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

Background

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

Methods

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

Results

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

Conclusion

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

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

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

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

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

Background

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

Purpose

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

Study Type

Prospective.

Population

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

Field Strength/Sequences

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

Assessment

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

Statistical Tests

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

Results

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

Conclusion

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

Level of Evidence

1

Technical Efficacy Stage

4

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

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

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

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

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSION

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

A thermally polarized 129 Xe phantom for quality assurance in multi-center hyperpolarized gas MRI studies

PURPOSE

Hyperpolarized ¹²⁹ Xe MR is increasingly being adopted worldwide, but no standards exist for assessing or comparing performance at different ¹²⁹ Xe imaging centers. Therefore, we sought to develop a thermally polarized xenon phantom assembly, approximating the size of a human torso, along with an associated imaging protocol to enable rapid quality-assurance imaging.

METHODS

MR-compatible pressure vessels, with an internal volume of 5.85 L, were constructed from pressure-rated, engineering grade PE4710 high-density polyethylene. They were filled with a mixture of 61% natural xenon and 39% oxygen to approximately 11.6 bar and placed in a loader shell filled with a 0.56% saline solution to mimic the human chest. Imaging employed a 2D spoiled gradient-echo sequence using non-slice-selective excitation (TR/TE = 750/6.13 ms, flip angle = 74°, FOV = 40 × 440 mm, matrix = 64 × 32, bandwidth = 30 Hz/pixel, averages = 4), resulting in a 1.6 min acquisition. System characterization and imaging were performed at 8 different MRI centers.

RESULTS

At 3 Telsa, ¹²⁹ Xe in the pressure vessels was characterized by T₁ = 580.5 ± 8.3 ms, linewidth = 0.21 ppm, and chemical shift = +10.2 ppm. The phantom assembly was used to obtain transmit voltage calibrations and 2D and 3D images across multiple coil and scanner configurations at 8 sites. Across the 5 sites that employed a standard flexible chest coil, the SNR was 12.4 ± 1.8.

CONCLUSION

The high-density polyethylene pressure vessels filled with thermally polarized xenon and associated loader shell combine to form a phantom assembly that enables spectroscopic and imaging acquisitions that can be used for testing, quality assurance, and performance tracking-capabilities essential for standardizing hyperpolarized ¹²⁹ Xe MRI within and across institutions.

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

The spectral parameters of hyperpolarized ¹²⁹ Xe exchanging between airspaces, interstitial barrier, and red blood cells (RBCs) are sensitive to pulmonary pathophysiology. This study sought to evaluate whether the dynamics of ¹²⁹ Xe spectroscopy provide additional insight, with particular focus on quantifying cardiogenic oscillations in the RBC resonance. ¹²⁹ Xe spectra were dynamically acquired in eight healthy volunteers and nine subjects with idiopathic pulmonary fibrosis (IPF). ¹²⁹ Xe FIDs were collected every 20 ms (T_E  = 0.932 ms, 512 points, dwell time = 32 μs, flip angle ≈ 20°) during a 16 s breathing maneuver. The FIDs were pre-processed using the spectral improvement by Fourier thresholding technique (SIFT) and fit in the time domain to determine the airspace, interstitial barrier, and RBC spectral parameters. The RBC and gas resonances were fit to a Lorentzian lineshape, while the barrier was fit to a Voigt lineshape to account for its greater structural heterogeneity. For each complex resonance the amplitude, chemical shift, linewidth(s), and phase were calculated. The time-averaged spectra confirmed that the RBC to barrier amplitude ratio (RBC:barrier ratio) and RBC chemical shift are both reduced in IPF subjects. Their temporal dynamics showed that all three ¹²⁹ Xe resonances are affected by the breathing maneuver. Most notably, several RBC spectral parameters exhibited prominent oscillations at the cardiac frequency, and their peak-to-peak variation differed between IPF subjects and healthy volunteers. In the IPF cohort, oscillations were more prominent in the RBC amplitude (16.8 ± 5.2 versus 9.7 ± 2.9%; P = 0.008), chemical shift (0.43 ± 0.33 versus 0.083 ± 0.05 ppm; P < 0.001), and phase (7.7 ± 5.6 versus 1.4 ± 0.8°; P < 0.001). Dynamic ¹²⁹ Xe spectroscopy is a simple and sensitive tool that probes the temporal variability of gas exchange and may prove useful in discerning the underlying causes of its impairment.

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.