Improving hyperpolarized 129Xe ADC mapping in pediatric and adult lungs with uncertainty propagation. (2nd November 2021)

• Record Type: Journal Article
• Authors: Bdaiwi, Abdullah S.; Niedbalski, Peter J.; Hossain, Md M.; Willmering, Matthew M.; Walkup, Laura L.; Wang, Hui; Thomen, Robert P.; Ruppert, Kai; Woods, Jason C.; Cleveland, Zackary I.
• Is Part Of: NMR in biomedicine. Volume 35:Number 3(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-11-02
• Subjects: ADC -- b‐value -- diffusion -- hyperpolarized 129Xe -- lung -- SNR; Nuclear magnetic resonance -- Periodicals; Magnetic Resonance Spectroscopy -- Periodicals; 574
• DOI: 10.1002/nbm.4639
• Languages: English
• ISSNs: 0952-3480

Abstract

Abstract : Rationale: Hyperpolarized (HP) 129 Xe‐MRI provides non‐invasive methods to quantify lung function and structure, with the 129 Xe 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 129 Xe ADC measurements have not been rigorously investigated. Here, we introduce an analytical model to predict the experimental uncertainty of 129 Xe 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 ( b max ), number of b ‐values ( N b ), number of phase encoding lines ( N ph ), 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 129 Xe ADC, 32 healthy subjects (age 6‐40 years) underwent diffusion‐weighted 129 Xe 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 ( ϵ ADC ) . As such, experimental parameters that impact non‐equilibrium 129 Xe magnetization necessarily impact the resulting ϵ ADC . The values of diffusion encoding parameters ( N b and b max ) that minimize ϵ ADC strongly depend on the underlying ADC value, resulting in a global minimum for ϵ ADC . Bayesian fitting outperformed other methods (error < 5%) for estimating ADC. The whole‐lung mean 129 Xe ADC of healthy subjects increased with age at a rate of 1.75 × 10 −4 cm 2 /s/yr ( p = 0.001). Conclusions: HP 129 Xe 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 : Lung microstructure dimensions can be quantified via the 129 Xe apparent diffusion coefficient (ADC), providing insights into lung growth and disease pathophysiology—for example, emphysema severity. To maximize the utility of this method, an analytical model was derived to predict ADC uncertainty as a function of acquisition parameters and ADC itself. An approximate lower bound on the in vivo ADC was obtained by measuring 129 Xe ADC as a function of age in healthy subjects as young as 6 years.