Exploring the Use of Continuous RF Irradiation for Xenon Polarization Transfer Contrast Imaging
Year: 2021
Type: conference-abstract
Abstract: Introduction: Xenon-polarization Transfer Contrast (XTC) imaging is a powerful technique for quantifying exchange rates between gas-and dissolved-phase xenon compartments, and involves a series of radiofrequency (RF) saturation pulses applied to either the tissue/plasma (TP) or red blood cell (RBC) resonance.Although increasing the number of pulses and their spacing generates greater contrast, it also increases acquisition time.In an effort to reduce acquisition time, particularly for free-breathing imaging protocols, we explored the feasibility of continuous RF irradiation for faster, more efficient saturation in healthy and emphysematous rats.Methods: Imaging was performed in 3 healthy and 1 emphysema-induced male Wistar rat.Animals were ventilated with room air, then switched to a mixture of 80% hyperpolarized xenon, 20% oxygen for 4 wash-in breaths, followed by imaging during a 10 second breath-hold.XTC data sets were obtained from three consecutive gas-phase (GP) images.For continuous RF acquisitions, a series of rectangular saturation pulses with varying lengths (25-100 ms) and flip angles (1,800°-54,000°) was applied on-resonance (213 ppm for RBC or 199 ppm for TP) in-between the second pair of images to generate contrast, and off-resonance (-213 ppm for RBC or -199 ppm for TP) between the first pair as a control for apparent T1 decay and any inadvertent GP magnetization loss.Depolarization ratio maps were calculated from the ratio of these images.For pulsed saturations, 60 8 ms Gaussian 180° RF pulses with 30 ms spacing were used.Results: To demonstrate consistency and RF amplifier stability, very large flip angles were evaluated.As seen in Figure 1a, saturations with the same flip angle per pulse length at various flip angle and pulse length combinations produced similar depolarization levels.In Figure 1b, discrete (60 8 ms 180° pulses with 30 ms spacing) and continuous (10 100 ms pulses at 18°/ms) saturation schemes are compared in an emphysema rat model.The distributions were consistent between the two methods, with average depolarization ratios of 0.42 ± 0.18 and 0.49 ± 0.18 for TP and 0.70 ± 0.16 and 0.75 ± 0.16 for RBC, respectively.However, the continuous saturation scheme significantly reduced overall acquisition time by 45%, displaying significant promise for use in free-breathing imaging protocols, especially with further optimizations of imaging and saturation parameters.Conclusion: XTC MRI with continuous RF irradiation produces depolarization distributions similar to those obtained with pulsed saturation schemes, but with a significant time advantage that decreases required breath-hold durations.
Authors: F. Amzajerdian, T.S. Achekzai, L. Loza, H. Hamedani, Y. Xin, I. Duncan, S. Kadlecek, K. Ruppert, R.R. Rizi
Language: English
Subfield: Atomic and Molecular Physics, and Optics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: gold
OpenAlex ID: W4252668327