Continuous-wave Electron Paramagnetic Resonance Imaging Research Articles

Accurate reconstruction of 4D spectral–spatial images from sparse-view data in continuous-wave EPRI

In continuous-wave electron paramagnetic resonance imaging (CW EPRI), data are collected generally at densely sampled views sufficient for achieving accurate reconstruction of a four dimensional spectral–spatial (4DSS) image by use of the conventional filtered-backprojection (FBP) algorithm. It is desirable to minimize the scan time by collection of data only at sparsely sampled views, referred to as sparse-view data. Interest thus remains in investigation of algorithms for accurate reconstruction of 4DSS images from sparse-view data collected for potentially enabling fast data acquisition in CW EPRI. In this study, we investigate and demonstrate optimization-based algorithms for accurate reconstruction of 4DSS images from sparse-view data. Numerical studies using simulated and real sparse-view data acquired in CW EPRI are conducted that reveal, in terms of image visualization and physical-parameter estimation, the potential of the algorithms developed for yielding accurate 4DSS images from sparse-view data in CW EPRI. The algorithms developed may be exploited for enabling sparse-view scans with minimized scan time in CW EPRI for yielding 4DSS images of quality comparable to, or better than, that of the FBP reconstruction from data collected at densely sampled views.

4D-image reconstruction directly from limited-angular-range data in continuous-wave electron paramagnetic resonance imaging

Objective: We investigate and develop optimization-based algorithms for accurate reconstruction of four-dimensional (4D)-spectral-spatial (SS) images directly from data collected over limited angular ranges (LARs) in continuous-wave (CW) electron paramagnetic resonance imaging (EPRI). Methods: Basing on a discrete-to-discrete data model devised in CW EPRI employing the Zeeman-modulation (ZM) scheme for data acquisition, we first formulate the image reconstruction problem as a convex, constrained optimization program that includes a data fidelity term and also constraints on the individual directional total variations (DTVs) of the 4D-SS image. Subsequently, we develop a primal–dual-based DTV algorithm, simply referred to as the DTV algorithm, to solve the constrained optimization program for achieving image reconstruction from data collected in LAR scans in CW-ZM EPRI. Results: We evaluate the DTV algorithm in simulated- and real-data studies for a variety of LAR scans of interest in CW-ZM EPRI, and visual and quantitative results of the studies reveal that 4D-SS images can be reconstructed directly from LAR data, which are visually and quantitatively comparable to those obtained from data acquired in the standard, full-angular-range (FAR) scan in CW-ZM EPRI. Conclusion: An optimization-based DTV algorithm is developed for accurately reconstructing 4D-SS images directly from LAR data in CW-ZM EPRI. Future work includes the development and application of the optimization-based DTV algorithm for reconstructions of 4D-SS images from FAR and LAR data acquired in CW EPRI employing schemes other than the ZM scheme. Significance: The DTV algorithm developed may be exploited potentially for enabling and optimizing CW EPRI with minimized imaging time and artifacts by acquiring data in LAR scans.

Brain Redox Imaging Using In Vivo Electron Paramagnetic Resonance Imaging and Nitroxide Imaging Probes

Reactive oxygen species (ROS) are produced by living organisms as a result of normal cellular metabolism. Under normal physiological conditions, oxidative damage is prevented by the regulation of ROS by the antioxidant network. However, increased ROS and decreased antioxidant defense may contribute to many brain disorders, such as stroke, Parkinson’s disease, and Alzheimer’s disease. Noninvasive assessment of brain redox status is necessary for monitoring the disease state and the oxidative damage. Continuous-wave electron paramagnetic resonance (CW-EPR) imaging using redox-sensitive imaging probes, such as nitroxides, is a powerful method for visualizing the redox status modulated by oxidative stress in vivo. For conventional CW-EPR imaging, however, poor signal-to-noise ratio, low acquisition efficiency, and lack of anatomic visualization limit its ability to achieve three-dimensional redox mapping of small rodent brains. In this review, we discuss the instrumentation and coregistration of EPR images to anatomical images and appropriate nitroxide imaging probes, all of which are needed for a sophisticated in vivo EPR imager for all rodents. Using new EPR imaging systems, site-specific distribution and kinetics of nitroxide imaging probes in rodent brains can be obtained more accurately, compared to previous EPR imaging systems. We also describe the redox imaging studies of animal models of brain disease using newly developed EPR imaging.

Effect of body temperature on the pharmacokinetics of a triarylmethyl-type paramagnetic contrast agent used in EPR oximetry.

Pharmacokinetics of the tri[8-carboxy-2,2,6,6-tetrakis(2-hydroxymethyl)benzo[1,2-d:4,5-d']bis(1,3)dithio-4-yl]methyl radical (Oxo63) after a single bolus and/or continuous intravenous infusion was investigated in tumor-bearing C3H mice with or without body temperature control while under anesthesia. The in vivo time course of Oxo63 in blood was measured using X-band electron paramagnetic resonance spectroscopy. Distribution of Oxo63 in normal muscle and tumor tissues was obtained using a surface coil resonator and a 700-MHz electron paramagnetic resonance spectrometer. The whole-body distribution of Oxo63 was obtained by 300-MHz continuous-wave electron paramagnetic resonance imaging. The high-resolution 300-MHz time-domain electron paramagnetic resonance imaging was also carried out to probe the distribution of Oxo63. Urination of mice was retarded at low body temperature, causing the concentration of Oxo63 in blood to attain high levels. However, the concentration of Oxo63 in tumor tissue was lower with no control of body temperature than active body temperature control. The nonsystemized blood flow in the tumor tissues may pool Oxo63 at lower body temperature. Pharmacokinetics of the contrast agent were found to be significantly affected by body temperature of the experimental animal, and can influence the probe distribution and the image patterns. Magn Reson Med 79:1212-1218, 2018. © Published 2017. This article is a U.S. Government work and is in the public domain in the USA.

Parallel image-acquisition in continuous-wave electron paramagnetic resonance imaging with a surface coil array: Proof-of-concept experiments

This article describes a feasibility study of parallel image-acquisition using a two-channel surface coil array in continuous-wave electron paramagnetic resonance (CW-EPR) imaging. Parallel EPR imaging was performed by multiplexing of EPR detection in the frequency domain. The parallel acquisition system consists of two surface coil resonators and radiofrequency (RF) bridges for EPR detection. To demonstrate the feasibility of this method of parallel image-acquisition with a surface coil array, three-dimensional EPR imaging was carried out using a tube phantom. Technical issues in the multiplexing method of EPR detection were also clarified. We found that degradation in the signal-to-noise ratio due to the interference of RF carriers is a key problem to be solved.

Sequential CW-EPR image acquisition with 760-MHz surface coil array

This paper describes the development of a surface coil array that consists of two inductively coupled surface-coil resonators, for use in continuous-wave electron paramagnetic resonance (CW-EPR) imaging at 760 MHz. To make sequential EPR image acquisition possible, we decoupled the surface coils using PIN-diode switches, to enable the shifting of the resonators resonance frequency by more than 200 MHz. To assess the effectiveness of the surface coil array in CW-EPR imaging, two-dimensional images of a solution of nitroxyl radicals were measured with the developed coil array. Compared to equivalent single coil acquired images, we found the visualized area to be extended approximately 2-fold when using the surface coil array. The ability to visualize larger regions of interest through the use of a surface coil array, may offer great potential in future EPR imaging studies.

Improvement of temporal resolution for three-dimensional continuous-wave electron paramagnetic resonance imaging

This paper describes improved temporal resolution for three-dimensional (3D) continuous-wave electron paramagnetic resonance (EPR) imaging. To improve temporal resolution, the duration of magnetic filed scanning that is used to obtain an EPR spectrum for each projection was reduced to 40 ms. The Helmholtz coil pair for field scanning was driven by triangular waves. The uniform distribution of projections was also used to reduce the number of projections for 3D image reconstruction. The reduction reaction of 4-hydroxy-2,2,6,6-tetramethyl-piperidinooxy with ascorbic acid was visualized by improved 3D EPR imaging techniques with a temporal resolution of 5.8 s.

Targeted-ROI imaging in electron paramagnetic resonance imaging

Electron paramagnetic resonance imaging (EPRI) is a technique that has been used for in vivo oxygen imaging of small animals. In continuous wave (CW) EPRI, the measurement can be interpreted as a sampled 4D Radon transform of the image function. The conventional filtered-backprojection (FBP) algorithm has been used widely for reconstructing images from full knowledge of the Radon transform acquired in CW EPRI. In practical applications of CW EPRI, one often is interested in information only in a region of interest (ROI) within the imaged subject. It is desirable to accurately reconstruct an ROI image only from partial knowledge of the Radon transform because acquisition of the partial data set can lead to considerable reduction of imaging time. The conventional FBP algorithm cannot, however, reconstruct accurate ROI images from partial knowledge of the Radon transform of even dimension. In this work, we describe two new algorithms, which are referred to as the backprojection filtration (BPF) and minimum-data filtered-backprojection (MDFBP) algorithms, for accurate ROI-image reconstruction from a partial Radon transform (or, truncated Radon transform) in CW EPRI. We have also performed numerical studies in the context of ROI-image reconstruction of a synthetic 2D image with density similar to that found in a small animal EPRI. This demonstrates both the inadequacy of the conventional FBP algorithm and the success of BPF and MDFBP algorithms in ROI reconstruction. The proposed ROI imaging approach promises a means to substantially reduce image acquisition time in CW EPRI.

Feasibility study of superresolution continuous-wave electron paramagnetic resonance imaging

In this letter, we report that superresolution continuous-wave electron paramagnetic resonance (cw-EPR) imaging is feasible for enhancing spatial resolution in images of unpaired electrons. We demonstrate one-dimensional superresolution EPR imaging for phantoms of 1,1-diphenyl-2-picrylhydrazyl (DPPH) powder with a 650MHz cw-EPR imager. The spatial resolution was improved up to fivefold with iterative deconvolution techniques. Our superresolution EPR imaging includes two-stage postprocessing, i.e., noniterative deconvolution for measured EPR spectra and iterative deconvolution processing for a blurred EPR image profile with the point spread function of the low-pass window function being applied.

Systematic approach to cutoff frequency selection in continuous-wave electron paramagnetic resonance imaging

This article describes a systematic method for determining the cutoff frequency of the low-pass window function that is used for deconvolution in two-dimensional continuous-wave electron paramagnetic resonance (EPR) imaging. An evaluation function for the criterion used to select the cutoff frequency is proposed, and is the product of the effective width of the point spread function for a localized point signal and the noise amplitude of a resultant EPR image. The present method was applied to EPR imaging for a phantom, and the result of cutoff frequency selection was compared with that based on a previously reported method for the same projection data set. The evaluation function has a global minimum point that gives the appropriate cutoff frequency. Images with reasonably good resolution and noise suppression can be obtained from projections with an automatically selected cutoff frequency based on the present method.

Continuous-wave NMR imaging of solids.

Current pulsed nuclear magnetic resonance methods of imaging samples such as solids with short spin-spin relaxation times are restricted to use with T2 values longer than approximately 10 microseconds. In the present study a method of imaging ultra-short T2 samples using continuous- wave, swept-field NMR is presented that, in principle, will be able to overcome this restriction. The technique is identical to that used in continuous-wave electron paramagnetic resonance imaging of paramagnetic species and involves irradiating the sample continuously with a radiofrequency excitation in the presence of a strong stationary magnetic field gradient. When the main magnetic field is swept over a suitable range, the variation of the NMR absorption signal with applied magnetic field yields a one-dimensional projection of the object under study along the gradient direction. Two- or three-dimensional image data sets may be reconstructed from projections that are obtained by applying the gradient in different directions. Signal-to-noise ratio can be improved by modulating the magnetic field and employing a lock-in amplifier to recover signal variations at the audio modulation frequency. Preliminary experiments were performed using a 7 Tesla magnet and a 300 MHz continuous-wave radiofrequency bridge with lock-in detection. The apparatus is described and the results of pilot experiments that employed vulcanized rubber samples are presented. The ability of the technique to detect short T2 samples was demonstrated by the presence of a background signal from the Perspex former of the birdcage resonator used for signal reception.