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Purpose: Hyperpolarized 129Xe MRI benefits from non‐Cartesian acquisitions that sample k‐space efficiently and rapidly. However, their reconstructions are complex and burdened by decay processes unique to hyperpolarized gas. Currently used gridded reconstructions are prone to artifacts caused by magnetization decay and are ill‐suited for undersampling. We present a compressed sensing (CS) reconstruction approach that incorporates magnetization decay in the forward model, thereby producing images with increased sharpness and contrast, even in undersampled data. Methods: Radio‐frequency, T1, and T2*$$ {\mathrm{T}}_2^{\ast } $$ decay processes were incorporated into the forward model and solved using iterative methods including CS. The decay‐modeled reconstruction was validated in simulations and then tested in 2D/3D‐spiral ventilation and 3D‐radial gas‐exchange MRI. Quantitative metrics including apparent‐SNR and sharpness were compared between gridded, CS, and twofold undersampled CS reconstructions. Observations were validated in gas‐exchange data collected from 15 healthy and 25 post‐hematopoietic‐stem‐cell‐transplant participants. Results: CS reconstructions in simulations yielded images with threefold increases in accuracy. CS increased sharpness and contrast for ventilation in vivo imaging and showed greater accuracy for undersampled acquisitions. CS improved gas‐exchange imaging, particularly in the dissolved‐phase where apparent‐SNR improved, and structure was made discernable. Finally, CS showed repeatability in important global gas‐exchange metrics including median dissolved‐gas signal ratio and median angle between real/imaginary components. Conclusion: A non‐Cartesian CS reconstruction approach that incorporates hyperpolarized 129Xe decay processes is presented. This approach enables improved image sharpness, contrast, and overall image quality in addition to up‐to threefold undersampling. This contribution benefits all hyperpolarized gas MRI through improved accuracy and decreased scan durations. [ABSTRACT FROM AUTHOR]

Purpose: To mitigate signal variations caused by inhomogeneous RF and magnetization decay in hyperpolarized 129Xe ventilation images using flip‐angle maps generated from sequential 2D spiral ventilation images acquired in a breath‐hold. Images and correction maps were compared with those obtained using conventional, 2D gradient‐recalled echo. Theory and Methods: Analytical expressions to predict signal intensity and uncertainty in flip‐angle measurements were derived from the Bloch equations and validated by simulations and phantom experiments. Imaging in 129Xe phantoms and human subjects (1 healthy, 1 cystic fibrosis) was performed using 2D gradient‐recalled echo and spiral. For both sequences, consecutive images were acquired with the same slice position during a breath‐hold (Cartesian scan time = 15 s; spiral scan time = 5 s). The ratio of these images was used to calculate flip‐angle maps and correct intensity inhomogeneities in ventilation images. Results: Mean measured flip angle showed excellent agreement with the applied flip angle in simulations (R2 = 0.99) for both sequences. Mean measured flip angle agreed well with the globally applied flip angle (∼15% difference) in 129Xe phantoms and in vivo imaging using both sequences. Corrected images displayed reduced coil‐dependent signal nonuniformity relative to uncorrected images. Conclusions: Flip‐angle maps were obtained using sequentially acquired, 2D spiral, 129Xe ventilation images. Signal intensity variations caused by RF‐coil inhomogeneity can be corrected by acquiring sequential single‐breath ventilation images in less than 5‐s scan time. Thus, this method can be used to remove undesirable heterogeneity while preserving physiological effects on the signal distribution. [ABSTRACT FROM AUTHOR]

Purpose: Xenon‐129 (129Xe) gas‐exchange MRI is a pulmonary‐imaging technique that provides quantitative metrics for lung structure and function and is often compared to pulmonary‐function tests. Unlike such tests, it does not normalize to predictive values based on demographic variables such as age. Many sites have alluded to an age dependence in gas‐exchange metrics; however, a procedure for normalizing metrics has not yet been introduced. Theory: We model healthy reference values for 129Xe gas‐exchange MRI against age using generalized additive models for location, scale, and shape (GAMLSS). GAMLSS takes signal data from an aggregated heathy‐reference cohort and fits a distribution with flexible median, variation, skewness, and kurtosis to predict age‐dependent centiles. This approach mirrors methods by the Global Lung Function Initiative for modeling pulmonary‐function test data and applies it to binning methods widely used by the 129Xe MRI community to interpret and quantify gas‐exchange data. Methods: Ventilation, membrane‐uptake, red blood cell transfer, and red blood cell:membrane gas‐exchange metrics were collected on 30 healthy subjects over an age range of 5 to 68 years. A GAMLSS model was fit against age and compared against widely used linear and generalized‐linear binning 129Xe MRI analysis schemes. Results: All 4 gas‐exchange metrics had significant skewness, and membrane‐uptake had significant kurtosis compared to a normal distribution. Age has significant impact on distribution parameters. GAMLSS‐binning produced narrower bins compared to the linear and generalized‐linear binning schemes and distributed signal data closer to a normal distribution. Conclusion: The proposed "proof‐of‐concept" GAMLSS‐binning approach can improve diagnostic accuracy of 129Xe gas‐exchange MRI by providing a means of modeling voxel distribution data against age. [ABSTRACT FROM AUTHOR]

Background: 129Xe gas‐transfer MRI provides regional measures of pulmonary gas exchange in adults and separates xenon in interstitial lung tissue/plasma (barrier) from xenon in red blood cells (RBCs). The technique has yet to be demonstrated in pediatric populations or conditions. Purpose/Hypothesis: To perform an exploratory analysis of 129Xe gas‐transfer MRI in children. Study Type: Prospective. Population: Seventy‐seven human volunteers (38 males, age = 17.7 ± 15.1 years, range 5–68 years, 16 healthy). Four pediatric disease cohorts. Field Strength/Sequence: 3‐T, three‐dimensional‐radial one‐point Dixon Fast Field Echo (FFE) Ultrashort Echo Time (UTE). Assessment: Breath hold compliance was assessed by quantitative signal‐to‐noise and dynamic metrics. Whole‐lung means and standard deviations were extracted from gas‐transfer maps. Gas‐transfer metrics were investigated with respect to age and lung disease. Clinical pulmonary function tests were retrospectively acquired for reference lung disease severity. Statistical Tests: Wilcoxon rank‐sum tests to compare age and disease cohorts, Wilcoxon signed‐rank tests to compare pre‐ and post‐breath hold vitals, Pearson correlations between age and gas‐transfer metrics, and limits of normal with a binomial exact test to compare fraction of subjects with abnormal gas‐transfer. P ≤ 0.05 was considered significant. Results: Eighty percentage of pediatric subjects successfully completed 129Xe gas‐transfer MRI. Gas‐transfer parameters differed between healthy children and adults, including ventilation (0.75 and 0.67) and RBC:barrier ratio (0.31 and 0.46) which also correlated with age (ρ = −0.76, 0.57, respectively). Bone marrow transplant subjects had impaired ventilation (90% of reference) and increased dissolved 129Xe standard deviation (242%). Bronchopulmonary dysplasia subjects had decreased barrier‐uptake (69%). Cystic fibrosis subjects had impaired ventilation (91%) and increased RBC‐transfer (146%). Lastly, childhood interstitial lung disease subjects had increased ventilation heterogeneity (113%). Limits of normal provided detection of abnormalities in additional gas‐transfer parameters. Data Conclusion: Pediatric 129Xe gas‐transfer MRI was adequately successful and gas‐transfer metrics correlated with age. Exploratory analysis revealed abnormalities in a variety of pediatric obstructive and restrictive lung diseases. Level of Evidence: 2 Technical Efficacy Stage: 2 [ABSTRACT FROM AUTHOR]

Rationale: Hyperpolarized (HP) 129Xe‐MRI provides non‐invasive methods to quantify lung function and structure, with the 129Xe apparent diffusion coefficient (ADC) being a well validated measure of alveolar airspace size. However, the experimental factors that impact the precision and accuracy of HP 129Xe ADC measurements have not been rigorously investigated. Here, we introduce an analytical model to predict the experimental uncertainty of 129Xe ADC estimates. Additionally, we report ADC dependence on age in healthy pediatric volunteers. Methods: An analytical expression for ADC uncertainty was derived from the Stejskal‐Tanner equation and simplified Bloch equations appropriate for HP media. Parameters in the model were maximum b‐value (bmax), number of b‐values (Nb), number of phase encoding lines (Nph), flip angle and the ADC itself. This model was validated by simulations and phantom experiments, and five fitting methods for calculating ADC were investigated. To examine the lower range for 129Xe ADC, 32 healthy subjects (age 6‐40 years) underwent diffusion‐weighted 129Xe MRI. Results: The analytical model provides a lower bound on ADC uncertainty and predicts that decreased signal‐to‐noise ratio yields increases in relative uncertainty (ϵADC). As such, experimental parameters that impact non‐equilibrium 129Xe magnetization necessarily impact the resulting ϵADC. The values of diffusion encoding parameters (Nb and bmax) that minimize ϵADC strongly depend on the underlying ADC value, resulting in a global minimum for ϵADC. Bayesian fitting outperformed other methods (error < 5%) for estimating ADC. The whole‐lung mean 129Xe ADC of healthy subjects increased with age at a rate of 1.75 × 10−4 cm2/s/yr (p = 0.001). Conclusions: HP 129Xe diffusion MRI can be improved by minimizing the uncertainty of ADC measurements via uncertainty propagation. Doing so will improve experimental accuracy when measuring lung microstructure in vivo and should allow improved monitoring of regional disease progression and assessment of therapy response in a range of lung diseases. [ABSTRACT FROM AUTHOR]