Comparison of pulse sequences for R1-based electron paramagnetic resonance oxygen imaging

Abstract

Electron paramagnetic resonance (EPR) spin-lattice relaxation (SLR) oxygen imaging has proven to be an indispensable tool for assessing oxygen partial pressure in live animals. EPR oxygen images show remarkable oxygen accuracy when combined with high precision and spatial resolution. Developing more effective means for obtaining SLR rates is of great practical, biological and medical importance. In this work we compared different pulse EPR imaging protocols and pulse sequences to establish advantages and areas of applicability for each method. Tests were performed using phantoms containing spin probes with oxygen concentrations relevant to in vivo oxymetry. We have found that for small animal size objects the inversion recovery sequence combined with the filtered backprojection reconstruction method delivers the best accuracy and precision. For large animals, in which large radio frequency energy deposition might be critical, free induction decay and three pulse stimulated echo sequences might find better practical usage.

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

“…An analogous technique in MRI using 19 F‐labeled tracers has been developed and validated in studying tumor oxygenation . In this study, we present another strategy for R 1 ‐based EPR oximetry that combines the high‐resolution capability of SPI by a single pulse sequence and the effect of interpulse delays and R 1 on the steady state magnetization for rapid signal averaging . This approach is applicable to a wide range of R 1 values, is independent of differences in the concentration of the contrast agent as originally demonstrated by Epel and Halpern, and has a lower specific absorption rate (SAR) .…”

“…In this study, we present another strategy for R 1 ‐based EPR oximetry that combines the high‐resolution capability of SPI by a single pulse sequence and the effect of interpulse delays and R 1 on the steady state magnetization for rapid signal averaging . This approach is applicable to a wide range of R 1 values, is independent of differences in the concentration of the contrast agent as originally demonstrated by Epel and Halpern, and has a lower specific absorption rate (SAR) . Oxo71, a deuterated Oxo63 probe, was assessed for: 1) the linear relationship between R 1 and pO 2 , 2) R 1 mapping of a phantom by EPR imaging, and 3) in vivo oximetry of a mouse tumor based on a R 1 map.…”

Concept of Phase Cycling in Pulsed Magnetic Resonance Using Sinusoidal Magnetic Field Modulation

“…(15), P is the modulation period. The second approach can be implemented in EPR imaging, where several τ values are used to determine single exponential decay rates [20] in a given voxel. In addition, if the relaxation times are sufficiently long, B pp can be chosen to be smaller than the resonator bandwidth.…”

In Vivo pO2 Imaging of Tumors

“…It produces images on a Cartesian grid with a much larger number of acquisitions than tomographic imaging. 68 As a result, it is less efficient than filtered back projection, but freer from artifact imposed by radial k -space coverage.…”