In vivo regional ventilation mapping using fluorinated gas MRI with an x-centric FGRE method

Volume-Controlled 19 F MR Ventilation Imaging of Fluorinated Gas

BACKGROUND

¹⁹ F MRI of inhaled gas tracers has developed into a promising tool for pulmonary diagnostics. Prior to clinical use, the intersession repeatability of acquired ventilation parameters must be quantified and maximized.

PURPOSE

To evaluate repeatability of static and dynamic ¹⁹ F ventilation parameters and correlation with predicted forced expiratory volume in 1 second (FEV₁ %pred) with and without inspiratory volume control.

STUDY TYPE

Prospective.

POPULATION

A total of 30 healthy subjects and 26 patients with chronic obstructive pulmonary disease (COPD).

FIELD STRENGTH/SEQUENCE

Three-dimensional (3D) gradient echo pulse sequence with golden-angle stack-of-stars k-space encoding at 1.5 T.

ASSESSMENT

All study participants underwent ¹⁹ F ventilation MRI over eight breaths with inspiratory volume control (w VC) and without inspiratory volume control (w/o VC), which was repeated within 1 week. Ventilated volume percentage (VVP), fractional ventilation (FV), and wash-in time (WI) were computed. Lung function testing was conducted on the first visit.

STATISTICAL TESTS

Correlation between imaging and FEV₁ %pred was measured using Pearson correlation coefficient (r). Differences in imaging parameters between first and second visit were analyzed using paired t-test. Repeatability was quantified using intraclass correlation coefficient (ICC) and coefficient of variation (CoV). Minimum detectable effect size (MDES) was calculated with a power analysis for study size n = 30 and a power of 0.8. All hypotheses were tested with a significance level of 5% two sided.

RESULTS

Strong and moderate linear correlations with FEV₁ %pred for COPD patients were found in almost all imaging parameters. The ICC w VC exceeds the ICC w/o VC for all imaging parameters. CoV was significantly lower w VC for initial VVP in COPD patients, FV, CoV FV, WI and standard deviation (SD) of WI. MDES of all imaging parameters were smaller w VC.

DATA CONCLUSION

¹⁹ F gas wash-in MRI with inspiratory volume control increases the correlation and repeatability of imaging parameters with lung function testing.

EVIDENCE LEVEL

2 TECHNICAL EFFICACY: Stage 2.

Feasibility of Dynamic Inhaled Gas MRI-Based Measurements Using Acceleration Combined with the Stretched Exponential Model

Dynamic inhaled gas (³He/¹²⁹Xe/¹⁹F) MRI permits the acquisition of regional fractional-ventilation which is useful for detecting gas-trapping in lung-diseases such as lung fibrosis and COPD. Deninger's approach used for analyzing the wash-out data can be substituted with the stretched-exponential-model (SEM) because signal-intensity is attenuated as a function of wash-out-breath in ¹⁹F lung imaging. Thirteen normal-rats were studied using ³He/¹²⁹Xe and ¹⁹F MRI and the ventilation measurements were performed using two 3T clinical-scanners. Two Cartesian-sampling-schemes (Fast-Gradient-Recalled-Echo/X-Centric) were used to test the proposed method. The fully sampled dynamic wash-out images were retrospectively under-sampled (acceleration-factors (AF) of 10/14) using a varying-sampling-pattern in the wash-out direction. Mean fractional-ventilation maps using Deninger's and SEM-based approaches were generated. The mean fractional-ventilation-values generated for the fully sampled k-space case using the Deninger method were not significantly different from other fractional-ventilation-values generated for the non-accelerated/accelerated data using both Deninger and SEM methods (p > 0.05 for all cases/gases). We demonstrated the feasibility of the SEM-based approach using retrospective under-sampling, mimicking AF = 10/14 in a small-animal-cohort from the previously reported dynamic-lung studies. A pixel-by-pixel comparison of the Deninger-derived and SEM-derived fractional-ventilation-estimates obtained for AF = 10/14 (≤16% difference) has confirmed that even at AF = 14, the accuracy of the estimates is high enough to consider this method for prospective measurements.

Application of a 2D frequency encoding sectoral approach to hyperpolarized 129Xe MRI at low field

Inhaled hyperpolarized ¹²⁹Xe MRI is a non-invasive and radiation risk free lung imaging method, which can directly measure the business unit of the lung where gas exchange occurs: the alveoli and acinar ducts (lung function). Currently, three imaging approaches have been demonstrated to be useful for hyperpolarized ¹²⁹Xe MR in lungs: Fast Gradient Recalled Echo (FGRE), Radial Projection Reconstruction (PR), and spiral/cones. Typically, non-Cartesian acquisitions such as PR and spiral/cones require specific data post-processing, such as interpolating, regridding, and density-weighting procedures for image reconstruction, which often leads to smoothing effects and resolution degradation. On the other hand, Cartesian methods such as FGRE are not short-echo time (TE) methods; they suffer from imaging gradient-induced diffusion-weighting of the k-space center, and employ a significant number of radio-frequency (RF) pulses. Due to the non-renewable magnetization of the hyperpolarized media, the use of a large number of RF pulses (FGRE/PR) required for full k-space coverage is a significant limitation, especially for low field (<0.5 T) hyperpolarized gas MRI. We demonstrate an ultra-fast, purely frequency-encoded, Cartesian pulse sequence called Frequency-Encoding Sectoral (FES), which takes advantage of the long T₂* of hyperpolarized ¹²⁹Xe gas at low field strength (0.074 T). In contrast to PR/FGRE, it uses a much smaller number of RF pulses, and consequently maximizes image Signal-to-Noise Ratio (SNR) while shortening acquisition time. Additionally, FES does not suffer from non-uniform T₂* decay leading to image blurring; a common issue with interleaved spirals/cones. The Cartesian k-space coverage of the proposed FES method does not require specific k-space data post-processing, unlike PR/FGRE and spiral/cones methods. Proton scans were used to compare the FES sequence to both FGRE and Phase Encoding Sectoral, in terms of their SNR values and imaging efficiency estimates. Using FES, proton and hyperpolarized ¹²⁹Xe images were acquired from a custom hollow acrylic phantom (0.04L) and two normal rats (¹²⁹Xe only), utilizing both single-breath and multiple-breath schemes. For the ¹²⁹Xe phantom images, the apparent diffusion coefficient, T₁, and T₂* relaxation maps were acquired and generated. Blurring due to the T₂* decay and B₀ field variation were simulated to estimate dependence of the image resolution on the duration of the data acquisition windows (i.e. sector length), and temperature-induced resonance frequency shift from the low field magnet hardware.

Reproducibility of 19 F-MR ventilation imaging in healthy volunteers

Purpose

To assess the reproducibility of percentage ventilated lung volume (%VV) measurements in healthy volunteers acquired by fluorine (¹⁹F)‐MRI of inhaled perfluoropropane, implemented at two research sites.

Methods

In this prospective, ethically approved study, 40 healthy participants were recruited (May 2018‐June 2019) to one of two research sites. Participants underwent a single MRI scan session on a 3T scanner, involving periodic inhalation of a 79% perfluoropropane/21% oxygen gas mixture. Each gas inhalation session lasted about 30 seconds, consisting of three deep breaths of gas followed by a breath‐hold. Four ¹⁹F‐MR ventilation images were acquired per participant, each separated by approximately 6 minutes. The value of %VV was determined by registering separately acquired ¹H images to ventilation images before semi‐automated image segmentation, performed independently by two observers. Reproducibility of %VV measurements was assessed by components of variance, intraclass correlation coefficients, coefficients of variation (CoV), and the Dice similarity coefficient.

Results

The MRI scans were well tolerated throughout, with no adverse events. There was a high degree of consistency in %VV measurements for each participant (CoV_observer1 = 0.43%; CoV_observer2 = 0.63%), with overall precision of %VV measurements determined to be within ± 1.7% (95% confidence interval). Interobserver agreement in %VV measurements revealed a high mean Dice similarity coefficient (SD) of 0.97 (0.02), with only minor discrepancies between observers.

Conclusion

We demonstrate good reproducibility of %VV measurements in a group of healthy participants using ¹⁹F‐MRI of inhaled perfluoropropane. Our methods have been successfully implemented across two different study sites, supporting the feasibility of performing larger multicenter clinical studies.

Flow Volume Loop and Regional Ventilation Assessment Using Phase-Resolved Functional Lung (PREFUL) MRI: Comparison With 129 Xenon Ventilation MRI and Lung Function Testing

BACKGROUND

Regional flow volume loop ventilation-weighted noncontrast-enhanced proton lung MRI in free breathing has emerged as a novel technique for assessment of regional lung ventilation, but has yet not been validated with ¹²⁹ Xenon MRI (¹²⁹ Xe-MRI), a direct visualization of ventilation in healthy volunteers, cystic fibrosis (CF), and chronic obstructive pulmonary disease (COPD) patients.

PURPOSE

To compare regional ventilation and regional flow volume loops measured by noncontrast-enhanced ventilation-weighted phase-resolved functional lung MRI (PREFUL-MRI) with ¹²⁹ Xe-MRI ventilation imaging and with lung function test parameters.

STUDY TYPE

Retrospective study.

POPULATION

Twenty patients with COPD, eight patients with CF, and six healthy volunteers.

FIELD STRENGTH/SEQUENCE

PREFUL and ¹²⁹ Xe-MRI gradient echo sequences were acquired at 1.5T.

ASSESSMENT

Coronal slices of PREFUL-MRI (free breathing) and ¹²⁹ Xe-MRI (single breath-hold) were acquired on the same day, matched by their ventrodorsal position and coregistered for evaluation. Ventilation defect percentage (VDP) was calculated based on regional ventilation (RV), regional flow volume loops (RFVL), or ¹²⁹ Xe-MRI with two different threshold methods. A combined VDP was calculated for RV and RFVL. Additionally, lung function testing was performed (such as the forced expiratory volume in 1 second [FEV₁ ]) was used.

STATISTICAL TESTS

The obtained parameters were compared using Wilcoxon tests, correlated using Spearman's correlation coefficient (r), and agreement between PREFUL and ¹²⁹ Xe-MRI parameters was assessed using Bland-Altman analysis and Dice coefficients.

RESULTS

VDP measured by PREFUL and ¹²⁹ Xe were significantly correlated with both thresholding techniques (r = 0.62-0.69, P < 0.05 for all) and with lung function test parameters. Combined RV and RFVL PREFUL defect maps correlated with lung function testing (eg, with FEV₁ r = -0.87 P < 0.05), and showed better regional agreement to ¹²⁹ Xe-MRI ventilation defects (Dice coefficient defect 0.413) with significantly higher VDP values (10.2 ± 27.3, P = 0.04) than either PREFUL defect map alone.

DATA CONCLUSION

Combined RV and RFVL PREFUL defect maps likely increase sensitivity to mild airway obstruction with increased VDP values compared to ¹²⁹ Xe-MRI, and correlate strongly with lung function test parameters.

LEVEL OF EVIDENCE

3 TECHNICAL EFFICACY STAGE: 2.

Evaluation of fluorine-19 magnetic resonance imaging of the lungs using octafluorocyclobutane in a rat model

PURPOSE

To test octafluorocyclobutane (OFCB) as an inhalation contrast agent for fluorine-19 MRI of the lung, and to compare the image quality of OFCB scans with perfluoropropane (PFP) scans THEORY AND METHODS: After normalizing for the number of signal averages, a theoretical comparison between the OFCB signal-to-noise ratio (SNR) and PFP SNR predicted the average SNR advantage of 90% using OFCB during gradient echo imaging. The OFCB relaxometry was conducted using single-voxel spectroscopy and spin-echo imaging. A comparison of OFCB and PFP SNRs was performed in vitro and in vivo. Five healthy Sprague-Dawley rats were imaged during single breath-hold and continuous breathing using a Philips Achieva 3.0T MRI scanner (Philips, Andover, MA). The scan time was constant for both gases. Statistical comparison between PFP and OFCB scans was conducted using a paired t test and by calculating the Bayes factor.

RESULTS

Spin-lattice (T₁ ) and effective spin-spin ( T 2 ∗ ) relaxation time constants of the pure OFCB gas were determined as 28.5 ± 1.2 ms and 10.5 ± 1.8 ms, respectively. Mixing with 21% of oxygen decreased T₁ by 30% and T 2 ∗ by 20%. The OFCB in vivo images showed 73% higher normalized SNR on average compared with images acquired using PFP. The statistical significance was shown by both paired t test and calculated Bayes factors. The experimental results agree with theoretical calculations within the error of the relaxation parameter measurements.

CONCLUSION

The quality of the lung images acquired using OFCB was significantly better compared with PFP scans. The OFCB images had higher a SNR and were artifact-free.

An 8-element Tx/Rx array utilizing MEMS detuning combined with 6 Rx loops for 19 F and 1 H lung imaging at 1.5T

PURPOSE

To firstly improve the attainable image SNR of ¹⁹ F and ¹ H C₃ F₈ lung imaging at 1.5 tesla using an 8-element transmit/receive (Tx/Rx) flexible vest array combined with a 6-element Rx-only array, and to secondly evaluate microelectromechanical systems for switching the array elements between the 2 resonant frequencies.

METHODS

The Tx efficiency and homogeneity of the 8-element array were measured and simulated for ¹ H imaging in a cylindrical phantom and then evaluated for in vivo ¹⁹ F/¹ H imaging. The added improvement provided by the 6-element Rx-only array was quantified through simulation and measurement and compared to the ultimate SNR. It was verified through the measurement of isolation that microelectromechanical systems switches provided broadband isolation of Tx/Rx circuitry such that the ¹⁹ F tuned Tx/Rx array could be effectively used for both ¹⁹ F and ¹ H nuclei.

RESULTS

For ¹ H imaging, the measured Tx efficiency/homogeneity (mean ± percent SD; 6.79 μ T / kW ± 26 % ) was comparable to that simulated ( 7.57 μ T / kW ± 20 % ). The 6 additional Rx-only loops increased the mean Rx sensitivity when compared to the 8-element array by a factor of 1.41× and 1.45× in simulation and measurement, respectively. In regions central to the thorax, the simulated SNR of the 14-element array achieves ≥70% of the ultimate SNR when including noise from the matching circuits and preamplifiers. A measured microelectromechanical systems switching speed of 12 µs and added minimum 22 dB of isolation between Tx and Rx were sufficient for Tx/Rx switching in this application.

CONCLUSION

The described single-tuned array driven at ¹⁹ F and ¹ H, utilizing microelectromechanical systems technology, provides excellent results for ¹⁹ F and ¹ H dual-nuclear lung ventilation imaging.

1 H-guided reconstruction of 19 F gas MRI in COPD patients

PURPOSE

To reduce acquisition time and improve image quality and robustness of ventilation assessment in a single breath-hold using ¹ H-guided reconstruction of fluorinated gas (¹⁹ F) MRI.

METHODS

Reconstructions constraining total variation in the image domain, L1 norm in the wavelet domain, and directional total variation between ¹⁹ F and ¹ H images were compared in order to accelerate ¹⁹ F ventilation imaging using retrospectively undersampled data from a healthy volunteer. Using the optimal constrained reconstruction in 8 patients with chronic obstructive pulmonary disease (16-seconds breath-hold), ventilation maps of various acceleration factors (2-fold to 13-fold) were compared with maps of the full data set using the Dice coefficient, difference in volume defect percentage and overlap percentage, as well as hyperpolarized ¹²⁹ Xe gas MRI.

RESULTS

The reconstruction constraining total variation and directional total variation simultaneously performed best in the healthy volunteer (RMS error = 0.07, structural similarity index = 0.77) for a measurement time of 2 seconds. Using the same reconstruction in the patients with chronic obstructive pulmonary disease, the Dice coefficient of defect volumes was 0.86 ± 0.05, the mean difference in volume defect percentage was -1.0 ± 1.7 percentage points, and the overlap percentage was 87% ± 2% for a measurement time of 6 seconds. Between volume defect percentage of ¹⁹ F and ¹²⁹ Xe, a linear correlation (r = 0.75; P = .03) was found, with ¹⁹ F volume defect percentage being significantly higher (mean difference = 11%; P = .04).

CONCLUSION

¹ H-guided reconstruction of pulmonary ¹⁹ F gas MRI enables reduction of acquisition time while maintaining image quality and robustness of functional parameters.

Dynamic susceptibility contrast 19 F-MRI of inhaled perfluoropropane: a novel approach to combined pulmonary ventilation and perfusion imaging

PURPOSE

To assess alveolar perfusion by applying dynamic susceptibility contrast MRI to ¹⁹ F-MRI of inhaled perfluoropropane (PFP). We hypothesized that passage of gadolinium-based contrast agent (GBCA) through the pulmonary microvasculature would reduce magnetic susceptibility differences between water and gas components of the lung, elevating the T 2 ∗ of PFP.

METHODS

Lung-representative phantoms were constructed of aqueous PFP-filled foams to characterize the impact of aqueous/gas phase magnetic susceptibility differences on PFP T 2 ∗ . Aqueous phase magnetic susceptibility was modulated by addition of different concentrations of GBCA. In vivo studies were performed to measure the impact of intravenously administered GBCA on the T 2 ∗ of inhaled PFP in mice (7.0 Tesla) and in healthy volunteers (3.0 Tesla).

RESULTS

Perfluoropropane T 2 ∗ was sensitive to modulation of magnetic susceptibility difference between gas and water components of the lung, both in phantom models and in vivo. Negation of aqueous/gas phase magnetic susceptibility difference was achieved in lung-representative phantoms and in mice, resulting in a ~2 to 3× elevation in PFP T 2 ∗ (3.7 to 8.5 ms and 0.7 to 2.6 ms, respectively). Human studies demonstrated a transient elevation of inhaled PFP T 2 ∗ (1.50 to 1.64 ms) during passage of GBCA bolus through the lung circulation, demonstrating sensitivity to lung perfusion.

CONCLUSION

We demonstrate indirect detection of a GBCA in the pulmonary microvasculature via changes to the T 2 ∗ of gas phase PFP within directly adjacent alveoli. This approach holds potential for assessing alveolar perfusion by dynamic susceptibility contrast ¹⁹ F-MRI of inhaled PFP, with concurrent assessment of lung ventilation properties, relevant to lung physiology and disease.

Optimized and accelerated 19 F-MRI of inhaled perfluoropropane to assess regional pulmonary ventilation

PURPOSE

To accelerate ¹⁹ F-MR imaging of inhaled perfluoropropane using compressed sensing methods, and to optimize critical scan acquisition parameters for assessment of lung ventilation properties.

METHODS

Simulations were performed to determine optimal acquisition parameters for maximal perfluoropropane signal-to-noise ratio (SNR) in human lungs for a spoiled gradient echo sequence. Optimized parameters were subsequently employed for ¹⁹ F-MRI of inhaled perfluoropropane in a cohort of 11 healthy participants using a 3.0 T scanner. The impact of 1.8×, 2.4×, and 3.0× undersampling ratios on ¹⁹ F-MRI acquisitions was evaluated, using both retrospective and prospective compressed sensing methods.

RESULTS

3D spoiled gradient echo ¹⁹ F-MR ventilation images were acquired at 1-cm isotropic resolution within a single breath hold. Mean SNR was 11.7 ± 4.1 for scans acquired within a single breath hold (duration = 18 s). Acquisition of ¹⁹ F-MRI scans at shorter scan durations (4.5 s) was also demonstrated as feasible. Application of both retrospective (n = 8) and prospective (n = 3) compressed sensing methods demonstrated that 1.8× acceleration had negligible impact on qualitative image appearance, with no statistically significant change in measured lung ventilated volume. Acceleration factors of 2.4× and 3.0× resulted in increasing differences between fully sampled and undersampled datasets.

CONCLUSION

This study demonstrates methods for determining optimal acquisition parameters for ¹⁹ F-MRI of inhaled perfluoropropane and shows significant reduction in scan acquisition times (and thus participant breath hold duration) by use of compressed sensing.

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.

Optimization of steady-state free precession MRI for lung ventilation imaging with 19 F C3 F8 at 1.5T and 3T

PURPOSE

To optimize 19 F imaging pulse sequences for perfluoropropane (C3 F8 ) gas human lung ventilation MRI considering intrinsic in vivo relaxation parameters at both 1.5T and 3T.

METHODS

Optimization of the imaging parameters for both 3D spoiled gradient (SPGR) and steady-state free precession (SSFP) 19 F imaging sequences with inhaled 79% C3 F8% and 21% oxygen was performed. Phantom measurements were used to validate simulations of SNR. In vivo parameter mapping and sequence optimization and comparison was performed by imaging the lungs of a healthy adult volunteer. T1 and T2* mapping was performed in vivo to optimize sequence parameters for in vivo lung MRI. The performance of SSFP and SPGR was then evaluated in vivo at 1.5T and 3T.

RESULTS

The in vivo T2* of C3 F8 was shown to be dependent upon lung inflation level (2.04 ms ± 36% for residual volume and 3.14 ms ± 28% for total lung capacity measured at 3T), with lower T2* observed near the susceptibility interfaces of the diaphragm and around pulmonary blood vessels. Simulation and phantom measurements indicate that a factor of ~2-3 higher SNR can be achieved with SSFP when compared with optimized SPGR. In vivo lung imaging showed a 1.7 factor of improvement in SNR achieved at 1.5T, while the theoretical improvement at 3T was not attained due to experimental SAR constraints, shorter in vivo T1 , and B0 inhomogeneity.

CONCLUSION

SSFP imaging provides increased SNR in lung ventilation imaging of C3 F8 demonstrated at 1.5T with optimized SSFP similar to the SNR that can be obtained at 3T with optimized SPGR.

Advanced pulmonary MRI to quantify alveolar and acinar duct abnormalities: Current status and future clinical applications

There are serious clinical gaps in our understanding of chronic lung disease that require novel, sensitive, and noninvasive in vivo measurements of the lung parenchyma to measure disease pathogenesis and progressive changes over time as well as response to treatment. Until recently, our knowledge and appreciation of the tissue changes that accompany lung disease has depended on ex vivo biopsy and concomitant histological and stereological measurements. These measurements have revealed the underlying pathologies that drive lung disease and have provided important observations about airway occlusion, obliteration of the terminal bronchioles and airspace enlargement, or fibrosis and their roles in disease initiation and progression. ex vivo tissue stereology and histology are the established gold standards and, more recently, micro-computed tomography (CT) measurements of ex vivo tissue samples has also been employed to reveal new mechanistic findings about the progression of obstructive lung disease in patients. While these approaches have provided important understandings using ex vivo analysis of excised samples, recently developed hyperpolarized noble gas MRI methods provide an opportunity to noninvasively measure acinar duct and terminal airway dimensions and geometry in vivo, and, without radiation burden. Therefore, in this review we summarize emerging pulmonary MRI morphometry methods that provide noninvasive in vivo measurements of the lung in patients with bronchopulmonary dysplasia and chronic obstructive pulmonary disease, among others. We discuss new findings, future research directions, as well as clinical opportunities to address current gaps in patient care and for testing of new therapies. Level of Evidence: 5 Technical Efficacy: Stage 5 J. Magn. Reson. Imaging 2019;50:28-40.

Comparison of MEMS switches and PIN diodes for switched dual tuned RF coils

PURPOSE

To evaluate the performance of micro-electromechanical systems (MEMS) switches against PIN diodes for switching a dual-tuned RF coil between 19 F and 1 H resonant frequencies for multi-nuclear lung imaging.

METHODS

A four-element fixed-phase and amplitude transmit-receive RF coil was constructed to provide homogeneous excitation across the lungs, and to serve as a test system for various switching methods. The MR imaging and RF performance of the coil when switched between the 19 F and 1 H frequencies using MEMS switches, PIN diodes and hardwired configurations were compared.

RESULTS

The performance of the coil with MEMS or PIN diode switching was comparable in terms of RF measurements, transmit efficiency and image SNR on both 19 F and 1 H nuclei. When the coil was not switched to the resonance frequency of the respective nucleus being imaged, reductions in the transmit efficiency were observed of 32% at the 19 F frequency and 12% at the 1 H frequency. The coil provides transmit field homogeneity of ±12.9% at the 1 H frequency and ±14.4% at the 19 F frequency in phantoms representing the thorax with the air space of the lungs filled with perfluoropropane gas.

CONCLUSION

MEMS and PIN diodes were found to provide comparable performance in on-state configuration, while MEMS were more robust in off-state high-powered operation (>1 kW), providing higher isolation and requiring a lower DC switching voltage than is needed for reverse biasing of PIN diodes. In addition, clear benefits of switching between the 19 F and 1 H resonances were demonstrated, despite the proximity of their Larmor frequencies.

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.

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.

A hybrid multibreath wash-in wash-out lung function quantification scheme in human subjects using hyperpolarized 3 He MRI for simultaneous assessment of specific ventilation, alveolar oxygen tension, oxygen uptake, and air trapping

PURPOSE

To present a method for simultaneous acquisition of alveolar oxygen tension (PA O2 ), specific ventilation (SV), and apparent diffusion coefficient (ADC) of hyperpolarized (HP) gas in the human lung, allowing reinterpretation of the PA O2 and SV maps to produce a map of oxygen uptake (R).

METHOD

An imaging scheme was designed with a series of identical normoxic HP gas wash-in breaths to measure ADC, SV, PA O2 , and R in less than 2 min. Signal dynamics were fit to an iterative recursive model that regionally solved for these parameters. This measurement was successfully performed in 12 subjects classified in three healthy, smoker, and chronic obstructive pulmonary disease (COPD) cohorts.

RESULTS

The overall whole lung ADC, SV, PA O2 , and R in healthy, smoker, and COPD subjects was 0.20 ± 0.03 cm2 /s, 0.39 ± 0.06,113 ± 2 Torr, and 1.55 ± 0.35 Torr/s, respectively, in healthy subjects; 0.21 ± 0.03 cm2 /s, 0.33 ± 0.06, 115.9 ± 4 Torr, and 0.97 ± 0.2 Torr/s, respectively, in smokers; and 0.25 ± 0.06 cm2 /s, 0.23 ± 0.08, 114.8 ± 6.0Torr, and 0.94 ± 0.12 Torr/s, respectively, in subjects with COPD. Hetrogeneity of SV, PA O2 , and R were indicators of both smoking-related changes and disease, and the severity of the disease correlated with the degree of this heterogeneity. Subjects with symptoms showed reduced oxygen uptake and specific ventilation.

CONCLUSION

High-resolution, nearly coregistered and quantitative measures of lung function and structure were obtained with less than 1 L of HP gas. This hybrid multibreath technique produced measures of lung function that revealed clear differences among the cohorts and subjects and were confirmed by correlations with global lung measurements. Magn Reson Med 78:611-624, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Using Hyperpolarized 129Xe MRI to Quantify the Pulmonary Ventilation Distribution

RATIONALE AND OBJECTIVES

Ventilation heterogeneity is impossible to detect with spirometry. Alternatively, pulmonary ventilation can be imaged three-dimensionally using inhaled ¹²⁹Xe magnetic resonance imaging (MRI). To date, such images have been quantified primarily based on ventilation defects. Here, we introduce a robust means to transform ¹²⁹Xe MRI scans such that the underlying ventilation distribution and its heterogeneity can be quantified.

MATERIALS AND METHODS

Quantitative ¹²⁹Xe ventilation MRI was conducted in 12 younger (24.7 ± 5.2 years) and 10 older (62.2 ± 7.2 years) healthy individuals, as well as in 9 younger (25.9 ± 6.4 yrs) and 10 older (63.2 ± 6.1 years) asthmatics. The younger healthy population was used to establish a reference ventilation distribution and thresholds for six intensity bins. These bins were used to display and quantify the ventilation defect region (VDR), the low ventilation region (LVR), and the high ventilation region (HVR).

RESULTS

The ventilation distribution in young subjects was roughly Gaussian with a mean and standard deviation of 0.52 ± 0.18, resulting in VDR = 2.1 ± 1.3%, LVR = 15.6 ± 5.4%, and HVR = 17.4 ± 3.1%. Older healthy volunteers exhibited a significantly right-skewed distribution (0.46 ± 0.20, P = 0.034), resulting in significantly increased VDR (7.0 ± 4.8%, P = 0.008) and LVR (24.5 ± 11.5%, P = 0.025). In the asthmatics, VDR and LVR increased in the older population, and HVR was significantly reduced (13.5 ± 4.6% vs 18.9 ± 4.5%, P = 0.009). Quantitative ¹²⁹Xe MRI also revealed altered ventilation heterogeneity in response to albuterol in two asthmatics with normal spirometry.

CONCLUSIONS

Quantitative ¹²⁹Xe MRI provides a robust and objective means to display and quantify the pulmonary ventilation distribution, even in subjects who have airway function impairment not appreciated by spirometry.

Fractional ventilation mapping using inert fluorinated gas MRI in rat models of inflammation and fibrosis

The purpose of this study was to extend established methods for fractional ventilation mapping using (19) F MRI of inert fluorinated gases to rat models of pulmonary inflammation and fibrosis. In this study, five rats were instilled with lipopolysaccharide (LPS) in the lungs two days prior to imaging, six rats were instilled with bleomycin in the lungs two weeks prior to imaging and an additional four rats were used as controls. (19) F MR lung imaging was performed at 3 T with rats continuously breathing a mixture of sulfur hexafluoride and O2 . Fractional ventilation maps were obtained using a wash-out approach, by switching the breathing mixture to pure O2 , and acquiring images following each successive wash-out breath. The mean fractional ventilation (r) was 0.29 ± 0.05 for control rats, 0.23 ± 0.10 for LPS-instilled rats and 0.19 ± 0.03 for bleomycin-instilled rats. Bleomycin-instilled rats had a significantly decreased mean r value compared with controls (P = 0.010). Although LPS-instilled rats had a slightly reduced mean r value, this trend was not statistically significant (P = 0.556). Fractional ventilation gradients were calculated in the anterior/posterior (A/P) direction, and the mean A/P gradient was -0.005 ± 0.008 cm(-1) for control rats, 0.013 ± 0.005 cm(-1) for LPS-instilled rats and 0.009 ± 0.018 cm(-1) for bleomycin-instilled rats. Fractional ventilation gradients were significantly different for control rats compared with LPS-instilled rats only (P = 0.016). The ventilation gradients calculated from control rats showed the expected gravitational relationship, while ventilation gradients calculated from LPS- and bleomycin-instilled rats showed the opposite trend. Histology confirmed that LPS-instilled rats had a significantly elevated alveolar wall thickness, while bleomycin-instilled rats showed signs of substantial fibrosis. Overall, (19)F MRI may be able to detect the effects of pulmonary inflammation and fibrosis using a simple and inexpensive imaging approach that can potentially be translated to humans.

Fluorine-19 MRI Contrast Agents for Cell Tracking and Lung Imaging

Fluorine-19 (¹⁹F)-based contrast agents for magnetic resonance imaging stand to revolutionize imaging-based research and clinical trials in several fields of medical intervention. First, their use in characterizing in vivo cell behavior may help bring cellular therapy closer to clinical acceptance. Second, their use in lung imaging provides novel noninvasive interrogation of the ventilated airspaces without the need for complicated, hard-to-distribute hardware. This article reviews the current state of ¹⁹F-based cell tracking and lung imaging using magnetic resonance imaging and describes the link between the methods across these fields and how they may mutually benefit from solutions to mutual problems encountered when imaging ¹⁹F-containing compounds, as well as hardware and software advancements.

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.