Reduction of ringing artifacts induced by diaphragm drifting in free-breathing dynamic pulmonary MRI using 3D koosh-ball acquisition.

PURPOSE: To reduce the ringing artifacts of the motion-resolved images in free-breathing dynamic pulmonary MRI. METHODS: A golden-step based interleaving (GSI) technique was proposed to reduce ringing artifacts induced by diaphragm drifting. The pulmonary MRI data were acquired using a superior-inferior navigated 3D radial UTE sequence in an interleaved manner during free breathing. Successive interleaves were acquired in an incoherent fashion along the polar direction. Four-dimensional images were reconstructed from the motion-resolved k-space data obtained by retrospectively binning. The reconstruction algorithms included standard nonuniform fast Fourier transform (NUFFT), Voronoi-density-compensated NUFFT, extra-dimensional UTE, and motion-state weighted motion-compensation reconstruction. The proposed interleaving technique was compared with a conventional sequential interleaving (SeqI) technique on a phantom and eight subjects. RESULTS: The quantified ringing artifacts level in the motion-resolved image is positively correlated with the quantified nonuniformity level of the corresponding k-space. The nonuniformity levels of the end-expiratory and end-inspiratory k-space binned from GSI data (0.34 ± 0.07, 0.33 ± 0.05) are significantly lower with statistical significance (p < 0.05) than that binned from SeqI data (0.44 ± 0.11, 0.42 ± 0.12). Ringing artifacts are substantially reduced in the dynamic images of eight subjects acquired using the proposed technique in comparison with that acquired using the conventional SeqI technique. CONCLUSION: Ringing artifacts in the motion-resolved images induced by diaphragm drifting can be reduced using the proposed GSI technique for free-breathing dynamic pulmonary MRI. This technique has the potential to reduce ringing artifacts in free-breathing liver and kidney MRI based on full-echo interleaved 3D radial acquisition.

Assessment of Lung Nodule Detection and Lung CT Screening Reporting and Data System Classification Using Zero Echo Time Pulmonary MRI.

BACKGROUND: The detection rate of lung nodules has increased considerably with CT as the primary method of examination, and the repeated CT examinations at 3 months, 6 months or annually, based on nodule characteristics, have increased the radiation exposure of patients. So, it is urgent to explore a radiation-free MRI examination method that can effectively address the challenges posed by low proton density and magnetic field inhomogeneities. PURPOSE: To evaluate the potential of zero echo time (ZTE) MRI in lung nodule detection and lung CT screening reporting and data system (lung-RADS) classification, and to explore the value of ZTE-MRI in the assessment of lung nodules. STUDY TYPE: Prospective. POPULATION: 54 patients, including 21 men and 33 women. FIELD STRENGTH/SEQUENCE: Chest CT using a 16-slice scanner and ZTE-MRI at 3.0T based on fast gradient echo. ASSESSMENT: Nodule type (ground-glass nodules, part-solid nodules, and solid nodules), lung-RADS classification, and nodule diameter (manual measurement) on CT and ZTE-MRI images were recorded. STATISTICAL TESTS: The percent of concordant cases, Kappa value, intraclass correlation coefficient (ICC), Wilcoxon signed-rank test, Spearman's correlation, and Bland-Altman. The p-value <0.05 is considered significant. RESULTS: A total of 54 patients (age, 54.8 ± 11.9 years; 21 men) with 63 nodules were enrolled. Compared with CT, the total nodule detection rate of ZTE-MRI was 85.7%. The intermodality agreement of ZTE-MRI and CT lung nodules type evaluation was substantial (Kappa = 0.761), and the intermodality agreement of ZTE-MRI and CT lung-RADS classification was moderate (Kappa = 0.592). The diameter measurements between ZTE-MRI and CT showed no significant difference and demonstrated a high degree of interobserver (ICC = 0.997-0.999) and intermodality (ICC = 0.956-0.985) agreements. DATA CONCLUSION: The measurement of nodule diameter by pulmonary ZTE-MRI is similar to that by CT, but the ability of lung-RADS to classify nodes from MRI images still requires further research. LEVEL OF EVIDENCE: 2 TECHNICAL EFFICACY: Stage 2.

Motion-compensated low-rank reconstruction for simultaneous structural and functional UTE lung MRI.

PURPOSE: Three-dimensional UTE MRI has shown the ability to provide simultaneous structural and functional lung imaging, but it is limited by respiratory motion and relatively low lung parenchyma SNR. The purpose of this paper is to improve this imaging by using a respiratory phase-resolved reconstruction approach, named motion-compensated low-rank reconstruction (MoCoLoR), which directly incorporates motion compensation into a low-rank constrained reconstruction model for highly efficient use of the acquired data. THEORY AND METHODS: The MoCoLoR reconstruction is formulated as an optimization problem that includes a low-rank constraint using estimated motion fields to reduce the rank, optimizing over both the motion fields and reconstructed images. The proposed reconstruction along with XD and motion state-weighted motion-compensation (MostMoCo) methods were applied to 18 lung MRI scans of pediatric and young adult patients. The data sets were acquired under free-breathing and without sedation with 3D radial UTE sequences in approximately 5 min. After reconstruction, they went through ventilation analyses. Performance across reconstruction regularization and motion-state parameters were also investigated. RESULTS: The in vivo experiments results showed that MoCoLoR made efficient use of the data, provided higher apparent SNR compared with state-of-the-art XD reconstruction and MostMoCo reconstructions, and yielded high-quality respiratory phase-resolved images for ventilation mapping. The method was effective across the range of patients scanned. CONCLUSION: The motion-compensated low-rank regularized reconstruction approach makes efficient use of acquired data and can improve simultaneous structural and functional lung imaging with 3D-UTE MRI. It enables the scanning of pediatric patients under free-breathing and without sedation.

Dynamic mode decomposition of dynamic MRI for assessment of pulmonary ventilation and perfusion.

PURPOSE: To introduce dynamic mode decomposition (DMD) as a robust alternative for the assessment of pulmonary functional information from dynamic non-contrast-enhanced acquisitions. METHODS: Pulmonary fractional ventilation and normalized perfusion maps were obtained using DMD from simulated phantoms as well as in vivo dynamic acquisitions of healthy volunteers at 1.5T. The performance of DMD was compared with conventional Fourier decomposition (FD) and matrix pencil (MP) methods in estimating functional map values. The proposed method was evaluated based on estimated signal amplitude in functional maps across varying number of measurements. RESULTS: Quantitative assessments performed on phantoms and in vivo measurements indicate that DMD is capable of successfully obtaining pulmonary functional maps. Specifically, compared to FD and MP methods, DMD is able to reduce variations in estimated amplitudes across different number of measurements. This improvement is evident in the fractional ventilation and normalized perfusion maps obtain from phantom simulations with frequency variations and noise, as well as in the maps obtained from in vivo measurements. CONCLUSIONS: A robust method for accurately estimating pulmonary ventilation and perfusion related signal changes in dynamic acquisitions is presented. The proposed method uses DMD to obtain functional maps reliably, while reducing amplitude variations caused by differences in number of measurements.

A Dual-Channel Deep Learning Approach for Lung Cavity Estimation From Hyperpolarized Gas and Proton MRI.

BACKGROUND: Hyperpolarized gas MRI can quantify regional lung ventilation via biomarkers, including the ventilation defect percentage (VDP). VDP is computed from segmentations derived from spatially co-registered functional hyperpolarized gas and structural proton (1 H)-MRI. Although acquired at similar lung inflation levels, they are frequently misaligned, requiring a lung cavity estimation (LCE). Recently, single-channel, mono-modal deep learning (DL)-based methods have shown promise for pulmonary image segmentation problems. Multichannel, multimodal approaches may outperform single-channel alternatives. PURPOSE: We hypothesized that a DL-based dual-channel approach, leveraging both 1 H-MRI and Xenon-129-MRI (129 Xe-MRI), can generate LCEs more accurately than single-channel alternatives. STUDY TYPE: Retrospective. POPULATION: A total of 480 corresponding 1 H-MRI and 129 Xe-MRI scans from 26 healthy participants (median age [range]: 11 [8-71]; 50% females) and 289 patients with pulmonary pathologies (median age [range]: 47 [6-83]; 51% females) were split into training (422 scans [88%]; 257 participants [82%]) and testing (58 scans [12%]; 58 participants [18%]) sets. FIELD STRENGTH/SEQUENCE: 1.5-T, three-dimensional (3D) spoiled gradient-recalled 1 H-MRI and 3D steady-state free-precession 129 Xe-MRI. ASSESSMENT: We developed a multimodal DL approach, integrating 129 Xe-MRI and 1 H-MRI, in a dual-channel convolutional neural network. We compared this approach to single-channel alternatives using manually edited LCEs as a benchmark. We further assessed a fully automatic DL-based framework to calculate VDPs and compared it to manually generated VDPs. STATISTICAL TESTS: Friedman tests with post hoc Bonferroni correction for multiple comparisons compared single-channel and dual-channel DL approaches using Dice similarity coefficient (DSC), average boundary Hausdorff distance (average HD), and relative error (XOR) metrics. Bland-Altman analysis and paired t-tests compared manual and DL-generated VDPs. A P value < 0.05 was considered statistically significant. RESULTS: The dual-channel approach significantly outperformed single-channel approaches, achieving a median (range) DSC, average HD, and XOR of 0.967 (0.867-0.978), 1.68 mm (37.0-0.778), and 0.066 (0.246-0.045), respectively. DL-generated VDPs were statistically indistinguishable from manually generated VDPs (P = 0.710). DATA CONCLUSION: Our dual-channel approach generated LCEs, which could be integrated with ventilated lung segmentations to produce biomarkers such as the VDP without manual intervention. EVIDENCE LEVEL: 4. TECHNICAL EFFICACY: Stage 1.

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.

Phase-cycled balanced SSFP imaging for non-contrast-enhanced functional lung imaging.

PURPOSE: To introduce phase-cycled balanced SSFP (bSSFP) acquisition as an alternative in Fourier decomposition MRI for improved robustness against field inhomogeneities. METHODS: Series 2D dynamic lung images were acquired in 5 healthy volunteers at 1.5 T and 3 T using bSSFP sequence with multiple RF phase increments and compared with conventional single RF phase increment acquisitions. The approach was evaluated based on functional map homogeneity analysis, while ensuring image and functional map quality by means of SNR and contrast-to-noise ratio analyses. RESULTS: At both field strengths, functional maps obtained with phase-cycled acquisitions displayed improved robustness against local signal losses compared with single-phase acquisitions. The coefficient of variation (mean ± SD, across volunteers) measured in the ventilation maps resulted in 29.7 ± 2.6 at 1.5 T and 37.5 ± 3.1 at 3 T for phase-cycled acquisitions, compared with 39.9 ± 5.2 at 1.5 T and 49.5 ± 3.7 at 3 T for single-phase acquisitions, indicating a significant improvement ( p<0.05$$ p<0.05 $$ ) in ventilation map homogeneity. CONCLUSIONS: Phase-cycled bSSFP acquisitions improve robustness against field inhomogeneity artifacts and significantly improve ventilation map homogeneity at both field strengths. As such, phase-cycled bSSFP may serve as a robust alternative in lung function assessments.

Examining lung microstructure using 19 F MR diffusion imaging in COPD patients.

PURPOSE: To examine the time-dependent diffusion of fluorinated (19 F) gas in human lungs for determination of surface-to-volume ratio in comparison to results from hyperpolarized 129 Xe and lung function testing in healthy volunteers and patients with chronic obstructive pulmonary disease. METHODS: Diffusion of fluorinated gas in the short-time regime was measured using multiple gradient-echo sequences with a single pair of trapezoidal gradient pulses. Pulmonary surface-to-volume ratio was calculated using a first-order approximation of the time-dependent diffusion in a study with 20 healthy volunteers and 22 patients with chronic obstructive pulmonary disease. The repeatability after 7 days as well as the correlation with hyperpolarized 129 Xe diffusion MRI and lung function testing was analyzed. RESULTS: Using 19 F diffusion MRI, the median surface-to-volume ratio is significantly decreased in chronic obstructive pulmonary disease patients (S/V = 126 cm-1 [87-144 cm-1 ]) compared with healthy volunteers (S/V = 164 cm-1 [160-84 cm-1 ], p < 0.0001). No significant difference was found between measurements within 7 days for healthy (p = 0.88, median coefficient of variation = 4.3%) and diseased subjects (p = 0.58, median coefficient of variation= 6.7%). Linear correlations were found with S/V from 129 Xe diffusion MRI (r = 0.85, p = 0.001) and the forced expiratory volume in 1 second (r = 0.68, p < 0.0001). CONCLUSION: Examination of lung microstructure using time-dependent diffusion measurement of inhaled 19 F is feasible, repeatable, and correlates with established measurements.

Extreme MRI: Large-scale volumetric dynamic imaging from continuous non-gated acquisitions.

PURPOSE: To develop a framework to reconstruct large-scale volumetric dynamic MRI from rapid continuous and non-gated acquisitions, with applications to pulmonary and dynamic contrast-enhanced (DCE) imaging. THEORY AND METHODS: The problem considered here requires recovering 100 gigabytes of dynamic volumetric image data from a few gigabytes of k-space data, acquired continuously over several minutes. This reconstruction is vastly under-determined, heavily stressing computing resources as well as memory management and storage. To overcome these challenges, we leverage intrinsic three-dimensional (3D) trajectories, such as 3D radial and 3D cones, with ordering that incoherently cover time and k-space over the entire acquisition. We then propose two innovations: (a) A compressed representation using multiscale low-rank matrix factorization that constrains the reconstruction problem, and reduces its memory footprint. (b) Stochastic optimization to reduce computation, improve memory locality, and minimize communications between threads and processors. We demonstrate the feasibility of the proposed method on DCE imaging acquired with a golden-angle ordered 3D cones trajectory and pulmonary imaging acquired with a bit-reversed ordered 3D radial trajectory. We compare it with "soft-gated" dynamic reconstruction for DCE and respiratory-resolved reconstruction for pulmonary imaging. RESULTS: The proposed technique shows transient dynamics that are not seen in gating-based methods. When applied to datasets with irregular, or non-repetitive motions, the proposed method displays sharper image features. CONCLUSIONS: We demonstrated a method that can reconstruct massive 3D dynamic image series in the extreme undersampling and extreme computation setting.

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.

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.

Nonuniform Fourier-decomposition MRI for ventilation- and perfusion-weighted imaging of the lung.

PURPOSE: To improve the robustness of pulmonary ventilation- and perfusion-weighted imaging with Fourier decomposition (FD) MRI in the presence of respiratory and cardiac frequency variations by replacing the standard fast Fourier transform with the more general nonuniform Fourier transform. THEORY AND METHODS: Dynamic coronal single-slice MRI of the thorax was performed in 11 patients and 5 healthy volunteers on a 1.5T whole-body scanner using a 2D ultra-fast balanced steady-state free-precession sequence with temporal resolutions of 4-9 images/s. For the proposed nonuniform Fourier-decomposition (NUFD) approach, the original signal with variable physiological frequencies that was acquired with constant sampling rate was retrospectively transformed into a signal with (ventilation or perfusion) frequency-adapted sampling rate. For that purpose, frequency tracking was performed with the synchro-squeezed wavelet transform. Ventilation- and perfusion-weighted NUFD amplitude and signal delay maps were generated and quantitatively compared with regularly sampled FD maps based on their signal-to-noise ratio (SNR). RESULTS: Volunteers and patients showed statistically significant increases of SNR in frequency-adapted NUFD results compared to regularly sampled FD results. For ventilation data, the mean SNR increased by 43.4%±25.3% and 24.4%±31.9% in volunteers and patients, respectively; for perfusion data, SNR increased by 93.0%±36.1% and 75.6%±62.8% . Two patients showed perfusion signal in pulmonary areas with NUFD that could not be imaged with FD. CONCLUSION: This study demonstrates that using nonuniform Fourier transform in combination with frequency tracking can significantly increase SNR and reduce frequency overlaps by collecting the signal intensity onto single frequency bins.

Feasibility of quantitative regional ventilation and perfusion mapping with phase-resolved functional lung (PREFUL) MRI in healthy volunteers and COPD, CTEPH, and CF patients.

PURPOSE: In this feasibility study, a phase-resolved functional lung imaging postprocessing method for extraction of dynamic perfusion (Q) and ventilation (V) parameters using a conventional 1H lung MRI Fourier decomposition acquisition is introduced. METHODS: Time series of coronal gradient-echo MR images with a temporal resolution of 288 to 324 ms of two healthy volunteers, one patient with chronic thromboembolic hypertension, one patient with cystic fibrosis, and one patient with chronic obstructive pulmonary disease were acquired at 1.5 T. Using a sine model to estimate cardiac and respiratory phases of each image, all images were sorted to reconstruct full cardiac and respiratory cycles. Time to peak (TTP), V/Q maps, and fractional ventilation flow-volume loops were calculated. RESULTS: For the volunteers, homogenous ventilation and perfusion TTP maps (V-TTP, Q-TTP) were obtained. The chronic thromboembolic hypertension patient showed increased perfusion TTP in hypoperfused regions in visual agreement with dynamic contrast-enhanced MRI, which improved postpulmonary endaterectomy surgery. Cystic fibrosis and chronic obstructive pulmonary disease patients showed a pattern of increased V-TTP and Q-TTP in regions of hypoventilation and decreased perfusion. Fractional ventilation flow-volume loops of the chronic obstructive pulmonary disease patient were smaller in comparison with the healthy volunteer, and showed regional differences in visual agreement with functional small airways disease and emphysema on CT. CONCLUSIONS: This study shows the feasibility of phase-resolved functional lung imaging to gain quantitative information regarding regional lung perfusion and ventilation without the need for ultrafast imaging, which will be advantageous for future clinical translation. Magn Reson Med 79:2306-2314, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Pulmonary MRI of neonates in the intensive care unit using 3D ultrashort echo time and a small footprint MRI system.

PURPOSE: To determine the feasibility of pulmonary magnetic resonance imaging (MRI) of neonatal lung structures enabled by combining two novel technologies: first, a 3D radial ultrashort echo time (UTE) pulse sequence capable of high spatial resolution full-chest imaging in nonsedated quiet-breathing neonates; and second, a unique, small-footprint 1.5T MRI scanner design adapted for neonatal imaging and installed within the neonatal intensive care unit (NICU). MATERIALS AND METHODS: Ten patients underwent MRI within the NICU, in accordance with an approved Institutional Review Board protocol. Five had clinical diagnoses of bronchopulmonary dysplasia (BPD), and five had putatively normal lung function. Pulmonary imaging was performed at 1.5T using 3D radial UTE and standard 3D fast gradient recalled echo (FGRE). Diagnostic quality, presence of motion artifacts, and apparent severity of lung pathology were evaluated by two radiologists. Quantitative metrics were additionally used to evaluate lung parenchymal signal. RESULTS: UTE images showed significantly higher signal in lung parenchyma (P < 0.0001) and fewer apparent motion artifacts compared to FGRE (P = 0.046). Pulmonary pathology was more severe in patients diagnosed with BPD relative to controls (P = 0.001). Infants diagnosed with BPD also had significantly higher signal in lung parenchyma, measured using UTE, relative to controls (P = 0.002). CONCLUSION: These results demonstrate the technical feasibility of pulmonary MRI in free-breathing, nonsedated infants in the NICU at high, isotropic resolutions approaching that achievable with computed tomography (CT). There is potential for pulmonary MRI to play a role in improving how clinicians understand and manage care of neonatal and pediatric pulmonary diseases. J. Magn. Reson. Imaging 2016. LEVEL OF EVIDENCE: 2 J. Magn. Reson. Imaging 2017;45:463-471.

Quantification of neonatal lung parenchymal density via ultrashort echo time MRI with comparison to CT.

PURPOSE: To demonstrate that ultrashort echo time (UTE) magnetic resonance imaging (MRI) can achieve computed tomography (CT)-like quantification of lung parenchyma in free-breathing, non-sedated neonates. Because infant CTs are used sparingly, parenchymal disease evaluation via UTE MRI has potential for translational impact. MATERIALS AND METHODS: Two neonatal control cohorts without suspected pulmonary morbidities underwent either a research UTE MRI (n = 5; 1.5T) or a clinically-ordered CT (n = 9). Whole-lung means and anterior-posterior gradients of UTE-measured image intensity (arbitrary units, au, normalized to muscle) and CT-measured density (g/cm3 ) were compared (Mann-Whitney U-test). Separately, a diseased neonatal cohort (n = 5) with various pulmonary morbidities underwent both UTE MRI and CT. UTE intensity and CT density were compared with Spearman correlations within ∼33 anatomically matched regions of interest (ROIs) in each diseased subject, spanning low- to high-density tissues. Radiological classifications were evaluated in all ROIs, with mean UTE intensities and CT densities compared in each classification. RESULTS: In control subjects, whole-lung UTE intensities (0.51 ± 0.04 au) were similar to CT densities (0.44 ± 0.09 g/cm3 ) (P = 0.062), as were UTE (0.021 ± 0.020 au/cm) and CT (0.034 ± 0.024 [g/cm3 ]/cm) anterior-posterior gradients (P = 0.351). In diseased subjects' ROIs, significant correlations were observed between UTE and CT (P ≤0.007 in each case). Relative differences between UTE and CT were small in all classifications (4-25%). CONCLUSION: These results demonstrate a strong association between UTE image intensity and CT density, both between whole-lung tissue in control patients and regional radiological pathologies in diseased patients. This indicates the potential for UTE MRI to longitudinally evaluate neonatal pulmonary disease and to provide visualization of pathologies similar to CT, without sedation/anesthesia or ionizing radiation. LEVEL OF EVIDENCE: 3 Technical Efficacy: Stage 2 J. Magn. Reson. Imaging 2017;46:992-1000.

Long T2 suppression in native lung 3-D imaging using k-space reordered inversion recovery dual-echo ultrashort echo time MRI.

OBJECTIVE: Long T2 species can interfere with visualization of short T2 tissue imaging. For example, visualization of lung parenchyma can be hindered by breathing artifacts primarily from fat in the chest wall. The purpose of this work was to design and evaluate a scheme for long T2 species suppression in lung parenchyma imaging using 3-D inversion recovery double-echo ultrashort echo time imaging with a k-space reordering scheme for artifact suppression. MATERIALS AND METHODS: A hyperbolic secant (HS) pulse was evaluated for different tissues (T1/T2). Bloch simulations were performed with the inversion pulse followed by segmented UTE acquisition. Point spread function (PSF) was simulated for a standard interleaved acquisition order and a modulo 2 forward-reverse acquisition order. Phantom and in vivo images (eight volunteers) were acquired with both acquisition orders. Contrast to noise ratio (CNR) was evaluated in in vivo images prior to and after introduction of the long T2 suppression scheme. RESULTS: The PSF as well as phantom and in vivo images demonstrated reduction in artifacts arising from k-space modulation after using the reordering scheme. CNR measured between lung and fat and lung and muscle increased from -114 and -148.5 to +12.5 and 2.8 after use of the IR-DUTE sequence. Paired t test between the CNRs obtained from UTE and IR-DUTE showed significant positive change (p < 0.001 for lung-fat CNR and p = 0.03 for lung-muscle CNR). CONCLUSION: Full 3-D lung parenchyma imaging with improved positive contrast between lung and other long T2 tissue types can be achieved robustly in a clinically feasible time using IR-DUTE with image subtraction when segmented radial acquisition with k-space reordering is employed.

Pulmonary CT and MRI phenotypes that help explain chronic pulmonary obstruction disease pathophysiology and outcomes.

Pulmonary x-ray computed tomographic (CT) and magnetic resonance imaging (MRI) research and development has been motivated, in part, by the quest to subphenotype common chronic lung diseases such as chronic obstructive pulmonary disease (COPD). For thoracic CT and MRI, the main COPD research tools, disease biomarkers are being validated that go beyond anatomy and structure to include pulmonary functional measurements such as regional ventilation, perfusion, and inflammation. In addition, there has also been a drive to improve spatial and contrast resolution while at the same time reducing or eliminating radiation exposure. Therefore, this review focuses on our evolving understanding of patient-relevant and clinically important COPD endpoints and how current and emerging MRI and CT tools and measurements may be exploited for their identification, quantification, and utilization. Since reviews of the imaging physics of pulmonary CT and MRI and reviews of other COPD imaging methods were previously published and well-summarized, we focus on the current clinical challenges in COPD and the potential of newly emerging MR and CT imaging measurements to address them. Here we summarize MRI and CT imaging methods and their clinical translation for generating reproducible and sensitive measurements of COPD related to pulmonary ventilation and perfusion as well as parenchyma morphology. The key clinical problems in COPD provide an important framework in which pulmonary imaging needs to rapidly move in order to address the staggering burden, costs, as well as the mortality and morbidity associated with COPD.

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.

Imaging of Cystic Fibrosis and Pediatric Bronchiectasis.

1. CT is superior to pulmonary function tests and chest radiography for the assessment and monitoring of cystic fibrosis (CF)-related lung disease and, also, of pediatric bronchiectasis not caused by CF (hereafter referred to as non-CF bronchiectasis). 2. Low-dose CT protocols that impart radiation doses similar to those used in chest radiography are feasible for the surveillance of patients with bronchiectasis. 3. Chest radiography is still most commonly used as the first-line imaging examination of choice for the assessment of acute complications related to bronchiectasis. 4. Pulmonary MRI, with or without the use of inhaled hyperpolarized gas, can be performed to obtain functional information, and, in dedicated centers, it may yield imaging results comparable to those obtained by CT. 5. Gastrointestinal and pancreaticobiliary manifestations of CF are observed with greater frequency in adults, because of increased life expectancy.

Clinical use of oxygen-enhanced T1 mapping MRI of the lung: reproducibility and impact of closed versus loose fit oxygen delivery system.

PURPOSE: To evaluate the reproducibility of oxygen-enhanced magnetic resonance imaging (MRI), and the influence of different gas delivery methods, in a clinical environment. MATERIALS AND METHODS: Twelve healthy volunteers were examined on two visits with an inversion recovery snapshot fast low angle shot sequence on a 1.5 T system. Coronal slices were obtained breathing room air as well as 100% oxygen with a flow rate of 15 L/min. For oxygen delivery a standard nontight face mask and a full closed air-cushion face mask were used. T1 relaxation times and the oxygen transfer function (OTF) were calculated. RESULTS: The mean T1 values did not change significantly between the two visits (P > 0.05). The T1 values breathing 100% oxygen obtained using the full closed mask were significantly lower (1093 ± 38 msec; P < 0.05) compared to the standard mask (1157 ± 52 msec). Accordingly, the OTF was significantly higher for the full closed mask (P < 0.05). The OTF changed significantly on the second visit using the standard mask (P < 0.05). The full closed mask showed lower interindividual variation for both the T1 values (3.5% vs. 4.5%) as well as the OTF (12.4% vs. 22.0%) and no difference of the OTF on the second visit (P > 0.05). CONCLUSION: Oxygen-enhanced T1 mapping MRI produces reproducible data when using a full closed face mask.