B1 and magnetization decay correction for hyperpolarized 129 Xe lung imaging using sequential 2D spiral acquisitions

Abstract

Purpose: To mitigate signal variations caused by inhomogeneous RF and magnetization decay in hyperpolarized ¹²⁹ Xe 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 ¹²⁹ Xe 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 ¹²⁹ Xe 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, ¹²⁹ Xe 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.

Keywords: B1 inhomogeneity; bias field correction; flip-angle maps; hyperpolarized 129Xe; lung ventilation images; spiral.