Object dependent sweep width reduction with spectral-spatial EPR imaging

For spectral-spatial EPR imaging, prior knowledge about the spatial support of an imaged object can be exploited in two ways. We can shrink the spatial field of view (FOV) to closely wrap the object in a sphere or reduce the sweep width in a projection dependent fashion. Use of a smaller spatial FOV with the same number of samples enhances spatial resolution by reducing voxel volume at the expense of signal-to-noise and a consequent degraded line-width resolution. We have developed another approach to define sweep width that prunes away the portions of the projection sweep with no signal. This reduces data acquisition time for the continuous wave (CW) EPR image proportional to the sweep width reduction. This method also avoids voxel volume reduction. Using the reduced-sweep method, we decreased the data acquisition time by 20% maintaining spatial and linewidth resolution.     

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.     

A parametric imaging approach for the segmentation of ultrasound data

When an ultrasonic examination is performed, a segmentation tool would often be very useful, either for the detection of pathologies, the early diagnosis of cancer or the follow-up of the lesions. Such a tool must be both reliable and accurate. However, because of the relatively reduced quality of ultrasound images due to the speckled texture, the segmentation of ultrasound data is a difficult task. We have previously proposed to tackle the problem using a multiresolution Bayesian region-based algorithm. For computation time purposes, a multiresolution version has been implemented. In order to improve the quality of the segmentation, we propose to perform the segmentation not only from the envelope image but to combine more information about the properties of the tissues in the segmentation process. Several acoustical parameters have thus been computed, either directly from the images or from the radio-frequency (RF) signal.

In a previous study, two parametric images were involved in the segmentation process. The parameter represented the integrated backscatter (IBS) and the mean central frequency (MCF), which is a measurement related to the attenuation of ultrasound waves in the media. In this study, parameters representative of the scattering conditions in the tissue are evaluated in the multiparametric segmentation process. They are extracted from the K-distribution (,b) and the Nakagami distribution (m,Ω) and are related to the local density of scatterers (,m), the size of the scatterers (b) and the backscattering properties of the medium (Ω).

The acoustical features are calculated locally on a sliding window. This procedure allows to built parametric mapping representing the particular characteristics of the medium. To test the influence of the acoustical parameters in the segmentation process, a set of numerical phantoms has been computed using the Field software developed by J.A. Jensen. Each phantom consists in two regions with two different acoustical properties: the density of scatterers and the scattering amplitude. From both the simulated RF signals and envelope images, the parameters have been computed; their relevance to represent a particular characteristic of the medium is evaluated. The segmentation has been processed for each phantom. The ability of each parameter to improve the segmentation results is validated. A agar–gel phantom has also been created, in order to test the accuracy of the parameters in conditions closer to the in vivo ultrasound imaging. This phantom contains four inclusions with different concentrations of silica. A B&K ultrasound device provides the RF data. The quantification of the segmentation quality is based on the rate of correctly classified pixels and it has been computed for all the parameters either from the field images or the phantom images. The large improvement in the segmentation results obtained reveals that the multiparametric segmentation scheme proposed in this study can be a reliable tool for the processing of noisy ultrasound data.     

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.     

A multifrequency EPR approach to travertine characterisation

The understanding of processes that give rise to travertine deposits is important. This is so because of its widespread use as decorative material, but more so in environmental studies due to the significance, by proxy, of travertine in climatology. In this paper, a multifrequency EPR spectroscopy study of the behaviour of an ubiquitary vicariant of Ca in calcite, Mn(II), is presented. EPR spectra were obtained from a natural sample at 9.5 (X-band), 95, 190, and 285GHz, and interpreted through numerical simulation. An analysis of the distribution of the zero-field splitting interaction revealed the source of some unexpected spectral features in the width of the lines in the X-band. By contrast, the homogeneous broadening plays only a minor role. Moreover, field-dependent anisotropies of the Zeeman and hyperfine tensors were observed at higher frequency. On the basis of results garnered in this study, the ZFS interaction of Mn(II) has been ascribed to the microstructural anomalies of the Mn(II) distribution in calcite. This may be considered as the fingerprint of the physical-chemical conditions at the time of travertine deposition. As a consequence, X-band EPR spectroscopy represents a specific tool to investigate the genesis, and to check the homogeneity of Mn(II) distribution in travertines as well as in other calcite-based materials.     

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.     

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.     

一种参数摄动的混沌异结构同步方法

研究了参数摄动情形下的混沌异结构同步问题,基于Lyapunov稳定性定理并结合范数理论给出了系统参数摄动下实现混沌异结构同步的一个充分条件,为同步控制器的设计提供了一般方法.只要两混沌系统维数相等,状态变量可测,就可利用所提方法实现系统参数摄动下的异结构同步,并能够保证在同步实现后同步控制量伴随误差变量一同收敛至零.该方法鲁棒性强,适用范围广,通过对混沌系统、超混沌系统的同步仿真,证实了该方法的有效性. 关键词： 混沌 超混沌 同步 Lyapunov函数     

A simple approach to T2 imaging in MRI

A simple method for obtaining images whose contrast depends only on T₂ is described and tested both on phantoms and in vivo. The method works reliably and effectively under clinically realistic operating conditions using standard imaging protocols. It can result in a substantial reduction in imaging times for T₂ weighted images.     

A practical approach to ultrasonic imaging using diffraction tomography

A technique for ultrasonic imaging based on the theory of diffraction tomography is presented. The method utilizes a fixed, circular configuration of transmitters and detectors. This configuration was selected because it avoids many practical limitations associated with the design of a medical imaging device. Practical considerations also motivated the inclusion of effects associated with the transmitter beam pattern rather than pursuing the more conventional approach in which plane-wave illumination is required. In addition, the problem of separately imaging both density and compressibility variations is considered.     

A novel approach to separating EPR lines arising from species with different transition moments

In pulsed EPR, spectral contributions from several species in one sample can be separated based on different EPR transition probabilities. This is usually done by monitoring the Rabi nutations in a 2D experiment. By using long pulses, the FID and echo shapes of species with different transition probabilities differ significantly, including temporal shifts of the observed echo signals in a two-pulse ESE experiment. These shifts can be used to disentangle spectral components in a 1D field-swept ESE experiment by choosing an appropriate detection time. This approach is demonstrated by experiments on a sample containing Mn(2+) and Cr(3+) centers as well as on an exchange-coupled Mn(III)/Mn(IV) system with Mn(2+) contaminations.     

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.     

Constructive approach to logics of physical systems: Application to EPR case

A constructive approach to logics of physical systems, according to which families of propositions about physical systems are not defined in an axiomatic way, but are built up in the course of experiments, is proposed. Several ways of joining Boolean algebras of propositions obtained in single experiments are studied. The proposed approach is applied to study families of propositions encountered in EPR-type experiments. Two examples of such experimental families of EPR propositions are studied and they are compared with two theoretical families of EPR propositions in the literature.     

A loop resonator for slice-selective in vivo EPR imaging in rats

A loop resonator was developed for 300 MHz continuous-wave electron paramagnetic resonance (CW-EPR) spectroscopy and imaging in live rats. A single-turn loop (55 mm in diameter) was used to provide sufficient space for the rat body. Efficiency for generating a radiofrequency magnetic field of 38 microT/W(1/2) was achieved at the center of the loop. For the resonator itself, an unloaded quality factor of 430 was obtained. When a 350 g rat was placed in the resonator at the level of the lower abdomen, the quality factor decreased to 18. The sensitive volume in the loop was visualized with a bottle filled with an aqueous solution of the nitroxide spin probe 3-carbamoyl-2,2,5,5-tetramethyl-3-pyrrolin-1-yloxy (3-CP). The resonator was shown to enable EPR imaging in live rats. Imaging was performed for 3-CP that had been infused intravenously into the rat and its distribution was visualized within the lower abdomen.     

A novel approach to the aperture windowing in medical imaging

A new technique is proposed to improve the lateral resolution in the conventional B-mode imaging systems, which enables a simple array aperture windowing in the transmitting mode. Amplitude shaping is performed without modifying the transmitting voltage of the array elements, but only varying the excitation pulse length from one element to another. This method presents some attractive practical advantages, and the reduction of the sidelobe energy is comparable to that attainable with a conventional aperture windowing. Parametric plots are given, which transform an amplitude apodization into a 'time apodization' for any type of transducer array.