B1 and magnetization decay correction for hyperpolarized 129Xe lung imaging using sequential 2D spiral acquisitions
Abdullah S. Bdaiwi, Mariah L. Costa, Joseph W. Plummer, Matthew M. Willmering, Laura L. Walkup, Zackary I. Cleveland
Abstract
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 (R 2 = 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.