Standard-based method for proton-electron double resonance imaging of oxygen

Abstract

Proton-electron double resonance imaging (PEDRI) has been utilized for indirect determination of oxygen concentrations in aqueous samples and living systems. Due to the complexity of the problem, there are seven oxygen related parameters that need to be measured to determine the distribution of oxygen. We present an improved approach in which image intensities from only two PEDRI acquisitions with different EPR irradiation powers are required to determine the distribution of a paramagnetic probe and oxygen in an analyzed sample. This is achieved using three reference samples with known concentrations of a paramagnetic probe and oxygen placed inside the resonator together with the measurement sample. An EPR-off image, which has low signal intensity at low magnetic field (0.02 T) is not required for the calculations, significantly reducing the total time of the experiments and the noise while enhancing the accuracy of these oxygen measurements. The Finland trityl radical was used as the paramagnetic probe and oxygen concentrations could be accurately measured and imaged over the physiological range from 0 to 240 μM.

Figure 1

Structure of the Finland Trityl radical.

Figure 2

(A) Schematic representation of the phantom: reference solutions are denoted as 1, 2 and 3; analyzed sample is denoted as S. (B) Image acquired with EPR power P_A = 0.8 W. Reference samples are denoted as 1, 2 and 3. All tubes were placed inside a bigger tube, 30 mm diameter, filled with 0.09% NaCl in water. Dashed line schematically shows the location of the tube border, not visible because saline solution surrounding the sample tubes has very small signal intensity comparing to the enhanced signals. The MRI acquisition parameters are: TR, 2 s; TE, 30 ms; matrix, 128 × 128; field of view (FOV), 64 × 64 mm; slice thickness, 4 mm; acquisition time, 4.3 min; NMR frequency, 841.5 kHz. EPR irradiation time 4.3 min. (C) Image acquired with EPR power P_B = 3.2 W. (D) Probe concentration map. (E) Oxygen concentration map. (F) MRI image of the phantom. The MRI acquisition parameters are: TR, 2 s; TE, 30 ms; matrix, 128 × 128; field of view (FOV), 64 × 64 mm; slice thickness, 20 mm; acquisition time, 4.3 min; NMR frequency, 841.5 kHz.

Figure 3

2D fast spin echo (FSE) pulse sequence, showing 2 out of 16 180-degree refocusing pulses. Number of echoes per RF excitation, n = 16; the number of RF excitations is 4.

Figure 4

Histograms of the distribution of the probe and oxygen concentrations in the analyzed sample.

Figure 5

Enhancement factor (EF), defined as I/I₀, as a function of applied EPR power. Solid line: c_R, 2 mM; dashed and dotted lines: c_R, 1 mM. Samples represented by the solid and dashed lines contained no oxygen. In the third sample oxygen was equilibrated with air. The MRI acquisition parameters are: TR, 2 s; TE, 26 ms; matrix, 128 × 128; field of view (FOV), 64 × 64 mm; slice thickness, 20 mm; acquisition time, 16 sec; NMR frequency, 841.5 kHz. EPR irradiation time, 16.4 sec; EPR frequency, 554 MHz.