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This work focuses on the reconstruction of two and three dimensional images of the concentration of paramagnetic species from electron paramagnetic resonance (EPR) measurements. A direct operator, modeling how the measurements are related to the paramagnetic sample to be imaged, is derived in the continuous framework taking into account the physical phenomena at work during the acquisition process. Then, this direct operator is discretized to closely take into account the discrete nature of the measurements and provide an explicit link between them and the discrete image to be reconstructed. A variational inverse problem with total variation regularization is formulated and an efficient resolvant scheme is implemented. The setting of the reconstruction parameters is thoroughly studied and facilitated thanks to the introduction of appropriate normalization factors. Moreover, an a contrario algorithm is proposed to derive the optimal resolution at which the data should be acquired. Finally, an in-depth experimental study over real EPR datasets is done to illustrate the potential and limitations of the presented image reconstruction model.

Nitroxide compounds have been used as redox-sensitive imaging probes by electron paramagnetic resonance (EPR) for assessing oxidative stress in vivo. Fast redox reactions of nitroxide radicals are favorable for assessment of higher redox sensitivity; however, a variety of nitroxides have not been trialed for use as imaging probes due to their very rapid in vivo reduction, which cannot be captured at the slow operation speed of existing EPR imagers. To overcome this limitation, we improved our EPR system to provide a stable and highly sensitive imaging operation. We challenged the improved EPR imager to perform three-dimensional (3D) EPR imaging of mouse brain using two useful nitroxide imaging probes, 4-hydroxy-2,2,6,6-tetramethylpiperidine 1-oxyl (Tempol) and 2,6-dispiro-4',4"-dipyrane-piperidine-4-one-N-oxyl (DiPy). The second-order rate constant of DiPy with ascorbic acid is 10 times larger than that of Tempol. The improved EPR imager obtained clear 3D EPR images of mouse brain and demonstrated that Tempol could exist with an unpaired electron. The imager also successfully obtained 3D EPR images of mouse head after administration of DiPy. As 126 projections can be acquired in a period of 6 s, 3D EPR imaging can visualize the sequential process of DiPy entering the brain, being distributed within the brain, and being reduced within the brain. These improvements to the EPR imager will enable useful nitroxide imaging probes that were previously unsuitable as imaging probes due to their rapid reduction to be considered for use for sensitive redox assessment in an in vivo system.

The multi-modal imaging contrast agent has been attracted an increased interest to the tumor detecting and diagnosis in these recent years since the multi-modal imaging technology combines the adva...

The imaging of the biodistribution and pharmacokinetics is critical in understanding the complexity of drug delivery mechanisms, the status of the disease, and the monitoring of the treatment progr...

Honey bees play a crucial role in the nature by pollinating wild flowers. Over the past years, there has been an increasing concern regarding the honey bee colony decline. Pesticides or environmental effects targeting the biochemistry of insect chitin and cuticle coating may be in part responsible for honey bee pathologies. We here propose the use of electron paramagnetic resonance imaging (EPRI) as a tool to image the melanin-chitin complexes as part of the exoskeleton of the honey bee. EPRI at 9.65 GHz was applied on intact freeze-dried bees. The imaging data were collected on the melanin peak. High-resolution images revealed that this compound is extensively distributed in the periphery of the animal, data consistent with the localization in the cuticle of the bee. While EPR of melanin has been so far explored in the context of melanoma characterization, it may offer new opportunities in research on honey bees and other insects.

X-band (9.4 GHz) electron paramagnetic resonance (EPR) and electron paramagnetic resonance imaging (EPRI) were used for elucidating structural aspects of the stratum corneum (SC). We found that psoriasis vulgaris (PV)-SC has a less-ordered structure than that of the control SC, indicating the abnormal architecture of PV-SC. Different spectra were observed for the PV-SC. The three-line spectral pattern suggests that the 5-doxylstearic acid (5-DSA) is mobile or less rigid in the SC. The simulated order parameter (S0) value obtained for 5-DSA in the SC was approximately 0.20. The statistical analysis suggests that the value of the PV-SC is significantly smaller than that of the control (p < 0.01). Thus, we suggest that this EPR assay is of great use for evaluating SC function. In addition, the EPRI of various SC samples provides a useful image concerning SC status. A strong red image was observed for the PV skin. No red lesion region was observed in the control. The EPR images differentiated various sizes and number distribution concerning the disordered states in the SC.

The drug-nitroxide radical hybrid-compound 7-N-((2,2,5,5-tetramethylpyrrolidine-1-yloxy(PROXYL))-3-yl-methyl)theophylline (3) was synthesized by coupling 7-N-tosyltheophylline with 3-hydroxymethyl-PROXYL, HMP). The stability of 3 relative to that of HMP was examined in the presence of the anti-oxidant, ascorbic acid (AsA). The initial reduction rate constants of 3 and HMP were 11.9±5.3 and 6.1±5.2 M-1 min-1, respectively. In the presence of glutathione (GSH), these constants increased slightly to 22.3±6.8 and 9.1±2.4 M-1 min-1, respectively. Two-dimensional cranial electron paramagnetic resonance imaging of mice intravenously injected with 3 via the tail vein revealed that probe 3 enters the mouse brain by passing through the blood-brain barrier (BBB).

Radiation treatment success and high tumor oxygenation and success have been known to be highly correlated. This suggests that radiation therapy guided by images of tumor regions with low oxygenation, oxygen-guided radiation therapy (OGRT) may be a promising enhancement of cancer radiation treatment. Before applying the technique to human subjects, OGRT needs to be tested in animals, most easily in rodents. Electron paramagnetic resonance imaging provides quantitative maps of tissue and tumor oxygen in rodents with 1 mm spatial resolution and 1 torr pO2 resolution at low oxygen levels. The difficulty of using mouse models is their small size and that of their tumors. To overcome this we used XRAD225Cx micro-CT/ therapy system and 3D printed conformal blocks. Radiation is delivered first to a uniform 15% tumor control dose for the whole tumor and then a boost dose to either hypoxic tumor regions or equal volumes of well oxygenated tumor. Delivery of the booster dose used a multiple beam angles to deliver radiation beams whose shape conforms to that of all hypoxic regions or fully avoids those regions. To treat/avoid all hypoxic regions we used individual radiation blocks 3D-printed from acrylonitrile butadiene styrene polymer infused with tungsten particles fabricated immediately after imaging to determine regions with pO2 less than 10 torr. Preliminary results demonstrate the efficacy of the radiation treatment with hypoxic boosts with syngeneic FSa fibrosarcoma tumors in the legs of C3H mice.

Hypoxia is considered one of the microenvironmental factors associated with the malignant nature of glioblastoma. Thus, evaluating intratumoural distribution of hypoxia would be useful for therapeutic planning as well as assessment of its effectiveness during the therapy. Electron paramagnetic resonance imaging (EPRI) is an imaging technique which can generate quantitative maps of oxygen in vivo using the exogenous paramagnetic compound, triarylmethyl and monitoring its line broadening caused by oxygen. In this study, the feasibility of EPRI for assessment of oxygen distribution in the glioblastoma using orthotopic U87 and U251 xenograft model is examined. Heterogeneous distribution of pO2 between 0 and 50 mmHg was observed throughout the tumours except for the normal brain tissue. U251 glioblastoma was more likely to exhibit hypoxia than U87 for comparable tumour size (median pO2; 29.7 and 18.2 mmHg, p = .028, in U87 and U251, respectively). The area with pO2 under 10 mmHg on the EPR oximetry (HF...

Hypoxia is considered one of the microenvironmental factors associated with the malignant nature of glioblastoma. Thus, evaluating intratumoural distribution of hypoxia would be useful for therapeutic planning as well as assessment of its effectiveness during the therapy. Electron paramagnetic resonance imaging (EPRI) is an imaging technique which can generate quantitative maps of oxygen in vivo using the exogenous paramagnetic compound, triarylmethyl and monitoring its line broadening caused by oxygen. In this study, the feasibility of EPRI for assessment of oxygen distribution in the glioblastoma using orthotopic U87 and U251 xenograft model is examined. Heterogeneous distribution of pO2 between 0 and 50 mmHg was observed throughout the tumours except for the normal brain tissue. U251 glioblastoma was more likely to exhibit hypoxia than U87 for comparable tumour size (median pO2; 29.7 and 18.2 mmHg, p = .028, in U87 and U251, respectively). The area with pO2 under 10 mmHg on the EPR oximetry (HF10) showed a good correlation with pimonidazole staining among tumours with evaluated size. In subcutaneous xenograft model, irradiation was relatively less effective for U251 compared with U87. In conclusion, EPRI is a feasible method to evaluate oxygen distribution in the brain tumour.

New hybrid radiotherapy treatment systems combining an MRI scanner with a source of ionizing radiation are being introduced in the clinic. The strong magnetic fields of MRI considerably affect radiation dose distributions, especially at tissue-air interfaces due to the electron return effect (ERE). Experimental investigation of the ERE within a sub-millimeter thick surface layer is still highly challenging. In the present work, we examine and quantify the magnetic field induced perturbations of dose distributions within a 0.5 mm layer surrounding millimeter-size air cavities by applying electron paramagnetic resonance imaging (EPRI). Air-filled fused quartz tubes (inner diameter 3 or 4 mm) mimic small air cavities and serve as model systems. The tubes were irradiated inside a PMMA phantom by a 6 MV photon beam. The irradiations were performed in the presence or absence of a transverse, magnetic field providing a magnetic field strength of 1.0 Tesla. The spatial distributions of radiation induced paramagnetic defects in the quartz tubes were subsequently determined by applying field-swept echo-detected EPRI and were then converted to relative dose distributions. The transverse magnetic field leads to considerable local dose enhancements and reductions (up to 35%) with respect to the mean dose within the quartz tubes. The experimentally determined dose distributions are in good quantitative agreement with Monte Carlo radiation transport simulations. The results of this work demonstrate the feasibility of field-swept echo-detected EPRI to measure magnetic field induced perturbations of dose distributions within a sub-millimeter thick surface layer at the dosimeter-air interface. ispartof: PHYSICS IN MEDICINE AND BIOLOGY vol:64 issue:17 ispartof: location:England status: published

Low frequency electron paramagnetic resonance imaging is a powerful tool to non-invasively measure the physiological status of tumors. Here, we report on the design and functionality of a rapid scan and pulse table-top imaging spectrometer based around an arbitrary waveform generator and 25mm cross-loop resonator operating at 700 MHz. Two and four-dimensional rapid scan spectral-spatial images are presented. This table-top imager is a prototype for future pre-clinical imagers.

Magnetic resonance is a physical phenomenon related to the possession of either electron or nuclear nonzero spin of molecules. It is the base of two important analytic methods, the application of which in biological and clinical research is completely different, despite this common basis, namely nuclear magnetic resonance (NMR) and electron paramagnetic (EPR, also called spin—ESR) resonance. Starting from this common physical root, this chapter compares the magnetic resonance-based, nuclear and electron-related techniques of imaging—NMR and EPR imaging (NMRI versus EPRI), focusing on the latter. EPRI is characterized from the very general perspective and described in detail including the basic modalities of the technique—continuous wave (CW), pulse or time-domain mode, and rapid scan (RS) EPRI. The description is supplemented with a handful of technical, software, and chemical details important for researchers, engineers, IT specialists and, hopefully, physicians who consider application of these techniques, equipment, programs, and “EPR reagents” in their work. The most recent biological and clinical aspects of EPRI are analyzed with a particular care, and pictured on the background of other related and not related research techniques, making the chapter a universal, interdisciplinary compendium of the common EPRI know-how.

In this part we shall outline the challenges one faces while developing time-domain radiofrequency (RF) EPR imaging spectrometer for in vivo studies. Time-domain or FT-EPR is quite a different animal compared to the CW modality. The evolution of FT-EPR instrumentation at the National Cancer Institute, NIH, USA and representative examples of application in cancer research are outlined in this article.

Electron paramagnetic resonance imaging (EPRI) can yield information about the 3-dimensional (3D) spatial distribution of the unpaired-electron-spin density from which the spatial distribution of oxygen concentration within tumor tissue, referred to as the oxygen image or electron paramagnetic resonance (EPR) image in this work, can be derived. Existing algorithms for reconstruction of EPR images often require data collected at a large number of densely sampled projection views, resulting in a prolonged data-acquisition time and consequently numerous practical challenges especially to in vivo animal EPRI. Therefore, a strong interest exists in shortening data-acquisition time through reducing the number of data samples collected in EPRI, and one approach is to acquire data at a reduced number of sparsely distributed projection views from which existing algorithms may reconstruct images with prominent artifacts. In this work, we investigate and develop an optimization-based technique for image reconstruction from data collected at sparsely sampled projection views for reducing scanning time in EPRI. Specifically, we design a convex optimization program in which the EPR image of interest is formulated as a solution and then tailor the Chambolle-Pock (CP) primal-dual algorithm to reconstruct the image by solving the convex optimization program. Using computer-simulated EPRI data from numerical phantoms and real EPRI data collected from physical phantoms, we perform studies on the verification and characterization of the optimization-based technique for EPR image reconstruction. Results of the studies suggest that the technique may yield accurate EPR images from data collected at sparsely distributed projection views, thus potentially enabling fast EPRI with reduced acquisition time.

The advent of hybrid scanners, combining complementary modalities, has revolutionized the application of advanced imaging technology to clinical practice and biomedical research. In this project, we investigated the melding of two complementary, functional imaging methods: positron emission tomography (PET) and electron paramagnetic resonance imaging (EPRI). PET radiotracers can provide important information about cellular parameters, such as glucose metabolism. While EPR probes can provide assessment of tissue microenvironment, measuring oxygenation and pH, for example. Therefore, a combined PET/EPRI scanner promises to provide new insights not attainable with current imagers by simultaneous acquisition of multiple components of tissue microenvironments. To explore the simultaneous acquisition of PET and EPR images, a prototype system was created by combining two existing scanners. Specifically, a silicon photomultiplier (SiPM)-based PET scanner ring designed as a portable scanner was combined with an EPRI scanner designed for the imaging of small animals. The ability of the system to obtain simultaneous images was assessed with a small phantom consisting of four cylinders containing both a PET tracer and EPR spin probe. The resulting images demonstrated the ability to obtain contemporaneous PET and EPR images without cross-modality interference. Given the promising results from this initial investigation, the next step in this project is the construction of the next generation pre-clinical PET/EPRI scanner for multi-parametric assessment of physiologically-important parameters of tissue microenvironments.

A thorough investigation of biomimetic polydopamine (PDA) by Electron Paramagnetic Resonance (EPR) is shown. In addition, temperature dependent spectroscopic EPR data are presented in the range 3.8-300 K. Small discrepancies in magnetic susceptibility behavior are observed between previously reported melanin samples. These variations were attributed to thermally acitivated processes. More importantly, EPR spatial-spatial 2D imaging of polydopamine radicals on a phantom is presented for the first time. In consequence, a new possible application of polydopamine as EPR imagining marker is addressed.

Electron paramagnetic resonance imaging (EPRI) provides 3D images of absolute oxygen concentration (pO2) in vivo with excellent spatial and pO2 resolution. When investigating such physiologic parameters in living animals, the situation is inherently dynamic. Improvements in temporal resolution and experimental versatility are necessary to properly study such a system. Uniformly distributed projections result in efficient use of data for image reconstruction. This has dictated current methods such as equal-solid-angle (ESA) spacing of projections. However, acquisition sequencing must still be optimized to achieve uniformity throughout imaging. An object-independent method for uniform acquisition of projections, using the ESA uniform distribution for the final set of projections, is presented. Each successive projection maximizes the distance in the gradient space between itself and prior projections. This maximally spaced projection sequencing (MSPS) method improves image quality for intermediate images reconstructed from incomplete projection sets, enabling useful real-time reconstruction. This method also provides improved experimental versatility, reduced artifacts, and the ability to adjust temporal resolution post factum to best fit the data and its application. The MSPS method in EPRI provides the improvements necessary to more appropriately study a dynamic system.

Tetrathiatriarylmethyl (TAM) radicals are favourite spin labels for electron paramagnetic resonance imaging (EPRI). In this work, we report a straightforward synthesis of new TAM radicals bound to an Arg-Gly-Asp (RGD)-peptido-mimetic known to be a cell-targeting agent. These new radicals are stable in solution, and sensitive to oxygen, and their experimental EPR data is consistent with density functional theory (DFT) calculations.

This article describes an improved method for suppressing image artifacts in the visualization of 14 N- and 15 N-labeled nitroxyl radicals in a single image scan using electron paramagnetic resonance (EPR). The purpose of this work was to solve the problem of asymmetric EPR absorption spectra in spectral processing. A hybrid function of Gaussian and Lorentzian lineshapes was used to perform spectral line-fitting to successfully separate the two kinds of nitroxyl radicals. This approach can process the asymmetric EPR absorption spectra of the nitroxyl radicals being measured, and can suppress image artifacts due to spectral asymmetry. With this improved visualization method and a 750-MHz continuous-wave EPR imager, a temporal change in the distributions of a two-phase paraffin oil and water/glycerin solution system was visualized using lipophilic and hydrophilic nitroxyl radicals, i.e., 2-(14-carboxytetradecyl)-2-ethyl-4,4-dimethyl-3-oxazolidinyloxy (16-DOXYL stearic acid) and 4-hydroxyl-2,2,6,6-tetramethylpiperidine-d 17 -1- 15 N-1-oxyl (TEMPOL-d 17 - 15 N). The results of the two-phase separation experiment verified that reasonable artifact suppression could be achieved by the present method that deals with asymmetric absorption spectra in the EPR imaging of 14 N- and 15 N-labeled nitroxyl radicals.

Purpose Electron paramagnetic resonance imaging has surfaced as a promising noninvasive imaging modality that is capable of imaging tissue oxygenation. Due to extremely short spin-spin relaxation times, electron paramagnetic resonance imaging benefits from single-point imaging and inherently suffers from limited spatial and temporal resolution, preventing localization of small hypoxic tissues and differentiation of hypoxia dynamics, making accelerated imaging a crucial issue. Methods In this study, methods for accelerated single-point imaging were developed by combining a bilateral k-space extrapolation technique with model-based reconstruction that benefits from dense sampling in the parameter domain (measurement of the T2* decay of a free induction delay). In bilateral kspace extrapolation, more k-space samples are obtained in a sparsely sampled region by bilaterally extrapolating data from temporally neighboring k-spaces. To improve the accuracy of T2* estimation, a principal component analysis-based method was implemented. Results In a computer simulation and a phantom experiment, the proposed methods showed its capability for reliable T2* estimation with high acceleration (8-fold, 15-fold, and 30-fold accelerations for 61×61×61, 95×95×95, and 127×127×127 matrix, respectively). Conclusion By applying bilateral k-space extrapolation and model-based reconstruction, improved scan times with higher spatial resolution can be achieved in the current single-point electron paramagnetic resonance imaging modality. Magn Reson Med 73:1692–1701, 2015. © 2014 Wiley Periodicals, Inc.

PURPOSE: The purpose of this study was to develop an X-Band electron paramagnetic resonance imaging protocol for visualization of oxidative stress in biopsies.METHODS: The developed electron parama ...

A scalable synthetic route for 15N-labeled 4-oxo-2,2,6,6-tetraethylpiperidine nitroxide (15N-TEEPONE) is described. This 15N-labeled nitroxide is suitable for electron paramagnetic resonance imaging of brain, and its higher sensitivity compared with that of its 14N-counterpart is an important advantage of the labeled derivative.

In this part we shall outline the challenges one faces while developing time-domain radiofrequency (RF) EPR imaging spectrometer for in vivo studies. Time-domain or FT-EPR is quite a different animal compared to the CW modality. The evolution of FT-EPR instrumentation at the National Cancer Institute, NIH, USA and representative examples of application in cancer research are outlined in this article.

Purpose EPR spectroscopy promises quantitative images of important physiologic markers of animal tumors and normal tissues, such as pO2, pH, and thiol redox status. These parameters of tissue function are conveniently reported by tailored nitroxides. For defining tumor physiology, it is vital that nitroxides are selectively localized in tumors relative to normal tissue. Furthermore, these paramagnetic species should be specifically taken up by cells of the tumor, thereby reporting on both the site of tumor formation and the physiological status of the tissue. This study investigates the tumor localization of the novel nitroxide, cis-3,4-di(acetoxymethoxycarbonyl)-2,2,5,5-tetramethyl-1-pyrrolidinyloxyl 3 relative to the corresponding di-acid 4.

Electron paramagnetic resonance imaging (EPRI) has been used to noninvasively provide 3D images of absolute oxygen concentration (pO2) in small animals. These oxygen images are well resolved both spatially (~1 mm) and in pO2 (1-3 mmHg). EPRI preclinical images of pO2 have demonstrated extremely promising results for various applications investigating oxygen related physiologic and biologic processes as well as the dependence of various disease states on pO2, such as the role of hypoxia in cancer. Recent developments have been made that help to progress EPRI towards the eventual goal of human application. For example, a bimodal crossed-wire surface coil has been developed. Very preliminary tests demonstrated a 20 dB isolation between transmit and receive for this coil, with an anticipated additional 20 dB achievable. This could potentially be used to image local pO2 in human subjects with superficial tumors with EPRI. Local excitation and detection will reduce the specific absorption rate limitations on images and eliminate any possible power deposition concerns. Additionally, a large 9 mT EPRI magnet has been constructed which can fit and provide static main and gradient fields for imaging local anatomy in an entire human. One potential obstacle that must be overcome in order to use EPRI to image humans is the approved use of the requisite EPRI spin probe imaging agent (trityl). While nontoxic, EPRI trityl spin probes have been injected intravenously when imaging small animals, and require relatively high total body injection doses that would not be suitable for human imaging applications. Work has been done demonstrating the alternative use of intratumoral (IT) injections, which can reduce the amount of trityl required for imaging by a factor of 2000- relative to a whole body intravenous injection. The development of a large magnet that can accommodate human subjects, the design of a surface coil for imaging of superficial pO2, and the reduction of required spin probe using IT injections all are crucial steps towards the eventual use of EPRI to image pO2 in human subjects. In the future this can help investigate the oxygenation status of superficial tumors (e.g., breast tumors). The ability to image pO2 in humans has many other potential applications to diseases such as peripheral vascular disease, heart disease, and stroke.

Malignant melanoma is a tumor characterized by the uncontrolled proliferation of melanocytes, mainly in skin, but also in eyes. Its incidence is rising each year. To improve the diagnosis and treatment of the tumor, it is essential to develop new effective methods to early detect and characterize melanoma. Previously, we demonstrated in a single-shot study that it was possible to map free radicals of melanin pigments using an electron paramagnetic resonance (EPR)-based method. Furthermore, we demonstrated that X-Band (9 GHz) EPR spectrometry was an accurate tool to assess the growth stage of a pigmented tumor. The aim of the present study was to investigate the ability of EPR imaging to detect and localize melanin pigments inside melanin phantoms, B16 melanoma tumor models and resected human melanomas. We show that EPR can provide an accurate image of synthetic samples, both in terms of shape and size, with errors always lower than 10% compared to the real size. Regarding melanoma studies, the ability of EPR imaging to map accurately the melanoma was depending on the concentration of melanin in the sample, which is proportional to the growth stage of the tumor and the consequent signal-to-noise ratio (SNR) provided by the EPR signal intensity. This led us to define an operational concept, considering SNR and interferences with other EPR signals, to determine when EPR imaging was feasible.

In vivo or ex vivo electron paramagnetic resonance imaging (EPRI) is a powerful technique for determining the spatial distribution of free radicals and other paramagnetic species in living organs and tissues. However, applications of EPRI have been limited by long projection acquisition times and the consequent fact that rapid gated EPRI was not possible. Hence in vivo EPRI typically provided only time-averaged information. In order to achieve direct gated EPRI, a fast EPR acquisition scheme was developed to decrease EPR projection acquisition time down to 10–20 ms, along with corresponding software and instrumentation to achieve fast gated EPRI of the isolated beating heart with submillimeter spatial resolution in as little as 2–3 min. Reconstructed images display temporal and spatial variations of the free-radical distribution, anatomical structure, and contractile function within the rat heart during the cardiac cycle. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.

Electron paramagnetic resonance imaging (EPRI) can be used to noninvasively and quantitatively obtain three-dimensional maps of tumor pO2. The paramagnetic tracer triarylmethyl (TAM), a substituted trityl radical moiety, is not toxic to animals and provides narrow isotropic spectra, which is ideal for in vivo EPR imaging experiments. From the oxygen-induced spectral broadening of TAM, pO2 maps can be derived using EPRI. The instrumentation consists of an EPRI spectrometer and 7T magnetic resonance imaging (MRI) system both operating at a common radiofrequency of 300 MHz. Anatomic images obtained by MRI can be overlaid with pO2 maps obtained from EPRI. With imaging times of less than 3 min, it was possible to monitor the dynamics of oxygen changes in tumor and distinguish chronically hypoxic regions from acutely hypoxic regions. In this article, the principles of pO2 imaging with EPR and some relevant examples of tumor imaging are reviewed.

The suitability of 3-carboxy-2,2,5,5-tetra( 2 H 3 )methyl-[4- 2 H( 1 H)]-3-pyrroline-(1- 15 N)-1-oxyl and its non-deuterated 14 N-containing nitroxide analogue for L-band EPR in vivo imaging has been demonstrated. Both nitroxides exhibit low rates of reduction with ascorbate and slow decay in mice blood and plasma and, according to the EPR data, accumulate in the liver of a living mice within 10–70 min after the injection via tail vein, and then collect in the bladder.

Electron paramagnetic resonance imaging using nitroxides is a powerful method for visualizing the redox status modulated by oxidative stress in vivo. Typically, however, data acquisition times have been too slow to obtain a sufficient number of projections for three-dimensional images, when using continuous wave-electron paramagnetic resonance imager in small rodents, using nitroxides with comparatively short T(2) and a half-life values. Because of improvements in imagers that enable rapid data-acquisition, the feasibility of three-dimensional electron paramagnetic resonance imaging with good quality in mice was tested with nitroxides. Three-dimensional images of mice were obtained at an interval of 15 sec under field scanning of 0.3 sec and with 46 projections in the case of strong electron paramagnetic resonance signals. Three-dimensional electron paramagnetic resonance images of a blood brain barrier-permeable nitroxide, 3-hydroxymethyl-2,2,5,5-tetramethylpyrrolidine-1-oxyl, in the mouse head clearly showed that 3-hydroxymethyl-2,2,5,5-tetramethylpyrrolidine-1-oxyl was distributed within brain tissues, and this was confirmed by MRI observations. Based on the pharmacokinetics of nitroxides in mice, half-life mapping was demonstrated in an ischemia-reperfusion model mouse brain. Inhomogeneous half-lives were clearly mapped pixel-by-pixel in mouse head under oxidative stress by the improved continuous wave-electron paramagnetic resonance imager noninvasively.

Electron paramagnetic resonance imaging (EPRI) using nitroxides is an emergent imaging method for studying in vivo physiology, including O(2) distribution in various tissues. Such imaging capabilities would allow O(2) mapping in tumors and in different brain regions following hypoxia or drug abuse. We have recently demonstrated that the anion of 3-carboxy-2,2,5,5-tetramethyl-1-pyrrolidinyloxyl (2) can be entrapped in brain tissue to quantitate O(2) concentration in vivo. To increase the sensitivity of O(2) measurement by EPR imaging, we synthesized 3-carboxy-2,2,5,5-tetra((2)H(3))methyl-1-(3,4,4-(2)H(3),1-(15)N)pyrrolidinyloxyl (7). EPR spectroscopic measurements demonstrate that this fully isotopically substituted nitroxide markedly improves signal-to-noise ratio and, therefore, the sensitivity of EPR imaging. The new isotopically substituted nitroxide shows increased sensitivity to changes in O(2) concentration, which will enable more accurate O(2) measurement in tissues using EPRI.

Electron paramagnetic resonance imaging (EPRI) is a new modality for visualizing O2 distribution in tissues, such as the brain after stroke or after administration of drugs of abuse. We have recently shown that 3-acetoxymethoxycarbonyl-2,2,5,5-tetramethyl-1-pyrrolidinyloxyl [1] is a pro-imaging agent that can cross the blood–brain barrier. After hydrolysis by esterases, the anion of 3-carboxy-2,2,5,5-tetramethyl-1-tetramethyl-1-pyrrolidinyloxyl [2] is trapped in brain tissue. In this study, we investigated the feasibility of using this to map the changes of O2 concentration in mouse brain after focal ischemia. The decrease in tissue O2 concentration in the ischemic region of mouse brain was clearly visualized by EPRI. The hypoxic zone mapped by EPRI was spatially well correlated with the infarction area in the brain imaged by diffusion-weighted magnetic resonance imaging (MRI). Finally, we observed a decrease in the size of the hypoxic region when the mouse breathed higher levels of O2. This finding suggests that EPRI with specifically designed nitroxides is a promising imaging modality for visualizing O2 distribution in brain tissue, especially in an ischemic brain. We believe that this imaging method can be used for monitoring the effects of therapeutic intervention aimed at enhancing brain O2 supply, which is crucial in minimizing brain injury after stroke.

Electron paramagnetic resonance imaging (EPRI) is a relatively recent imaging technique, which provides potentially multidimensional imaging of the spatial distribution of paramagnetic species. Thanks to the use of stable spin probes, low frequency EPR imaging has recently allowed the use of large tissue samples or whole animals in vivo in the field of biology and medicine. It is normally necessary to introduce prior intravenous or intramuscular infusion of stable or slowly metabolizable non-toxic water-soluble paramagnetic materials, or stable implantable particulate materials as spin probes into the system. The classification and research progress of spin probes at present were described briefly. Three important potential approaches in water-soluble paramagnetic materials design including deuterated, site-specific and macromolecular conjugated nitroxides were also investigated.

Purpose To estimate the in vivo intracerebral reducing ability after acute stress in adolescent rats subjected to early neonatal isolation (NI), by performing temporal electron paramagnetic resonance imaging (EPRI) of the brain. Materials and Methods An EPRI system operating at an EPR frequency of 700 MHz was used. The intracerebral reducing ability was estimated based on the halflife of the EPR signal of the blood–brain barrier (BBB)-permeable nitroxide radical. The NI treatment was performed for a period of one hour per day over postnatal days 2–9. Six-week-old rats were exposed to acute stress (immobilization for 90 minutes) prior to the EPRI study. Results Depletion of the intracerebral reducing ability caused by the acute stress was observed; however, this depletion phenomenon did not occur in animals that were not subjected to NI. Conclusion The results obtained in this study prove that NI induces cerebral vulnerability to acute stress in adolescence. J. Magn. Reson. Imaging 2006. © 2006 Wiley-Liss, Inc.

Glioblastoma is the most frequent primary malignant brain tumor and its malignant nature is closely associated with hypoxia. Thus, it would be useful for treatment of glioblastomas to evaluate intratumoral distribution of hypoxia for determining the therapeutic planning as well as assessment of its effectiveness during the therapy. Electron paramagnetic resonance imaging (EPRI), is an imaging technique which can generate quantitative maps of oxygen in vivo using the exogenous paramagnetic compound, triarylmethyl. In this study, we examined the feasibility of EPRI for assessment of oxygen distribution in the orthotopic glioblastoma models of U251 and U87. Heterogeneous distribution of pO2 between 0 and 50 mmHg was observed throughout the tumors except for the normal brain tissue. U251 glioblastoma was more likely to be exposed to hypoxia than U87 (median pO2; 29.7 mmHg and 18.2 mmHg, P = 0.028, in U87 and U251, respectively). The ratio of hypoxic fraction where pO2 was less than 10 mmHg showed a good correlation with pimonidazole-positive region. In subcutaneous xenograft model, irradiation was relatively less effective for U251 compared to U87, suggesting the high population of hypoxic cells. In conclusion, EPRI is a feasible method to evaluate oxygen distribution in the brain tumor.

Chronic hypoxia strongly affects the malignant state and resistance to therapy for tumours. Transient hypoxia has been hypothesized, but not proven to be more deleterious. Electron paramagnetic resonance imaging (EPRI) provides non-invasive, quantitative imaging of static pO2 in vivo. Dynamic EPRI produces pO2 movies, enabling non-invasive assessment of in vivo pO2 changes, such as transient hypoxia. Recent developments have been made to enable Dynamic EPRI. Maximally spaced projection sequencing has been implemented to allow for more accurate and versatile acquisition of EPRI data when studying dynamic systems. Principal component analysis filtering has been employed to enhance SNR. Dynamic EPRI studies will provide temporally resolved oxygen movies necessary to perform in vivo studies of physiologically relevant pO2 changes in tumours. These oxygen movies will allow for the localization/quantification of transient hypoxia and will therefore help to disentangle the relationship between chronic and transient hypoxia, in order to better understand their roles in therapeutic optimization and outcome.

Electron paramagnetic resonance imaging, EPRI, is an emerging in vivo imaging modality that holds a unique promise of quantitatively mapping tissue oxygen distribution noninvasively. Unlike magnetic resonance imaging (MRI), EPRI requires the use of nontoxic biologically compatible spin probes. Many tumors are characterized by a hypoxic core that is highly resistant to radiation and chemotherapeutic treatment. EPRI enables fast noninvasive assessment of tumor hypoxia, re-oxygenation, and tissue redox status. The two main approaches to EPRI are continuous wave, CW electron paramagnetic resonance (EPR) spectroscopy, and time-domain EPR spectroscopy, FT-EPR. The present article describes the current state of the art of radiofrequency CW and time-domain EPR imaging.

Recently, it has been shown that rat hearts subjected to global ischemia generate nitric oxide (NO) and that a significant portion of it is generated by the reduction of nitrite under the acidic and reducing conditions that occur during myocardial ischemia [Zweier, Wang, Samouilov, Kuppusamy, Nature Med. 1, 804-809 (1995)]. In the present study it is further attempted to map the spatial distributions of the NO generation in the ischemic myocardium using L-band electron paramagnetic resonance imaging. Rat hearts were loaded with 10 mM nitrite and subjected to global no-flow ischemia, during which time a series of three-dimensional spatial electron paramagnetic resonance (EPR) images of the distribution of NO were obtained using the NO trap bis(N-methyl-D-glucamine dithiocarbamate)iron(II). The images clearly showed that NO is formed throughout the myocardium. Kinetic experiments showed that maximum NO generation and trapping occur at the midmyocardium and spread out to endocardium and epicardium of the left ventricle. The magnitude of generation in the RV myocardium is four- to fivefold lower than in the LV.

The recent development of a bi-modality magnetic resonance imaging/electron paramagnetic resonance imaging (MRI/EPRI) platform has enabled longitudinal monitoring of both tumor oxygenation and redox status in murine cancer models. The current study used this imaging platform to test the hypothesis that a more reducing tumor microenvironment accompanies the development of tumor hypoxia. To test this, the redox status of the tumor was measured using Tempol as a redox‑sensitive MRI contrast agent, and tumor hypoxia was measured with Oxo63, which is an oxygen-sensitive EPRI spin probe. Images were acquired every 1-2 days in mice bearing SCCVII tumors. The median pO(2) decreased from 14 mmHg at 7 days after tumor implantation to 7 mmHg at 15 days after implantation. Additionally, the hypoxic fraction, defined as the percentage of the tumor that exhibited a pO(2)10 mmHg, increased with tumor size (from 10% at 500 mm(3) to 60% at 3,500 mm(3)). The rate of Tempol reduction increased as a function of tumor volume (0.4 min(-1) at 500 mm(3) to 1.7 min(-1) at 3,500 mm(3)), suggesting that the tumor microenvironment became more reduced as the tumor grew. The results show that rapid Tempol reduction correlates with decreased tumor oxygenation, and that the Tempol decay rate constant may be a surrogate marker for tumor hypoxia.

The article presents a modification to the spline deconvolution technique. The method was developed within the framework of electron paramagnetic resonance imaging and is particularly useful in cases of low signal‐to‐noise ratio (S/N) and when the deconvolution kernel is a large fraction (about 50%) of the studied function. The modified spline deconvolution method works directly in the signal space and does not require a low‐pass filter that causes loss of information and limits the resolution as is the case of Fourier deconvolution. The comparison was done with Fourier deconvolution and was based on three parameters that account for resolution, noise, and proportion stability. Experimental tests were done using two nitroxide‐free radicals of different linewidths (0.05 and 0.16 mT, respectively) at different gradient values (between 0.05 and 0.2 mT/m). Modified spline deconvolution gave consistently better results in all the three parameters, particularly at the lower gradient values. For this reason the method is useful in in vivo experiments where the low S/N does not permit use of high gradient values.

Custom disposable patient immobilization systems that conform to the patient’s body contours are commonly used to facilitate accurate repeated patient setup for imaging and treatment in radiation therapy. However, in small-animal imaging, immobilization is often overlooked or done in a way that is not conducive to reproducible positioning. This has a negative impact on the potential for accurate analysis of serial or multimodality imaging. We present the use of vinyl polysiloxane dental impression material for immobilization of mice for imaging. Four different materials were examined to identify any potential artifacts using magnetic resonance techniques. A water phantom placed inside the cast was used at 4.7 T with magnetic resonance imaging and showed no effect at the center of the image when compared with images without the cast. A negligible effect was seen near the ends of the coil. Each material had no detectable signal using electron paramagnetic resonance imaging at 9 mT. The use of dental material also greatly enhances the use of fiducial markers that can be embedded in the mold. Therefore, image registration is simplified as the immobilization of the animal and fiducials together helps in translating from one image coordinate system to another.

This paper presents an apparatus that permits synchronization of the signal acquisition from a moving sample in electron spin resonance imaging experiments. The apparatus has been designed to observe a perfused beating heart and includes all the necessary apparatus to stimulate the heart and to synchronize the signal acquisition on this stimulus. The apparatus is completed by a low-frequency electron paramagnetic resonance bridge operated at 1.5 GHz, a re-entrant cavity for the sample and air coils to obtain the field gradients. Tests were carried out on a phantom consisting of a thin-walled silicon rubber tube of the approximate dimensions of a rat heart. The phantom movements are obtained by inflating and deflating the tube with a peristaltic pump that can be reversed by computer. The acquisitions are taken in synchronization with the pump positions. The hardware used to drive the pump and to synchronize the acquisitions is also described.

A nested uniform sampling over the sphere is presented. The set of sampling points contains a sequence of nested subsets, each providing an approximately uniform covering of the sphere. The proposed Matryoshka covering of the sphere allows multiresolution 3-D imaging. Simulation and electron paramagnetic resonance imaging results illustrate its application to 3-D tomography.