Application of a 2D frequency encoding sectoral approach to hyperpolarized 129 Xe MRI at low field.

Inhaled hyperpolarized ¹²⁹ Xe MRI is a non-invasive and radiation risk free lung imaging method, which can directly measure the business unit of the lung where gas exchange occurs: the alveoli and acinar ducts (lung function). Currently, three imaging approaches have been demonstrated to be useful for hyperpolarized ¹²⁹ Xe MR in lungs: Fast Gradient Recalled Echo (FGRE), Radial Projection Reconstruction (PR), and spiral/cones. Typically, non-Cartesian acquisitions such as PR and spiral/cones require specific data post-processing, such as interpolating, regridding, and density-weighting procedures for image reconstruction, which often leads to smoothing effects and resolution degradation. On the other hand, Cartesian methods such as FGRE are not short-echo time (TE) methods; they suffer from imaging gradient-induced diffusion-weighting of the k-space center, and employ a significant number of radio-frequency (RF) pulses. Due to the non-renewable magnetization of the hyperpolarized media, the use of a large number of RF pulses (FGRE/PR) required for full k-space coverage is a significant limitation, especially for low field (<0.5 T) hyperpolarized gas MRI. We demonstrate an ultra-fast, purely frequency-encoded, Cartesian pulse sequence called Frequency-Encoding Sectoral (FES), which takes advantage of the long T ₂ * of hyperpolarized ¹²⁹ Xe gas at low field strength (0.074 T). In contrast to PR/FGRE, it uses a much smaller number of RF pulses, and consequently maximizes image Signal-to-Noise Ratio (SNR) while shortening acquisition time. Additionally, FES does not suffer from non-uniform T ₂ * decay leading to image blurring; a common issue with interleaved spirals/cones. The Cartesian k-space coverage of the proposed FES method does not require specific k-space data post-processing, unlike PR/FGRE and spiral/cones methods. Proton scans were used to compare the FES sequence to both FGRE and Phase Encoding Sectoral, in terms of their SNR values and imaging efficiency estimates. Using FES, proton and hyperpolarized ¹²⁹ Xe images were acquired from a custom hollow acrylic phantom (0.04L) and two normal rats ( ¹²⁹ Xe only), utilizing both single-breath and multiple-breath schemes. For the ¹²⁹ Xe phantom images, the apparent diffusion coefficient, T ₁ , and T ₂ * relaxation maps were acquired and generated. Blurring due to the T ₂ * decay and B ₀ field variation were simulated to estimate dependence of the image resolution on the duration of the data acquisition windows (i.e. sector length), and temperature-induced resonance frequency shift from the low field magnet hardware.
Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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