MRI of the lungs using hyperpolarized noble gases

Toward Lung Ventilation Imaging Using Hyperpolarized Diethyl Ether Gas Contrast Agent

Hyperpolarized 129Xe gas was FDA-approved as an inhalable contrast agent for magnetic resonance imaging of a wide range of pulmonary diseases in December 2022. Despite the remarkable success in clinical research settings, the widespread clinical translation of HP 129Xe gas faces two critical challenges: the high cost of the relatively low-throughput hyperpolarization equipment and the lack of 129Xe imaging capability on clinical MRI scanners, which have narrow-bandwidth electronics designed only for proton (1H) imaging. To solve this translational grand challenge of gaseous hyperpolarized MRI contrast agents, here we demonstrate the utility of batch-mode production of proton-hyperpolarized diethyl ether gas via heterogeneous pairwise addition of parahydrogen to ethyl vinyl ether. An approximately 0.1-liter bolus of hyperpolarized diethyl ether gas was produced in 1 second and injected in excised rabbit lungs. Lung ventilation imaging was performed using sub-second 2D MRI with up to 2×2 mm2 in-plane resolution using a clinical 0.35 T MRI scanner without any modifications. This feasibility demonstration paves the way for the use of inhalable diethyl ether as a gaseous contrast agent for pulmonary MRI applications using any clinical MRI scanner.

Deep Learning Approaches for Quantifying Ventilation Defects in Hyperpolarized Gas Magnetic Resonance Imaging of the Lung: A Review

This paper provides an in-depth overview of Deep Neural Networks and their application in the segmentation and analysis of lung Magnetic Resonance Imaging (MRI) scans, specifically focusing on hyperpolarized gas MRI and the quantification of lung ventilation defects. An in-depth understanding of Deep Neural Networks is presented, laying the groundwork for the exploration of their use in hyperpolarized gas MRI and the quantification of lung ventilation defects. Five distinct studies are examined, each leveraging unique deep learning architectures and data augmentation techniques to optimize model performance. These studies encompass a range of approaches, including the use of 3D Convolutional Neural Networks, cascaded U-Net models, Generative Adversarial Networks, and nnU-net for hyperpolarized gas MRI segmentation. The findings highlight the potential of deep learning methods in the segmentation and analysis of lung MRI scans, emphasizing the need for consensus on lung ventilation segmentation methods.

Assessment of Pulmonary Ventilation Using 3D Ventilation Flow-Weighted and Ventilation-Weighted Maps From 3D Ultrashort Echo Time (UTE) MRI

BACKGROUND

Three-dimensional (3D) ventilation flow-weighted (VFW) maps together with 3D ventilation-weighted (VW) maps may help to better assess pulmonary function.

PURPOSE

To investigate the use of 3D VFW and VW maps for evaluating pulmonary ventilation function.

STUDY TYPE

Prospective.

POPULATION

Two patients (one male, 85 years old; one female, 64 years old) with chronic obstructive pulmonary disease (COPD) and nine healthy subjects (all male; 23-27 years).

FIELD STRENGTH/SEQUENCE

3-T, 3D radial UTE imaging.

ASSESSMENT

3D VFW and VW maps were calculated from 3D UTE MRI by voxel-wise subtraction of respiratory phase images. Their validation was tested in nine healthy volunteers using slow/deep and fast/shallow breathing conditions. Additional validation was performed by comparison with single photon emission computed tomography (SPECT) ventilation maps of one healthy participant. For comparison, gravity dependence of anterior-posterior regional ventilation was assessed by one-dimensional plot of the mean signal intensity for each coronal slice. Structural similarity index measure was also calculated. Finally, VW maps and VFW maps of two COPD patients were evaluated for emphysema lesions with reference to CT images.

STATISTICAL TESTS

Wilcoxon sign-rank tests for regional Ventilation and Ventilation flow, analysis of variance, post-hoc t-tests and Bonferroni correction, coefficient of variation, Kullback-Liebler divergence. A P-value <0.05 was considered statistically significant.

RESULTS

The validation of 3D VFW and VW maps was shown by statistically significant differences in ventilation flow and ventilation between the breathing conditions. Additionally, UTE-MRI and SPECT-based ventilation maps showed gravitational dependence in the anteroposterior direction. When applied to patients with COPD, the use of 3D VFW and VW maps was able to differentiate between two patients with different phenotypes.

DATA CONCLUSION

The use of 3D VFW and VW maps can provide regional information on ventilation function and potentially contribute to assessment of COPD subtypes and disease progression.

EVIDENCE LEVEL

2 TECHNICAL EFFICACY: Stage 1.

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.

Emerging Imaging Modalities for Functional Assessment of Donor Lungs Ex Vivo

The severe shortage of functional donor lungs that can be offered to recipients has been a major challenge in lung transplantation. Innovative ex vivo lung perfusion (EVLP) and tissue engineering methodologies are now being developed to repair damaged donor lungs that are deemed unsuitable for transplantation. To assess the efficacy of donor lung reconditioning methods intended to rehabilitate rejected donor lungs, monitoring of lung function with improved spatiotemporal resolution is needed. Recent developments in live imaging are enabling non-destructive, direct, and longitudinal modalities for assessing local tissue and whole lung functions. In this review, we describe how emerging live imaging modalities can be coupled with lung tissue engineering approaches to promote functional recovery of ex vivo donor lungs.

Relationship between Lung and Brain Injury in COVID-19 Patients: A Hyperpolarized 129Xe-MRI-based 8-Month Follow-Up

Although the lungs are the primary organ involved, increasing evidence supports the neuroinvasive potential of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). This study investigates the potential relationship between coronavirus disease (COVID-19)-related deterioration of brain structure and the degree of damage to lung function. Nine COVID-19 patients were recruited in critical condition from Jin Yin-tan Hospital (Wuhan, China) who had been discharged between 4 February and 27 February 2020. The demographic, clinical, treatment, and laboratory data were extracted from the electronic medical records. All patients underwent chest CT imaging, ¹²⁹Xe gas lung MRI, and ¹H brain MRI. Four of the patients were followed up for 8 months. After nearly 12 months of recovery, we found no significant difference in lung ventilation defect percentage (VDP) between the COVID-19 group and the healthy group (3.8 ± 2.1% versus 3.7 ± 2.2%) using ¹²⁹Xe MRI, and several lung-function-related parameters—such as gas–blood exchange time (T)—showed improvement (42.2 ms versus 32.5 ms). Combined with ¹H brain MRI, we found that the change in gray matter volume (GMV) was strongly related to the degree of pulmonary function recovery—the greater the increase in GMV, the higher degree of pulmonary function damage.

Improving hyperpolarized 129 Xe ADC mapping in pediatric and adult lungs with uncertainty propagation

RATIONALE

Hyperpolarized (HP) ¹²⁹ Xe-MRI provides non-invasive methods to quantify lung function and structure, with the ¹²⁹ Xe apparent diffusion coefficient (ADC) being a well validated measure of alveolar airspace size. However, the experimental factors that impact the precision and accuracy of HP ¹²⁹ Xe ADC measurements have not been rigorously investigated. Here, we introduce an analytical model to predict the experimental uncertainty of ¹²⁹ Xe ADC estimates. Additionally, we report ADC dependence on age in healthy pediatric volunteers.

METHODS

An analytical expression for ADC uncertainty was derived from the Stejskal-Tanner equation and simplified Bloch equations appropriate for HP media. Parameters in the model were maximum b-value (b_max ), number of b-values (N_b ), number of phase encoding lines (N_ph ), flip angle and the ADC itself. This model was validated by simulations and phantom experiments, and five fitting methods for calculating ADC were investigated. To examine the lower range for ¹²⁹ Xe ADC, 32 healthy subjects (age 6-40 years) underwent diffusion-weighted ¹²⁹ Xe MRI.

RESULTS

The analytical model provides a lower bound on ADC uncertainty and predicts that decreased signal-to-noise ratio yields increases in relative uncertainty (ϵADC) . As such, experimental parameters that impact non-equilibrium ¹²⁹ Xe magnetization necessarily impact the resulting ϵADC . The values of diffusion encoding parameters (N_b and b_max ) that minimize ϵADC strongly depend on the underlying ADC value, resulting in a global minimum for ϵADC . Bayesian fitting outperformed other methods (error < 5%) for estimating ADC. The whole-lung mean ¹²⁹ Xe ADC of healthy subjects increased with age at a rate of 1.75 × 10^-4  cm² /s/yr (p = 0.001).

CONCLUSIONS

HP ¹²⁹ Xe diffusion MRI can be improved by minimizing the uncertainty of ADC measurements via uncertainty propagation. Doing so will improve experimental accuracy when measuring lung microstructure in vivo and should allow improved monitoring of regional disease progression and assessment of therapy response in a range of lung diseases.

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.

Hyperpolarization of 15N in an amino acid derivative

Hyperpolarization is a nuclear magnetic resonance (NMR) technique which can be used to significantly enhance the signal in NMR experiments. In recent years, the possibility to enhance the NMR signal of heteronuclei by the use of para-hydrogen induced polarization (PHIP) has gained attention, especially in the area of possible applications in magnetic resonance imaging (MRI). Herein we introduce a way to synthesize a fully deuterated, ¹⁵N labelled amino acid derivative and the possibility to polarize the ¹⁵N by means of hydrogenation with para-hydrogen to a polarization level of 0.18%. The longevity of the polarization with a longitudinal relaxation time of more than a minute can allow for the observation of dynamic processes and metabolic imaging in vivo. In addition, we observe the phenomenon of proton–deuterium exchange with a homogeneous catalyst leading to signal enhanced allyl moeities in the precursor.

A perdeuterated, ¹⁵N-labeled derivative of the amino acid glycine has been synthesized and polarized by means of para-hydrogen induced polarization (PHIP).

Cardiopulmonary bypass for resection of pancoast tumor with mediastinal extension and involvement of right bronchial branch: A case report

Introduction

and importance: Pneumonectomy is commonly associated with cardiopulmonary complications. Pneumonectomy in Pancoast tumor with mediastinal extension and no metastasis could be successful and efficient.

Case presentation

Herein, we report a successful pneumonectomy of a 54-year-old man with pancoast tumor along with the involvement of mediastinal space including right hilum of the lung, right bronchial, inferior vena cava vein and pericardium.

Clinical discussion

Based on the bronchoscopy and biopsy, the complete involvement of right bronchial tree was reported and non-small cell carcinoma was diagnosed in pathology.

Conclusion

To reduce the complications of the surgery, the cardiopulmonary pump machine was used during the operation.

Spontaneous Enhancement of Magnetic Resonance Signals Using a RASER

Nuclear magnetic resonance is usually drastically limited by its intrinsically low sensitivity: Only a few spins contribute to the overall signal. To overcome this limitation, hyperpolarization methods were developed that increase signals several times beyond the normal/thermally polarized signals. The ideal case would be a universal approach that can signal enhance the complete sample of interest in solution to increase detection sensitivity. Here, we introduce a combination of para-hydrogen enhanced magnetic resonance with the phenomenon of the RASER: Large signals of para-hydrogen enhanced molecules interact with the magnetic resonance coil in a way that the signal is spontaneously converted into an in-phase signal. These molecules directly interact with other compounds via dipolar couplings and enhance their signal. We demonstrate that this is not only possible for solvent molecules but also for an amino acid.

Measurement of a129Xe transverse relaxation rate without the influence of Rb polarization-induced magnetic gradient

We demonstrate the measurement of the transverse spin relaxation rate of Xe without the influence of the Rb polarization-induced magnetic field gradient. The optical pumping beam and probe beam turn on and off repeatedly during the measurement to reduce the relaxation originating from the Rb polarization-induced magnetic field gradient. During the absence of the optical pumping beam, the nuclear spin of the noble gas atom does not experience the alkali-polarization-induced magnetic field gradient so that the transverse spin relaxation rate (1/T₂) decreases. When the duty cycle reaches zero, the measured transverse relaxation time T₂ approaches the longitudinal spin relaxation time T₁. Our method helps in roughly estimating the longitudinal spin relaxation time with a single measurement of the free induction decay.

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.

Magnetic resonance microscopy of samples with translational symmetry with FOVs smaller than sample size

In MRI, usually the Field of View (FOV) has to cover the entire object. If this condition is not fulfilled, an infolding image artifact is observed, which suppresses visualization. In this study it is shown that for samples with translational symmetry, i.e., those consisting of identical objects in periodic unit cells, the FOV can be reduced to match the unit cell which enables imaging of an average object, of which the signal is originated from all unit cells of the sample, with no punishment by a loss in signal-to-noise ratio (SNR). This theoretical prediction was confirmed by experiments on a test sample with a 7 × 7 mm² unit cell arranged in a 3 × 3 matrix which was scanned by the spin-echo and by single point imaging methods. Effects of experimental imperfections in size and orientation mismatch between FOV and unit cell were studied as well. Finally, this method was demonstrated on a 3D periodic sample of tablets, which yielded well-resolved images of moisture distribution in an average tablet, while single tablet imaging provided no results. The method can be applied for SNR increase in imaging of any objects with inherently low signals provided they can be arranged in a periodic structure.

Acquisition strategies for spatially resolved magnetic resonance detection of hyperpolarized nuclei

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

Imaging the Injured Lung: Mechanisms of Action and Clinical Use

Acute respiratory distress syndrome (ARDS) consists of acute hypoxemic respiratory failure characterized by massive and heterogeneously distributed loss of lung aeration caused by diffuse inflammation and edema present in interstitial and alveolar spaces. It is defined by consensus criteria, which include diffuse infiltrates on chest imaging-either plain radiography or computed tomography. This review will summarize how imaging sciences can inform modern respiratory management of ARDS and continue to increase the understanding of the acutely injured lung. This review also describes newer imaging methodologies that are likely to inform future clinical decision-making and potentially improve outcome. For each imaging modality, this review systematically describes the underlying principles, technology involved, measurements obtained, insights gained by the technique, emerging approaches, limitations, and future developments. Finally, integrated approaches are considered whereby multimodal imaging may impact management of ARDS.

4DCT Ventilation Map Construction Using Biomechanics-base Image Registration and Enhanced Air Segmentation

Current lung radiation therapy (RT) treatment planning algorithms used in most centers assume homogeneous lung function. However, co-existing pulmonary dysfunctions present in many non-small cell lung cancer (NSCLC) patients, particularly smokers, cause regional variations in both perfusion and ventilation, leading to inhomogeneous lung function. An adaptive RT treatment planning that deliberately avoids highly functional lung regions can potentially reduce pulmonary toxicity and morbidity. The ventilation component of lung function can be measured using a variety of techniques. Recently, 4DCT ventilation imaging has emerged as a cost-effective and accessible method. Current 4DCT ventilation calculation methods, including the intensity-based and Jacobian models, suffer from inaccurate estimations of air volume distribution and unreliability of intensity-based image registration algorithms. In this study, we propose a novel method that utilizes a biomechanical model-based registration along with an accurate air segmentation algorithm to calculate 4DCT ventilation maps. The results show a successful development of ventilation maps using the proposed method.

Hyperpolarized 129 Xe imaging of embryonic stem cell-derived alveolar-like macrophages in rat lungs: proof-of-concept study using superparamagnetic iron oxide nanoparticles

PURPOSE

To measure regional changes in hyperpolarized ¹²⁹ Xe MRI signal and apparent transverse relaxation ( T 2 ∗ ) because of instillation of SPION-labeled alveolar-like macrophages (ALMs) in the lungs of rats and compare to histology.

METHODS

MRI was performed in 6 healthy mechanically ventilated rats before instillation, as well as 5 min and 1 h after instillation of 4 million SPION-labeled ALMs into either the left or right lung. T 2 ∗ maps were calculated from 2D multi-echo data at each time point and changes in T 2 ∗ were measured and compared to control rats receiving 4 million unlabeled ALMs. Histology of the ex vivo lungs was used to compare the regional MRI findings with the locations of the SPION-labeled ALMs.

RESULTS

Regions of signal loss were observed immediately after instillation of unlabeled and SPION-labeled ALMs and persisted at least 1 h in the case of the SPION-labeled ALMs. This was reflected in the measurements of T 2 ∗ . One hour after the instillation of SPION-labeled ALMs, the T 2 ∗ decreased to 54.0 ± 7.0% of the baseline, compared to a full recovery to baseline after the instillation of unlabeled ALMs. Histology confirmed the co-localization of SPION-labeled ALMs with regions of signal loss and T 2 ∗ decreases for each rat.

CONCLUSION

Hyperpolarized ¹²⁹ Xe MRI can detect the presence of SPION-labeled ALMs in the airways 1 h after instillation. This approach is promising for targeting and tracking of stem cells for the treatment of lung disease.

Rapid 13C NMR hyperpolarization delivered from para-hydrogen enables the low concentration detection and quantification of sugars

The monosaccharides glucose and fructose are rapidly detected and quantified by ¹³C NMR in conjunction with the hyperpolarisation method signal amplification by reversible exchange-relay.

Monosaccharides, such as glucose and fructose, are important to life. In this work we highlight how the rapid delivery of improved ¹³C detectability for sugars by nuclear magnetic resonance (NMR) can be achieved using the para-hydrogen based NMR hyperpolarization method SABRE-Relay (Signal Amplification by Reversible Exchange-Relay). The significant ¹³C signal enhancements of 250 at a high field of 9.4 T, and 3100 at a low field of 1 T, enable the detection of trace amounts of these materials as well as the quantification of their tautomeric makeup. Using studies on ¹³C and ²H isotopically labelled agents we demonstrate how hyperpolarization lifetime (T₁) values can be extended, and how singlet states with long lifetimes can be created. The precise quantification of d-glucose-¹³C₆-d₇ at the millimolar concentration level is shown to be possible within minutes in conjunction with a linear hyperpolarized response as a function of concentration. In addition to the measurements using labelled materials, low concentration detection is also illustrated for millimolar samples with natural abundance ¹³C where isomeric form quantification can be achieved with a single transient.

Hyperpolarization of 15N-pyridinium and 15N-aniline derivatives by using parahydrogen: new opportunities to store nuclear spin polarization in aqueous media

We introduce ¹⁵N quaternary pyridinium as moiety that can be NMR-signal-enhanced by several orders of magnitudes and allows for long-term storage of the so gained hyperpolarization in water.

Hyperpolarization techniques hold the promise to improve the sensitivity of magnetic resonance imaging (MRI) contrast agents by over 10 000-fold. Among these techniques, para-hydrogen induced polarization (PHIP) allows for generating contrast agents within seconds. Typical hyperpolarized contrast agents are traceable for 2–3 minutes only, thus prolonging tracking-times holds great importance for the development of new ways to diagnose and monitor diseases. Here, we report on the design of perdeuterated ¹⁵N-containing molecules with longitudinal relaxation times (T₁) of several minutes. T₁ is a measure for how long hyperpolarization can be stored. In particular, we introduce two new hyperpolarizable families of compounds that we signal enhanced with para-hydrogen: tert-amine aniline derivatives and a quaternary pyridinium compound with ¹⁵N-T₁ of about 8 minutes. Especially the latter compound has great potential for applicability since we achieved ¹⁵N-polarization up to 8% and the pyridinium motif is contained in a variety of drug molecules and is also used in drug delivery systems.

On the application of balanced steady-state free precession to MR microscopy

OBJECTIVE

The applicability of the balanced steady-state free precession (bSSFP) sequence to the field of MR microscopy was investigated, since the potentially high SNR makes bSSFP attractive. However, particularly at ultra-high magnetic fields, a number of constraints emerge: the frequency sensitivity of the bSSFP signal, the duty cycle of the imaging gradients, and the intrinsic diffusion attenuation of the steady state due to the imaging gradients.

MATERIALS AND METHODS

Optimization of the bSSFP sequence was performed on three imaging systems (7 T and 9.4 T) suited for MR microscopy. Since biological samples are often imaged in the very proximity of materials from sample containers/holder or devices such as electrodes, several microscopy phantoms representing such circumstances were fabricated and examined with 3D bSSFP.

RESULTS

Artifact-free microscopic bSSFP images could be obtained with voxel sizes down to 16 µm × 16 µm × 78 µm and with an SNR gain of 25% over standard gradient echo images.

CONCLUSION

With appropriate choice of phantom materials, optimization of the flip angle to the diffusion-attenuated steady state and protocols considering duty-cycle limitations, bSSFP can be a valuable tool in MR microscopy.

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 Lung Ventilation Quantification Using Fluorinated (19F) Gas Magnetic Resonance Imaging

RATIONALE AND OBJECTIVES

To assess the repeatability of global and regional lung ventilation quantification in both healthy subjects and patients with chronic obstructive pulmonary disease (COPD) using fluorinated (¹⁹F) gas washout magnetic resonance (MR) imaging in free breathing.

MATERIAL AND METHODS

In this prospective institutional review board-approved study, 12 healthy nonsmokers and eight COPD patients were examined with ¹⁹F dynamic gas washout MR imaging in free breathing and with lung function testing. Measurements were repeated within 2 weeks. Lung ventilation was quantified using ¹⁹F gas washout time. Repeatability was analyzed for the total lung and on a regional basis using the coefficient of variation (COV) and Bland-Altman plots.

RESULTS

In healthy subjects and COPD patients, a good repeatability was found for lung ventilation quantification using dynamic ¹⁹F gas washout MR imaging on a global (COV < 8%) and regional (COV < 15%) level. Gas washout time was significantly increased in the COPD group compared to the healthy subjects.

CONCLUSION

¹⁹F gas washout MR imaging provides a good repeatability of lung ventilation quantification and appears to be sensitive to early changes of regional lung function alterations such as normal aging.

Production of highly concentrated and hyperpolarized metabolites within seconds in high and low magnetic fields

We introduce two experiments that allow for the rapid production of hyperpolarized metabolites. More than 50% ¹³C polarization in 50 mM concentrations is achieved. This can be translated to portable low field NMR devices.

Charting the human respiratory tract with airborne nanoparticles: evaluation of the Airspace Dimension Assessment technique

Airspace Dimension Assessment (AiDA) is a technique to assess lung morphology by measuring lung deposition of inhaled nanoparticles. Nanoparticles deposit in the lungs predominately by diffusion, and average diffusion distances, corresponding to effective airspace radii ( rAiDA), can be inferred from measurements of particle recovery after varied breath holds. Also, particle recovery after a 0-s breath hold ( R0) may hold information about the small conducting airways. This study investigates rAiDA at different volumetric sample depths in the lungs of healthy subjects. Measurements were performed with 50-nm polystyrene nanospheres on 19 healthy subjects aged 17–67 yr. Volumetric sample depths ranged from 200 to 5,000 ml and breath-hold times from 5 to 20 s. At the examined volumetric sample depths, rAiDA values ranged from ~200–600 μm, which correspond to dimensions of the bronchiolar and the gas-exchanging regions of the lungs. R0 decreased with volumetric sample depth and showed more intersubject variation than rAiDA. Correlations were found between the AiDA parameters, anthropometry, and lung function tests, but not between rAiDA and R0. For repeated measurements on 3 subjects over an 18-mo period, rAiDA varied on average within ± 7 μm (± 2.4%). The results indicate that AiDA has potential as an efficient new in vivo technique to assess individual lung properties. The information obtained by such measurements may be of value for lung diagnostics, especially for the distal lungs, which are challenging to examine directly by other means.

NEW & NOTEWORTHY This is the first study to measure effective airspace radii ( rAiDA) at volumetric sample depths 200–5,000 ml in healthy subjects by Airspace Dimension Assessment (AiDA). Observed rAiDA were 200–600 μm, which corresponds to airspaces for the bronchiolar and the gas-exchanging regions around airway generation 14–17. rAiDA correlated with lung function tests and anthropometry. Measurements of rAiDA on 3 subjects over 11–18 mo were within ± 7 μm.

Patient-Specific Computational Simulations of Hyperpolarized 3He MRI Ventilation Defects in Healthy and Asthmatic Subjects

Combined, medical imaging data and respiratory computer simulations may facilitate novel insight into pulmonary disease phenotypes, including the structure/function relationships within the airways. This integration may ultimately enable improved classification and treatment of asthma. Severe asthma (15% of asthmatics) is particularly challenging to treat, as these patients do not respond well to inhaled therapeutics.

METHODS

This study combines medical image data with patient-specific computational models to predict gas distributions and airway mechanics in healthy and asthmatic subjects. We achieve this by integrating segmental volume defect percent (SVDP), measured from hyperpolarized 3He MRI and CT images, to create models of patient-specific gas flow within the conducting airways. Predicted and measured SVDP distributions are achieved when the prescribed resistances are increased systematically.

RESULTS

Because of differences in airway morphology and regional function, airway resistances and flow structures varied between the asthmatic subjects. Specifically, while mean SVDP was similar between the severe asthmatics (4.30±5.22 versus 3.54±5.98%), one subject exhibited abnormal flow structures, high near wall flow gradients, and enhanced conducting airway resistances (17.3E-3versus 1.1E-3 cmH2O-s/mL) in comparison to the other severe asthmatic subject.

CONCLUSION

By coupling medical imaging data with computer simulations, we provide detailed insight into pathological flow characteristics and airway mechanics in asthmatics, beyond what could be inferred independently.

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

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

Over 50 % 1H and 13C Polarization for Generating Hyperpolarized Metabolites-A para-Hydrogen Approach

para‐Hydrogen‐induced polarization (PHIP) is a method to rapidly generate hyperpolarized compounds, enhancing the signal of nuclear magnetic resonance (NMR) experiments by several thousand‐fold. The hyperpolarization of metabolites and their use as contrast agents in vivo is an emerging diagnostic technique. High degrees of polarization and extended polarization lifetime are necessary requirements for the detection of metabolites in vivo. Here, we present pulsed NMR methods for obtaining hyperpolarized magnetization in two metabolites. We demonstrate that the hydrogenation with para‐hydrogen of perdeuterated vinyl acetate allows us to create hyperpolarized ethyl acetate with close to 60 % ¹H two‐spin order. With nearly 100 % efficiency, this order can either be transferred to ¹H in‐phase magnetization or ¹³C magnetization of the carbonyl function. Close to 60 % polarization is experimentally verified for both nuclei. Cleavage of the ethyl acetate precursor in a 20 s reaction yields ethanol with approximately 27 % ¹H polarization and acetate with around 20 % ¹³C polarization. This development will open new opportunities to generate metabolic contrast agents in less than one minute.

Pulsed Magnetic Resonance to Signal-Enhance Metabolites within Seconds by utilizing para-Hydrogen

Diseases such as Alzheimer's and cancer have been linked to metabolic dysfunctions, and further understanding of metabolic pathways raises hope to develop cures for such diseases. To broaden the knowledge of metabolisms in vitro and in vivo, methods are desirable for direct probing of metabolic function. Here, we are introducing a pulsed nuclear magnetic resonance (NMR) approach to generate hyperpolarized metabolites within seconds, which act as metabolism probes. Hyperpolarization represents a magnetic resonance technique to enhance signals by over 10 000-fold. We accomplished an efficient metabolite hyperpolarization by developing an isotopic labeling strategy for generating precursors containing a favorable nuclear spin system to add para-hydrogen and convert its two-spin longitudinal order into enhanced metabolite signals. The transfer is performed by an invented NMR experiment and 20 000-fold signal enhancements are achieved. Our technique provides a fast way of generating hyperpolarized metabolites by using para-hydrogen directly in a high magnetic field without the need for field cycling.

Brain Imaging Using Hyperpolarized 129Xe Magnetic Resonance Imaging

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

Free-breathing Dynamic 19F Gas MR Imaging for Mapping of Regional Lung Ventilation in Patients with COPD

Purpose To quantify regional lung ventilation in patients with chronic obstructive pulmonary disease (COPD) by using free-breathing dynamic fluorinated (fluorine 19 [¹⁹F]) gas magnetic resonance (MR) imaging. Materials and Methods In this institutional review board-approved prospective study, 27 patients with COPD were examined by using breath-hold ¹⁹F gas wash-in MR imaging during inhalation of a normoxic fluorinated gas mixture (perfluoropropane) and by using free-breathing dynamic ¹⁹F gas washout MR imaging after inhalation of the gas mixture was finished for a total of 25-30 L. Regional lung ventilation was quantified by using volume defect percentage (VDP), washout time, number of breaths, and fractional ventilation (FV). To compare different lung function parameters, Pearson correlation coefficient and Fisher z transformation were used, which were corrected for multiple comparisons with the Bonferroni method. Results Statistically significant correlations were observed for all evaluated lung function test parameters compared with median and interquartile range of ¹⁹F washout parameters. An inverse linear correlation of median number of breaths (r = -0.82; P < .0001) and median washout times (r = -0.77; P < .0001) with percentage predicted of forced expiratory volume in 1 second (FEV₁) was observed; correspondingly median FV (r = 0.86; P < .0001) correlated positively with percentage predicted FEV₁. Comparing initial with late phase, median VDP of all subjects decreased from 49% (25th-75th percentile, 35%-62%) to 6% (25th-75th percentile, 2%-10%; P < .0001). VDP at the beginning of the gas wash-in phase (VDP_initial) significantly correlated with percentage predicted FEV₁ (r = -0.74; P = .0028) and FV (r = 0.74; P = .0002). Median FV was significantly increased in ventilated regions (11.1% [25th-75th percentile, 6.8%-14.5%]) compared with the defect regions identified by VDP_initial (5.8% [25th-75th percentile, 4.0%-7.4%]; P < .0001). Conclusion Quantification of regional lung ventilation by using dynamic ¹⁹F gas washout MR imaging in free breathing is feasible at 1.5 T even in obstructed lung segments. ^© RSNA, 2017 Online supplemental material is available for this article.

Optically polarized 3He

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

Assessment of pulmonary microstructural changes by hyperpolarized 129Xe diffusion-weighted imaging in an elastase-instilled rat model of emphysema

BACKGROUND

The purpose of this study was to explore the feasibility of hyperpolarized ¹²⁹Xe diffusion-weighted imaging (DWI) for the evaluation of pulmonary microstructural changes in the presence of pancreatic porcine elastase (PPE)-induced pulmonary emphysema rat model.

METHODS

Sixteen male Sprague-Dawley (SD) rats were randomly divided into two groups, the emphysema model group and control group. Experimental emphysematous models were made by instilling elastase into rat lungs of model group, the control group were instilled with isodose saline. Hyperpolarized ¹²⁹Xe magnetic resonance imaging (MRI) and histology were performed in all 16 rats after 30 days. DWIs were performed on a Bruker 7.0 T micro MRI, and the apparent diffusion coefficients (ADCs) were measured in all rats. Mean linear intercepts (MLIs) of pulmonary alveoli were measured on histology. The statistical analyses were performed about the correlation between the mean ADC of hyperpolarized ¹²⁹Xe in the whole lung and MLI of pulmonary histology metric.

RESULTS

The pulmonary emphysematous model was successfully confirmed by the histology and all scans were also successful. The ADC value of ¹²⁹Xe in the model group (0.0313±0.0005 cm²/s) was significantly increased compared with that of the control group (0.0288±0.0007 cm²/s, P<0.0001). Morphological differences such as MLI of pulmonary alveoli were observed between the two groups, the MLI of pulmonary alveoli in model group significantly increased (91±5 µm) than that of control group (50±3 µm, P<0.0001). Furthermore, the ADCs was moderately correlated with MLIs (r=0.724, P<0.01).

CONCLUSIONS

These results indicate that ¹²⁹Xe ADC value can quantitatively reflect the alveolar space enlargement and it is a promising biomarker for the detection of pulmonary emphysema.

Detection of regional radiation-induced lung injury using hyperpolarized 129Xe chemical shift imaging in a rat model involving partial lung irradiation: Proof-of-concept demonstration

PURPOSE

The purpose of this work was to use magnetic resonance imaging (MRI) of hyperpolarized (HP) 129Xe dissolved in pulmonary tissue (PT) and red blood cells (RBCs) to detect regional changes to PT structure and perfusion in a partial-lung rat model of radiation-induced lung injury and compare with histology.

METHODS AND MATERIALS

The right medial region of the lungs of 6 Sprague-Dawley rats was irradiated (20 Gy, single-fraction). A second nonirradiated cohort served as the control group. Imaging was performed 4 weeks after irradiation to quantify intensity and heterogeneity of PT and RBC 129Xe signals. Imaging findings were correlated with measures of PT and RBC distribution.

RESULTS

Asymmetric (right vs left) changes in 129Xe signal intensity and heterogeneity were observed in the irradiated cohort but were not seen in the control group. PT signal was observed to increase in intensity and heterogeneity and RBC signal was observed to increase in heterogeneity in the irradiated right lungs, consistent with histology.

CONCLUSION

Regional changes to PT and RBC 129Xe signals are detectable 4 weeks following partial-lung irradiation in rats.

Thoracic CT-MRI coregistration for regional pulmonary structure-function measurements of obstructive lung disease

PURPOSE

Recent pulmonary imaging research has revealed that in patients with chronic obstructive pulmonary disease (COPD) and asthma, structural and functional abnormalities are spatially heterogeneous. This novel information may help optimize treatment in individual patients, monitor interventional efficacy, and develop new treatments. Moreover, by automating the measurement of regional biomarkers for the 19 different anatomical lung segments, there is an opportunity to embed imaging biomarkers into clinically acceptable clinical workflows and improve lung disease clinical care. Therefore, to exploit the regional structure-function information provided by thoracic imaging, and as a first step toward this goal, our objective was to develop a fully automated registration pipeline for thoracic x-ray computed tomography (CT) and inhaled gas functional magnetic resonance imaging (MRI) whole lung and segmental structure-function biomarkers.

METHODS

Thirty-five patients including 15 severe, poorly controlled asthmatics and 20 COPD patients [classified according to the global initiative for chronic obstructive lung disease (GOLD) criteria)] provided written informed consent to a study protocol approved by Health Canada and underwent pulmonary function tests, MRI, and CT during a single 2-hour visit. Using this diverse patient dataset, we developed and evaluated a joint deformable registration approach to simultaneously coregister CT with both ¹ H and ³ He MRI by enforcing the similarity of the deformation fields from the two individual registrations. We derived a simpler model that was equivalent to the original challenging optimization problem through variational analysis and the simpler model gave rise to an efficient numerical solver that was parallelized on a graphics processing unit. The coregistered CT-³ He MRI and whole lung/segmental lung masks were used to generate whole lung and segmental ³ He MRI ventilation defect percent (VDP). To estimate fiducial localization reproducibility, a single observer manually identified 109 pairs of CT and ³ He MRI fiducials for 35 patient images on five separate occasions and determined the fiducial localization error (FLE). CT-³ He MRI registration accuracy was evaluated using the target registration error (TRE). Whole lung VDP generated using the algorithm was compared with VDP generated using a previously validated semiautomated approach and computational efficiency was evaluated using run time.

RESULTS

In 35 patients including 15 with severe asthma and 20 with COPD, mean forced expiratory volume in 1 s (FEV₁ ) was 63±24%_pred and FEV₁ /forced vital capacity (FVC) was 54 ± 17%. FLE was 0.16 mm and 0.34 mm for ³ He MRI and CT, respectively. TRE was 4.5 ± 2.0 mm, 4.0 ± 1.7 mm, 4.8 ± 2.3 mm for asthma, COPD GOLD II, and GOLD III groups, respectively, with a mean of 4.4 ± 2.0 mm for the entire dataset. TRE was significantly improved for joint CT-¹ H/³ He MRI registration compared with CT-¹ H MRI rigid registration (P < 0.0001). Whole lung VDP generated using the pipeline was not significantly different (P = 0.37) compared to a semiautomated method with which it was strongly correlated (r = 0.93, P < 0.0001). The fully automated pipeline required 11 ± 0.4 min to generate whole lung and segmental VDP.

CONCLUSIONS

For a diverse group of patients with COPD and asthma, whole lung and segmental VDP was measured using an automated lung image analysis pipeline which provides a way to incorporate lung functional biomarkers into clinical research and patient care.

Fluid-structure interaction including volumetric coupling with homogenised subdomains for modeling respiratory mechanics

In this article, a novel approach is presented for combining standard fluid-structure interaction with additional volumetric constraints to model fluid flow into and from homogenised solid domains. The proposed algorithm is particularly interesting for investigations in the field of respiratory mechanics as it enables the mutual coupling of airflow in the conducting part and local tissue deformation in the respiratory part of the lung by means of a volume constraint. In combination with a classical monolithic fluid-structure interaction approach, a comprehensive model of the human lung can be established that will be useful to gain new insights into respiratory mechanics in health and disease. To illustrate the validity and versatility of the novel approach, three numerical examples including a patient-specific lung model are presented. The proposed algorithm proves its capability of computing clinically relevant airflow distribution and tissue strain data at a level of detail that is not yet achievable, neither with current imaging techniques nor with existing computational models. Copyright © 2016 John Wiley & Sons, Ltd.

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.

NMR Hyperpolarization Techniques of Gases

Nuclear spin polarization can be significantly increased through the process of hyperpolarization, leading to an increase in the sensitivity of nuclear magnetic resonance (NMR) experiments by 4-8 orders of magnitude. Hyperpolarized gases, unlike liquids and solids, can often be readily separated and purified from the compounds used to mediate the hyperpolarization processes. These pure hyperpolarized gases enabled many novel MRI applications including the visualization of void spaces, imaging of lung function, and remote detection. Additionally, hyperpolarized gases can be dissolved in liquids and can be used as sensitive molecular probes and reporters. This Minireview covers the fundamentals of the preparation of hyperpolarized gases and focuses on selected applications of interest to biomedicine and materials science.

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.

Gas-Phase Hydrogenation with Parahydrogen Over Immobilized Vaska’s Complex

Generating parahydrogen-induced polarization (PHIP) of nuclear spins with immobilized transition metal complexes as hydrogenation catalysts allows one to produce pure hyperpolarized substances, which can open new revolutionary perspectives for PHIP applications. A major drawback of immobilized complexes is their low stability under reaction conditions. In the present work we studied an immobilized iridium complex, Ir/SiO2 P, synthesized by a covalent anchoring of Vaska’s complex on phospine-modified silica gel. This complex was used to obtain hyperpolarized gasses in the gas phase hydrogenation of propene, propyne and 1-butyne with parahydrogen in PASADENA and ALTADENA experiments. It was found that, in contrast to other immobilized complexes, Ir/SiO2 P is stable under reaction conditions at up to 140°C, and the reduction of iridium does not occur according to XPS analysis. Moreover, the application of Ir/SiO2 P catalyst allowed us to generate continuous flow of hyperpolarized propene and 1-butene with (300–500)-fold NMR signal enhancement which is significantly higher than commonly observed for most supported metal catalysts. The shape of polarized propene signals in PASADENA experiment has indicated that parahydrogen addition to propyne occurs non-stereospecifically, i.e. PHIP was observed for all protons of the vinyl fragment of propene. The analysis of the polarized signals has shown that syn pairwise addition dominates, which was confirmed by spectra simulations. It was found that storage of Ir/SiO2 P under Ar atmosphere leads to a decrease in PHIP amplitude and an increase in the activity of the catalyst. This observation is discussed in terms of the interaction of Ir/SiO2 P with trace amounts of oxygen in Ar, leading to partial oxidation of triphenylphosphine ligand to triphenylphosphine oxide accompanied by the activation of the immobilized complex. It was also found that the interaction of Ir/SiO2 P with alkenes likely leads to formation of stable monohydride complexes, decreasing the production of PHIP in hydrogenations. At the same time, stable substrate complexes are likely formed in alkyne hydrogenations, leading to a significant decrease in the monohydride complex formation and to an increased production of PHIP.