A parametric approach to spectral-spatial EPR imaging

Continuous wave electron paramagnetic resonance imaging for in vivo mapping of spin distribution and spectral shape requires rapid data acquisition. A spectral-spatial imaging technique is presented that provides an order of magnitude reduction in acquisition time, compared to iterative tomographic reprojection. The proposed approach assumes that spectral shapes in the sample are well-approximated by members from a parametric family of functions. A model is developed for the spectra measured with magnetic field modulation. Parameters defining the spin distribution and spectral shapes are then determined directly from the measurements using maximum a posteriori probability estimation. The approach does not suffer approximation error from limited sweep width of the main magnetic field and explicitly incorporates the variability in signal-to-noise ratio versus strength of magnetic field gradient. The processing technique is experimentally demonstrated on a one-dimensional phantom containing a nitroxide spin label with constant g-factor. Using an L-band EPR spectrometer, spectral shapes and spin distribution are accurately recovered from two projections and a spectral window which is comparable to the maximum linewidth of the sample.     

Spatially uniform sampling in 4-D EPR spectral-spatial imaging

Four-dimensional EPR imaging involves a computationally intensive inversion of the sampled Radon transform. Conventionally, N-dimensional reconstructions have been carried out with N-1 stages of 2-D backprojection to exploit a dimension-dependent reduction in execution time. The huge data size of 4-D EPR imaging demands the use of a 3-stage reconstruction each consisting of 2-D backprojections. This gives three orders of magnitude reduction in computation relative to a single stage 4-D filtered backprojection. The multi-stage reconstruction, however, requires a uniform angular sampling that yields an inefficient distribution of gradient directions. We introduce a solution that involves acquisition of projections uniformly distributed in solid angle and reconstructs in three 2-D stages with the spatial uniform solid angle data set converted to uniform linear angular projections using 2-D interpolation. Images were taken from the two sampling schemes to compare the spatial resolution and the line width resolution. The degradation in the image quality due to the additional interpolation was small, and we achieved approximately 30% reduction in data acquisition time.     

Simulation of 4D spectral-spatial EPR images

Electron paramagnetic resonance imaging (EPRI) can be modeled by the forward projection of a 4D synthetic spectral-spatial phantom. We developed a simulation tool for EPRI and carried out a quantitative comparison between simulation and experiment, focusing on the signal and noise characteristics. The signal height in the simulation was compared to that in the experimental projections at gradients of different magnitudes and directions. We investigated the noise power spectrum of an EPR imager and incorporated it into the simulation. The signal and noise modeling of the simulation achieved the same performance as the EPR imager. Using this simulation, various sampling schemes were tried to find an optimized parameter set under the customized noise model of this EPR imager.     

Simple data acquisition method for multi-dimensional EPR spectral-spatial imaging using a combination of constant-time and projection-reconstruction modalities

A combination of the constant-time spectral-spatial imaging (CTSSI) modality and projection-reconstruction modality was tested to simplify data acquisition for multi-dimensional CW EPR spectral-spatial imaging. In this method, 3D spectral-spatial image data were obtained by simple repetition of conventional 2D CW imaging process, except that the field gradient amplitude was incremented in constant steps in each repetition. The data collection scheme was no different from the conventional CW imaging system for spectral-spatial data acquisition. No special equipment and/or rewriting of existing software were required. The data acquisition process for multi-dimensional spectral-spatial imaging is consequently simplified. There is also no “missing-angle” issue because the CTSSI modality was employed to reconstruct 2D spectral-spatial images. Extra reconstruction processes to obtain higher spatial dimensions were performed using a conventional projection-reconstruction modality. This data acquisition technique can be applied to any conventional CW EPR (spatial) imaging system for multi-dimensional spectral-spatial imaging.     

Temperature dependent exchange narrowing of line width in EPR on interacting donors in germanium and silicon

An expression has been derived for the temperature dependent exchange integral for a pair of interacting shallow donors substituted in a dielectric material. This has been achieved by taking into account the phonon induced mixing of excited orbital states with the ground orbital states of the impurity centres. The results have been used to explain the temperature narrowing of the EPR line width observed by some workers in phosphorus doped germanium at low temperatures. Good agreement between theory and experimental results seems to favour the proposed mechanism in preference to one of motional narrowing due to hopping.     

Progressive EPR imaging with adaptive projection acquisition

Continuous wave electron paramagnetic resonance imaging (EPRI) of living biological systems requires rapid acquisition and visualization of free radical images. In the commonly used multiple-stage back-projection image reconstruction algorithm, the EPR image cannot be reconstructed until a complete set of projections is collected. If the data acquisition is incomplete, the previously acquired incomplete data set is no longer useful. In this work, a 3-dimensional progressive EPRI technique was implemented based on inverse Radon transform in which a 3-dimensional EPR image is acquired and reconstructed gradually from low resolution to high resolution. An adaptive data acquisition strategy is proposed to determine the significance of projections and acquire them in an order from the most significant to the least significant. The image acquisition can be terminated at any time if further collection of projections does not improve the image resolution distinctly, providing flexibility to trade image quality with imaging time. The progressive imaging technique was validated using computer simulations as well as imaging experiments. The adaptive acquisition uses 50-70% less projections as compared to the regular acquisition. In conclusion, adaptive data acquisition with progressive image reconstruction should be very useful for the accelerated acquisition and visualization of free radical distribution.     

Optimization of multiplexed holographic gratings in PQ-PMMA for spectral-spatial imaging filters

Holographic gratings formed in thick phenanthrenquinone- (PQ-) doped poly(methyl methacrylate) (PMMA) can be made to have narrowband spectral and spatial transmittance filtering properties. We present the design and performance of angle-multiplexed holographic filters formed in PQ-PMMA at 488 nm and reconstructed with a LED operated at approximately 630 nm. The dark delay time between exposure and the preillumination exposure of the polymer prior to exposure of the holographic area are varied to optimize the diffraction efficiency of multiplexed holographic filters. The resultant holographic filters can enhance the performance of four-dimensional spatial-spectral imaging systems. The optimized filters are used to simultaneously sample spatial and spectral information at five different depths separated by 50 microm within biological tissue samples.     

Optimization of magnetic field sweep and field modulation amplitude for continuous-wave EPR oximetry

For continuous-wave electron paramagnetic resonance spectroscopy, what settings of magnetic field sweep width and field modulation amplitude yield the best accuracy in estimated linewidth? Statistical bounds on estimation error presented in this work provide practical guidance: set the sweep width and modulation amplitude to 8 and 4 times the half-width half-maximum linewidth, Γ, respectively. For unknown linewidths in the range [Γ(min),Γ(max)] the worst-case estimation error is minimized by using settings designed for Γ(max). The analysis assumes a Lorentzian lineshape and a constant modulation amplitude across the extent of the irradiated paramagnetic probe. The analytical guidelines are validated using L-band spectroscopy with a particulate LiNc-BuO probe.     

Biochemical EPR imaging of skin

EPR Imaging (EPRI) of spin labels is a powerful method for investigating skin and can give information about biochemical processes which are involved in numerous skin diseases. Furthermore it enables the non invasive investigation of the liberation, penetration and distribution of spin labelled drugs. The basis of these measurements is spectral spatial EPR imaging employing modulated field gradients and simultaneous field scans (MOSS). A skin region (?=6 mm) was treated with a 10 μl spin label solution (1 mM). EPR spectra of 128 points were recorded in 128 spatial planes resulting in a 128×128 image matrix. A spatial resolution of better than 10 μm can be obtained for a spectral line width of 0.1 mT and a gradient of 4 Tm^?1.In vivo imaging on mammalian skin can be performed by employing surface coils at S-band frequencies, 3 GHz.     

Design of a permanent magnet with a mechanical sweep suitable for variable-temperature continuous-wave and pulsed EPR spectroscopy

A magnetic system is introduced which consists of three nested rings of permanent magnets of a Halbach dipolar layout and is capable for EPR spectroscopy. Two of the rings can be rotated independently to adjust the magnetic flux in the center and even allow for mechanical field sweeps. The presented prototype achieves a magnetic flux range of 0.0282–0.3013 T with a minimal sweep of 0.15 mT and homogeneity of about 10⁻³.First applications with CW and pulsed Mims ENDOR as well as ESEEM experiments on a sample of a glycine single crystal doped with 1% copper nitrate demonstrate that flux range, sweep accuracy and homogeneity of this prototype is sufficient for EPR experiments on most solid samples.Together with a recently improved design magnets can be build which could serve as compact and easily transportable replacement of standard electromagnets with negligible consumption of power or coolants.     

EPR imaging using T1 selectivity

Electron-spin-echo-detected EPR using an inversion-recovery three-pulse sequence permits EPR imaging selectivity based on electron spin longitudinal relaxation times. The feasibility is demonstrated with samples of coal, irradiated quartz, nitroxyl radicals, and galvinoxyl radicals.     

Two-dimensional imaging by the continuous-wave EPR

To be able to perform a two-dimensional study of free radical distribution by the continuous-wave electron paramagnetic resonance method in the X-band, special coils producing a magnetic gradient of 8 G/mm have been designed and constructed. The EPR spectra recorded for this gradient were subjected to the procedure of deconvolution in order to elicit information on the concentration of the radical distribution. The data obtained were used as the source file of the program reconstructing the image. The reconstruction was based on the iterative simultaneous algebraic reconstruction technique (Andersen A.H., Kak A.C.: Ultrason. Imag. 6, 81–94, 1984). The quality of the generated images depends on the angle of the sample axis to the gradient direction set by a goniometer and on the deconvolution procedures applied. The first tests on artificially generated phantoms indicated a dependence of the obtained images on the magnetic field gradients applied. The determined spatial distribution of radicals has confirmed their uniform distribution in the sample. The preliminary tests were performed for diphenyl-picrylhydrazyl. Having proved the reliability of the method, analogous measurements were also performed for plyphenylene sulphide PPS-V1 and indicated a homogeneous distribution of radicals in the whole volume of the sample. The images obtained confirmed the uniform distribution of the radicals.     

Kinetic measurements using EPR imaging with a modulated field gradient

EPR imaging with modulated field gradient was applied for the investigation of fast diffusion processes. Three different imaging methods are possible: spectral-temporal, spatio-temporal, and spectral-spatial imaging. The time resolution is on the order of seconds and the spatial resolution is in the micrometer region. The efficiency of this imaging technique is demonstrated for the penetration of the spin probe Tempol in the skin of hairless mice biopsies. The skin is normally protected against the penetration of water soluble substances by the horny layer, a resistive thin lipophilic layer. Overcoming this horny layer for water soluble ingredients is one of the main practical problems for the topical application of pharmaceutics which could be investigated by EPR imaging. Different images represent the penetration behavior of the water soluble Tempol in the skin after treatment with the penetration enhancer DMSO (Dimethylsulfoxide) and after removing the horny layer.     

Temperature dependent EPR of octahedral S-state ions in garnets

Temperature dependence of EPR of the octahedral Fe³⁺ and Mn²⁺ ions in four different garnets is studied between 77 K and the room temperature. The observed change of the spin hamiltonian parametersB ₂ ⁰ ,B ₄ ⁰ ,B ₄ ³ is of the order of several %, while theg-factor is temperature independent within the experimental errors. The role of thermal expansion and the effect of phonons on the temperature variation of B ₂ ⁰ is analyzed. It is shown that the inhomogeneous thermal expansion may account for theB ₂ ⁰ (T) dependence observed.The authors wish to thank Dr. B. V.Mill of Moscow State University (USSR) for supplying the single crystals of Na₃Sc₂V₃O₁₂, Ca₃Al₂Ge₃O₁₂ and Ca₃Ga₂Ge₃O₁₂ garnets.     

Pulsed EPR imaging of nitroxides in mice

Nitroxides, unlike trityl radicals, have shorter T₂s which until now were not detectable in vivo by a time-domain pulsed Electron Paramagnetic Resonance (EPR) spectrometer at 300 MHz since their phase memory times were shorter than the spectrometer recovery times. In the current version of the time-domain EPR spectrometer with improved spectrometer recovery times, the feasibility of detecting signals from nitroxide radicals was tested. Among the nitroxides evaluated, deuterated ¹⁵N-Tempone (¹⁵N-PDT) was found to have the longest T₂. The signal intensity profile as a function of concentration of these agents was evaluated and a biphasic behavior was observed; beyond a nitroxide concentration of 1.5 mM, signal intensity was found to decrease as a result of self-broadening. Imaging experiments were carried out with ¹⁵N-PDT in solutions equilibrated with 0%, 5%, 10%, and 21% oxygen using the single point imaging (SPI) modality in EPR. The image intensity in these tubes was found to depend on the oxygen concentration which in turn influences the T₂ of ¹⁵N-PDT. In vivo experiments were demonstrated with ¹⁵N-PDT in anesthetized mice where the distribution and metabolism of ¹⁵N-PDT could be monitored. This study, for the first time shows the capability to image a cell-permeable nitroxide in mice using pulsed EPR in the SPI modality.     

EPR spectroscopy and imaging of TEMPO-labeled magnetite nanoparticles

The article presents results of a study of TEMPO-labeled polymer coated superparamagnetic iron(II,III) oxide nanoparticles using both Electron Paramagnetic Resonance (EPR) spectroscopy and Electron Paramagnetic Resonance imaging technique (EPRI). The X-band (9.4 GHz) EPR spectroscopy was used to investigate the behavior of TEMPO-labeled polymer coated magnetite nanoparticles in different conditions (temperature and orientation in magnetic field). The broad line, which comes from the core of Fe₃O₄ nanoparticles, shows anisotropy. This signal broadens with decreasing temperature, its intensity increases with increasing temperature and the g factor decreases with increasing temperature. The shape of the signal from nitroxide radical strongly depends on temperature. When temperature is higher than 200 K, a narrow triplet appears, but when it is lower than 200 K the signal consists of broad asymmetric lines. Analysis of the signal allowed characterization of the motion of the spin label attached to nanoparticles. Values of anisotropy parameter ɛ and rotational correlation time τ_c were calculated for TEMPO in the fast rotation regime.The ability of TEMPO-labeled PEG coated magnetite nanoparticles to diffuse within the hydrogel medium was also investigated. The EPR imaging of nanoparticles diffusion in hydrogel was made at room temperature using an EPR L-band (1 GHz) spectrometer. EPRI has been proved effective for evaluation of changes in the spatial distribution of nanoparticles in the sample.