Hyperpolarized helium-3 magnetic resonance lung imaging of non-sedated infants and young children: a proof-of-concept study

Initial feasibility and challenges of hyperpolarized 129 Xe MRI in neonates with bronchopulmonary dysplasia

PURPOSE

The underlying functional and microstructural lung disease in neonates who are born preterm (bronchopulmonary dysplasia, BPD) remains poorly characterized. Moreover, there is a lack of suitable techniques to reliably assess lung function in this population. Here, we report our preliminary experience with hyperpolarized 129 Xe MRI in neonates with BPD.

METHODS

Neonatal intensive care patients with established BPD were recruited (N = 9) and imaged at a corrected gestational age of median:40.7 (range:37.1, 44.4) wk using a 1.5T neonatal scanner. 2D 129 Xe ventilation and diffusion-weighted images and dissolved phase spectroscopy were acquired, alongside 1 H 3D radial UTE. 129 Xe images were acquired during a series of short apneic breath-holds (˜3 s). 1 H UTE images were acquired during tidal breathing. Ventilation defects were manually identified and qualitatively compared to lung structures on UTE. ADCs were calculated on a voxel-wise basis. The signal ratio of the 129 Xe red blood cell (RBC) and tissue membrane (M) resonances from spectroscopy was determined.

RESULTS

Spiral-based 129 Xe ventilation imaging showed good image quality and sufficient sensitivity to detect mild ventilation abnormalities in patients with BPD. 129 Xe ADC values were elevated above that expected given healthy data in older children and adults (median:0.046 [range:0.041, 0.064] cm2 s-1 ); the highest value obtained from an extremely pre-term patient. 129 Xe spectroscopy revealed a low RBC/M ratio (0.14 [0.06, 0.21]).

CONCLUSION

We have demonstrated initial feasibility of 129 Xe lung MRI in neonates. With further data, the technique may help guide management of infant lung diseases in the neonatal period and beyond.

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.

Bronchopulmonary dysplasia from chest radiographs to magnetic resonance imaging and computed tomography: adding value

Bronchopulmonary dysplasia (BPD) is a common long-term complication of preterm birth. The chest radiograph appearance and survivability have evolved since the first description of BPD in 1967 because of improved ventilation and clinical strategies and the introduction of surfactant in the early 1990s. Contemporary imaging care is evolving with the recognition that comorbidities of tracheobronchomalacia and pulmonary hypertension have a great influence on outcomes and can be noninvasively evaluated with CT and MRI techniques, which provide a detailed evaluation of the lungs, trachea and to a lesser degree the heart. However, echocardiography remains the primary modality to evaluate and screen for pulmonary hypertension. This review is intended to highlight the important findings that chest radiograph, CT and MRI can contribute to precision diagnosis, phenotyping and prognosis resulting in optimal management and therapeutics.

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.

Hyperpolarized 129Xe magnetic resonance spectroscopy in a rat model of bronchopulmonary dysplasia

Premature infants often require mechanical ventilation and oxygen therapy, which can result in bronchopulmonary dysplasia (BPD), characterized by developmental arrest and impaired lung function. Conventional clinical methods for assessing the prenatal lung are not adequate for the detection and assessment of long-term health risks in infants with BPD, highlighting the need for a noninvasive tool for the characterization of lung microstructure and function. Theoretical diffusion models, like the model of xenon exchange (MOXE), interrogate alveolar gas exchange by predicting the uptake of inert hyperpolarized (HP) ¹²⁹Xe gas measured with HP ¹²⁹Xe magnetic resonance spectroscopy (MRS). To investigate HP ¹²⁹Xe MRS as a tool for noninvasive characterization of pulmonary microstructural and functional changes in vivo, HP ¹²⁹Xe gas exchange data were acquired in an oxygen exposure rat model of BPD that recapitulates the fewer and larger distal airways and pulmonary vascular stunting characteristics of BPD. Gas exchange parameters from MOXE, including airspace mean chord length (L_m), apparent hematocrit in the pulmonary capillaries (HCT), and pulmonary capillary transit time (t_x), were compared with airspace mean axis length and area density (MAL and ρ_A) and percentage area of tissue and air (PTA and PAA) from histology. L_m was significantly larger in the exposed rats (P = 0.003) and correlated with MAL, ρ_A, PTA, and PAA (0.59<|ρ|<0.66 and P < 0.05). Observed increase in HCT (P = 0.012) and changes in t_x are also discussed. These findings support the use of HP ¹²⁹Xe MRS for detecting fewer, enlarged distal airways in this rat model of BPD, and potentially in humans.

129Xe MRI as a measure of clinical disease severity for pediatric asthma

BACKGROUND

Measurement of regional lung ventilation with hyperpolarized ¹²⁹Xe magnetic resonance imaging (¹²⁹Xe MRI) in pediatric asthma is poised to advance our understanding of disease mechanisms and pathophysiology in a disorder with diverse clinical phenotypes. ¹²⁹Xe MRI has not been investigated in a pediatric asthma cohort.

OBJECTIVE

We hypothesized that ¹²⁹Xe MRI is feasible and can demonstrate ventilation defects that relate to and predict clinical severity in a pediatric asthma cohort.

METHODS

Thirty-seven children (13 with severe asthma, 8 with mild/moderate asthma, 16 age-matched healthy controls) aged 6 to 17 years old were imaged with ¹²⁹Xe MRI. Ventilation defect percentage (VDP) and image reader score were calculated and compared with clinical measures at baseline and at follow-up.

RESULTS

Children with asthma had higher VDP (P = .002) and number of defects per image slice (defects/slice) (P = .0001) than children without asthma. Children with clinically severe asthma had significantly higher VDP and number of defects/slice than healthy controls. Children with asthma who had a higher number of defects/slice had a higher rate of health care utilization (r = 0.48; P = .03) and oral corticosteroid use (r = 0.43; P = .05) at baseline. Receiver-operating characteristic analysis demonstrated that the VDP and number of defects/slice were predictive of increased health care utilization, asthma, and severe asthma. VDP correlated with FEV₁ (r = -0.35; P = .04) and FEV₁/forced vital capacity ratio (r = -0.41; P = .01).

CONCLUSIONS

¹²⁹Xe MRI correlates with asthma severity, health care utilization, and oral corticosteroid use. Because delineation of clinical severity is often difficult in children, ¹²⁹Xe MRI may be an important biomarker for severity, with potential to identify children at higher risk for exacerbations and improve outcomes.

Modern pulmonary imaging of bronchopulmonary dysplasia

Bronchopulmonary dysplasia (BPD) is a complex and serious cardiopulmonary morbidity in infants who are born preterm. Despite advances in clinical care, BPD remains a significant source of morbidity and mortality, due in large part to the increased survival of extremely preterm infants. There are few strong early prognostic indicators of BPD or its later outcomes, and evidence for the usage and timing of various interventions is minimal. As a result, clinical management is often imprecise. In this review, we highlight cutting-edge methods and findings from recent pulmonary imaging research that have high translational value. Further, we discuss the potential role that various radiological modalities may play in early risk stratification for development of BPD and in guiding treatment strategies of BPD when employed in varying severities and time-points throughout the neonatal disease course.

Pediatric Lung MRI: Currently Available and Emerging Techniques

OBJECTIVE. The purpose of this article is to review currently available and emerging techniques for pediatric lung MRI for general radiologists. CONCLUSION. MRI is a radiation-free alternative to CT, and clearly understanding the strengths and limitations of established and emerging techniques of pediatric lung MRI can allow practitioners to select and combine the optimal techniques, apply them in clinical practice, and potentially improve early diagnostic accuracy and patient management.

Current state of the art MRI for the longitudinal assessment of cystic fibrosis

Pulmonary MRI can now provide high-resolution images that are sensitive to early disease and specific to inflammation in cystic fibrosis (CF) lung disease. With specificity and function limited via computed tomography (CT), there are significant advantages to MRI. Many of the modern MRI techniques can be performed throughout life, and can be employed to understand changes over time, in addition to quantification of treatment response. Proton density and T1 /T2 contrast images can be obtained within a single breath-hold, providing depiction of structural abnormalities and active inflammation. Modern radial and/or spiral ultrashort echo-time (UTE) techniques rival CT in resolution for depiction and quantification of structure, for both airway and parenchymal abnormalities. Contrast perfusion MRI techniques are now utilized routinely to visualize changes in pulmonary and bronchial circulation that routinely occur in CF lung disease, and noncontrast techniques are moving closer to clinical translation. Functional information can be obtained from noncontrast proton images alone, using techniques such as Fourier decomposition. Hyperpolarized-gas MRI, increasingly using 129 Xe, is now becoming more widespread and has been demonstrated to have high sensitivity to early airway obstruction in CF via ventilation MRI. The sensitivity of 129 Xe MRI promises future use in personalized medicine, management of early CF lung disease, and in future clinical trials. By combining structural and functional techniques, with or without hyperpolarized gases, regional structure-function relationships can be obtained, giving insight into the pathophysiology of disease and improved clinical management. This article reviews the modern MRI techniques that can routinely be employed for CF lung disease in nearly any large medical center. Level of Evidence: 4 Technical Efficacy Stage: 5 J. Magn. Reson. Imaging 2019.

Will Airway Gene Therapy for Cystic Fibrosis Improve Lung Function? New Imaging Technologies Can Help Us Find Out

The promise of genetic therapies has turned into reality in recent years, with new first-line treatments for fatal diseases now available to patients. The development and testing of genetic therapies for respiratory diseases such as cystic fibrosis (CF) has also progressed. The addition of gene editing to the genetic agent toolbox, and its early success in other organ systems, suggests we will see rapid expansion of gene correction options for CF in the future. Although substantial progress has been made in creating techniques and genetic agents that can be highly effective for CF correction in vitro, physiologically relevant functional in vivo changes have been largely prevented by poor delivery efficiency within the lungs. Somewhat hidden from view, however, is the absence of reliable, accurate, detailed, and noninvasive outcome measures that can detect subtle disease and treatment effects in the lungs of humans or animal models. The ability to measure the fundamental function of the lung-ventilation, the effective transport of air throughout the lung-has been constrained by the available measurement technologies. Without sensitive measurement methods, it is difficult to quantify the effectiveness of genetic therapies for CF. The mainstays of lung health assessment are spirometry, which cannot provide adequate disease localization and is not sensitive enough to detect small early changes in disease; and computed tomography, which provides structural rather than functional information. Magnetic resonance imaging using hyperpolarized gases is increasingly useful for lung ventilation assessment, and it removes the radiation risk that accompanies X-ray methods. A new lung imaging technique, X-ray velocimetry, can now offer highly detailed regional lung ventilation information well suited to the diagnosis, treatment, and monitoring needs of CF lung disease, particularly after the application of genetic therapies. In this review, we discuss the options now available for imaging-based lung function measurement in the generation and use of genetic and other therapies for treating CF lung disease.

Expanding Applications of Pulmonary MRI in the Clinical Evaluation of Lung Disorders: Fleischner Society Position Paper

Pulmonary MRI provides structural and quantitative functional images of the lungs without ionizing radiation, but it has had limited clinical use due to low signal intensity from the lung parenchyma. The lack of radiation makes pulmonary MRI an ideal modality for pediatric examinations, pregnant women, and patients requiring serial and longitudinal follow-up. Fortunately, recent MRI techniques, including ultrashort echo time and zero echo time, are expanding clinical opportunities for pulmonary MRI. With the use of multicoil parallel acquisitions and acceleration methods, these techniques make pulmonary MRI practical for evaluating lung parenchymal and pulmonary vascular diseases. The purpose of this Fleischner Society position paper is to familiarize radiologists and other interested clinicians with these advances in pulmonary MRI and to stratify the Society recommendations for the clinical use of pulmonary MRI into three categories: (a) suggested for current clinical use, (b) promising but requiring further validation or regulatory approval, and (c) appropriate for research investigations. This position paper also provides recommendations for vendors and infrastructure, identifies methods for hypothesis-driven research, and suggests opportunities for prospective, randomized multicenter trials to investigate and validate lung MRI methods.

Safety of repeated hyperpolarized helium 3 magnetic resonance imaging in pediatric asthma patients

BACKGROUND

Hyperpolarized helium 3 magnetic resonance imaging (3He MRI) is useful for investigating pulmonary physiology of pediatric asthma, but a detailed assessment of the safety profile of this agent has not been performed in children.

OBJECTIVE

To evaluate the safety of 3He MRI in children and adolescents with asthma.

MATERIALS AND METHODS

This was a retrospective observational study. 3He MRI was performed in 66 pediatric patients (mean age 12.9 years, range 8-18 years, 38 male, 28 female) between 2007 and 2017. Fifty-five patients received a single repeated examination and five received two repeated examinations. We assessed a total of 127 3He MRI exams. Heart rate, respiratory rate and pulse oximetry measured oxygen saturation (SpO2) were recorded before, during (2 min and 5 min after gas inhalation) and 1 h after MRI. Blood pressure was obtained before and after MRI. Any subjective symptoms were also noted. Changes in vital signs were tested for significance during the exam and divided into three subject age groups (8-12 years, 13-15 years, 16-18 years) using linear mixed-effects models.

RESULTS

There were no serious adverse events, but three minor adverse events (2.3%; headache, dizziness and mild hypoxia) were reported. We found statistically significant increases in heart rate and SpO2 after 3He MRI. The youngest age group (8-12 years) had an increased heart rate and a decreased respiratory rate at 2 min and 5 min after 3H inhalation, and an increased SpO2 post MRI.

CONCLUSION

The use of 3He MRI is safe in children and adolescents with asthma.

Alveolar Airspace Size in Healthy and Diseased Infant Lungs Measured via Hyperpolarized 3He Gas Diffusion Magnetic Resonance Imaging

BACKGROUND

Alveolar development and lung parenchymal simplification are not well characterized in vivo in neonatal patients with respiratory morbidities, such as bronchopulmonary dysplasia (BPD). Hyperpolarized (HP) gas diffusion magnetic resonance imaging (MRI) is a sensitive, safe, nonionizing, and noninvasive biomarker for measuring airspace size in vivo but has not yet been implemented in young infants.

OBJECTIVE

This work quantified alveolar airspace size via HP gas diffusion MRI in healthy and diseased explanted infant lung specimens, with comparison to histological morphometry.

METHODS

Lung specimens from 8 infants were obtained: 7 healthy left upper lobes (0-16 months, post-autopsy) and 1 left lung with filamin-A mutation, closely representing BPD lung disease (11 months, post-transplantation). Specimens were imaged using HP 3He diffusion MRI to generate apparent diffusion coefficients (ADCs) as biomarkers of alveolar airspace size, with comparison to mean linear intercept (Lm) via quantitative histology.

RESULTS

Mean ADC and Lm were significantly increased throughout the diseased specimen (ADC = 0.26 ± 0.06 cm2/s, Lm = 587 ± 212 µm) compared with healthy specimens (ADC = 0.14 ± 0.03 cm2/s, Lm = 133 ± 37 µm; p < 1 × 10-7); increased values reflect enlarged airspaces. Mean ADCs in healthy specimens were significantly correlated to Lm (r = 0.69, p = 0.041).

CONCLUSIONS

HP gas diffusion MRI is sensitive to healthy and diseased regional alveolar airspace size in infant lungs, with good comparison to quantitative histology in ex vivo specimens. This work demonstrates the translational potential of gas MRI techniques for in vivo assessment of normal and abnormal alveolar development in neonates with pulmonary disease.

Structural lung abnormalities on computed tomography correlate with asthma inflammation in bronchoscopic alveolar lavage fluid

Objective: Image scoring systems have been developed to assess the severity of specific lung abnormalities in patients diagnosed with various pulmonary diseases except for asthma. A comprehensive asthma imaging scoring system may identify specific abnormalities potentially linking these to inflammatory phenotypes.Methods: Computed tomography (CT) images of 88 children with asthma (50 M/38 F, mean age 7.8 ± 5.4 years) acquired within 12 months of bronchoscopic alveolar lavage fluid (BALF) sampling that assessed airway inflammation cell types were reviewed along with CT images of 49 controls (27 M/22 F, mean age 3.4 ± 2.2 years). Images were scored using a comprehensive scoring system to quantify bronchiectasis (BR), bronchial wall thickening (BWT), ground glass opacity, mucus plugging (MP), consolidations, linear densities (LD), and air trapping (AT). Each category was scored 0-2 in each of six lobar regions (with lingula separated from left upper lobe).Results: Absolute average overall scores of the controls and children with asthma were 0.72 ± 1.59 and 5.39 ± 5.83, respectively (P < 0.0001). Children with asthma scored significantly higher for BR (N = 20, 0.33 ± 0.80, P = 0.0002), BWT (N = 28, 0.72 ± 1.40, P < 0.0001), MP (N = 28, 0.37 ± 1.12, P = 0.0052), consolidation (N = 31, 0.67 ± 1.22, P < 0.0001), LD (N = 58, 1.12 ± 1.44, P < 0.0001), and AT (N = 52, 1.78 ± 2.31, P < 0.0001). There was a significant difference between the BR score of children with positive inflammatory response in BALF (N = 53) and those who were negative for airway inflammation cells (0.14 ± 0.36, P = 0.040).Conclusions: Significant lung structural abnormalities were readily identified on CT of children with asthma, with image differentiation of those with an inflammatory response on BALF. Chest imaging demonstrates potential as a noninvasive clinical tool for additional characterization of asthma phenotypes.

Hyperpolarized Gas Magnetic Resonance Imaging of Pediatric Cystic Fibrosis Lung Disease

Conventional pulmonary function tests appear normal in early cystic fibrosis (CF) lung disease. Therefore, new diagnostic approaches are required that can detect CF lung disease in children and monitor treatment response. Hyperpolarized (HP) gas (¹²⁹Xe and ³He) magnetic resonance imaging (MRI) is a powerful, emergent tool for mapping regional lung function and may be well suited for studying pediatric CF. HP gas MRI is well tolerated, reproducible, and it can be performed longitudinally without the need for ionizing radiation. In particular, quantification of the distribution of ventilation, or ventilation defect percent (VDP), has been shown to be a sensitive indicator of CF lung disease and correlates well with pulmonary function tests. This article presents the current state of CF diagnosis and treatment and describes the potential role of HP gas MRI for detection of early CF lung disease and following the effects of interventions. The typical HP gas imaging workflow is described, along with a discussion of image analysis to calculate VDP, dosing considerations, and the reproducibility of VDP. The potential use of VDP as an outcome measure in CF is discussed, by considering the correlation with pulmonary function measures, preliminary interventional studies, and case studies involving longitudinal imaging and pulmonary exacerbations. Finally, emerging HP gas imaging techniques such as multiple breath washout imaging are introduced, followed by a discussion of future directions. Overall, HP gas MRI biomarkers are expected to provide sensitive outcome measures that can be used in disease surveillance as well as interventional studies involving novel CF therapies.

Structural and Functional Pulmonary Magnetic Resonance Imaging in Pediatrics-From the Neonate to the Young Adult

The clinical imaging modalities available to investigate pediatric pulmonary conditions such as bronchopulmonary dysplasia, cystic fibrosis, and asthma are limited primarily to chest x-ray radiograph and computed tomography. As the challenges that historically limited the application of magnetic resonance imaging (MRI) to the lung have been overcome, its clinical potential has greatly expanded. In this review article, recent advances in pulmonary MRI including ultrashort echo time and hyperpolarized-gas MRI techniques are discussed with an emphasis on pediatric research and translational applications.

"Structure-Function Imaging of Lung Disease Using Ultrashort Echo Time MRI"

RATIONALE AND OBJECTIVES

The purpose of this review is to acquaint the reader with recent advances in ultrashort echo time (UTE) magnetic resonance imaging (MRI) of the lung and its implications for pulmonary MRI when used in conjunction with functional MRI technique.

MATERIALS AND METHODS

We provide an overview of recent technical advances of UTE and explore the advantages of combined structure-function pulmonary imaging in the context of restrictive and obstructive pulmonary diseases such as idiopathic pulmonary fibrosis (IPF) and cystic fibrosis (CF).

RESULTS

UTE MRI clearly shows the lung parenchymal changes due to IPF and CF. The use of UTE MRI, in conjunction with established functional lung MRI in chronic lung diseases, will serve to mitigate the need for computed tomography in children.

CONCLUSION

Current limitations of UTE MRI include long scan times, poor delineation of thin-walled structures (e.g. cysts and reticulation) due to limited spatial resolution, low signal to noise ratio, and imperfect motion compensation. Despite these limitations, UTE MRI can now be considered as an alternative to multidetector computed tomography for the longitudinal follow-up of the morphological changes from lung diseases in neonates, children, and young adults, particularly as a complement to the unique functional capabilities of MRI.

Hyperpolarised 129Xe magnetic resonance imaging to monitor treatment response in children with cystic fibrosis

Pulmonary magnetic resonance imaging using hyperpolarised 129Xe gas (XeMRI) can quantify ventilation inhomogeneity by measuring the percentage of unventilated lung volume (ventilation defect per cent (VDP)). While previous studies have demonstrated its sensitivity for detecting early cystic fibrosis (CF) lung disease, the utility of XeMRI to monitor response to therapy in CF is unknown. The aim of this study was to assess the ability of XeMRI to capture treatment response in paediatric CF patients undergoing inpatient antibiotic treatment for a pulmonary exacerbation.

15 CF patients aged 8–18 years underwent XeMRI, spirometry, plethysmography and multiple-breath nitrogen washout at the beginning and end of inpatient treatment of a pulmonary exacerbation. VDP was calculated from XeMRI images obtained during a static breath hold using semi-automated k-means clustering and linear binning approaches.

XeMRI was well tolerated. VDP, lung clearance index and the forced expiratory volume in 1 s all improved with treatment; however, response was not uniform in individual patients. Of all outcome measures, VDP showed the largest relative improvement (−42.1%, 95% CI −52.1–−31.9%, p<0.0001).

These data support further investigation of XeMRI as a tool to capture treatment response in CF lung disease.

Neonatal Pulmonary Magnetic Resonance Imaging of Bronchopulmonary Dysplasia Predicts Short-Term Clinical Outcomes

RATIONALE

Bronchopulmonary dysplasia (BPD) is a serious neonatal pulmonary condition associated with premature birth, but the underlying parenchymal disease and trajectory are poorly characterized. The current National Institute of Child Health and Human Development (NICHD)/NHLBI definition of BPD severity is based on degree of prematurity and extent of oxygen requirement. However, no clear link exists between initial diagnosis and clinical outcomes.

OBJECTIVES

We hypothesized that magnetic resonance imaging (MRI) of structural parenchymal abnormalities will correlate with NICHD-defined BPD disease severity and predict short-term respiratory outcomes.

METHODS

A total of 42 neonates (20 severe BPD, 6 moderate, 7 mild, 9 non-BPD control subjects; 40 ± 3-wk postmenstrual age) underwent quiet-breathing structural pulmonary MRI (ultrashort echo time and gradient echo) in a neonatal ICU-sited, neonatal-sized 1.5 T scanner, without sedation or respiratory support unless already clinically prescribed. Disease severity was scored independently by two radiologists. Mean scores were compared with clinical severity and short-term respiratory outcomes. Outcomes were predicted using univariate and multivariable models, including clinical data and scores.

MEASUREMENTS AND MAIN RESULTS

MRI scores significantly correlated with severities and predicted respiratory support at neonatal ICU discharge (P < 0.0001). In multivariable models, MRI scores were by far the strongest predictor of respiratory support duration over clinical data, including birth weight and gestational age. Notably, NICHD severity level was not predictive of discharge support.

CONCLUSIONS

Quiet-breathing neonatal pulmonary MRI can independently assess structural abnormalities of BPD, describe disease severity, and predict short-term outcomes more accurately than any individual standard clinical measure. Importantly, this nonionizing technique can be implemented to phenotype disease, and has potential to serially assess efficacy of individualized therapies.

Assessment of the influence of lung inflation state on the quantitative parameters derived from hyperpolarized gas lung ventilation MRI in healthy volunteers

In this study, the effect of lung volume on quantitative measures of lung ventilation was investigated using MRI with hyperpolarized ³He and ¹²⁹Xe. Six volunteers were imaged with hyperpolarized ³He at five different lung volumes [residual volume (RV), RV + 1 liter (1L), functional residual capacity (FRC), FRC + 1L, and total lung capacity (TLC)], and three were also imaged with hyperpolarized ¹²⁹Xe. Imaging at each of the lung volumes was repeated twice on the same day with corresponding ¹H lung anatomical images. Percent lung ventilated volume (%VV) and variation of signal intensity [heterogeneity score (H_score)] were evaluated. Increased ventilation heterogeneity, quantified by reduced %VV and increased H_score, was observed at lower lung volumes with the least ventilation heterogeneity observed at TLC. For ³He MRI data, the coefficient of variation of %VV was <1.5% and <5.5% for H_score at all lung volumes, while for ¹²⁹Xe data the values were 4 and 10%, respectively. Generally, %VV generated from ¹²⁹Xe images was lower than that seen from ³He images. The good repeatability of ³He %VV found here supports prior publications showing that percent lung-ventilated volume is a robust method for assessing global lung ventilation. The greater ventilation heterogeneity observed at lower lung volumes indicates that there may be partial airway closure in healthy lungs and that lung volume should be carefully considered for reliable longitudinal measurements of %VV and H_score. The results suggest that imaging patients at different lung volumes may help to elucidate obstructive disease pathophysiology and progression. NEW & NOTEWORTHY We present repeatability data of quantitative metrics of lung function derived from hyperpolarized helium-3, xenon-129, and proton anatomical images acquired at five lung volumes in volunteers. Increased regional ventilation heterogeneity at lower lung inflation levels was observed in the lungs of healthy volunteers.