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A new electron paramagnetic resonance (EPR) method, rapid scan EPR (RS-EPR), is demonstrated for 2D spectral spatial imaging which is superior to the traditional continuous wave (CW) technique and opens new venues for in vivo imaging. Results are demonstrated at 250 MHz, but the technique is applicable at any frequency.
We demonstrate a superior method of 2D spectral-spatial imaging of stable radical reporter molecules at 250 MHz using rapid-scan electron-paramagnetic-resonance (RS-EPR), which can provide quantitative information under in vivo conditions on oxygen concentration, pH, redox status and concentration of signaling molecules (i.e., OH•, NO•). The RS-EPR technique has a higher sensitivity, improved spatial resolution (1 mm), and shorter acquisition time in comparison to the standard continuous wave (CW) technique. A variety of phantom configurations have been tested, with spatial resolution varying from 1 to 6 mm, and spectral width of the reporter molecules ranging from 16 µT (160 mG) to 5 mT (50 G). A cross-loop bimodal resonator decouples excitation and detection, reducing the noise, while the rapid scan effect allows more power to be input to the spin system before saturation, increasing the EPR signal. This leads to a substantially higher signal-to-noise ratio than in conventional CW EPR experiments.
Relative to other medical imaging modalities, electron paramagnetic resonance imaging (EPRI) is uniquely able to quantitatively image physiological properties including pH1-3, pO24-7, temperature8, perfusion and viability of tissues9, microviscosity and ease of diffusion of small molecules10 and oxidative stress11. Estimation of the ease of disulfide cleavage by glutathione (GSH) in tissue and cells12,13 can report on redox status. For in vivo imaging, EPR in the frequency range between 250 MHz and 1 GHz is chosen because these frequencies provide sufficient depth of tissu....
1. Setup of the Rapid Scan Coil Driver at 250 MHz
The product of the experiment is a set of projections that are reconstructed into two-dimensional (one spectral, one spatial) images with a false color scale to represent signal amplitude. Deep blue denotes baseline where no signal is present, green is low amplitude and red is highest. Slices along the x-axis (spectral dimension) depict the EPR signal (EPR transition) on a magnetic field axis. Along the y-axis (spatial dimension), separation between signals corresponds to the physical spa.......
Rapid-scan signals have higher frequency components than CW, and require a larger resonator bandwidth depending on linewidths, relaxation times, and the speed of the rapid-scans. The bandwidth required for a given experiment is based upon the linewidth and the scan rate of the magnetic field (Equation 2). Depending on the relaxation times of the probe under study (T2 and T2*), and the scan rate, oscillations can appear on the trailing edge of the signal. For nitroxide radicals with T2 ~50.......
We have nothing to disclose.
Partial support of this work by NIH grants NIBIB EB002807 and CA177744 (GRE and SSE) and P41 EB002034 to GRE, Howard J. Halpern, PI, and by the University of Denver is gratefully acknowledged. Mark Tseytlin was supported by NIH R21 EB022775, NIH K25 EB016040, NIH/NIGMS U54GM104942. The authors are grateful to Valery Khramtsov, now at the University of West Virginia, and Illirian Dhimitruka at the Ohio State University for synthesis of the pH sensitive TAM radicals, and to Gerald Rosen and Joseph Kao at the University of Maryland for synthesis of the mHCTPO, proxyl, BMPO and nitronyl radicals.
....Name | Company | Catalog Number | Comments |
4-oxo-2,2,6,6-tetra(2H3)methyl-1-(3,3,5,5-2H4,1-15N)piperdinyloxyl (15N PDT) | CDN Isotopes | M-2327 | 98% atom 15N, 98 % atom D, Quebec Canada |
4-1H-3-carbamoyl-2,2,5,5-tetra(2H3)methyl-3-pyrrolinyloxyl (15N mHCTPO) | N/A | N/A | Synthesized at U.Maryland and described in Reference 29 |
3-carboxy-2,2,5,5-tetra(2H3)methyl-1-(3,4,4-2H3,1-15N)pyrrolidinyloxyl (15N Proxyl) | N/A | N/A | Synthesized at U.Maryland and described in reference 25 |
4 mm Quartz EPR Tubes | Wilmad Glass | 707-SQ-100M | |
4-oxo-2,2,6,6-tetra(2H3)methyl-1-(3,3,5,5-2H4)piperdinyloxyl (14N PDT) | CDN Isotopes | D-2328 | 98% atom D, Quebec Canada |
pH sensitive trityl radical (aTAM4) | Ohio State University | N/A | Synthesized at Ohio State University and described in reference 26 |
Potassum Phosphate, Monobasic | J.T. Baker Chemicals | 1-3246 | |
6 mm Quartz EPR Tubes | Wilmad Glass | Q-5M-6M-0-250/RB | |
8 mm Quartz EPR Tubes | Wilmad Glass | Q-7M-8M-0-250/RB | |
5-tert-butoxycarbonyl-5-methyl-1-pyrroline-N-oxide (BMPO) | N/A | N/A | Synthesized at U.Maryland and described in reference 30 |
Hydrogen Peroxide | Sigma Aldrich | H1009 SIGMA | 30% |
16 mm Quartz EPR tube | Wilmad Glass | 16-7PP-11QTZ | |
Medium Pressure 450 W UV lamp | Hanovia | 679-A36 | Fairfield, NJ |
L-Glutathione, reduced | Sigma Aldrich | G470-5 | |
Nitronyl | NA | N/A | Synthesized at U.Maryland and described in reference 31 |
Sodium Hydroxide | J.T. Baker Chemicals | 1-3146 |
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