Advanced InversionRecovery Single Point Imaging for High Spatial Resolution and Quantitative Oxygen Imaging

The partial pressure of oxygen (pO₂) in tissue is a tightly regulated physiological quantity. Hypoxia is a hallmark of many diseases, and changes in tissue pO₂ underline critical developmental, metabolic, and physiological processes. Electron paramagnetic resonance oxygen imaging (EPROI) has emerged as the most precise technique to image pO₂ in vitro and in vivo. EPROI uses linear relationship between the relaxation rates (R₁ or R₂) of the spin probe OXO71 and pO₂.

The current gold standard for EPROI is inversion recovery electron spin echo (IRESE) sequence with radial k-space acquisition scheme, which is fast, provides high pO₂ resolution using T₁ imaging, but suffers from low spatial resolution, especially at higher oxygen concentrations. The other method, single-point imaging (SPI), which uses Cartesian k-space acquisition scheme and free-induction decay (FID) readout, provides higher spatial resolution but has lower signal-to-noise ratio (SNR) and lower pO₂ accuracy, especially for T₂* imaging. In this work, we introduce advanced inversion-recovery single-point imaging (AIR-SPI), which combines inversion-recovery SPI (IR-SPI) with multi-point FID averaging in the spatial domain. We evaluate the performance of AIR-SPI in phantoms and in vivo imaging of a fibrosarcoma (FSA) tumor in a mouse leg.

We demonstrate 2-8-fold SNR improvement and higher spatial resolution for AIR-SPI compared to IR-SPI and IRESE, especially for high oxygen concentrations. We also demonstrate higher pO₂ precision for AIR-SPI.

AIR-SPI offers improvements in image SNR, spatial, and oxygen resolution over all existing EPR oxygen imaging methods, without increasing imaging time. Importantly, it uses smaller RF power than echo-based methods, which is crucial for scaling EPROI to human applications.