Quantitative 1H and hyperpolarized 3He magnetic resonance imaging: Comparison in chronic obstructive pulmonary disease and healthy never-smokers

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.

Hyperpolarized 3 helium MRI measured apparent diffusion coefficient and its correlations with pulmonary functions tests in patients with chronic obstructive pulmonary disease: A meta-analysis

BACKGROUND

This study evaluates role of hyperpolarized ³ helium (³ He) MRI measured apparent diffusion coefficient (ADC) in examining pulmonary function of chronic obstructive pulmonary disease (COPD) patients.

METHODS

After literature search in electronic databases, studies were selected by following precise eligibility criteria. Meta-analyses were performed to estimate mean difference in ADC between COPD patients and healthy individuals and to seek correlations between lung ADC and pulmonary function. Metaregression analyses were performed to seek relationships between ADC and age, gender, BMI, cigarette pack-years, and pulmonary function tests.

RESULTS

Twenty-five studies (622 COPD patients and 469 healthy controls) were included. Lung ADC was 0.402 (95% confidence interval [CI]: 0.374, 0.429) in COPD patients and 0.228 (95% CI: 0.205, 0.252) in healthy individuals (mean difference 0.160 [95% CI: 0.127, 0.193]; p < 0.001). In metaregression, age (coefficient: 0.006; p = 0.004), pack-years (coefficient: 0.005; p = 0.018), forced expiratory volume in 1 s (FEV1)/forced vital capacity (FVC) ratio (coefficient: -1.815; p = 0.007), percent predicted diffusion capacity of carbon monoxide (DLCO) (coefficient: -0.004; p = 0.008), and percent predicted inspiratory capacity (coefficient: -0.004; p = 0.012) were significantly associated with ADC in COPD patients. In meta-analysis of correlation coefficients, ADC was significantly correlated with FEV1 (r = -0.62; p < 0.00001), FEV1/FVC (r = -0.80; p < 0.00001), DLCO (r = -0.85; p < 0.00001), functional residual capacity (r = 0.71; p < 0.00001), reserve volume (r = 0.53; p = 0.0001), and emphysema index (r = 0.89; p < 0.00001).

CONCLUSION

Hyperpolarized ³ He MRI measured ADC was higher in COPD patients than in healthy individuals and was inversely associated with FEV1, FEV1/FVC, DLCO, and inspiratory capacity.

Current Advances in COPD Imaging

OBJECTIVE

To review the recent advances in available technologies for imaging COPD and present the novel optical coherence tomography (OCT) airway imaging technology.

MATERIALS AND METHODS

This is an unstructured review of published evidence of available pulmonary imaging technologies along with a demonstration of state-of-the-art OCT imaging technology of in vivo human and animal airways.

RESULTS

Advanced imaging techniques such as Magnetic Resonance (MR) imaging using hyperoloarized noble gases, micro-Computed Tomography (micro-CT), and OCT aim to further our understanding of COPD. Lung densitometry can aid in identifying an exacerbation prone phenotype which may have implications for targeting specific therapies to these individuals. MR ventilation scans have the ability to provide a functional and regional distribution of airflow obstruction offering insight into the airway and parenchymal changes induced by COPD. Micro-CT gives a near microscopic view of the terminal bronchioles and alveoli permitting study of the microarchitecture of the lung ex vivo. Optical coherence tomography can visualize the microstructure of the airway walls (epithelium, smooth muscle, blood vessels, cartilage) permitting real time in vivo as well as longitudinal evaluation of airway changes in patients with COPD.

CONCLUSION

Advanced imaging techniques play a vital role in expanding our current understanding of COPD.

Fast dynamic ventilation MRI of hyperpolarized 129 Xe using spiral imaging

Purpose

To develop and optimize a rapid dynamic hyperpolarized ¹²⁹Xe ventilation (DXeV) MRI protocol and investigate the feasibility of capturing pulmonary signal‐time curves in human lungs.

Theory and Methods

Spiral k‐space trajectories were designed with the number of interleaves N_int = 1, 2, 4, and 8 corresponding to voxel sizes of 8 mm, 5 mm, 4 mm, and 2.5 mm, respectively, for field of view = 15 cm. DXeV images were acquired from a gas‐flow phantom to investigate the ability of N_int = 1, 2, 4, and 8 to capture signal‐time curves. A finite element model was constructed to investigate gas‐flow dynamics corroborating the experimental signal‐time curves. DXeV images were also carried out in six subjects (three healthy and three chronic obstructive pulmonary disease subjects).

Results

DXeV images and numerical modelling of signal‐time curves permitted the quantification of temporal and spatial resolutions for different numbers of spiral interleaves. The two‐interleaved spiral (N_int = 2) was found to be the most time‐efficient to obtain DXeV images and signal‐time curves of whole lungs with a temporal resolution of 624 ms for 13 slices. Signal‐time curves were well matched in three healthy volunteers. The Spearman's correlations of chronic obstructive pulmonary disease subjects were statistically different from three healthy subjects (P < 0.05).

Conclusion

The N_int = 2 spiral demonstrates the successful acquisition of DXeV images and signal‐time curves in healthy subjects and chronic obstructive pulmonary disease patients. Magn Reson Med 79:2597–2606, 2018. © 2017 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Fast Determination of Flip Angle and T1 in Hyperpolarized Gas MRI During a Single Breath-Hold

MRI of hyperpolarized media, such as (129)Xe and (3)He, shows great potential for clinical applications. The optimal use of the available spin polarization requires accurate flip angle calibrations and T1 measurements. Traditional flip angle calibration methods are time-consuming and suffer from polarization losses during T1 relaxation. In this paper, we propose a method to simultaneously calibrate flip angles and measure T1 in vivo during a breath-hold time of less than 4 seconds. We demonstrate the accuracy, robustness and repeatability of this method and contrast it with traditional methods. By measuring the T1 of hyperpolarized gas, the oxygen pressure in vivo can be calibrated during the same breath hold. The results of the calibration have been applied in variable flip angle (VFA) scheme to obtain a stable steady-state transverse magnetization. Coupled with this method, the ultra-short TE (UTE) and constant VFA (CVFA) schemes are expected to give rise to new applications of hyperpolarized media.

Advances in functional and structural imaging of the human lung using proton MRI

The field of proton lung MRI is advancing on a variety of fronts. In the realm of functional imaging, it is now possible to use arterial spin labeling (ASL) and oxygen-enhanced imaging techniques to quantify regional perfusion and ventilation, respectively, in standard units of measurement. By combining these techniques into a single scan, it is also possible to quantify the local ventilation-perfusion ratio, which is the most important determinant of gas-exchange efficiency in the lung. To demonstrate potential for accurate and meaningful measurements of lung function, this technique was used to study gravitational gradients of ventilation, perfusion, and ventilation-perfusion ratio in healthy subjects, yielding quantitative results consistent with expected regional variations. Such techniques can also be applied in the time domain, providing new tools for studying temporal dynamics of lung function. Temporal ASL measurements showed increased spatial-temporal heterogeneity of pulmonary blood flow in healthy subjects exposed to hypoxia, suggesting sensitivity to active control mechanisms such as hypoxic pulmonary vasoconstriction, and illustrating that to fully examine the factors that govern lung function it is necessary to consider temporal as well as spatial variability. Further development to increase spatial coverage and improve robustness would enhance the clinical applicability of these new functional imaging tools. In the realm of structural imaging, pulse sequence techniques such as ultrashort echo-time radial k-space acquisition, ultrafast steady-state free precession, and imaging-based diaphragm triggering can be combined to overcome the significant challenges associated with proton MRI in the lung, enabling high-quality three-dimensional imaging of the whole lung in a clinically reasonable scan time. Images of healthy and cystic fibrosis subjects using these techniques demonstrate substantial promise for non-contrast pulmonary angiography and detailed depiction of airway disease. Although there is opportunity for further optimization, such approaches to structural lung imaging are ready for clinical testing.