Pulmonary MRI with hyperpolarized xenon-129 demonstrates novel alterations in gas transfer across the air-blood barrier in asthma.

BACKGROUND: Individuals with asthma can vary widely in clinical presentation, severity, and pathobiology. Hyperpolarized xenon-129 (Xe129) MRI is a novel imaging method to provide 3-D mapping of both ventilation and gas exchange in the human lung. PURPOSE: To evaluate the functional changes in adults with asthma as compared to healthy controls using Xe129 MRI. METHODS: All subjects (20 controls and 20 asthmatics) underwent lung function measurements and Xe129 MRI on the same day. Outcome measures included the pulmonary ventilation defect and transfer of inspired Xe129 into two soluble compartments: tissue and blood. Ten asthmatics underwent Xe129 MRI before and after bronchodilator to test whether gas transfer measures change with bronchodilator effects. RESULTS: Initial analysis of the results revealed striking differences in gas transfer measures based on age, hence we compared outcomes in younger (n = 24, ≤ 35 years) versus older (n = 16, > 45 years) asthmatics and controls. The younger asthmatics exhibited significantly lower Xe129 gas uptake by lung tissue (Asthmatic: 0.98% ± 0.24%, Control: 1.17% ± 0.12%, P = 0.035), and higher Xe129 gas transfer from tissue to the blood (Asthmatic: 0.40 ± 0.10, Control: 0.31% ± 0.03%, P = 0.035) than the younger controls. No significant difference in Xe129 gas transfer was observed in the older group between asthmatics and controls (P > 0.05). No significant change in Xe129 transfer was observed before and after bronchodilator treatment. CONCLUSIONS: By using Xe129 MRI, we discovered heterogeneous alterations of gas transfer that have associations with age. This finding suggests a heretofore unrecognized physiological derangement in the gas/tissue/blood interface in young adults with asthma that deserves further study.

Hyperpolarized 129Xe MRI, 99mTc scintigraphy, and SPECT in lung ventilation imaging: a quantitative comparison.

RATIONALE AND OBJECTIVES: The current clinical standard for functional imaging of patients with lung ailments is nuclear medicine scintigraphy and Single Photon Emission Computed Tomography (SPECT) which detect the gamma decay of inhaled radioactive tracers. Hyperpolarized (HP) Xenon-129 MRI (XeMRI) of the lungs has recently been FDA approved and provides similar functional images of the lungs with higher spatial resolution than scintigraphy and SPECT. Here we compare Technetium-99m (99mTc) diethylene-triamine-pentaacetate scintigraphy and SPECT with HP XeMRI in healthy controls, asthma, and chronic obstructive pulmonary disorder (COPD) patients. MATERIALS AND METHODS: 59 subjects, healthy, with asthma, and with COPD, underwent 99mTc scintigraphy/SPECT, standard spirometry, and HP XeMRI. XeMRI and SPECT images were registered for direct voxel-wise signal comparisons. Images were also compared using ventilation defect percentage (VDP), and a standard 6-compartment method. VDP calculated from XeMRI and SPECT images was compared to spirometry. RESULTS: Median Pearson correlation coefficient for voxel-wise signal comparison was 0.698 (0.613-0.782) between scintigraphy and XeMRI and 0.398 (0.286-0.502) between SPECT and XeMRI. Correlation between VDP measures was r = 0.853, p < 0.05. VDP separated asthma and COPD from the control group and was significantly correlated with FEV1, FEV1/FVC, and FEF 25-75. CONCLUSION: HP XeMRI provides equivalent information to 99mTc SPECT and standard spirometry measures. Additionally, XeMRI is non-invasive, hence it could be used for longitudinal studies for evaluating emerging treatment for lung ailments.

Acquiring Hyperpolarized 129Xe Magnetic Resonance Images of Lung Ventilation.

Hyperpolarized 129Xe MRI comprises a unique array of structural and functional lung imaging techniques. Technique standardization across sites is increasingly important given the recent FDA approval of 129Xe as an MR contrast agent and as interest in 129Xe MRI increases among research and clinical institutions. Members of the 129Xe MRI Clinical Trials Consortium (Xe MRI CTC) have agreed upon best practices for each of the key aspects of the 129Xe MRI workflow, and these recommendations are summarized in a recent publication. This work provides practical information to develop an end-to-end workflow for collecting 129Xe MR images of lung ventilation according to the Xe MRI CTC recommendations. Preparation and administration of 129Xe for MR studies will be discussed and demonstrated, with specific topics including choice of appropriate gas volumes for entire studies and for individual MR scans, preparation and delivery of individual 129Xe doses, and best practices for monitoring subject safety and 129Xe tolerability during studies. Key MR technical considerations will also be covered, including pulse sequence types and optimized parameters, calibration of 129Xe flip angle and center frequency, and 129Xe MRI ventilation image analysis.

Lung Volume Dependence and Repeatability of Hyperpolarized 129Xe MRI Gas Uptake Metrics in Healthy Volunteers and Participants with COPD.

Purpose: To assess the effect of lung volume on measured values and repeatability of xenon 129 (129Xe) gas uptake metrics in healthy volunteers and participants with chronic obstructive pulmonary disease (COPD). Materials and Methods: This Health Insurance Portability and Accountability Act-compliant prospective study included data (March 2014-December 2015) from 49 participants (19 with COPD [mean age, 67 years ± 9 (SD)]; nine women]; 25 older healthy volunteers [mean age, 59 years ± 10; 20 women]; and five young healthy women [mean age, 23 years ± 3]). Thirty-two participants underwent repeated 129Xe and same-breath-hold proton MRI at residual volume plus one-third forced vital capacity (RV+FVC/3), with 29 also undergoing one examination at total lung capacity (TLC). The remaining 17 participants underwent imaging at TLC, RV+FVC/3, and residual volume (RV). Signal ratios between membrane, red blood cell (RBC), and gas-phase compartments were calculated using hierarchical iterative decomposition of water and fat with echo asymmetry and least-squares estimation (ie, IDEAL). Repeatability was assessed using coefficient of variation and intraclass correlation coefficient, and volume relationships were assessed using Spearman correlation and Wilcoxon rank sum tests. Results: Gas uptake metrics were repeatable at RV+FVC/3 (intraclass correlation coefficient = 0.88 for membrane/gas; 0.71 for RBC/gas, and 0.88 for RBC/membrane). Relative ratio changes were highly correlated with relative volume changes for membrane/gas (r = -0.97) and RBC/gas (r = -0.93). Membrane/gas and RBC/gas measured at RV+FVC/3 were significantly lower in the COPD group than the corresponding healthy group (P ≤ .001). However, these differences lessened upon correction for individual volume differences (P = .23 for membrane/gas; P = .09 for RBC/gas). Conclusion: Dissolved-phase 129Xe MRI-derived gas uptake metrics were repeatable but highly dependent on lung volume during measurement.Keywords: Blood-Air Barrier, MRI, Chronic Obstructive Pulmonary Disease, Pulmonary Gas Exchange, Xenon Supplemental material is available for this article © RSNA, 2023.

Concomitant field compensation of spiral turbo spin-echo at 0.55 T.

OBJECTIVE: Diagnostic-quality neuroimaging methods are vital for widespread clinical adoption of low field MRI. Spiral imaging is an efficient acquisition method that can mitigate the reduced signal-to-noise ratio at lower field strengths. As concomitant field artifacts are worse at lower field, we propose a generalizable quadratic gradient-field nulling as an echo-to-echo compensation and apply it to spiral TSE at 0.55 T. MATERIALS AND METHODS: A spiral in-out TSE acquisition was developed with a compensation for concomitant field variation between spiral interleaves, by adding bipolar gradients around each readout to minimize phase differences at each refocusing pulse. Simulations were performed to characterize concomitant field compensation approaches. We demonstrate our proposed compensation method in phantoms and (n = 8) healthy volunteers at 0.55 T. RESULTS: Spiral read-outs with integrated spoiling demonstrated strong concomitant field artifacts but were mitigated using the echo-to-echo compensation. Simulations predicted a decrease of concomitant field phase RMSE between echoes of 42% using the proposed compensation. Spiral TSE improved SNR by 17.2 ± 2.3% compared to reference Cartesian acquisition. DISCUSSION: We demonstrated a generalizable approach to mitigate concomitant field artifacts for spiral TSE acquisitions via the addition of quadratic-nulling gradients, which can potentially improve neuroimaging at low-field through increased acquisition efficiency.

Hyperpolarized Xenon-129: A New Tool to Assess Pulmonary Physiology in Patients with Pulmonary Fibrosis.

PURPOSE: The existing tools to quantify lung function in interstitial lung diseases have significant limitations. Lung MRI imaging using inhaled hyperpolarized xenon-129 gas (129Xe) as a contrast agent is a new technology for measuring regional lung physiology. We sought to assess the utility of the 129Xe MRI in detecting impaired lung physiology in usual interstitial pneumonia (UIP). MATERIALS AND METHODS: After institutional review board approval and informed consent and in compliance with HIPAA regulations, we performed chest CT, pulmonary function tests (PFTs), and 129Xe MRI in 10 UIP subjects and 10 healthy controls. RESULTS: The 129Xe MRI detected highly heterogeneous abnormalities within individual UIP subjects as compared to controls. Subjects with UIP had markedly impaired ventilation (ventilation defect fraction: UIP: 30 ± 9%; healthy: 21 ± 9%; p = 0.026), a greater amount of 129Xe dissolved in the lung interstitium (tissue-to-gas ratio: UIP: 1.45 ± 0.35%; healthy: 1.10 ± 0.17%; p = 0.014), and impaired 129Xe diffusion into the blood (RBC-to-tissue ratio: UIP: 0.20 ± 0.06; healthy: 0.28 ± 0.05; p = 0.004). Most MRI variables had no correlation with the CT and PFT measurements. The elevated level of 129Xe dissolved in the lung interstitium, in particular, was detectable even in subjects with normal or mildly impaired PFTs, suggesting that this measurement may represent a new method for detecting early fibrosis. CONCLUSION: The hyperpolarized 129Xe MRI was highly sensitive to regional functional changes in subjects with UIP and may represent a new tool for understanding the pathophysiology, monitoring the progression, and assessing the effectiveness of treatment in UIP.

Low-field MRI: A report on the 2022 ISMRM workshop.

In March 2022, the first ISMRM Workshop on Low-Field MRI was held virtually. The goals of this workshop were to discuss recent low field MRI technology including hardware and software developments, novel methodology, new contrast mechanisms, as well as the clinical translation and dissemination of these systems. The virtual Workshop was attended by 368 registrants from 24 countries, and included 34 invited talks, 100 abstract presentations, 2 panel discussions, and 2 live scanner demonstrations. Here, we report on the scientific content of the Workshop and identify the key themes that emerged. The subject matter of the Workshop reflected the ongoing developments of low-field MRI as an accessible imaging modality that may expand the usage of MRI through cost reduction, portability, and ease of installation. Many talks in this Workshop addressed the use of computational power, efficient acquisitions, and contemporary hardware to overcome the SNR limitations associated with low field strength. Participants discussed the selection of appropriate clinical applications that leverage the unique capabilities of low-field MRI within traditional radiology practices, other point-of-care settings, and the broader community. The notion of "image quality" versus "information content" was also discussed, as images from low-field portable systems that are purpose-built for clinical decision-making may not replicate the current standard of clinical imaging. Speakers also described technical challenges and infrastructure challenges related to portability and widespread dissemination, and speculated about future directions for the field to improve the technology and establish clinical value.

Concomitant magnetic-field compensation for 2D spiral-ring turbo spin-echo imaging at 0.55T and 1.5T.

PURPOSE: To develop 2D turbo spin-echo (TSE) imaging using annular spiral rings (abbreviated "SPRING-RIO TSE") with compensation of concomitant gradient fields and B0 inhomogeneity at both 0.55T and 1.5T for fast T2 -weighted imaging. METHODS: Strategies of gradient waveform modifications were implemented in SPRING-RIO TSE for compensation of self-squared concomitant gradient terms at the TE and across echo spacings, along with reconstruction-based corrections to simultaneously compensate for the residual concomitant gradient and B0 field induced phase accruals along the readout. The signal pathway disturbance caused by time-varying and spatially dependent concomitant fields was simulated, and echo-to-echo phase variations before and after sequence-based compensation were compared. Images from SPRING-RIO TSE with no compensation, with compensation, and Cartesian TSE were also compared via phantom and in vivo acquisitions. RESULTS: Simulation showed how concomitant fields affected the signal evolution with no compensation, and both simulation and phantom studies demonstrated the performance of the proposed sequence modifications, as well as the readout off-resonance corrections. Volunteer data showed that after full correction, the SPRING-RIO TSE sequence achieved high image quality with improved SNR efficiency (15%-20% increase), and reduced RF SAR (˜50% reduction), compared to the standard Cartesian TSE, presenting potential benefits, especially in regaining SNR at low-field (0.55T). CONCLUSION: Implementation of SPRING-RIO TSE with concomitant field compensation was tested at 0.55T and 1.5T. The compensation principles can be extended to correct for other trajectory types that are time-varying along the echo train and temporally asymmetric in TSE-based imaging.

A single-breath-hold protocol for hyperpolarized 129 Xe ventilation and gas exchange imaging.

Hyperpolarized 129 Xe MRI (Xe-MRI) is increasingly used to image the structure and function of the lungs. Because 129 Xe imaging can provide multiple contrasts (ventilation, alveolar airspace size, and gas exchange), imaging often occurs over several breath-holds, which increases the time, expense, and patient burden of scans. We propose an imaging sequence that can be used to acquire Xe-MRI gas exchange and high-quality ventilation images within a single, approximately 10 s, breath-hold. This method uses a radial one-point Dixon approach to sample dissolved 129 Xe signal, which is interleaved with a 3D spiral ("FLORET") encoding pattern for gaseous 129 Xe. Thus, ventilation images are obtained at higher nominal spatial resolution (4.2 × 4.2 × 4.2 mm3 ) compared with gas-exchange images (6.25 × 6.25 × 6.25 mm3 ), both competitive with current standards within the Xe-MRI field. Moreover, the short 10 s Xe-MRI acquisition time allows for 1 H "anatomic" images used for thoracic cavity masking to be acquired within the same breath-hold for a total scan time of about 14 s. Images were acquired using this single-breath method in 11 volunteers (N = 4 healthy, N = 7 post-acute COVID). For 11 of these participants, a separate breath-hold was used to acquire a "dedicated" ventilation scan and five had an additional "dedicated" gas exchange scan. The images acquired using the single-breath protocol were compared with those from dedicated scans using Bland-Altman analysis, intraclass correlation (ICC), structural similarity, peak signal-to-noise ratio, Dice coefficients, and average distance. Imaging markers from the single-breath protocol showed high correlation with dedicated scans (ventilation defect percent, ICC = 0.77, p = 0.01; membrane/gas, ICC = 0.97, p = 0.001; red blood cell/gas, ICC = 0.99, p < 0.001). Images showed good qualitative and quantitative regional agreement. This single-breath protocol enables the collection of essential Xe-MRI information within one breath-hold, simplifying scanning sessions and reducing costs associated with Xe-MRI.

Highly accelerated dynamic acquisition of 3D grid-tagged hyperpolarized-gas lung images using compressed sensing.

PURPOSE: To develop and test compressed sensing-based multiframe 3D MRI of grid-tagged hyperpolarized gas in the lung. THEORY AND METHODS: Applying grid-tagging RF pulses to inhaled hyperpolarized gas results in images in which signal intensity is predictably and sparsely distributed. In the present work, this phenomenon was used to produce a sampling pattern in which k-space is undersampled by a factor of approximately seven, yet regions of high k-space energy remain densely sampled. Three healthy subjects received multiframe 3D 3 He tagging MRI using this undersampling method. Images were collected during a single exhalation at eight timepoints spanning the breathing cycle from end-of-inhalation to end-of-exhalation. Grid-tagged images were used to generate 3D displacement maps of the lung during exhalation, and time-resolved maps of principal strains and fractional volume change were generated from these displacement maps using finite-element analysis. RESULTS: Tags remained clearly resolvable for 4-6 timepoints (5-8 s) in each subject. Displacement maps revealed noteworthy temporal and spatial nonlinearities in lung motion during exhalation. Compressive normal strains occurred along all three principal directions but were primarily oriented in the head-foot direction. Fractional volume changes displayed clear bilateral symmetry, but with the lower lobes displaying slightly higher change than the upper lobes in 2 of the 3 subjects. CONCLUSION: We developed a compressed sensing-based method for multiframe 3D MRI of grid-tagged hyperpolarized gas in the lung during exhalation. This method successfully overcomes previous challenges for 3D dynamic grid-tagging, allowing time-resolved biomechanical readouts of lung function to be generated.

Cytoreductive Surgical Treatment of Pleural Mesothelioma in a Porcine Model Using Magnetic-Resonance-Guided Focused Ultrasound Surgery (MRgFUS) and Radiofrequency Ablation (RFA).

A combination of surgery and chemotherapy is the most effective treatment available for Malignant Pleural Mesothelioma (MPM). However, both cause significant collateral damage and cannot eliminate residual microscopic disease. This investigation aimed to compare and determine the feasibility of utilizing Radiofrequency Ablation (RFA) and Magnetic-Resonance-guided Focused Ultrasound Surgery (MRgFUS) as alternative treatments for MPM. A large animal tumor model was developed in 13 Yorkshire female pigs using the MSTO211H cell line. Two pigs were initially used to determine the cyclosporine dose required for immunosuppression and tumor development. Subsequently, 11 other pigs underwent tumor development. Of these 11, 2 died during cell inoculation. Small tumor masses and adhesions were present in the other 9, indicating mesothelioma development. Five pigs then received RFA treatment, and 4 pigs received MRgFUS treatment. Tumor model development and effect of the two treatments were examined using MRI and by necropsy. RFA and MRgFUS both successfully ablated approximately the same sized area in the same treatment time. This study demonstrates that RFA and MRgFUS are feasible for tumor debulking, and while MRgFUS requires more pretreatment planning compared to RFA, MRgFUS is a completely noninvasive procedure.

SPRING-RIO TSE: 2D T2 -Weighted Turbo Spin-Echo brain imaging using SPiral RINGs with retraced in/out trajectories.

PURPOSE: To develop a new approach to 2D turbo spin -echo (TSE) imaging using annular spiral rings with a retraced in/out trajectory, dubbed "SPRING-RIO TSE", for fast T2 -weighted brain imaging at 3T. METHODS: A long spiral trajectory was split into annular segmentations that were then incorporated into a 2D TSE acquisition module to fully exploit the sampling efficiency of spiral rings. A retraced in/out trajectory strategy coupled with spiral-ring TSE was introduced to increase SNR, mitigate T2 -decay induced artifacts, and self-correct moderate off-resonance while maintaining the target TE and causing no scan time penalty. Model-based k-space estimation and semiautomatic off-resonance correction algorithms were implemented to minimize effects of k-space trajectory infidelity and B0 inhomogeneity, respectively. The resulting SPRING-RIO TSE method was compared to the original spiral-ring (abbreviated "SPRING") TSE and Cartesian TSE using simulations, and phantom and in vivo acquisitions. RESULTS: Simulation and phantom studies demonstrated the performance of the proposed SPRING-RIO TSE pulses sequence, as well as that of trajectory correction and off-resonance correction. Volunteer data showed that the proposed method achieves high-quality 2D T2 -weighted brain imaging with a higher scan efficiency (0:45 min/14 slices versus 1:31 min/14 slices), improved image contrast, and reduced specific absorption rate compared to conventional 2D Cartesian TSE. CONCLUSION: 2D T2 -weighted brain imaging using spiral-ring TSE was implemented and tested, providing several potential advantages over conventional 2D Cartesian TSE imaging.

Free-breathing self-gated continuous-IR spiral T1 mapping: Comparison of dual flip-angle and Bloch-Siegert B1-corrected techniques.

PURPOSE: To develop a B1-corrrected single flip-angle continuous acquisition strategy with free-breathing and cardiac self-gating for spiral T1 mapping, and compare it to a previous dual flip-angle technique. METHODS: Data were continuously acquired using a spiral-out trajectory, rotated by the golden angle in time. During the first 2 s, off-resonance Fermi RF pulses were applied to generate a Bloch-Siegert shift B1 map, and the subsequent data were acquired with an inversion RF pulse applied every 4 s to create a T1* map. The final T1 map was generated from the B1 and the T1* maps by using a look-up table that accounted for slice profile effects, yielding more accurate T1 values. T1 values were compared to those from inversion recovery (IR) spin echo (phantom only), MOLLI, SAturation-recovery single-SHot Acquisition (SASHA), and previously proposed dual flip-angle results. This strategy was evaluated in a phantom and 25 human subjects. RESULTS: The proposed technique showed good agreement with IR spin-echo results in the phantom experiment. For in-vivo studies, the proposed technique and the previously proposed dual flip-angle method were more similar to SASHA results than to MOLLI results. CONCLUSIONS: B1-corrected single flip-angle T1 mapping successfully acquired B1 and T1 maps in a free-breathing, continuous-IR spiral acquisition, providing a method with improved accuracy to measure T1 using a continuous Look-Locker acquisition, as compared to the previously proposed dual excitation flip-angle technique.

The Impact of Optic Nerve Movement on Intracranial Radiation Treatment.

Purpose: In radiotherapy, high radiation exposure to optic nerve (ON) can cause optic neuropathy or vision loss. In this study, we evaluated the pattern and extent of the ON movement using MRI, and investigated the potential dosimetric effect of this movement on radiotherapy. Methods: MRI was performed in multiple planes in 5 human subjects without optic pathway abnormalities to determine optic nerve motion in different scenarios. The subjects were requested to gaze toward five directions during MRI acquisitions, including neutral (straight forward), left/right (horizontal movement), and up/down (vertical movement). Subsequently, the measured displacement was applied to patients with peri-optic tumors to evaluate the potential dosimetric effect of this motion. Results: The motion of ON followed a nearly conical shape. By average, the anterior end of ONs moved with 10.8 ± 2.2 mm horizontally and 9.3 ± 0.8 mm vertically, while posterior end has negligible displacement. For patients who underwent stereotactic radiotherapy to a peri-optic tumors, the movement of ON in this measured range introduced non-negligible dosimetric effect. Conclusion: The range of motion of the anterior portions of the optic nerves is on the order of centimeters, which may need to be considered with extra attention during radiation therapy in treating peri-optic lesions.

Utilizing flip angle/TR equivalence to reduce breath hold duration in hyperpolarized 129 Xe 1-point Dixon gas exchange imaging.

PURPOSE: To reduce scan duration in hyperpolarized 129 Xe 1-point Dixon gas exchange imaging by utilizing flip angle (FA)/TR equivalence. METHODS: Images were acquired in 12 subjects (n = 3 radiation therapy, n = 1 unexplained dyspnea, n = 8 healthy) using both standard (TR = 15 ms, FA = 20°, duration = 15 s, 998 projections) and "fast" (TR = 5.4 ms, FA = 12°, duration = 11.3 s, 2100 projections) acquisition parameters. For the fast acquisition, 3 image sets were reconstructed using subsets of 1900, 1500, and 1000 projections. From the resulting ventilation, tissue ("barrier"), and red blood cell (RBC) images, image metrics and biomarkers were compared to assess agreement between methods. RESULTS: Images acquired using both FA/TR settings had similar qualitative appearance. There were no significant differences in SNR, image mean, or image SD between images. Moreover, the percentage of the lungs in "defect", "normal", and "high" bins for each image (ventilation, RBC, barrier) was not significantly different among the acquisition types. After registration, comparison of 3D image metrics (Dice, volume similarity, average distance) agreed well between bins. Images using 1000 projections for reconstruction had no significant differences from images using all projections. CONCLUSION: Using flip angle/TR equivalence, hyperpolarized 129 Xe gas exchange images can be acquired via the 1-point Dixon technique in as little as 6 s, compared to ~15 s for previously reported parameter settings. The resulting images from this accelerated scan have no significant differences from the standard method in qualitative appearance or quantitative metrics.

Protocols for multi-site trials using hyperpolarized 129 Xe MRI for imaging of ventilation, alveolar-airspace size, and gas exchange: A position paper from the 129 Xe MRI clinical trials consortium.

Hyperpolarized (HP) 129 Xe MRI uniquely images pulmonary ventilation, gas exchange, and terminal airway morphology rapidly and safely, providing novel information not possible using conventional imaging modalities or pulmonary function tests. As such, there is mounting interest in expanding the use of biomarkers derived from HP 129 Xe MRI as outcome measures in multi-site clinical trials across a range of pulmonary disorders. Until recently, HP 129 Xe MRI techniques have been developed largely independently at a limited number of academic centers, without harmonizing acquisition strategies. To promote uniformity and adoption of HP 129 Xe MRI more widely in translational research, multi-site trials, and ultimately clinical practice, this position paper from the 129 Xe MRI Clinical Trials Consortium (https://cpir.cchmc.org/XeMRICTC) recommends standard protocols to harmonize methods for image acquisition in HP 129 Xe MRI. Recommendations are described for the most common HP gas MRI techniques-calibration, ventilation, alveolar-airspace size, and gas exchange-across MRI scanner manufacturers most used for this application. Moreover, recommendations are described for 129 Xe dose volumes and breath-hold standardization to further foster consistency of imaging studies. The intention is that sites with HP 129 Xe MRI capabilities can readily implement these methods to obtain consistent high-quality images that provide regional insight into lung structure and function. While this document represents consensus at a snapshot in time, a roadmap for technical developments is provided that will further increase image quality and efficiency. These standardized dosing and imaging protocols will facilitate the wider adoption of HP 129 Xe MRI for multi-site pulmonary research.

Evaluation of Regional Lung Function in Pulmonary Fibrosis with Xenon-129 MRI.

Idiopathic pulmonary fibrosis, a pattern of interstitial lung disease, is often clinically unpredictable in its progression. This paper presents hyperpolarized Xenon-129 chemical shift imaging as a noninvasive, nonradioactive method of probing lung physiology as well as anatomy to monitor subtle changes in subjects with IPF. Twenty subjects, nine healthy and eleven IPF, underwent HP Xe-129 ventilation MRI and 3D-SBCSI. Spirometry was performed on all subjects before imaging, and DLCO and hematocrit were measured in IPF subjects after imaging. Images were post-processed in MATLAB and segmented using ANTs. IPF subjects exhibited, on average, higher Tissue/Gas ratios and lower RBC/Gas ratios compared with healthy subjects, and quantitative maps were more heterogeneous in IPF subjects. The higher ratios are likely due to fibrosis and thickening of the pulmonary interstitium. T2* relaxation was longer in IPF subjects and corresponded with hematocrit scores, although the mechanism is not well understood. A lower chemical shift in the red blood cell spectroscopic peak correlated well with a higher Tissue/RBC ratio and may be explained by reduced blood oxygenation. Tissue/RBC also correlated well, spatially, with areas of fibrosis in HRCT images. These results may help us understand the underlying mechanism behind gas exchange impairment and disease progression.

Image- versus histogram-based considerations in semantic segmentation of pulmonary hyperpolarized gas images.

PURPOSE: To characterize the differences between histogram-based and image-based algorithms for segmentation of hyperpolarized gas lung images. METHODS: Four previously published histogram-based segmentation algorithms (ie, linear binning, hierarchical k-means, fuzzy spatial c-means, and a Gaussian mixture model with a Markov random field prior) and an image-based convolutional neural network were used to segment 2 simulated data sets derived from a public (n = 29 subjects) and a retrospective collection (n = 51 subjects) of hyperpolarized 129Xe gas lung images transformed by common MRI artifacts (noise and nonlinear intensity distortion). The resulting ventilation-based segmentations were used to assess algorithmic performance and characterize optimization domain differences in terms of measurement bias and precision. RESULTS: Although facilitating computational processing and providing discriminating clinically relevant measures of interest, histogram-based segmentation methods discard important contextual spatial information and are consequently less robust in terms of measurement precision in the presence of common MRI artifacts relative to the image-based convolutional neural network. CONCLUSIONS: Direct optimization within the image domain using convolutional neural networks leverages spatial information, which mitigates problematic issues associated with histogram-based approaches and suggests a preferred future research direction. Further, the entire processing and evaluation framework, including the newly reported deep learning functionality, is available as open source through the well-known Advanced Normalization Tools ecosystem.