Anatomical, functional and metabolic imaging of radiation-induced lung injury using hyperpolarized MRI

Regional variations in hyperpolarized 129Xe lung MRI: Insights from CSI-CSSR and CSSR in healthy and irradiated rat models

PURPOSE

To compare pulmonary function metrics obtained with hyperpolarized xenon-129 (HXe) MRS, using chemical shift saturation recovery (CSSR) and CSI-CSSR, in healthy rats and a rat model of radiation-induced lung injury.

METHODS

HXe-MR data were acquired in two healthy rats and one rat with radiation-induced lung injury using whole-lung spectroscopy and CSI-CSSR techniques. The CSI-CSSR acquisitions were performed with both fixed TE and variable TE. Apparent alveolar septal wall thickness, gas transfer dynamics, and regional lung function were quantified and compared across acquisition methods. Spectral analysis included alignment of dissolved-phase frequency spectra using the membrane resonance as reference, segmentation of gas-phase (GP) frequency distribution, and characterization of gas uptake in the vasculature.

RESULTS

Complex GP line shapes were observed in rat lungs, necessitating pixel-wise CSI analysis and membrane resonance alignment for improved quantification. Notable differences in alveolar septal wall thickness, dissolved-phase GP ratios, and GP and red blood cell frequencies were found between acquisition techniques and lung conditions. CSI-CSSR provided unique insights into regional lung function, including the identification of distinct GP frequency zones potentially corresponding to different airway structures, and the ability to map relative xenon gas transport. Metrics from fixed-TE and variable-TE acquisitions usually differed by less than 10%, but the latter yielded a 20% SNR gain.

CONCLUSION

HXe-MRS and CSI-CSSR techniques provide similar but not universally interchangeable insights into lung function, particularly in the presence of pathology.

Spectrum of Imaging Patterns of Lung Cancer following Radiation Therapy

Radiation therapy using conventional or newer high-precision dose techniques, including three-dimensional conformal radiotherapy, intensity-modulated radiation therapy, stereotactic body radiation therapy, four-dimensional conformational radiotherapy, and proton therapy, is an important component of treating patients with lung cancer. Knowledge of the radiation technique used and the expected temporal evolution of radiation-induced lung injury, as well as patient-specific parameters such as previous radiotherapy, concurrent chemoradiotherapy, or immunotherapy, is important in image interpretation. This review discusses factors that affect the development and severity of radiation-induced lung injury and its radiological manifestations, as well as the differences between conventional and high-precision dose radiotherapy techniques.

Revealing a Third Dissolved-Phase Xenon-129 Resonance in Blood Caused by Hemoglobin Glycation

Hyperpolarized (HP) xenon-129 (¹²⁹Xe), when dissolved in blood, has two NMR resonances: one in red blood cells (RBC) and one in plasma. The impact of numerous blood components on these resonances, however, has not yet been investigated. This study evaluates the effects of elevated glucose levels on the chemical shift (CS) and T2* relaxation times of HP ¹²⁹Xe dissolved in sterile citrated sheep blood for the first time. HP ¹²⁹Xe was mixed with sheep blood samples premixed with a stock glucose solution using a liquid-gas exchange module. Magnetic resonance spectroscopy was performed on a 3T clinical MRI scanner using a custom-built quadrature dual-tuned ¹²⁹Xe/¹H coil. We observed an additional resonance for the RBCs (¹²⁹Xe-RBC1) for the increased glucose levels. The CS of ¹²⁹Xe-RBC1 and ¹²⁹Xe-plasma peaks did not change with glucose levels, while the CS of ¹²⁹Xe-RBC2 (original RBC resonance) increased linearly at a rate of 0.015 ± 0.002 ppm/mM with glucose level. ¹²⁹Xe-RBC1 T2* values increased nonlinearly from 1.58 ± 0.24 ms to 2.67 ± 0.40 ms. As a result of the increased glucose levels in blood samples, the novel additional HP ¹²⁹Xe dissolved phase resonance was observed in blood and attributed to the ¹²⁹Xe bound to glycated hemoglobin (HbA_1c).

Ulinastatin enhances autophagy against radiation-induced lung injury in mice

Background

To investigate the enhancement of autophagy by ulinastatin for protecting against radiation-induced lung injury (RILI) in mice.

Methods

Forty C57BL/6 mice were equally divided into (I) control (C), (II) irradiation (R), (III) ulinastatin (U), (IV) 3-methyladenine (3-MA) (M), and (V) ulinastatin plus 3-MA (U+M) groups. Three mice in each group were infected with adeno-associated virus (AAV) carrying green fluorescent protein (GFP)-1A/1B-light chain 3 (GFP-LC3) in the lung for the marker of autophagy. All mice in R, U, M and U+M groups were given chest irradiation (1 Gy/min, 12 min), following injection with normal saline in C and U groups, ulinastatin (500,000 IU/kg·d, i.p., 7 d) in U group, 3-MA (10 mg/kg·d, i.p., 7 d) in M group, and ulinastatin plus 3-MA in U+M group. The effects of ulinastatin on lung injury and autophagy were evaluated by electron microscope (EM), immunohistochemistry, mRNA expression levels of collagen alpha-1 (COL1A1), collagen alpha-2 (COL1A2), α-smooth muscle actin (α-SMA) and transforming growth factor β1 (TGF-β1), and protein levels of LC3, α-SMA, COL1A2, TGF-β1, matrix metalloproteinase-2 (MMP-2) and MMP-9.

Results

EM observation revealed that the radiation caused the injury of type I and II alveolar epithelial cells, which was improved by ulinastatin treatment associated with increased the numbers of autophagosomes. GFP-LC3 signals was significantly enhanced by ulinastatin detected by immune histochemical tests. At transcriptional and/or translational levels, ulinastatin significantly enhanced the expression levels of TGF-β1 and LC3 but reduced COL1A1, COL1A2, α-SMA, MMP-2 and MMP-9 after radiation-induced RILI.

Conclusions

Ulinastatin reduces RILI by enhancing autophagy, which might be a potential therapeutic drug in the protection against RILI.

Magnetic resonance imaging of the time course of hyperpolarized 129Xe gas exchange in the human lungs and heart

Purpose

To perform magnetic resonance imaging (MRI), human lung imaging, and quantification of the gas-transfer dynamics of hyperpolarized xenon-129 (HPX) from the alveoli into the blood plasma.

Materials and methods

HPX MRI with iterative decomposition of water and fat with echo asymmetry and least-square estimation (IDEAL) approach were used with multi-interleaved spiral k-space sampling to obtain HPX gas and dissolved phase images. IDEAL time-series images were then obtained from ten subjects including six normal subjects and four patients with pulmonary emphysema to test the feasibility of the proposed technique for capturing xenon-129 gas-transfer dynamics (XGTD). The dynamics of xenon gas diffusion over the entire lung was also investigated by measuring the signal intensity variations between three regions of interest, including the left and right lungs and the heart using Welch’s t test.

Results

The technique enabled the acquisition of HPX gas and dissolved phase compartment images in a single breath-hold interval of 8 s. The y-intersect of the XGTD curves were also found to be statistically lower in the patients with lung emphysema than in the healthy group (p < 0.05).

Conclusion

This time-series IDEAL technique enables the visualization and quantification of inhaled xenon from the alveoli to the left ventricle with a clinical gradient strength magnet during a single breath-hold, in healthy and diseased lungs.

Key Points

• The proposed hyperpolarized xenon-129 gas and dissolved magnetic resonance imaging technique can provide regional and temporal measurements of xenon-129 gas-transfer dynamics.

• Quantitative measurement of xenon-129 gas-transfer dynamics from the alveolar to the heart was demonstrated in normal subjects and pulmonary emphysema.

• Comparison of gas-transfer dynamics in normal subjects and pulmonary emphysema showed that the proposed technique appears sensitive to changes affecting the alveoli, pulmonary interstitium, and capillaries.

Electronic supplementary material

The online version of this article (10.1007/s00330-018-5853-9) contains supplementary material, which is available to authorized users.

Quantitative evaluation of pulmonary gas-exchange function using hyperpolarized 129 Xe CEST MRS and MRI

Hyperpolarized ¹²⁹ Xe gas MR has been a powerful tool for evaluating pulmonary structure and function due to the extremely high enhancement in spin polarization, the good solubility in the pulmonary parenchyma, and the excellent chemical sensitivity to its surrounding environment. Generally, the quantitative structural and functional information of the lung are evaluated using hyperpolarized ¹²⁹ Xe by employing the techniques of chemical shift saturation recovery (CSSR) and xenon polarization transfer contrast (XTC). Hyperpolarized ¹²⁹ Xe chemical exchange saturation transfer (Hyper-CEST) is another method for quantifying the exchange information of hyperpolarized ¹²⁹ Xe by using the exchange of xenon signals according to its different chemical shifts, and it has been widely used in biosensor studies in vitro. However, the feasibility of using hyperpolarized ¹²⁹ Xe CEST to quantify the pulmonary gas exchange function in vivo is still unclear. In this study, the technique of CEST was used to quantitatively evaluate the gas exchange in the lung globally and regionally via hyperpolarized ¹²⁹ Xe MRS and MRI, respectively. A new parameter, the pulmonary apparent gas exchange time constant (T_app ), was defined, and it increased from 0.63 s to 0.95 s in chronic obstructive pulmonary disease (COPD) rats (induced by cigarette smoke and lipopolysaccharide exposure) versus the controls with a significant difference (P = 0.001). Additionally, the spatial distribution maps of T_app in COPD rats' pulmonary parenchyma showed a regionally obvious increase compared with healthy rats. These results indicated that hyperpolarized ¹²⁹ Xe CEST MR was an effective method for globally and regionally quantifying the pulmonary gas exchange function, which would be helpful in diagnosing lung diseases that are related to gas exchange, such as COPD.

Simultaneous assessment of both lung morphometry and gas exchange function within a single breath-hold by hyperpolarized 129 Xe MRI

During the measurement of hyperpolarized ¹²⁹ Xe magnetic resonance imaging (MRI), the diffusion-weighted imaging (DWI) technique provides valuable information for the assessment of lung morphometry at the alveolar level, whereas the chemical shift saturation recovery (CSSR) technique can evaluate the gas exchange function of the lungs. To date, the two techniques have only been performed during separate breaths. However, the request for multiple breaths increases the cost and scanning time, limiting clinical application. Moreover, acquisition during separate breath-holds will increase the measurement error, because of the inconsistent physiological status of the lungs. Here, we present a new method, referred to as diffusion-weighted chemical shift saturation recovery (DWCSSR), in order to perform both DWI and CSSR within a single breath-hold. Compared with sequential single-breath schemes (namely the 'CSSR + DWI' scheme and the 'DWI + CSSR' scheme), the DWCSSR scheme is able to significantly shorten the breath-hold time, as well as to obtain high signal-to-noise ratio (SNR) signals in both DWI and CSSR data. This scheme enables comprehensive information on lung morphometry and function to be obtained within a single breath-hold. In vivo experimental results demonstrate that DWCSSR has great potential for the evaluation and diagnosis of pulmonary diseases.

Magnetic resonance imaging with hyperpolarized agents: methods and applications

In the past decade, hyperpolarized (HP) contrast agents have been under active development for MRI applications to address the twin challenges of functional and quantitative imaging. Both HP helium (³He) and xenon (¹²⁹Xe) gases have reached the stage where they are under study in clinical research. HP ¹²⁹Xe, in particular, is poised for larger scale clinical research to investigate asthma, chronic obstructive pulmonary disease, and fibrotic lung diseases. With advances in polarizer technology and unique capabilities for imaging of ¹²⁹Xe gas exchange into lung tissue and blood, HP ¹²⁹Xe MRI is attracting new attention. In parallel, HP ¹³C and ¹⁵N MRI methods have steadily advanced in a wide range of pre-clinical research applications for imaging metabolism in various cancers and cardiac disease. The HP [1-¹³C] pyruvate MRI technique, in particular, has undergone phase I trials in prostate cancer and is poised for investigational new drug trials at multiple institutions in cancer and cardiac applications. This review treats the methodology behind both HP gases and HP ¹³C and ¹⁵N liquid state agents. Gas and liquid phase HP agents share similar technologies for achieving non-equilibrium polarization outside the field of the MRI scanner, strategies for image data acquisition, and translational challenges in moving from pre-clinical to clinical research. To cover the wide array of methods and applications, this review is organized by numerical section into (1) a brief introduction, (2) the physical and biological properties of the most common polarized agents with a brief summary of applications and methods of polarization, (3) methods for image acquisition and reconstruction specific to improving data acquisition efficiency for HP MRI, (4) the main physical properties that enable unique measures of physiology or metabolic pathways, followed by a more detailed review of the literature describing the use of HP agents to study: (5) metabolic pathways in cancer and cardiac disease and (6) lung function in both pre-clinical and clinical research studies, concluding with (7) some future directions and challenges, and (8) an overall summary.

Magnetic resonance imaging of disease progression and resolution in a transgenic mouse model of pulmonary fibrosis

Pulmonary fibrosis contributes to morbidity and mortality in a range of diseases, and there are no approved therapies for reversing its progression. To understand the mechanisms underlying pulmonary fibrosis and assess potential therapies, mouse models are central to basic and translational research. Unfortunately, metrics commonly used to assess murine pulmonary fibrosis require animals to be grouped and euthanized, increasing experimental difficulty and cost. We examined the ability of magnetic resonance imaging (MRI) to noninvasively assess lung fibrosis progression and resolution in a doxycycline (Dox) regulatable, transgenic mouse model that overexpresses transforming growth factor-α (TGF-α) under control of a lung-epithelial-specific promoter. During 7 wk of Dox treatment, fibrotic lesions were readily observed as high-signal tissue. Mean weighted signal and percent signal volume were found to be the most robust MRI-derived measures of fibrosis, and these metrics correlated significantly with pleural thickness, histology scores, and hydroxyproline content (R = 0.75-0.89). When applied longitudinally, percent high signal volume increased by 1.5% wk-1 (P < 0.001) and mean weighted signal increased at a rate of 0.0065 wk-1 (P = 0.0062). Following Dox treatment, lesions partially resolved, with percent high signal volume decreasing by -3.2% wk-1 (P = 0.0034) and weighted mean signal decreasing at -0.015 wk-1 (P = 0.0028). Additionally, longitudinal MRI revealed dynamic remodeling in a subset of lesions, a previously unobserved behavior in this model. These results demonstrate MRI can noninvasively assess experimental lung fibrosis progression and resolution and provide unique insights into its pathobiology.

Quantification of regional early stage gas exchange changes using hyperpolarized (129)Xe MRI in a rat model of radiation-induced lung injury

PURPOSE

To assess the feasibility of hyperpolarized (HP) (129)Xe MRI for detection of early stage radiation-induced lung injury (RILI) in a rat model involving unilateral irradiation by assessing differences in gas exchange dynamics between irradiated and unirradiated lungs.

METHODS

The dynamics of gas exchange between alveolar air space and pulmonary tissue (PT), PT and red blood cells (RBCs) was measured using single-shot spiral iterative decomposition of water and fat with echo asymmetry and least-squares estimation images of the right and left lungs of two age-matched cohorts of Sprague Dawley rats. The first cohort (n = 5) received 18 Gy irradiation to the right lung using a (60)Co source and the second cohort (n = 5) was not irradiated and served as the healthy control. Both groups were imaged two weeks following irradiation when radiation pneumonitis (RP) was expected to be present. The gas exchange data were fit to a theoretical gas exchange model to extract measurements of pulmonary tissue thickness (LPT) and relative blood volume (VRBC) from each of the right and left lungs of both cohorts. Following imaging, lung specimens were retrieved and percent tissue area (PTA) was assessed histologically to confirm RP and correlate with MRI measurements.

RESULTS

Statistically significant differences in LPT and VRBC were observed between the irradiated and non-irradiated cohorts. In particular, LPT of the right and left lungs was increased approximately 8.2% and 5.0% respectively in the irradiated cohort. Additionally, VRBC of the right and left lungs was decreased approximately 36.1% and 11.7% respectively for the irradiated cohort compared to the non-irradiated cohort. PTA measurements in both right and left lungs were increased in the irradiated group compared to the non-irradiated cohort for both the left (P < 0.05) and right lungs (P < 0.01) confirming the presence of RP. PTA measurements also correlated with the MRI measurements for both the non-irradiated (r = 0.79, P < 0.01) and irradiated groups (r = 0.91, P < 0.01).

CONCLUSIONS

Regional RILI can be detected two weeks post-irradiation using HP (129)Xe MRI and analysis of gas exchange curves. This approach correlates well with histology and can potentially be used clinically to assess radiation pneumonitis associated with early RILI to improve radiation therapy outcomes.

Protective effects of ulinastatin and methylprednisolone against radiation-induced lung injury in mice

The effectiveness of ulinastatin and methylprednisolone in treating pathological changes in mice with radiation-induced lung injury (RILI) was evaluated. Forty C57BL/6 female mice received whole-chest radiation (1.5 Gy/min for 12 min) and were randomly allocated into Group R (single radiation, n = 10), Group U (ulinastatin treatment, n = 10), Group M (methylprednisolone treatment, n = 10), or Group UM (ulinastatin and methylprednisolone treatment, n = 10). Another 10 untreated mice served as controls (Group C). Pathological changes in lung tissue, pulmonary interstitial area density (PIAD) and expression levels of transforming growth factor β1 (TGF-β1) and tumor necrosis factor α (TNF-α) in lung tissue, serum and bronchoalveolar lavage fluid were determined. Alleviation of pathological changes in lung tissue was observed in Groups U, M and UM. Treatment with ulinastatin, methylprednisolone or both effectively delayed the development of fibrosis at 12 weeks after radiation. Ulinastatin, methylprednisolone or both could alleviate the radiation-induced increase in the PIAD (P < 0.05 or P < 0.01). Treatment with ulinastatin, methylprednisolone or both significantly reduced the expression of TNF-α, but not TGF-β1, at 9 weeks after radiation compared with Group R (P < 0.01). Ulinastatin and /: or methylprednisolone effectively decreased the level of TNF-α in lung tissue after RILI and inhibited both the inflammatory response and the development of fibrosis.

Differentiating inflamed and normal lungs by the apparent reaction rate constants of lactate dehydrogenase probed by hyperpolarized (13)C labeled pyruvate

BACKGROUND

Clinically translatable hyperpolarized (HP) (13)C-NMR can probe in vivo enzymatic reactions, e.g., lactate dehydrogenase (LDH)-catalyzed reaction by injecting HP (13)C-pyruvate into the subject, which is converted to (13)C labeled lactate by the enzyme. Parameters such as (13)C-lactate signals and lactate-to-pyruvate signal ratio are commonly used for analyzing the HP (13)C-NMR data. However, the biochemical/biological meaning of these parameters remains either unclear or dependent on experimental settings. It is preferable to quantify the reaction rate constants with a clearer physical meaning. Here we report the extraction of the kinetic parameters of the LDH reaction from HP (13)C-NMR data and investigate if they can be potential predictors of lung inflammation.

METHODS

Male Sprague-Dawley rats (12 controls, 14 treated) were used. One dose of bleomycin (2.5 U/kg) was administered intratracheally to the treatment group. The lungs were removed, perfused, and observed by the HP-NMR technique, where a HyperSense dynamic nuclear polarization system was used to generate the HP (13)C-pyruvate for injecting into the lungs. A 20 mm (1)H/(13)C dual-tuned coil in a 9.4-T Varian vertical bore NMR spectrometer was employed to acquire the (13)C spectral data every 1 s over a time period of 300 s using a non-selective, 15-degree radiofrequency pulse. The apparent rate constants of the LDH reaction and their ratio were quantified by applying ratiometric fitting analysis to the time series data of (13)C labeled pyruvate and lactate.

RESULTS

The apparent forward rate constant kp =(3.67±3.31)×10(-4) s(-1), reverse rate constant kl =(4.95±2.90)×10(-2) s(-1), rate constant ratio kp /kl =(7.53±5.75)×10(-3) for the control lungs; kp =(11.71±4.35)×10(-4) s(-1), kl =(9.89±3.89)×10(-2) s(-1), and kp /kl =(12.39±4.18)×10(-3) for the inflamed lungs at the 7(th) day post treatment. Wilcoxon rank-sum test showed that the medians of these kinetic parameters of the 7-day cohort were significantly larger than those of the control cohort (P<0.001, P=0.001, and P=0.019, respectively). The rate constants of individual lungs correlated significantly with the histology scores of neutrophils and organizing pneumonia foci but not macrophages. Both kp and kp /kl positively correlated with lactate labeling signals. No correlation was found between kl and lactate labeling signals.

CONCLUSIONS

The results indicate bleomycin-induced lung inflammation significantly increased both the forward and reverse reaction rate constants of LDH and their ratio at day-7 after bleomycin treatment.

Early stage radiation-induced lung injury detected using hyperpolarized (129) Xe Morphometry: Proof-of-concept demonstration in a rat model

PURPOSE

Radiation-induced lung injury (RILI) is still the major dose-limiting toxicity related to lung cancer radiation therapy, and it is difficult to predict and detect patients who are at early risk of severe pneumonitis and fibrosis. The goal of this proof-of-concept preclinical demonstration was to investigate the potential of hyperpolarized (129) Xe diffusion-weighted MRI to detect the lung morphological changes associated with early stage RILI.

METHODS

Hyperpolarized (129) Xe MRI was performed using eight different diffusion sensitizations (0.0-115 s/cm(2) ) in a small group of control rats (n = 4) and rats 2 wk after radiation exposure (n = 5). The diffusion-weighted images were used to obtain morphological estimates of the pulmonary parenchyma including external radius (R), internal radius (r), alveolar sleeve depth (h), and mean airspace chord length (Lm ). The histological mean linear intercept (MLI) were obtained for five control and five irradiated animals.

RESULTS

Mean R, r, and Lm were both significantly different (P < 0.02) in the irradiated rats (74 ± 17 µm, 43 ± 12 µm, and 54 ± 17 µm, respectively) compared with the control rats (100 ± 12 µm, 67 ± 10 µm, and 79 ± 12 µm, respectively). Changes in measured Lm values were consistent with changes in MLI values observed by histology.

CONCLUSIONS

Hyperpolarized (129) Xe MRI provides a way to detect and measure regional microanatomical changes in lung parenchyma in a preclinical model of RILI. Magn Reson Med 75:2421-2431, 2016. © 2015 Wiley Periodicals, Inc.