EPR-based oximetric imaging: a combination of single point-based spatial encoding and T₁ weighting

Area of Science:

• Magnetic Resonance Imaging
• Biophysics
• Medical Physics

Background:

• Electron Paramagnetic Resonance (EPR) oximetry is crucial for assessing tissue oxygen levels.
• Spin-lattice relaxation rate (R1) is a key parameter influenced by oxygen concentration.
• Trityl-based probes like Oxo71 are used for EPR oximetry.

Purpose of the Study:

• To evaluate the efficacy of R1-based time-domain EPR oximetry for in vivo applications.
• To compare the performance of R1-based oximetry with R2*-based methods in determining partial oxygen pressure (pO2).
• To assess the spatial and oxygen resolution of R1-based EPR oximetry in small animal models.

Main Methods:

• R1 dependence of Oxo71 on pO2 was measured using single-point imaging and rapid repetition.
• R1 was determined across 22 repetition times (2.1–40.0 µs) at 300 MHz.
• Simultaneous pO2 mapping using R1 and apparent spin-spin relaxation rate (R2*) was performed on phantoms with varying oxygen concentrations (0%, 2%, 5%).

Main Results:

• R1 and R2* derived pO2 maps correlated with known oxygen levels in phantom studies.
• R1-based estimation demonstrated superior pO2 resolution in low oxygen regions compared to R2*.
• In vivo R1-based oximetry in tumor xenografts showed homogeneous profiles in hypoxic areas, similar to R2* methods.
• Scan time for R1 mapping can be reduced by using fewer repetition times (e.g., 3 times between 4.0–12.0 µs).

Conclusions:

• R1-based EPR oximetry using a single-point imaging modality provides valuable spatial and oxygen resolution for small animal research.
• This technique offers improved accuracy in low pO2 environments, enhancing its applicability in preclinical studies.
• The method is efficient, with potential for reduced scan times, making it practical for in vivo assessments.