparable SNR values and thus appeared visually very similar. Nonetheless, the relatively small SNR differences among field strengths were statistically significant.

CONCLUSIONS

Hyperpolarized ³ He images of the human lung with similar image quality were obtained at three field strengths ranging from 0.43T and 1.5T. The decrease in susceptibility effects at lower fields that are reflected in longer T₂ and T2* values may be advantageous for optimizing pulse sequences inherently sensitive to such effects. The three-fold increase in T2* at lower field strength would allow lower receiver bandwidths, providing a concomitant decrease in noise and relative increase in SNR. Magn Reson Med 78:1458-1463, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Finite element modeling of (129)Xe diffusive gas exchange NMR in the human alveoli

Existing models of (129)Xe diffusive exchange for lung microstructural modeling with time-resolved MR spectroscopy data have considered analytical solutions to one-dimensional, homogeneous models of the lungs with specific assumptions about the alveolar geometry. In order to establish a model system for simulating the effects of physiologically-realistic changes in physical and microstructural parameters on (129)Xe exchange NMR, we have developed a 3D alveolar capillary model for finite element analysis. To account for the heterogeneity of the alveolar geometry across the lungs, we have derived realistic geometries for finite element analysis based on 2D histological samples and 3D micro-CT image volumes obtained from ex vivo biopsies of lung tissue from normal subjects and patients with interstitial lung disease. The 3D alveolar capillary model permits investigation of the impact of alveolar geometrical parameters and diffusion and perfusion coefficients on the in vivo measured (129)Xe CSSR signal response. The heterogeneity of alveolar microstructure that is accounted for in image-based models resulted in considerable alterations to the shape of the (129)Xe diffusive uptake curve when compared to 1D models. Our findings have important implications for the future design and optimization of (129)Xe MR experiments and in the interpretation of lung microstructural changes from this data.

Advances in Imaging and Automated Quantification of Pulmonary Diseases in Non-neoplastic Diseases

Histological examination has always been the gold standard for the detection and quantification of lung remodeling. However, this method has some limitations regarding the invasiveness of tissue acquisition. Quantitative imaging methods enable the acquisition of valuable information on lung structure and function without the removal of tissue from the body; thus, they are useful for disease identification and follow-up. This article reviews the various quantitative imaging modalities used currently for the non-invasive study of chronic obstructive pulmonary disease, asthma, and interstitial lung diseases. Some promising computer-aided diagnosis methods are also described.

MR Imaging Biomarkers in Oncology Clinical Trials

The authors discuss eight areas of quantitative MR imaging that are currently used (RECIST, DCE-MR imaging, DSC-MR imaging, diffusion MR imaging) in clinical trials or emerging (CEST, elastography, hyperpolarized MR imaging, multiparameter MR imaging) as promising techniques in diagnosing cancer and assessing or predicting response of cancer to therapy. Illustrative applications of the techniques in the clinical setting are summarized before describing the current limitations of the methods.

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

PURPOSE

To evaluate the reproducibility of indices of lung microstructure and function derived from ¹²⁹ Xe chemical shift saturation recovery (CSSR) spectroscopy in healthy volunteers and patients with chronic obstructive pulmonary disease (COPD), and to study the sensitivity of CSSR-derived parameters to pulse sequence design and lung inflation level.

METHODS

Preliminary data were collected from five volunteers on three occasions, using two implementations of the CSSR sequence. Separately, three volunteers each underwent CSSR at three different lung inflation levels. After analysis of these preliminary data, five COPD patients were scanned on three separate days, and nine age-matched volunteers were scanned three times on one day, to assess reproducibility.

RESULTS

CSSR-derived alveolar septal thickness (ST) and surface-area-to-volume (S/V) ratio values decreased with lung inflation level (P < 0.001; P = 0.057, respectively). Intra-subject standard deviations of ST were lower than the previously measured differences between volunteers and subjects with interstitial lung disease. The mean coefficient of variation (CV) values of ST were 3.9 ± 1.9% and 6.0 ± 4.5% in volunteers and COPD patients, respectively, similar to CV values for whole-lung carbon monoxide diffusing capacity. The mean CV of S/V in volunteers and patients was 14.1 ± 8.0% and 18.0 ± 19.3%, respectively.

CONCLUSION

¹²⁹ Xe CSSR presents a reproducible method for estimation of alveolar septal thickness. Magn Reson Med 77:2107-2113, 2017. © 2016 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Hyperpolarized 83Kr magnetic resonance imaging of alveolar degradation in a rat model of emphysema

Hyperpolarized ⁸³Kr surface quadrupolar relaxation (SQUARE) generates MRI contrast that was previously shown to correlate with surface-to-volume ratios in porous model surface systems. The underlying physics of SQUARE contrast is conceptually different from any other current MRI methodology as the method uses the nuclear electric properties of the spin I = 9/2 isotope ⁸³Kr. To explore the usage of this non-radioactive isotope for pulmonary pathophysiology, MRI SQUARE contrast was acquired in excised rat lungs obtained from an elastase-induced model of emphysema. A significant ⁸³Kr T₁ relaxation time increase in the SQUARE contrast was found in the elastase-treated lungs compared with the baseline data from control lungs. The SQUARE contrast suggests a reduction in pulmonary surface-to-volume ratio in the emphysema model that was validated by histology. The finding supports usage of ⁸³Kr SQUARE as a new biomarker for surface-to-volume ratio changes in emphysema.

Nondisruptive Dissolution of Hyperpolarized (129)Xe into Viscous Aqueous and Organic Liquid Crystalline Environments

Studies of hyperpolarized xenon-129 (hp-(129)Xe) in media such as liquid crystals and cell suspensions are in demand for applications ranging from biomedical imaging to materials engineering but have been hindered by the inability to bubble Xe through the desired media as a result of viscosity or perturbations caused by bubbles. Herein a device is reported that can be reliably used to dissolve hp-(129)Xe into viscous aqueous and organic samples without bubbling. This method is robust, requires small sample volumes (<60 μL), is compatible with existing NMR hardware, and is made from readily available materials. Experiments show that Xe can be introduced into viscous and aligned media without disrupting molecular order. We detected dissolved xenon in an aqueous liquid crystal that is disrupted by the shear forces of bubbling, and we observed liquid-crystal phase transitions in (MBBA). This tool allows an entirely new class of samples to be investigated by hyperpolarized-gas NMR spectroscopy.

Functional imaging of the lungs with gas agents

This review focuses on the state-of-the-art of the three major classes of gas contrast agents used in magnetic resonance imaging (MRI)-hyperpolarized (HP) gas, molecular oxygen, and fluorinated gas--and their application to clinical pulmonary research. During the past several years there has been accelerated development of pulmonary MRI. This has been driven in part by concerns regarding ionizing radiation using multidetector computed tomography (CT). However, MRI also offers capabilities for fast multispectral and functional imaging using gas agents that are not technically feasible with CT. Recent improvements in gradient performance and radial acquisition methods using ultrashort echo time (UTE) have contributed to advances in these functional pulmonary MRI techniques. The relative strengths and weaknesses of the main functional imaging methods and gas agents are compared and applications to measures of ventilation, diffusion, and gas exchange are presented. Functional lung MRI methods using these gas agents are improving our understanding of a wide range of chronic lung diseases, including chronic obstructive pulmonary disease, asthma, and cystic fibrosis in both adults and children.

Metallic nanoparticles as synthetic building blocks for cancer diagnostics: from materials design to molecular imaging applications

Metallic nanoparticles have been a matter of intense exploration within the last decade due to their potential to change the face of the medical world through their role as 'nanotheranostics'. Their envisaged capacity to act as synthetic platforms for a multimodal imaging approach to diagnosis and treatment of degenerative diseases, including cancer, remains a matter of lively debate. Certain synthetic metal-based nanomaterials, e.g. gold and iron oxide nanoparticles, are already in clinical use or under advanced preclinical investigations following in vitro and in vivo preclinical imaging success. We surveyed the recent publications landscape including those reported metallic nanoparticles having established applications in vivo, as well as some of the new metallic nanoparticles which, despite their potential as cancer nanodiagnostics, are currently awaiting in vivo evaluation. The objective of this review is to highlight the current metallic nanoparticles and/or alloys as well as their derivatives with multimodal imaging potential, focusing on their chemistry as a springboard to discussing their role in the future of nanomedicines design. We also highlight here some of the fundamentals of molecular and nano-imaging techniques of relevance to the metal-based colloids, alloys and metallic nanoparticles discerning their future prospects as cancer nanodiagnostics. The current approaches for metallic and alloy surface derivatisation, aiming to achieve functional and biocompatible materials for multimodal bioimaging applications, are discussed in order to bring about some new perspectives on the efficiency of metallic nanoparticles as synthetic scaffolds for imaging probe design and forecast their future use in medical imaging techniques (optical, CT, PET, SPECT and MRI).

Dose and pulse sequence considerations for hyperpolarized (129)Xe ventilation MRI

PURPOSE

The aim of this study was to evaluate the effect of hyperpolarized (129)Xe dose on image signal-to-noise ratio (SNR) and ventilation defect conspicuity on both multi-slice gradient echo and isotropic 3D-radially acquired ventilation MRI.

MATERIALS AND METHODS

Ten non-smoking older subjects (ages 60.8±7.9years) underwent hyperpolarized (HP) (129)Xe ventilation MRI using both GRE and 3D-radial acquisitions, each tested using a 71ml (high) and 24ml (low) dose equivalent (DE) of fully polarized, fully enriched (129)Xe. For all images SNR and ventilation defect percentage (VDP) were calculated.

RESULTS

Normalized SNR (SNRn), obtained by dividing SNR by voxel volume and dose was higher for high-DE GRE acquisitions (SNRn=1.9±0.8ml(-2)) than low-DE GRE scans (SNRn=0.8±0.2ml(-2)). Radially acquired images exhibited a more consistent, albeit lower SNRn (High-DE: SNRn=0.5±0.1ml(-2), low-DE: SNRn=0.5±0.2ml(-2)). VDP was indistinguishable across all scans.

CONCLUSIONS

These results suggest that images acquired using the high-DE GRE sequence provided the highest SNRn, which was in agreement with previous reports in the literature. 3D-radial images had lower SNRn, but have advantages for visual display, monitoring magnetization dynamics, and visualizing physiological gradients. By evaluating normalized SNR in the context of dose-equivalent formalism, it should be possible to predict (129)Xe dose requirements and quantify the benefits of more efficient transmit/receive coils, field strengths, and pulse sequences.

Functional lung avoidance for individualized radiotherapy (FLAIR): study protocol for a randomized, double-blind clinical trial

Background

Although radiotherapy is a key component of curative-intent treatment for locally advanced, unresectable non-small cell lung cancer (NSCLC), it can be associated with substantial pulmonary toxicity in some patients. Current radiotherapy planning techniques aim to minimize the radiation dose to the lungs, without accounting for regional variations in lung function. Many patients, particularly smokers, can have substantial regional differences in pulmonary ventilation patterns, and it has been hypothesized that preferential avoidance of functional lung during radiotherapy may reduce toxicity. Although several investigators have shown that functional lung can be identified using advanced imaging techniques and/or demonstrated the feasibility and theoretical advantages of avoiding functional lung during radiotherapy, to our knowledge this premise has never been tested via a prospective randomized clinical trial.

Methods/Design

Eligible patients will have Stage III NSCLC with intent to receive concurrent chemoradiotherapy (CRT). Every patient will undergo a pre-treatment functional lung imaging study using hyperpolarized ³He MRI in order to identify the spatial distribution of normally-ventilated lung. Before randomization, two clinically-approved radiotherapy plans will be devised for all patients on trial, termed standard and avoidance. The standard plan will be designed without reference to the functional state of the lung, while the avoidance plan will be optimized such that dose to functional lung is as low as reasonably achievable. Patients will then be randomized in a 1:1 ratio to receive either the standard or the avoidance plan, with both the physician and the patient blinded to the randomization results. This study aims to accrue a total of 64 patients within two years. The primary endpoint will be a pulmonary quality of life (QOL) assessment at 3 months post-treatment, measured using the functional assessment of cancer therapy–lung cancer subscale. Secondary endpoints include: pulmonary QOL at other time-points, provider-reported toxicity, overall survival, progression-free survival, and quality-adjusted survival.

Discussion

This randomized, double-blind trial will comprehensively assess the impact of functional lung avoidance on pulmonary toxicity and quality of life in patients receiving concurrent CRT for locally advanced NSCLC.

Trial registration

Clinicaltrials.gov identifier: NCT02002052.

Anatomical, functional and metabolic imaging of radiation-induced lung injury using hyperpolarized MRI

MRI of hyperpolarized (129)Xe gas and (13)C-enriched substrates (e.g. pyruvate) presents an unprecedented opportunity to map anatomical, functional and metabolic changes associated with lung injury. In particular, inhaled hyperpolarized (129)Xe gas is exquisitely sensitive to changes in alveolar microanatomy and function accompanying lung inflammation through decreases in the apparent diffusion coefficient (ADC) of alveolar gas and increases in the transfer time (T(tr)) of xenon exchange from the gas and into the dissolved phase in the lung. Furthermore, metabolic changes associated with hypoxia arising from lung injury may be reflected by increases in lactate-to-pyruvate signal ratio obtained by magnetic resonance spectroscopic imaging following injection of hyperpolarized [1-(13)C]pyruvate. In this work, the application of hyperpolarized (129)Xe and (13)C MRI to radiation-induced lung injury (RILI) is reviewed and results of ADC, T(tr) and lactate-to-pyruvate signal ratio changes in a rat model of RILI are summarized. These results are consistent with conventional functional (i.e. blood gases) and histological (i.e. tissue density) changes, and correlate significantly with inflammatory cell counts (i.e. macrophages). Hyperpolarized MRI may provide an earlier indication of lung injury associated with radiotherapy of thoracic tumors, potentially allowing adjustment of treatment before the onset of severe complications and irreversible fibrosis.

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

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

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

Magnetic-resonance spectroscopy and imaging using hyperpolarized xenon-129 show great potential for evaluation of the most important function of the human lung -- gas exchange. In particular, chemical shift saturation recovery (CSSR) xenon-129 spectroscopy provides important physiological information for the lung as a whole by characterizing the dynamic process of gas exchange, while dissolved-phase (DP) xenon-129 imaging captures the time-averaged regional distribution of gas uptake by lung tissue and blood. Herein, we present recent advances in assessing lung function using CSSR spectroscopy and DP imaging in a total of 45 subjects (23 healthy, 13 chronic obstructive pulmonary disease (COPD) and 9 asthma). From CSSR acquisitions, the COPD subjects showed red blood cell to tissue-plasma (RBC-to-TP) ratios below the average for the healthy subjects (p < 0.001), but significantly higher septal wall thicknesses as compared with the healthy subjects (p < 0.005); the RBC-to-TP ratios for the asthmatic subjects fell outside two standard deviations (either higher or lower) from the mean of the healthy subjects, although there was no statistically significant difference for the average ratio of the study group as a whole. Similarly, from the 3D DP imaging acquisitions, we found that all the ratios (TP to gas phase (GP), RBC to GP, RBC to TP) measured in the COPD subjects were lower than those from the healthy subjects (p < 0.05 for all ratios), while these ratios in the asthmatic subjects differed considerably between subjects. Despite having been performed at different lung inflation levels, the RBC-to-TP ratios measured by CSSR and 3D DP imaging were fairly consistent with each other, with a mean difference of 0.037 (ratios from 3D DP imaging larger). In ten subjects the RBC-to-GP ratios obtained from the 3D DP imaging acquisitions were also highly correlated with their diffusing capacity of the lung for carbon monoxide per unit alveolar volume ratios measured by pulmonary function testing (R = 0.91).

Biomedical imaging with hyperpolarized noble gases

Hyperpolarized noble gases (HNGs), polarized to approximately 50% or higher, have led to major advances in magnetic resonance (MR) imaging of porous structures and air-filled cavities in human subjects, particularly the lung. By boosting the available signal to a level about 100 000 times higher than that at thermal equilibrium, air spaces that would otherwise appear as signal voids in an MR image can be revealed for structural and functional assessments. This review discusses how HNG MR imaging differs from conventional proton MR imaging, how MR pulse sequence design is affected and how the properties of gas imaging can be exploited to obtain hitherto inaccessible information in humans and animals. Current and possible future imaging techniques, and their application in the assessment of normal lung function as well as certain lung diseases, are described.