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

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.

Т1 mapping of rat lungs in 19 F MRI using octafluorocyclobutane

PURPOSE

To demonstrate the feasibility of using octafluorocyclobutane (OFCB, c-C₄ F₈ ) for T₁ mapping of lungs in ¹⁹ F MRI.

METHODS

The study was performed at 7 T in three healthy rats and three rats with pulmonary hypertension. To increase the sensitivity of ¹⁹ F MRI, a bent-shaped RF coil with periodic metal strips structure was used. The double flip angle method was used to calculate normalized transmitting RF field (B_1n ⁺ ) maps and for correcting T₁ maps built with the variable flip angle (VFA) method. The ultrashort TE pulse sequence was applied for acquiring MR images throughout the study.

RESULTS

The dependencies of OFCB relaxation times on its partial pressure in mixtures with oxygen, air, helium, and argon were obtained. T₁ of OFCB linearly depended on its partial pressure with the slope of about 0.35 ms/kPa in the case of free diffusion. RF field inhomogeneity leads to distortion of T₁ maps built with the VFA method, and therefore to high standard deviation of T₁ in these maps. To improve the accuracy of the T₁ maps, the B_1n ⁺ maps were applied for VFA correction. This contributed to a 2-3-fold decrease in the SD of T₁ values in the corresponding maps compared with T₁ maps calculated without the correction. Three-dimensional T₁ maps were obtained, and the mean T₁ in healthy rat lungs was 35 ± 10 ms, and in rat lungs with pulmonary hypertension - 41 ± 9 ms.

CONCLUSION

OFCB has a spin-rotational relaxation mechanism and can be used for ¹⁹ F T₁ mapping of lungs. The calculated OFCB maps captured ventilation defects induced by edema.

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.

Multi-nuclear magnetic resonance spectroscopy: state of the art and future directions

With the development of heteronuclear fluorine, sodium, phosphorus, and other probes and imaging technologies as well as the optimization of magnetic resonance imaging (MRI) equipment and sequences, multi-nuclear magnetic resonance (multi-NMR) has enabled localize molecular activities in vivo that are central to a variety of diseases, including cardiovascular disease, neurodegenerative pathologies, metabolic diseases, kidney, and tumor, to shift from the traditional morphological imaging to the molecular imaging, precision diagnosis, and treatment mode. However, due to the low natural abundance and low gyromagnetic ratios, the clinical application of multi-NMR has been hampered. Several techniques have been developed to amplify the NMR sensitivity such as the dynamic nuclear polarization, spin-exchange optical pumping, and brute-force polarization. Meanwhile, a wide range of nuclei can be hyperpolarized, such as ²H, ³He, ¹³C, ¹⁵ N, ³¹P, and ¹²⁹Xe. The signal can be increased and allows real-time observation of biological perfusion, metabolite transport, and metabolic reactions in vivo, overcoming the disadvantages of conventional magnetic resonance of low sensitivity. HP-NMR imaging of different nuclear substrates provides a unique opportunity and invention to map the metabolic changes in various organs without invasive procedures. This review aims to focus on the recent applications of multi-NMR technology not only in a range of preliminary animal experiments but also in various disease spectrum in human. Furthermore, we will discuss the future challenges and opportunities of this multi-NMR from a clinical perspective, in the hope of truly bridging the gap between cutting-edge molecular biology and clinical applications.

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.

Imaging Biomarkers in Animal Models of Drug-Induced Lung Injury: A Systematic Review

For drug-induced interstitial lung disease (DIILD) translational imaging biomarkers are needed to improve detection and management of lung injury and drug-toxicity. Literature was reviewed on animal models in which in vivo imaging was used to detect and assess lung lesions that resembled pathological changes found in DIILD, such as inflammation and fibrosis. A systematic search was carried out using three databases with key words “Animal models”, “Imaging”, “Lung disease”, and “Drugs”. A total of 5749 articles were found, and, based on inclusion criteria, 284 papers were selected for final data extraction, resulting in 182 out of the 284 papers, based on eligibility. Twelve different animal species occurred and nine various imaging modalities were used, with two-thirds of the studies being longitudinal. The inducing agents and exposure (dose and duration) differed from non-physiological to clinically relevant doses. The majority of studies reported other biomarkers and/or histological confirmation of the imaging results. Summary of radiotracers and examples of imaging biomarkers were summarized, and the types of animal models and the most used imaging modalities and applications are discussed in this review. Pathologies resembling DIILD, such as inflammation and fibrosis, were described in many papers, but only a few explicitly addressed drug-induced toxicity experiments.

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.

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.

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.

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.