Beyond Spirometry: Linking Wasted Ventilation to Exertional Dyspnea in the Initial Stages of COPD

Exertional dyspnea, a key complaint of patients with chronic obstructive pulmonary disease (COPD), ultimately reflects an increased inspiratory neural drive to breathe. In non-hypoxemic patients with largely preserved lung mechanics - as those in the initial stages of the disease - the heightened inspiratory neural drive is strongly associated with an exaggerated ventilatory response to metabolic demand. Several lines of evidence indicate that the so-called excess ventilation (high ventilation-CO2 output relationship) primarily reflects poor gas exchange efficiency, namely increased physiological dead space. Pulmonary function tests estimating the extension of the wasted ventilation and selected cardiopulmonary exercise testing variables can, therefore, shed unique light on the genesis of patients' out-of-proportion dyspnea. After a succinct overview of the basis of gas exchange efficiency in health and inefficiency in COPD, we discuss how wasted ventilation translates into exertional dyspnea in individual patients. We then outline what is currently known about the structural basis of wasted ventilation in "minor/trivial" COPD vis-à-vis the contribution of emphysema versus a potential impairment in lung perfusion across non-emphysematous lung. After summarizing some unanswered questions on the field, we propose that functional imaging be amalgamated with pulmonary function tests beyond spirometry to improve our understanding of this deeply neglected cause of exertional dyspnea. Advances in the field will depend on our ability to develop robust platforms for deeply phenotyping (structurally and functionally), the dyspneic patients showing unordinary high wasted ventilation despite relatively preserved FEV1.

Phase-resolved MRI for measurement of pulmonary perfusion and ventilation defects in comparison with dynamic contrast-enhanced MRI and 129Xe MRI

INTRODUCTION

This meta-analysis aims to evaluate the agreement and correlation between phase-resolved functional lung MRI (PREFUL MRI) and dynamic contrast-enhanced (DCE) MRI in evaluating perfusion defect percentage (QDP), as well as the agreement between PREFUL MRI and 129Xe MRI in assessing ventilation defect percentage (VDP).

METHOD

A systematic search was conducted in the Medline, Embase and Cochrane Library databases to identify relevant studies comparing QDP and VDP measured by DCE MRI and 129Xe MRI compared with PREFUL MRI. Meta-analytical techniques were applied to calculate the pooled weighted bias, limits of agreement (LOA) and correlation coefficient. The publication bias was assessed using Egger's regression test, while heterogeneity was assessed using Cochran's Q test and Higgins I2 statistic.

RESULTS

A total of 399 subjects from 10 studies were enrolled. The mean difference and LOA were -2.31% (-8.01% to 3.40%) for QDP and 0.34% (-4.94% to 5.62%) for VDP. The pooled correlations (95% CI) were 0.65 (0.55 to 0.73) for QDP and 0.72 (0.61 to 0.80) for VDP. Furthermore, both QDP and VDP showed a negative correlation with forced expiratory volume in 1 s (FEV1). The pooled correlation between QDP and FEV1 was -0.51 (-0.74 to -0.18), as well as between VDP and FEV1 was -0.60 (-0.73 to -0.44).

CONCLUSIONS

PREFUL MRI is a promising imaging for the assessment of lung function, as it demonstrates satisfactory deviations and LOA when compared with DEC MRI and 129Xe MRI.

PROSPERO REGISTRATION NUMBER

CRD42023430847.

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

BACKGROUND

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

PURPOSE

To investigate the use of 3D VFCW 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 VFCW 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 VFCW 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 capacity, 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 VFCW and VW maps was shown by statistically significant differences in ventilation flow capacity 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 VFCW and VW maps was able to differentiate between two patients with different phenotypes.

DATA CONCLUSION

The use of 3D VFCW 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.

Phase-Resolved Functional Lung (PREFUL) MRI to Quantify Ventilation: Feasibility and Physiological Relevance in Severe Asthma

RATIONALE AND OBJECTIVES

Emergent evidence in several respiratory diseases supports translational potential for Phase-Resolved Functional Lung (PREFUL) MRI to spatially quantify ventilation but its feasibility and physiological relevance have not been demonstrated in patients with asthma. This study compares PREFUL-derived ventilation defect percent (VDP) in severe asthma patients to healthy controls and measures its responsiveness to bronchodilator therapy and relation to established measures of airways disease.

MATERIALS AND METHODS

Forty-one adults with severe asthma and seven healthy controls performed same-day free-breathing 1H MRI, 129Xe MRI, spirometry, and oscillometry. A subset of participants (n = 23) performed chest CT and another subset of participants with asthma (n = 19) repeated 1H MRI following the administration of a bronchodilator. VDP was calculated for both PREFUL and 129Xe MRI. Additionally, the percent of functional small airways disease was determined from CT parametric response maps (PRMfSAD).

RESULTS

PREFUL VDP measured pre-bronchodilator (19.1% [7.4-43.3], p = 0.0002) and post-bronchodilator (16.9% [6.1-38.4], p = 0.0007) were significantly greater than that of healthy controls (7.5% [3.7-15.5]) and was significantly decreased post-bronchodilator (from 21.9% [10.1-36.9] to 16.9% [6.1-38.4], p = 0.0053). PREFUL VDP was correlated with spirometry (FEV1%pred: r = -0.46, p = 0.0023; FVC%pred: r = -0.35, p = 0.024, FEV1/FVC: r = -0.46, p = 0.0028), 129Xe MRI VDP (r = 0.39, p = 0.013), and metrics of small airway disease (CT PRMfSAD: r = 0.55, p = 0.021; Xrs5 Hz: r = -0.44, p = 0.0046, and AX: r = 0.32, p = 0.044).

CONCLUSION

PREFUL-derived VDP is responsive to bronchodilator therapy in asthma and is associated with measures of airflow obstruction and small airway dysfunction. These findings validate PREFUL VDP as a physiologically relevant and accessible ventilation imaging outcome measure in asthma.

Free-breathing 3D phase-resolved functional lung MRI vs breath-hold hyperpolarized 129Xe ventilation MRI in patients with chronic obstructive pulmonary disease and healthy volunteers

OBJECTIVES

3D phase-resolved functional lung (PREFUL) MRI offers evaluation of pulmonary ventilation without inhalation of contrast agent. This study seeks to compare ventilation maps obtained from 3D PREFUL MRI with a direct ventilation measurement derived from 129Xe MRI in both patients with chronic obstructive pulmonary disease (COPD) and healthy volunteers.

METHODS

Thirty-one patients with COPD and 12 healthy controls underwent free-breathing 3D PREFUL MRI and breath-hold 129Xe MRI at 1.5 T. For both MRI techniques, ventilation defect (VD) maps were determined and respective ventilation defect percentage (VDP) values were computed. All parameters of both techniques were compared by Spearman correlation coefficient (r) and the differences between VDP values were quantified by Bland-Altman analysis and tested for significance using Wilcoxon signed-rank test. In a regional comparison of VD maps, spatial overlap and Sørensen-Dice coefficients of healthy and defect areas were computed.

RESULTS

On a global level, all 3D PREFUL VDP values correlated significantly to VDP measure derived by 129Xe ventilation imaging (all r > 0.65; all p < 0.0001). 129Xe VDP was significantly greater than 3D PREFUL derived VDPRVent (mean bias = 10.5%, p < 0.001) and VDPFVL-CM (mean bias = 11.3%, p < 0.0001) but not for VDPCombined (mean bias = 1.7%, p = 0.70). The total regional agreement of 129Xe and 3D PREFUL VD maps ranged between 60% and 63%.

CONCLUSIONS

Free-breathing 3D PREFUL MRI showed a strong correlation with breath-hold hyperpolarized 129Xe MRI regarding the VDP values and modest differences in the detection of VDs on a regional level.

CLINICAL RELEVANCE STATEMENT

3D PREFUL MRI correlated with 129Xe MRI, unveiling regional differences in COPD defect identification. This proposes 3D PREFUL MRI as a ventilation mapping surrogate, eliminating the need for extra hardware or inhaled gases.

KEY POINTS

Current non-invasive evaluation techniques for lung diseases have drawbacks; 129Xe MRI is limited by cost and availability. 3D PREFUL MRI correlated with 129Xe MRI, with regional differences in identifying COPD defects. 3D PREFUL MRI can provide ventilation mapping without the need for additional hardware or inhaled gases.

Sequence comparison of spoiled gradient echo and balanced steady-state free precession for pulmonary free-breathing proton MRI in patients and healthy volunteers: Correspondence, repeatability, and validation with dynamic contrast-enhanced MRI

Phase-resolved functional lung (PREFUL) MRI is a proton-based, contrast agent-free technique derived from the Fourier decomposition approach to measure regional ventilation and perfusion dynamics during free-breathing. Besides the necessity of extensive PREFUL postprocessing, the utilized MRI sequence must fulfill specific requirements. This study investigates the impact of sequence selection on PREFUL-MRI-derived functional parameters by comparing the standard spoiled gradient echo (SPGRE) sequence with a lung-optimized balanced steady-state free precession (bSSFP) sequence, thereby facilitating PREFULs clinical application in pulmonary disease assessment. This study comprised a prospective dataset of healthy volunteers and a retrospective dataset of patients with suspected chronic thromboembolic pulmonary hypertension. Both cohorts underwent PREFUL-MRI with both sequences to assess the correspondence of PREFUL ventilation and perfusion parameters (A). Additionally, healthy subjects were scanned a second time to evaluate repeatability (B), whereas patients received dynamic contrast-enhanced (DCE)-MRI, considered the perfusion gold standard for comparison with PREFUL-MRI (C). Signal-to-noise ratio (SNR), calculated from the unprocessed images, was compared alongside median differences of PREFUL-MRI-derived parameters using a paired Wilcoxon signed rank test. Further evaluations included calculation of the Pearson correlation, intraclass-correlation coefficient for repeatability assessment, and spatial overlap (SO) for regional comparison of PREFUL-MRI and DCE-MRI. bSSFP showed a clear SNR advantage over SPGRE (median: 23 vs. 9, p < 0.001). (A) Despite significant differences, parameter values were strongly correlated (r ≥ 0.75). After thresholding, binary maps showed high healthy overlap across both cohorts (SOHealthy > 86%) and high defect overlap in the patient cohort (SODefect ≥ 48%). (B) bSSFP demonstrated slightly higher repeatability across most parameters. (C) Both sequences demonstrated comparable correspondence to DCE-MRI, with SPGRE excelling in absolute quantification and bSSFP in spatial agreement. Although bSSFP showed superior SNR results, both sequences displayed spatial defect concordance and highly correlated PREFUL parameters with deviations regarding repeatability and alignment with DCE-MRI.

Estimating ventilation correlation coefficients in the lungs using PREFUL-MRI in chronic obstructive pulmonary disease patients and healthy adults

PURPOSE

Various parameters of regional lung ventilation can be estimated using phase-resolved functional lung (PREFUL)-MRI. The parameter "ventilation correlation coefficient (Vent-CC)" was shown advantageous because it assesses the dynamics of regional air flow. Calculating Vent-CC depends on a voxel-wise comparison to a healthy reference flow curve. This work examines the effect of placing a reference region of interest (ROI) in various lung quadrants or in different coronal slices. Furthermore, algorithms for automated ROI selection are presented and compared in terms of test-retest repeatability.

METHODS

Twenty-eight healthy subjects and 32 chronic obstructive pulmonary disease (COPD) patients were scanned twice using PREFUL-MRI. Retrospective analyses examined the homogeneity of air flow curves of various reference ROIs using cross-correlation. Vent-CC and ventilation defect percentage (VDP) calculated using various reference ROIs were compared using one-way analysis of variance (ANOVA). The coefficient of variation was calculated for Vent-CC and VDP when using different reference selection algorithms.

RESULTS

Flow-volume curves were highly correlated between ROIs placed at various lung quadrants in the same coronal slice (r > 0.97) with no differences in Vent-CC and VDP (ANOVA: p > 0.5). However, ROIs placed at different coronal slices showed lower correlation coefficients and resulted in significantly different Vent-CC and VDP values (ANOVA: p < 0.001). Vent-CC and VDP showed higher repeatability when calculated using the presented new algorithm.

CONCLUSION

In COPD and healthy cohorts, assessing regional ventilation dynamics using PREFUL-MRI in terms of the Vent-CC metric showed higher repeatability using a new algorithm for selecting a homogenous reference ROI from the same slice.

PREFUL MRI for Monitoring Perfusion and Ventilation Changes after Elexacaftor-Tezacaftor-Ivacaftor Therapy for Cystic Fibrosis: A Feasibility Study

Purpose To assess the feasibility of monitoring the effects of elexacaftor-tezacaftor-ivacaftor (ETI) therapy on lung ventilation and perfusion in people with cystic fibrosis (CF), using phase-resolved functional lung (PREFUL) MRI. Materials and Methods This secondary analysis of a multicenter prospective study was carried out between August 2020 and March 2021 and included participants 12 years or older with CF who underwent PREFUL MRI, spirometry, sweat chloride test, and lung clearance index assessment before and 8-16 weeks after ETI therapy. For PREFUL-derived ventilation and perfusion parameter extraction, two-dimensional coronal dynamic gradient-echo MR images were evaluated with an automated quantitative pipeline. T1- and T2-weighted MR images and PREFUL perfusion maps were visually assessed for semiquantitative Eichinger scores. Wilcoxon signed rank test compared clinical parameters and PREFUL values before and after ETI therapy. Correlation of parameters was calculated as Spearman ρ correlation coefficient. Results Twenty-three participants (median age, 18 years [IQR: 14-24.5 years]; 13 female) were included. Quantitative PREFUL parameters, Eichinger score, and clinical parameters (lung clearance index = 21) showed significant improvement after ETI therapy. Ventilation defect percentage of regional ventilation decreased from 18% (IQR: 14%-25%) to 9% (IQR: 6%-17%) (P = .003) and perfusion defect percentage from 26% (IQR: 18%-36%) to 19% (IQR: 13%-24%) (P = .002). Areas of matching normal (healthy) ventilation and perfusion increased from 52% (IQR: 47%-68%) to 73% (IQR: 61%-83%). Visually assessed perfusion scores did not correlate with PREFUL perfusion (P = .11) nor with ventilation-perfusion match values (P = .38). Conclusion The study demonstrates the feasibility of PREFUL MRI for semiautomated quantitative assessment of perfusion and ventilation changes in response to ETI therapy in people with CF. Keywords: Pediatrics, MR-Functional Imaging, Pulmonary, Lung, Comparative Studies, Cystic Fibrosis, Elexacaftor-Tezacaftor-Ivacaftor Therapy, Fourier Decomposition, PREFUL, Free-Breathing Proton MRI, Pulmonary MRI, Perfusion, Functional MRI, CFTR, Modulator Therapy, Kaftrio Clinical trial registration no. NCT04732910 Supplemental material is available for this article. © RSNA, 2024.

MR Perfusion Imaging of the Lung

Lung perfusion assessment is critical for diagnosing and monitoring a variety of respiratory conditions. MRI perfusion provides a radiation-free technique, making it an ideal choice for longitudinal imaging in younger populations. This review focuses on the techniques and applications of MRI perfusion, including contrast-enhanced (CE) MRI and non-CE methods such as arterial spin labeling (ASL), fourier decomposition (FD), and hyperpolarized 129-Xenon (129-Xe) MRI. ASL leverages endogenous water protons as tracers for a non-invasive measure of lung perfusion, while FD offers simultaneous measurements of lung perfusion and ventilation, enabling the generation of ventilation/perfusion mapsHyperpolarized 129-Xe MRI emerges as a novel tool for assessing regional gas exchange in the lungs. Despite the promise of MRI perfusion techniques, challenges persist, including competition with other imaging techniques and the need for additional validation and standardization. In conditions such as cystic fibrosis and lung cancer, MRI has displayed encouraging results, whereas in diseases like chronic obstructive pulmonary disease, further validation remains necessary. In conclusion, while MRI perfusion techniques hold immense potential for a comprehensive, non-invasive assessment of lung function and perfusion, their broader clinical adoption hinges on technological advancements, collaborative research, and rigorous validation.

Diagnostic accuracy of perfusion-weighted phase-resolved functional lung magnetic resonance imaging in patients with chronic pulmonary embolism

Purpose

This study aimed to evaluate the diagnostic performance of perfusion-weighted phase-resolved functional lung (PW-PREFUL) magnetic resonance imaging (MRI) in patients with chronic pulmonary embolism (CPE).

Materials and methods

This study included 86 patients with suspected chronic thromboembolic pulmonary hypertension (CTEPH), who underwent PREFUL MRI and ventilation/perfusion (V/Q) single-photon emission computed tomography/computed tomography (SPECT/CT). PREFUL MRI was performed at 1.5 T using a balanced steady-state free precession sequence during free breathing. Color-coded PW images and quantitative parameters were obtained by postprocessing. Meanwhile, V/Q SPECT/CT imaging was performed as a reference standard. Hypoperfused areas in the lungs were scored for each lobe and segment using V/Q SPECT/CT images and PW-PREFUL MR images, respectively. Normalized perfusion (QN) and perfusion defect percentage (QDP) were calculated for all slices. For intra- and interobserver variability, the MRI images were analyzed 2 months after the first analysis by the same radiologist and another radiologist (11 years of lung MRI experience) blinded to the results of the first reader.

Results

Of the 86 enrolled patients, 77 met the inclusion criteria (36 diagnosed with CPE using V/Q SPECT/CT and 41 diagnosed with non-CPE etiology). For the PW-PREFUL MRI, the sensitivity, specificity, accuracy, and positive and negative predictive values for the diagnosis of CPE were 97, 95, 96, 95, and 98% at the patient level; 91, 94, 93, 91, and 94% at the lobe level, and 85, 94, 92, 88, and 94% at the segment level, respectively. The detection of segmental and subsegmental hypoperfusion using PW-PREFUL MRI revealed a moderate agreement with V/Q SPECT/CT (κ = 0.65; 95% confidence interval: 0.61-0.68). The quantitative results indicated that the QN was lower in the CPE group than in the non-CPE group [median score (interquartile range, IQR) 6.3 (2.8-9.2) vs. 13.0 (8.8-16.7), p < 0.001], and the QDP was higher [median score (IQR) 33.8 (15.7-51.7) vs. 2.2 (1.4-2.9), p < 0.001].

Conclusion

PREFUL MRI could be an alternative test to detect CPE without requiring breath-hold, contrast agents, or ionizing radiation.

129 Xe and Free-Breathing 1 H Ventilation MRI in Patients With Cystic Fibrosis: A Dual-Center Study

BACKGROUND

Free-breathing 1 H ventilation MRI shows promise but only single-center validation has yet been performed against methods which directly image lung ventilation in patients with cystic fibrosis (CF).

PURPOSE

To investigate the relationship between 129 Xe and 1 H ventilation images using data acquired at two centers.

STUDY TYPE

Sequence comparison.

POPULATION

Center 1; 24 patients with CF (12 female) aged 9-47 years. Center 2; 7 patients with CF (6 female) aged 13-18 years, and 6 healthy controls (6 female) aged 21-31 years. Data were acquired in different patients at each center.

FIELD STRENGTH/SEQUENCE

1.5 T, 3D steady-state free precession and 2D spoiled gradient echo.

ASSESSMENT

Subjects were scanned with 129 Xe ventilation and 1 H free-breathing MRI and performed pulmonary function tests. Ventilation defect percent (VDP) was calculated using linear binning and images were visually assessed by H.M., L.J.S., and G.J.C. (10, 5, and 8 years' experience).

STATISTICAL TESTS

Correlations and linear regression analyses were performed between 129 Xe VDP, 1 H VDP, FEV1 , and LCI. Bland-Altman analysis of 129 Xe VDP and 1 H VDP was carried out. Differences in metrics were assessed using one-way ANOVA or Kruskal-Wallis tests.

RESULTS

129 Xe VDP and 1 H VDP correlated strongly with; each other (r = 0.84), FEV1 z-score (129 Xe VDP r = -0.83, 1 H VDP r = -0.80), and LCI (129 Xe VDP r = 0.91, 1 H VDP r = 0.82). Bland-Altman analysis of 129 Xe VDP and 1 H VDP from both centers had a bias of 0.07% and limits of agreement of -16.1% and 16.2%. Linear regression relationships of VDP with FEV1 were not significantly different between 129 Xe and 1 H VDP (P = 0.08), while 129 Xe VDP had a stronger relationship with LCI than 1 H VDP.

DATA CONCLUSION

1 H ventilation MRI shows large-scale agreement with 129 Xe ventilation MRI in CF patients with established lung disease but may be less sensitive to subtle ventilation changes in patients with early-stage lung disease.

EVIDENCE LEVEL

2 TECHNICAL EFFICACY: Stage 2.

Inter- and intravisit repeatability of free-breathing MRI in pediatric cystic fibrosis lung disease

PURPOSE

The purpose of this study is to assess the intra- and interscan repeatability of free-breathing phase-resolved functional lung (PREFUL) MRI in stable pediatric cystic fibrosis (CF) lung disease in comparison to static breath-hold hyperpolarized 129-xenon MRI (Xe-MRI) and pulmonary function tests.

METHODS

Free-breathing 1-hydrogen MRI and Xe-MRI were acquired from 15 stable pediatric CF patients and seven healthy age-matched participants on two visits, 1 month apart. Same-visit MRI scans were also performed on a subgroup of the CF patients. Following the PREFUL algorithm, regional ventilation (RVent) and regional flow volume loop cross-correlation maps were determined from the free-breathing data. Ventilation defect percentage (VDP) was determined from RVent maps (VDP_RVent ), regional flow volume loop cross-correlation maps (VDP_CC ), VDP_RVent ∪ VDP_CC , and multi-slice Xe-MRI. Repeatability was evaluated using Bland-Altman analysis, coefficient of repeatability (CR), and intraclass correlation.

RESULTS

Minimal bias and no significant differences were reported for all PREFUL MRI and Xe-MRI VDP parameters between intra- and intervisits (all P > 0.05). Repeatability of VDP_RVent , VDP_CC , VDP_RVent ∪ VDP_CC , and multi-slice Xe-MRI were lower between the two-visit scans (CR = 14.81%, 15.36%, 16.19%, and 9.32%, respectively) in comparison to the same-day scans (CR = 3.38%, 2.90%, 1.90%, and 3.92%, respectively). pulmonary function tests showed high interscan repeatability relative to PREFUL MRI and Xe-MRI.

CONCLUSION

PREFUL MRI, similar to Xe-MRI, showed high intravisit repeatability but moderate intervisit repeatability in CF, which may be due to inherent disease instability, even in stable patients. Thus, PREFUL MRI may be considered a suitable outcome measure for future treatment response studies.

Functional lung imaging using novel and emerging MRI techniques

Respiratory diseases are leading causes of death and disability in the world. While early diagnosis is key, this has proven difficult due to the lack of sensitive and non-invasive tools. Computed tomography is regarded as the gold standard for structural lung imaging but lacks functional information and involves significant radiation exposure. Lung magnetic resonance imaging (MRI) has historically been challenging due to its short T2 and low proton density. Hyperpolarised gas MRI is an emerging technique that is able to overcome these difficulties, permitting the functional and microstructural evaluation of the lung. Other novel imaging techniques such as fluorinated gas MRI, oxygen-enhanced MRI, Fourier decomposition MRI and phase-resolved functional lung imaging can also be used to interrogate lung function though they are currently at varying stages of development. This article provides a clinically focused review of these contrast and non-contrast MR imaging techniques and their current applications in lung disease.

Ventilation and perfusion MRI at a 0.35 T MR-Linac: feasibility and reproducibility study

BACKGROUND

Hybrid devices that combine radiation therapy and MR-imaging have been introduced in the clinical routine for the treatment of lung cancer. This opened up not only possibilities in terms of accurate tumor tracking, dose delivery and adapted treatment planning, but also functional lung imaging. The aim of this study was to show the feasibility of Non-uniform Fourier Decomposition (NuFD) MRI at a 0.35 T MR-Linac as a potential treatment response assessment tool, and propose two signal normalization strategies for enhancing the reproducibility of the results.

METHODS

Ten healthy volunteers (median age 28 ± 8 years, five female, five male) were repeatedly scanned at a 0.35 T MR-Linac using an optimized 2D+t balanced steady-state free precession (bSSFP) sequence for two coronal slice positions. Image series were acquired in normal free breathing with breaks inside and outside the scanner as well as deep and shallow breathing. Ventilation- and perfusion-weighted maps were generated for each image series using NuFD. For intra-volunteer ventilation map reproducibility, a normalization factor was defined based on the linear correlation of the ventilation signal and diaphragm position of each scan as well as the diaphragm motion amplitude of a reference scan. This allowed for the correction of signal dependency on the diaphragm motion amplitude, which varies with breathing patterns. The second strategy, which can be used for ventilation and perfusion, eliminates the dependency on the signal amplitude by normalizing the ventilation/perfusion maps with the average ventilation/perfusion signal within a selected region-of-interest (ROI). The position and size dependency of this ROI was analyzed. To evaluate the performance of both approaches, the normalized ventilation/perfusion-weighted maps were compared and the deviation of the mean ventilation/perfusion signal from the reference was calculated for each scan. Wilcoxon signed-rank tests were performed to test whether the normalization methods can significantly improve the reproducibility of the ventilation/perfusion maps.

RESULTS

The ventilation- and perfusion-weighted maps generated with the NuFD algorithm demonstrated a mostly homogenous distribution of signal intensity as expected for healthy volunteers regardless of the breathing maneuver and slice position. Evaluation of the ROI's size and position dependency showed small differences in the performance. Applying both normalization strategies improved the reproducibility of the ventilation by reducing the median deviation of all scans to 9.1%, 5.7% and 8.6% for the diaphragm-based, the best and worst performing ROI-based normalization, respectively, compared to 29.5% for the non-normalized scans. The significance of this improvement was confirmed by the Wilcoxon signed rank test with [Formula: see text] at [Formula: see text]. A comparison of the techniques against each other revealed a significant difference in the performance between best ROI-based normalization and worst ROI ([Formula: see text]) and between best ROI-based normalization and scaling factor ([Formula: see text]), but not between scaling factor and worst ROI ([Formula: see text]). Using the ROI-based approach for the perfusion-maps, the uncorrected deviation of 10.2% was reduced to 5.3%, which was shown to be significant ([Formula: see text]).

CONCLUSIONS

Using NuFD for non-contrast enhanced functional lung MRI at a 0.35 T MR-Linac is feasible and produces plausible ventilation- and perfusion-weighted maps for volunteers without history of chronic pulmonary diseases utilizing different breathing patterns. The reproducibility of the results in repeated scans significantly benefits from the introduction of the two normalization strategies, making NuFD a potential candidate for fast and robust early treatment response assessment of lung cancer patients during MR-guided radiotherapy.

Evaluation of image registration algorithms for 3D phase-resolved functional lung ventilation magnetic resonance imaging in healthy volunteers and chronic obstructive pulmonary disease patients

The purpose of the current study was to assess the influence of the registration algorithms on the repeatability of three-dimensional (3D) phase-resolved functional lung (PREFUL) ventilation magnetic resonance imaging (MRI). Twenty-three healthy volunteers and 10 patients with chronic obstructive pulmonary disease (COPD) underwent 3D PREFUL MRI during tidal breathing. The registration of dynamically acquired data to a fixed image was executed using single-step, stepwise, and group-oriented registration (GOREG) approaches. Advanced Normalization Tools (ANTs) and the Forsberg image-registration package were used for the registration. Image registration algorithms were tested for differences and evaluated by the repeatability analysis of ventilation parameters using coefficient of variation (CoV), intraclass-correlation coefficient, Bland-Altman plots, and correlation to spirometry. Also, the registration time and image quality were computed for all registration approaches. Very strong to strong correlations (r range: 0.917-0.999) were observed between ventilation parameters derived using various registration approaches. Median CoV values of the cross-correlation (CC) parameter were significantly lower (all p ≤ 0.0054) for ANTs GOREG compared with single-step and stepwise ANTs registration. The majority of comparisons between COPD patients and age-matched healthy volunteers showed agreement among the registration approaches. The repeatability of regional ventilation (RVent)-based ventilation defect percentage (VDP_RVent ) and VDP_CC was significantly higher (both p ≤ 0.0054) for Forsberg GOREG compared with ANTs GOREG. All 3D PREFUL-derived ventilation parameters correlated with forced expiratory volume in 1 s (FEV₁ ) and the FEV₁ / forced vital capacity (FVC) ratio (all |r| > 0.40, all p < 0.03). The image sharpness of RVent maps was statistically elevated (all p < 0.001) using GOREG compared with single-step and stepwise registration approaches using ANTs. The best computational performance was achieved with Forsberg GOREG. The GOREG scheme improves the repeatability and image quality of dynamic 3D PREFUL ventilation parameters. Registration time can be ~10-fold reduced to 9 min using the Forsberg method with equal or even improved repeatability and comparable PREFUL ventilation results compared with the ANTs method.

Pulmonary Dysfunction after Pediatric COVID-19

Background Long COVID occurs at a lower frequency in children and adolescents than in adults. Morphologic and free-breathing phase-resolved functional low-field-strength MRI may help identify persistent pulmonary manifestations after SARS-CoV-2 infection. Purpose To characterize both morphologic and functional changes of lung parenchyma at low-field-strength MRI in children and adolescents with post-COVID-19 condition compared with healthy controls. Materials and Methods Between August and December 2021, a cross-sectional clinical trial using low-field-strength MRI was performed in children and adolescents from a single academic medical center. The primary outcome was the frequency of morphologic changes at MRI. Secondary outcomes included MRI-derived functional proton ventilation and perfusion parameters. Clinical symptoms, the duration from positive reverse transcriptase-polymerase chain reaction test result, and serologic parameters were compared with imaging results. Nonparametric tests for pairwise and corrected tests for groupwise comparisons were applied to assess differences in healthy controls, recovered participants, and those with long COVID. Results A total of 54 participants after COVID-19 infection (mean age, 11 years ± 3 [SD]; 30 boys [56%]) and nine healthy controls (mean age, 10 years ± 3; seven boys [78%]) were included: 29 (54%) in the COVID-19 group had recovered from infection and 25 (46%) were classified as having long COVID on the day of enrollment. Morphologic abnormality was identified in one recovered participant. Both ventilated and perfused lung parenchyma (ventilation-perfusion [V/Q] match) was higher in healthy controls (81% ± 6.1) compared with the recovered group (62% ± 19; P = .006) and the group with long COVID (60% ± 20; P = .003). V/Q match was lower in patients with time from COVID-19 infection to study participation of less than 180 days (63% ± 20; P = .03), 180-360 days (63% ± 18; P = .03), and 360 days (41% ± 12; P < .001) as compared with the never-infected healthy controls (81% ± 6.1). Conclusion Low-field-strength MRI showed persistent pulmonary dysfunction in children and adolescents who recovered from COVID-19 and those with long COVID. Clinical trial registration no. NCT04990531 © RSNA, 2022 Supplemental material is available for this article. See also the editorial by Paltiel in this issue.

Assessment of lung ventilation of premature infants with bronchopulmonary dysplasia at 1.5 Tesla using phase-resolved functional lung magnetic resonance imaging

BACKGROUND

The most common chronic complication of preterm birth is bronchopulmonary dysplasia (BPD), widely referred to as chronic lung disease of prematurity. All current definitions rely on characterizing the disease based on respiratory support level and do not provide full understanding of the underlying cardiopulmonary pathophysiology.

OBJECTIVE

To evaluate a rapid functional lung imaging technique in premature infants and to quantitate pulmonary ventilation using 1.5 Tesla magnetic resonance imaging (MRI).

MATERIALS AND METHODS

We conducted a prospective MRI study of 12 premature infants in the neonatal intensive care unit (NICU) using the phase resolved functional lung MRI technique to calculate pulmonary ventilation parameters in preterm infants with and without BPD grade 0/1 (n = 6) and grade 2/3 (n = 6).

RESULTS

The total ventilation defect percentage showed a significant difference between groups (16.0% IQR (11.0%,18%) BPD grade 2/3 vs. 8.0% IQR (4.5%,9.0%) BPD grade 0/1, p = 0.01).

CONCLUSION

Phase-resolved functional lung MRI is feasible for assessment of ventilation defect percentages in preterm infants and shows regional variation in localized lung function in this population.

Artificially-generated consolidations and balanced augmentation increase performance of U-net for lung parenchyma segmentation on MR images

PURPOSE

To improve automated lung segmentation on 2D lung MR images using balanced augmentation and artificially-generated consolidations for training of a convolutional neural network (CNN).

MATERIALS AND METHODS

From 233 healthy volunteers and 100 patients, 1891 coronal MR images were acquired. Of these, 1666 images without consolidations were used to build a binary semantic CNN for lung segmentation and 225 images (187 without consolidations, 38 with consolidations) were used for testing. To increase CNN performance of segmenting lung parenchyma with consolidations, balanced augmentation was performed and artificially-generated consolidations were added to all training images. The proposed CNN (CNNBal/Cons) was compared to two other CNNs: CNNUnbal/NoCons-without balanced augmentation and artificially-generated consolidations and CNNBal/NoCons-with balanced augmentation but without artificially-generated consolidations. Segmentation results were assessed using Sørensen-Dice coefficient (SDC) and Hausdorff distance coefficient.

RESULTS

Regarding the 187 MR test images without consolidations, the mean SDC of CNNUnbal/NoCons (92.1 ± 6% (mean ± standard deviation)) was significantly lower compared to CNNBal/NoCons (94.0 ± 5.3%, P = 0.0013) and CNNBal/Cons (94.3 ± 4.1%, P = 0.0001). No significant difference was found between SDC of CNNBal/Cons and CNNBal/NoCons (P = 0.54). For the 38 MR test images with consolidations, SDC of CNNUnbal/NoCons (89.0 ± 7.1%) was not significantly different compared to CNNBal/NoCons (90.2 ± 9.4%, P = 0.53). SDC of CNNBal/Cons (94.3 ± 3.7%) was significantly higher compared to CNNBal/NoCons (P = 0.0146) and CNNUnbal/NoCons (P = 0.001).

CONCLUSIONS

Expanding training datasets via balanced augmentation and artificially-generated consolidations improved the accuracy of CNNBal/Cons, especially in datasets with parenchymal consolidations. This is an important step towards a robust automated postprocessing of lung MRI datasets in clinical routine.

Free-Breathing Low-Field MRI of the Lungs Detects Functional Alterations Associated With Persistent Symptoms After COVID-19 Infection

OBJECTIVES

With the COVID-19 pandemic, repetitive lung examinations have become necessary to follow-up symptoms and associated alterations. Low-field MRI, benefiting from reduced susceptibility effects, is a promising alternative for lung imaging to limit radiations absorbed by patients during CT examinations, which also have limited capability to assess functional alterations. The aim of this investigative study was to explore the functional abnormalities that free-breathing 0.55 T MRI in combination with the phase-resolved functional lung (PREFUL) analysis could identify in patients with persistent symptoms after COVID-19 infection.

MATERIALS AND METHODS

Seventy-four COVID-19 patients and 8 healthy volunteers were prospectively scanned in free-breathing with a balanced steady-state free-precession sequence optimized at 0.55 T, 5 months postinfection on average. Normalized perfusion (Q), fractional ventilation (FV), and flow-volume loop correlation (FVLc) maps were extracted with the PREFUL technique. Q, FV, and FVLc defects as well as defect overlaps between these metrics were quantified. Morphological turbo-spin-echo images were also acquired, and the extent of abnormalities was scored by a board-certified radiologist. To investigate the functional correlates of persistent symptoms, a recursive feature elimination algorithm was applied to find the most informative variables to detect the presence of persistent symptoms with a logistic regression model and a cross-validation strategy. All MRI metrics, sex, age, body mass index, and the presence of preexisting lung conditions were included.

RESULTS

The most informative variables to detect persistent symptoms were the percentage of concurrent Q and FVLc defects and of areas free of those defects. A detection accuracy of 71.4% was obtained with these 2 variables when fitting the model on the entire dataset. Although none of the single variables differed between patients with and without persistent symptoms ( P > 0.05), the combined score of these 2 variables did ( P < 0.02). This score also showed a consistent increase from healthy volunteers (7.7) to patients without persistent symptoms (8.2) and with persistent symptoms (8.6). The morphological abnormality score showed poor correlation with the functional parameters.

CONCLUSIONS

Functional pulmonary examinations using free-breathing 0.55 T MRI with PREFUL analysis revealed potential quantitative markers of impaired lung function in patients with persistent symptoms after COVID-19 infection, potentially complementing morphologic imaging. Future work is needed to explore the translational relevance and clinical implication of these findings.

A dual center and dual vendor comparison study of automated perfusion-weighted phase-resolved functional lung magnetic resonance imaging with dynamic contrast-enhanced magnetic resonance imaging in patients with cystic fibrosis

For sensitive diagnosis and monitoring of pulmonary disease, ionizing radiation-free imaging methods are of great importance. A noncontrast and free-breathing proton magnetic resonance imaging (MRI) technique for assessment of pulmonary perfusion is phase-resolved functional lung (PREFUL) MRI. Since there is no validation of PREFUL MRI across different centers and scanners, the purpose of this study was to compare perfusion-weighted PREFUL MRI with the well-established dynamic contrast-enhanced (DCE) MRI across two centers on scanners from two different vendors. Sixteen patients with cystic fibrosis (CF) (Center 1: 10 patients; Center 2: 6 patients) underwent PREFUL and DCE MRI at 1.5T in the same imaging session. Normalized perfusion-weighted values and perfusion defect percentage (QDP) values were calculated for the whole lung and three central slices (dorsal, central, ventral of the carina). Obtained parameters were compared using Pearson correlation, Spearman correlation, Bland-Altman analysis, Wilcoxon signed-rank test, and Wilcoxon rank-sum test. Moderate-to-strong correlations between normalized perfusion-weighted PREFUL and DCE values were found (posterior slice: r = 0.69, p < 0.01). Spatial overlap of PREFUL and DCE QDP maps showed an agreement of 79.4% for the whole lung. Further, spatial overlap values of Center 1 were not significantly different to those of Center 2 for the three central slices (p > 0.07). The feasibility of PREFUL MRI across two different centers and two different vendors was shown in patients with CF and obtained results were in agreement with DCE MRI.

PREFUL MRI Depicts Dual Bronchodilator Changes in COPD: A Retrospective Analysis of a Randomized Controlled Trial

Purpose

To assess whether dynamic ventilation and perfusion (Q) biomarkers derived by phase-resolved functional lung (PREFUL) MRI can measure treatment response to 14-day therapy with indacaterol-glycopyrronium (IND-GLY) and correlate to clinical outcomes including lung function, symptoms, and cardiac function in patients with chronic obstructive pulmonary disease (COPD), as determined by spirometry, body plethysmography, cardiac MRI, and dyspnea score measurements.

Materials and Methods

The cardiac left ventricular function in COPD (CLAIM) study enrolled patients aged 40 years or older with COPD, stable cardiovascular function, and hyperinflation (residual volume > 135% predicted). Dynamic MRI data of these patients were retrospectively analyzed using the PREFUL technique to assess the effect of 14-day IND-GLY treatment versus placebo on regional measurements of ventilation dynamics. After manual segmentation of the lung parenchyma, flow-volume loops of each voxel were correlated to an individualized reference flow-volume loop, creating a two-dimensional flow-volume loop correlation map (FVL-CM) as a measure of ventilation dynamics. Ventilation-perfusion match (VQM) was evaluated in combination with perfusion and regional ventilation (VQM_RVent) and with perfusion and the FVL-CM measurement (VQM_CM). For image and statistical analysis, the lung parenchyma was segmented as a region of interest by manually delineating the lung boundary and excluding the large (central) vessels for each section. Differences in ventilation, perfusion, and VQM between IND-GLY and placebo were compared using analysis of variance, with study treatment, patient, and period included as factors.

Results

Fifty patients (mean age, 64.3 years ± 7.65 [SD]; 35 men) were included in this analysis. IND-GLY significantly increased mean correlation as measured with FVL-CM versus that of placebo (least squares [LS] means treatment difference: 0.05 [95% CI: 0.03, 0.07]; P < .0001). Compared with placebo, IND-GLY increased mean Q (LS means treatment difference: 9.27 mL/min/100 mL [95% CI: 0.05, 18.49]; P = .049) and improved both VQM_CM and VQM_RVent (LS means treatment difference: 0.06 [95% CI: 0.03, 0.08]; P < .0001 and 0.05 [95% CI: 0.02, 0.08]; P = .001, respectively).

Conclusion

Regional ventilation dynamics and VQM measured by PREFUL MRI show treatment response in COPD.

Supplemental material is available for this article.

Clinical trial registration no. NTR6831

Keywords: MRI, COPD, Perfusion, Ventilation, Lung, Pulmonary

Published under a CC BY 4.0 license

Clinical Feasibility of Structural and Functional MRI in Free-Breathing Neonates and Infants

BACKGROUND

Evaluation of structural lung abnormalities with magnetic resonance imaging (MRI) has previously been shown to be predictive of clinical neonatal outcomes in preterm birth. MRI during free-breathing with phase-resolved functional lung (PREFUL) may allow for complimentary functional information without exogenous contrast.

PURPOSE

To investigate the feasibility of structural and functional pulmonary MRI in a cohort of neonates and infants with no cardiorespiratory disease. Macrovascular pulmonary blood flows were also evaluated.

STUDY TYPE

Prospective.

POPULATION

Ten term infants with no clinically defined cardiorespiratory disease were imaged. Infants recruited from the general population and neonatal intensive care unit (NICU) were studied.

FIELD STRENGTH/SEQUENCE

T₁ -weighted VIBE, T₂ -weighted BLADE uncorrected for motion. Ultrashort echo time (UTE) and 3D-flow data were acquired during free-breathing with self-navigation and retrospective reconstruction. Single slice 2D-gradient echo (GRE) images were acquired during free-breathing for PREFUL analysis. Imaging was performed at 3 T.

ASSESSMENT

T₁ , T₂ , and UTE images were scored according to the modified Ochiai scheme by three pediatric body radiologists. Ventilation/perfusion-weighted maps were extracted from free-breathing GRE images using PREFUL analysis. Ventilation and perfusion defect percent (VDP, QDP) were calculated from the segmented ventilation and perfusion-weighted maps. Time-averaged cardiac blood velocities from three-dimensional-flow were evaluated in major pulmonary arteries and veins.

STATISTICAL TEST

Intraclass correlation coefficient (ICC).

RESULTS

The ICC of replicate structural scores was 0.81 (95% CI: 0.45-0.95) across three observers. Elevated Ochiai scores, VDP, and QDP were observed in two NICU participants. Excluding these participants, mean ± standard deviation structural scores were 1.2 ± 0.8, while VDP and QDP were 1.0% ± 1.1% and 0.4% ± 0.5%, respectively. Main pulmonary arterial blood flows normalized to body surface area were 3.15 ± 0.78 L/min/m² .

DATA CONCLUSION

Structural and functional pulmonary imaging is feasible using standard clinical MRI hardware (commercial whole-body 3 T scanner, table spine array, and flexible thoracic array) in free-breathing infants.

EVIDENCE LEVEL

2 TECHNICAL EFFICACY: Stage 1.

Abnormal dynamic ventilation function of COVID-19 survivors detected by pulmonary free-breathing proton MRI

Objectives

To visualize and quantitatively assess regional lung function of survivors of COVID-19 who were hospitalized using pulmonary free-breathing ¹H MRI.

Methods

A total of 12 healthy volunteers and 27 COVID-19 survivors (62.4 ± 8.1 days between infection and image acquisition) were recruited in this prospective study and performed chest ¹H MRI acquisitions with free tidal breathing. Then, conventional Fourier decomposition ventilation (FD-V) and global fractional ventilation (FV_Global) were analyzed. Besides, a modified PREFUL (mPREFUL) method was developed to adapt to COVID-19 survivors and generate dynamic ventilation maps and parameters. All the ventilation maps and parameters were analyzed using Student’s t-test. Pearson’s correlation and a Bland-Altman plot between FV_Global and mPREFUL were analyzed.

Results

There was no significant difference between COVID-19 and healthy groups regarding a static FD-V map (0.47 ± 0.12 vs 0.42 ± 0.08; p = .233). However, mPREFUL demonstrated lots of regional high ventilation areas (high ventilation percentage (HVP): 23.7% ± 10.6%) existed in survivors. This regional heterogeneity (i.e., HVP) in survivors was significantly higher than in healthy volunteers (p = .003). The survivors breathed deeper (flow-volume loop: 5375 ± 3978 vs 1688 ± 789; p = .005), and breathed more air in respiratory cycle (total amount: 62.6 ± 19.3 vs 37.3 ± 9.9; p < .001). Besides, mPREFUL showed both good Pearson’s correlation (r = 0.74; p < .001) and Bland-Altman consistency (mean bias = −0.01) with FV_Global.

Conclusions

Dynamic ventilation imaging using pulmonary free-breathing ¹H MRI found regional abnormity of dynamic ventilation function in COVID-19 survivors.

Key Points

• Pulmonary free-breathing¹H MRI was used to visualize and quantitatively assess regional lung ventilation function of COVID-19 survivors.

• Dynamic ventilation maps generated from¹H MRI were more sensitive to distinguish the COVID-19 and healthy groups (total air amount: 62.6 ± 19.3 vs 37.3 ± 9.9; p < .001), compared with static ventilation maps (FD-V value: 0.47 ± 0.12 vs 0.42 ± 0.08; p = .233).

• COVID-19 survivors had larger regional heterogeneity (high ventilation percentage: 23.7% ± 10.6% vs 13.1% ± 7.9%; p = .003), and breathed deeper (flow-volume loop: 5375 ± 3978 vs 1688 ± 789; p = .005) than healthy volunteers.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00330-022-08605-w.

Validation of Phase-Resolved Functional Lung (PREFUL) Magnetic Resonance Imaging Pulse Wave Transit Time Compared to Echocardiography in Chronic Obstructive Pulmonary Disease

BACKGROUND

Phase-resolved functional lung (PREFUL) magnetic resonance imaging (MRI) pulmonary pulse wave transit time (pPTT) is a contrast agent free, vascular imaging biomarker, but has not been validated in chronic obstructive pulmonary disease (COPD).

PURPOSE

To validate PREFUL with echocardiographic pPTT as a reference standard and to compare arterial/venous pPTT mapping with spirometry and clinical parameters.

STUDY TYPE

Prospective.

POPULATION

Twenty-one patients (62% female) with COPD and 44 healthy participants (50% female).

FIELD STRENGTH/SEQUENCE

1.5 T; 2D-spoiled gradient-echo sequence.

ASSESSMENT

Three coronal PREFUL MRI slices, echocardiography, and spirometry including forced expiratory volume in 1 second (FEV1, liter) and predicted defined as FEV1 in% divided by the population average FEV1%, were performed. Pulmonary pulse transit time from the main artery to the microvasculature (PREFUL pPTT), to the right upper lobe vein (PREFUL pPTT_av , echo pPTT_av ), from microvasculature to right upper lobe vein (PREFUL_vein ) and the ratio of PREFUL pPTT to PREFUL pPTT_vein were calculated. Body mass index (BMI), Global Initiative for COPD (GOLD) stage 1-4, disease duration, and cigarette packs smoked per day multiplied by the smoked years (pack years) were computed.

STATISTICAL TESTS

Shapiro-Wilk-test, paired-two-sided-t-tests, Bland-Altman-analysis, coefficient of variation, Pearson ρ were applied, pPTT data were compared between 21 subjects from the 44 healthy subjects who were age- and sex-matched to the COPD cohort, P < 0.05 was considered statistically significant.

RESULTS

PREFUL pPTT_av significantly correlated with echo pPTT_av (ρ = 0.95) with 1.85 msec bias, 95% limits of agreement: 55.94 msec, -52.23 msec in all participants (P = 0.59). In the healthy participants, PREFUL and echo pPTT_av significantly correlated with age (ρ = 0.81, ρ = 0.78), FEV1 (ρ = -0.47, ρ = -0.34) and BMI (ρ = 0.56, ρ = 0.51). In COPD patients, PREFUL pPTT significantly correlated with FEV1 predicted (ρ = -0.59), GOLD (ρ = 0.53), disease duration (ρ = 0.54), and pack years (ρ = 0.49).

DATA CONCLUSION

Arteriovenous PTT measured by PREFUL MRI corresponds precisely to echocardiography and appears to be feasible even in severe COPD.

EVIDENCE LEVEL

1 TECHNICAL EFFICACY: Stage 2.

Repeatability of dynamic 3D phase-resolved functional lung (PREFUL) ventilation MR Imaging in patients with chronic obstructive pulmonary disease and healthy volunteers

BACKGROUND

A previous study has demonstrated the feasibility of 3D phase-resolved functional lung (PREFUL) MRI in healthy volunteers and patients with chronic pulmonary disease. Before clinical use, the repeatability of the ventilation parameters derived from 3D PREFUL MRI must be determined.

PURPOSE

To evaluate repeatability of 3D PREFUL and to compare with pulmonary functional lung testing (PFT).

STUDY TYPE

Prospective.

POPULATION

Fifty-three healthy subjects and 13 patients with chronic obstructive pulmonary disease (COPD).

FIELD STRENGTH/SEQUENCE

A prototype 3D stack-of-stars spoiled-gradient-echo sequence at 1.5 T.

ASSESSMENT

Study participants underwent repeated MRI examination (median time interval between scans COPD/healthy subjects [interquartile range]: 7/0 days [6-8/0-0 days]) and one PFT carried out at the time of the baseline MRI. For 3D PREFUL, regional ventilation (RVent) and flow-volume loops were computed and rated by cross-correlation (CC). Also, ventilation time-to-peak (VTTP) was computed. Ventilation defect percentage (VDP) maps were obtained for RVent and CC.

STATISTICAL TESTS

Repeatability of 3D PREFUL parameters was evaluated using Bland-Altman analysis, coefficient of variation (COV) and intraclass correlation coefficient (ICC). The relation between 3D PREFUL and PFT measures (forced expiratory volume in 1 second (FEV₁ ) and forced vital capacity (FVC) was assessed using the Pearson correlation coefficient (r).

RESULTS

In healthy subjects and COPD patients, no significant bias (all P range: 0.09-0.77) and a moderate to good repeatability of RVent, VTTP, and VDP_RVent were found (COV range: 0.1%-18.2%, ICC range: 0.51-0.88). For CC and VDP_CC moderate repeatability was found (COV range: 0.6%-43.6%, ICC: 0.38-0.60). CC, VDP_RVent , and VDP_CC showed a good correlation with FEV₁ (all |r| > 0.58, all P < 0.05) and FEV₁ /FVC ratio (all |r| > 0.62, all P < 0.05).

DATA CONCLUSION

3D PREFUL provided a good repeatability of RVent, VTTP, and VDP_RVent and moderate repeatability of CC and VDP_CC in healthy volunteers and COPD patients, and correlated well with FEV₁ and FEV₁ /FVC.

LEVEL OF EVIDENCE

2 TECHNICAL EFFICACY STAGE: 2.

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.