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.     

Uniform distribution of projection data for improved reconstruction quality of 4D EPR imaging

In continuous wave (CW) electron paramagnetic resonance imaging (EPRI), high quality of reconstruction in a limited acquisition time is a high priority. It has been shown for the case of 3D EPRI, that a uniform distribution of the projection data generally enhances reconstruction quality. In this work, we have suggested two data acquisition techniques for which the gradient orientations are more evenly distributed over the 4D acquisition space as compared to the existing methods. The first sampling technique is based on equal solid angle partitioning of 4D space, while the second technique is based on Fekete points estimation in 4D to generate a more uniform distribution of data. After acquisition, filtered backprojection (FBP) is applied to carry out the reconstruction in a single stage. The single-stage reconstruction improves the spatial resolution by eliminating the necessity of data interpolation in multi-stage reconstructions. For the proposed data distributions, the simulations and experimental results indicate a higher fidelity to the true object configuration. Using the uniform distribution, we expect about 50% reduction in the acquisition time over the traditional method of equal linear angle acquisition.     

Quasi Monte Carlo-based isotropic distribution of gradient directions for improved reconstruction quality of 3D EPR imaging

In continuous wave (CW) electron paramagnetic resonance imaging (EPRI), high quality of reconstructed image along with fast and reliable data acquisition is highly desirable for many biological applications. An accurate representation of uniform distribution of projection data is necessary to ensure high reconstruction quality. The current techniques for data acquisition suffer from nonuniformities or local anisotropies in the distribution of projection data and present a poor approximation of a true uniform and isotropic distribution. In this work, we have implemented a technique based on Quasi-Monte Carlo method to acquire projections with more uniform and isotropic distribution of data over a 3D acquisition space. The proposed technique exhibits improvements in the reconstruction quality in terms of both mean-square-error and visual judgment. The effectiveness of the suggested technique is demonstrated using computer simulations and 3D EPRI experiments. The technique is robust and exhibits consistent performance for different object configurations and orientations.     

Enhanced resolution for EPR imaging by two-step deblurring

The broad spectrum of spin probes used for electron paramagnetic resonance imaging (EPRI) result in poor spatial resolution of the reconstructed images. Conventional deconvolution procedures can enhance the resolution to some extent but obtaining high resolution EPR images is still a challenge. In this work, we have implemented and analyzed the performance of a postacquisition deblurring technique to enhance the spatial resolution of the EPR images. The technique consists of two steps; noniterative deconvolution followed by iterative deconvolution of the acquired projections which are then projected back using filtered backprojection (FBP) to reconstruct a high resolution image. Further, we have proposed an analogous technique for iterative reconstruction algorithms such as multiplicative simultaneous iterative reconstruction technique (MSIRT) which can be a method of choice for many applications. The performance of the suggested deblurring approach is evaluated using computer simulations and EPRI experiments. Results suggest that the proposed procedure is superior to the standard FBP and standard iterative reconstruction algorithms in terms of mean-square-error (MSE), spatial resolution, and visual judgment. Although the procedure is described for 2D imaging, it can be readily extended to 3D imaging.     

Fast 3D spatial EPR imaging using spiral magnetic field gradient

Electron paramagnetic resonance imaging (EPRI) provides direct detection and mapping of free radicals. The continuous wave (CW) EPRI technique, in particular, has been widely used in a variety of applications in the fields of biology and medicine due to its high sensitivity and applicability to a wide range of free radicals and paramagnetic species. However, the technique requires long image acquisition periods, and this limits its use for many in vivo applications where relatively rapid changes occur in the magnitude and distribution of spins. Therefore, there has been a great need to develop fast EPRI techniques. We report the development of a fast 3D CW EPRI technique using spiral magnetic field gradient. By spiraling the magnetic field gradient and stepping the main magnetic field, this approach acquires a 3D image in one sweep of the main magnetic field, enabling significant reduction of the imaging time. A direct one-stage 3D image reconstruction algorithm, modified for reconstruction of the EPR images from the projections acquired with the spiral magnetic field gradient, was used. We demonstrated using a home-built L-band EPR system that the spiral magnetic field gradient technique enabled a 4-7-fold accelerated acquisition of projections. This technique has great potential for in vivo studies of free radicals and their metabolism.     

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.     

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.     

Adaptive data acquisition algorithm in Raman distributed temperature measurement system

The sampling rate of data acquisition card (DAQ) employed in Raman distributed temperature sensor (RDTS) system is set to be constant, which exceeds 100 MHz for a higher spatial resolution of 1 m. The acquired data need to be averaged tens of thousands times to improve the signal-to-noise ratio (SNR). High-speed DAQ and cumulative average algorithm put forward higher request to the hardware device and the stability of data transmission between DAQ and computer. In this paper we propose a new adaptive data acquisition algorithm to regulate the sampling rate automatically according to the changes of data. The theoretical simulation of this algorithm in RDTS is analyzed in detail. The results indicate that the amount of computation is reduced efficiently and the measurement time is decreased correspondingly.     

在体剂量检测专用EPR数据采集与控制系统

研制了X波段在体剂量检测专用EPR谱仪的数据采集与控制系统,利用阿尔泰USB2812数据采集卡作为控制卡,在LabVIEW图形化编程开发环境中进行程序开发,实现了扫场电源的控制、微波模的显示、微波功率的控制和EPR信号的采集等功能. 数据处理程序可实现存储数据的查询、谱线基线校正和积分等基本功能,为下一步开展牙齿剂量试验研究提供了平台.     

基于图像数据封装包文的数据采集方法

针对如何在工业应用中找到一种实现周期更短和更低成本的图像数据采集方法进行了研究。通过分析以太网帧格式和JPEG图像数据特点,给出了一种对JPEG图像帧进行分片并对分片进行数据封装的方法,在此基础之上实现了基于图像数据封装包文的数据采集设计。最后在FPGA环境中对该设计进行了实现,此实现成功应用到了工程项目当中。实践表明,基于图像数据封装包文的数据采集方法能够在不添加外置存储器等复杂器件的情况下实现稳定的、实时的JPEG图像数据采集。     

基于PCI总线的多通道图像数据高速采集

根据立体测绘相机的数据传输要求,需要将多个图像数据源产生的异步图像数据传输和存储到计算机上,研究和实现了一种实用的完整解决方案。采用FPGA对多个图像数据进行组合,在FPGA内部进行一级缓存,将异步的图像数据源,变换成同步数据,经过独立的FIFO芯片对图像数据进行缓存,方便总线繁忙时对图像进行缓存;然后通过专用的PCI接口芯片,将图像数据传输到PCI总线上;使用DriverWorks进行驱动程序的设计,将数据存入内存中,通过应用程序显示及存储于硬盘中。     

Fast EPR imaging at 300 MHz using spinning magnetic field gradients

Electron paramagnetic resonance imaging (EPRI) technology has rapidly progressed in the last decade enabling many important applications in the fields of biology and medicine. At frequencies of 300-1200 MHz a range of in vivo applications have been performed. However, the requisite imaging time duration to acquire a given number of projections, limits the use of this technique in many in vivo applications where relatively rapid kinetics occur. Therefore, there has been a great need to develop approaches to accelerate EPRI data acquisition. We report the development of a fast low-frequency EPRI technique using spinning magnetic field gradients (SMFG). Utilizing a 300 MHz CW (continuous wave) EPRI system, SMFG enabled over 10-fold accelerated acquisition of image projections. 2D images with over 200 projections could be acquired in less than 3s and with 20s acquisitions good image quality was obtained on large aqueous free radical samples. This technique should be particularly useful for in vivo studies of free radicals and their metabolism.     

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.     

基于EPP协议的便携机数据采集系统

设计了基于EPP协议的便携机数据采集系统 ,给出了技术解决方案。方案具有软硬件设计简捷、无需插卡等优点 ,可使外设工作于PC并行口的多种模式下。解决了线阵CCD图像采集系统与笔记本电脑之间的数据通讯 ,达到了便携数据采集的目的。     

Evaluation of sub-microsecond recovery resonators for in vivo electron paramagnetic resonance imaging

Time-domain (TD) electron paramagnetic resonance (EPR) imaging at 300MHz for in vivo applications requires resonators with recovery times less than 1 micros after pulsed excitation to reliably capture the rapidly decaying free induction decay (FID). In this study, we tested the suitability of the Litz foil coil resonator (LCR), commonly used in MRI, for in vivo EPR/EPRI applications in the TD mode and compared with parallel coil resonator (PCR). In TD mode, the sensitivity of LCR was lower than that of the PCR. However, in continuous wave (CW) mode, the LCR showed better sensitivity. The RF homogeneity was similar in both the resonators. The axis of the RF magnetic field is transverse to the cylindrical axis of the LCR, making the resonator and the magnet co-axial. Therefore, the loading of animals, and placing of the anesthesia nose cone and temperature monitors was more convenient in the LCR compared to the PCR whose axis is perpendicular to the magnet axis.     

Regularized optimization (RO) reconstruction for oximetric EPR imaging

A new algorithm for EPR imaging oximetry is described and tested with experimental data for the case of one spatial and one spectral dimension. A single species with variable linewidth is assumed. Instead of creating a 2D image, two one-dimensional profiles are reconstructed: the concentration of the radical and the corresponding oxygen concentration, which reduces the dimensionality of the problem. The algorithm (i) seeks to minimize the discrepancy between experimental data and projections calculated from the profiles and (ii) uses Tikhonov regularization to constrain the smoothness of the results. This approach controllably smoothes profiles rather than the data, while preserving sharp features.     

Assessment of liver phagocytic activity using EPR spectrometry and imaging

The aim of the present study was to evaluate the usefulness of electron paramagnetic resonance (EPR) spectroscopy and imaging in assessing the phagocytic activity of the liver after administration of India ink. We conducted experiments on livers from control rodents and from rodents in which the Kupffer cell population had been depleted by pretreatment with gadolinium chloride. The EPR signal intensity recorded in liver homogenates was about two times lower in GdCl(3) treated rats than in control rats. EPR imaging carried out on precision-cut liver slices indicated a good correlation between the depletion of Kupffer cells and the EPR signal intensity.     

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.     

An optimized multislice acquisition sequence for the inversion-recovery MR imaging

An optimized multislice data acquisition scheme for inversion-recovery MR imaging is proposed and experimental results are presented. In this new scheme, instead of forming a set of multislice inversion-recovery sequences in series for a given phase encoding step, 180° inversion pulses corresponding to different slices are interwoven with the spin echo data acquisition sequence in an optimal way depending on the desired inversion-recovery time. For example, between the 180° inversion RF pulse and the spin-echo imaging sequence, a number of imaging and inversion sequences are inserted with different slice combinations, i.e., long inversion-recovery time is effectively utilized for the other slice pre-inversion and data acquisition. With the optimized sequence, imaging time has been reduced by as much as a factor of four compared with the existing methods.     

基于虚拟仪器技术的多路水声信号同步采集及处理平台设计

本文介绍了虚拟仪器技术在水声信号处理研究中的一个具体应用,利用LabVIEW语言编程,在NI公司高速数据采集硬件平台上实现了高采样率下多通道数据的同步采集、处理、记录功能,并在不使用信号处理机的情况下,利用主控单板机的单核CPU完成了32通道常规波束成形(CBF)的实时计算。同时,本文对非DSP硬件平台的实时信号处理实现方法进行了初步的研究。