High spatial resolution multisite EPR oximetry of transient focal cerebral ischemia in the rat

A Review of Low-Frequency EPR Technology for the Measurement of Brain pO2 and Oxidative Stress

EPR can uniquely measure paramagnetic species. Although commercial EPR was introduced in 1950s, the early studies were mostly restricted to chemicals in solution or cellular experiments using X-band EPR equipment. Due to its limited penetration (<1 mm), experiments with living animals were almost impossible. To overcome these difficulties, Swartz group, along with several other leaders in field, pioneered the technology of low frequency EPR (e.g., L-band, 1-2 GHz). The development of low frequency EPR and the associated probes have dramatically expanded the application of EPR technology into the biomedical research field, providing answers to important scientific questions by measuring specific parameters that are impossible or very difficult to obtain by other approaches. In this review, which is aimed at highlighting the seminal contribution from Swartz group over the last several decades, we will focus on the development of EPR technology that was designed to deal with the potential challenges arising from conducting EPR spectroscopy in living animals. The second half of the review will be concentrated on the application of low frequency EPR in measuring cerebral tissue pO₂ changes and oxidative stress in various physiological and pathophysiological conditions in the brain of animal disease models.

In vivo electron paramagnetic resonance oximetry and applications in the brain

Molecular oxygen (O₂) is essential to brain function and mechanisms necessary to regulate variations in delivery or utilization of O₂ are crucial to support normal brain homeostasis, physiology and energy metabolism. Any imbalance in cerebral tissue partial pressure of O₂ (pO₂) levels may lead to pathophysiological complications including increased reactive O₂ species generation leading to oxidative stress when tissue O₂ level is too high or too low. Accordingly, the need for oximetry methods, which assess cerebral pO₂in vivo and in real time, is imperative to understand the role of O₂ in various metabolic and disease states, including the effects of treatment and therapy options. In this review, we provide a brief overview of the common in vivo oximetry methodologies for measuring cerebral pO₂. We discuss the advantages and limitations of oximetry methodologies to measure cerebral pO₂in vivo followed by a more in-depth review of electron paramagnetic resonance oximetry spectroscopy and imaging using several examples of current electron paramagnetic resonance oximetry applications in the brain.

Contribution of Harold M. Swartz to In Vivo EPR and EPR Dosimetry

In 2015, we are celebrating half a century of research in the application of Electron Paramagnetic Resonance (EPR) as a biodosimetry tool to evaluate the dose received by irradiated people. During the EPR Biodose 2015 meeting, a special session was organized to acknowledge the pioneering contribution of Harold M. (Hal) Swartz in the field. The article summarizes his main contribution in physiology and medicine. Four emerging themes have been pursued continuously along his career since its beginning: (1) radiation biology; (2) oxygen and oxidation; (3) measuring physiology in vivo; and (4) application of these measurements in clinical medicine. The common feature among all these different subjects has been the use of magnetic resonance techniques, especially EPR. In this article, you will find an impressionist portrait of Hal Swartz with the description of the 'making of' this pioneer, a time-line perspective on his career with the creation of three National Institutes of Health-funded EPR centers, a topic-oriented perspective on his career with a description of his major contributions to Science, his role as a mentor and his influence on his academic children, his active role as founder of scientific societies and organizer of scientific meetings, and the well-deserved international recognition received so far.

Overmodulation of projections as signal-to-noise enhancement method in EPR imaging

A study concerning the image quality in electron paramagnetic resonance imaging in two-dimensional spatial experiments is presented. The aim of the measurements was to improve the signal-to-noise ratio (SNR) of the projections and the reconstructed image by applying modulation amplitude higher than the radical electron paramagnetic resonance linewidth. Data were gathered by applying four constant modulation amplitudes, where one was below 1/3 (Amod  = 0.04 mT) of the radical linewidth (ΔBpp  = 0.14 mT). Three other modulation amplitude values were used in this experiment, leading to undermodulated (Amod  < 1/3 ΔBpp), partially overmodulated (Amod  ~ 1/3 ΔBpp) and fully overmodulated (Amod  > > 1/3 ΔBpp) projections. The advantages of an applied overmodulation condition were demonstrated in the study performed on a phantom containing four shapes of 1.25 mM water solution of 2, 2, 6, 6-tetramethyl-1-piperidinyloxyl. It was shown that even when the overmodulated reference spectrum was used in the deconvolution procedure, as well as the projection itself, the phantom shapes reconstructed as images directly correspond to those obtained in undermodulation conditions. It was shown that the best SNR of the reconstructed images is expected for the modulation amplitude close to 1/3 of the projection linewidth, which is defined as the distance from the first maximum to the last minimum of the gradient-broadened spectrum. For higher modulation amplitude, the SNR of the reconstructed image is decreased, even if the SNR of the measured projection is increased.

Direct and Repeated Measurement of Heart and Brain Oxygenation Using In Vivo EPR Oximetry

Low level of oxygen (hypoxia) is a critical factor that defines the pathological consequence of several pathophysiologies, particularly ischemia, that usually occur following the blockage of a blood vessel in vital organs, such as brain and heart, or abnormalities in the microvasculature, such as peripheral vascular disease. Therefore, methods that can directly and repeatedly quantify oxygen levels in the brain and heart will significantly improve our understanding of ischemic pathologies. Importantly, such oximetry capability will facilitate the development of strategies to counteract low levels of oxygen and thereby improve outcome following stroke or myocardial infarction. In vivo electron paramagnetic resonance (EPR) oximetry has the capability to monitor tissue oxygen levels in real time. The method has largely been tested and used in experimental animals, although some clinical measurements have been performed. In this chapter, a brief overview of the methodology to repeatedly quantify oxygen levels in the brain and heart of experimental animal models, ranging from mice to swine, is presented. EPR oximetry requires a one-time placement of an oxygen-sensitive probe in the tissue of interest, while the rest of the procedure for reliable, accurate, and repeated measurements of pO2 (partial pressure of oxygen) is noninvasive and can be repeated as often as desired. A multisite oximetry approach can be used to monitor pO2 at many sites simultaneously. Building on significant advances in the application of EPR oximetry in experimental animal models, spectrometers have been developed for use in human subjects. Initial feasibility of pO2 measurement in solid tumors of patients has been successfully demonstrated.

Assessment of the changes in 9L and C6 glioma pO2 by EPR oximetry as a prognostic indicator of differential response to radiotherapy

Tumor hypoxia impedes the outcome of radiotherapy. As the extent of hypoxia in solid tumors varies during the course of radiotherapy, methods that can provide repeated assessment of tumor pO2 such as EPR oximetry may enhance the efficacy of radiotherapy by scheduling irradiations when the tumors are oxygenated. The repeated measurements of tumor pO2 may also identify responders, and thereby facilitate the design of better treatment plans for nonresponding tumors. We have investigated the temporal changes in the ectopic 9L and C6 glioma pO2 irradiated with single radiation doses less than 10 Gy by EPR oximetry. The 9L and C6 tumors were hypoxic with pO2 of approximately 5-9 mmHg. The pO2 of C6 tumors increased significantly with irradiation of 4.8-9.3 Gy. However, no change in the 9L tumor pO2 was observed. The irradiation of the oxygenated C6 tumors with a second dose of 4.8 Gy resulted in a significant delay in growth compared to hypoxic and 2 Gy × 5 treatment groups. The C6 tumors with an increase in pO2 of greater than 50% from the baseline of irradiation with 4.8 Gy (responders) had a significant tumor growth delay compared to nonresponders. These results indicate that the ectopic 9L and C6 tumors responded differently to radiotherapy. We propose that the repeated measurement of the oxygen levels in the tumors during radiotherapy can be used to identify responders and to design tumor oxygen guided treatment plans to improve the outcome.

Multisite EPR oximetry from multiple quadrature harmonics

Multisite continuous wave (CW) electron paramagnetic resonance (EPR) oximetry using multiple quadrature field modulation harmonics is presented. First, a recently developed digital receiver is used to extract multiple harmonics of field modulated projection data. Second, a forward model is presented that relates the projection data to unknown parameters, including linewidth at each site. Third, a maximum likelihood estimator of unknown parameters is reported using an iterative algorithm capable of jointly processing multiple quadrature harmonics. The data modeling and processing are applicable for parametric lineshapes under nonsaturating conditions. Joint processing of multiple harmonics leads to 2-3-fold acceleration of EPR data acquisition. For demonstration in two spatial dimensions, both simulations and phantom studies on an L-band system are reported.

Repeated assessment of orthotopic glioma pO(2) by multi-site EPR oximetry: a technique with the potential to guide therapeutic optimization by repeated measurements of oxygen

Tumor hypoxia plays a vital role in therapeutic resistance. Consequently, measurements of tumor pO(2) could be used to optimize the outcome of oxygen-dependent therapies, such as, chemoradiation. However, the potential optimizations are restricted by the lack of methods to repeatedly and quantitatively assess tumor pO(2) during therapies, particularly in gliomas. We describe the procedures for repeated measurements of orthotopic glioma pO(2) by multi-site electron paramagnetic resonance (EPR) oximetry. This oximetry approach provides simultaneous measurements of pO(2) at more than one site in the glioma and contralateral cerebral tissue. The pO(2) of intracerebral 9L, C6, F98 and U251 tumors, as well as contralateral brain, were measured repeatedly for five consecutive days. The 9L glioma was well oxygenated with pO(2) of 27-36 mm Hg, while C6, F98 and U251 glioma were hypoxic with pO(2) of 7-12mm Hg. The potential of multi-site EPR oximetry to assess temporal changes in tissue pO(2) was investigated in rats breathing 100% O(2). A significant increase in F98 tumor and contralateral brain pO(2) was observed on day 1 and day 2, however, glioma oxygenation declined on subsequent days. In conclusion, EPR oximetry provides the capability to repeatedly assess temporal changes in orthotopic glioma pO(2). This information could be used to test and optimize the methods being developed to modulate tumor hypoxia. Furthermore, EPR oximetry could be potentially used to enhance the outcome of chemoradiation by scheduling treatments at times of increase in glioma pO(2).

Hypoxic stress and cancer: imaging the axis of evil in tumor metastasis

Tumors emerge as a result of the sequential acquisition of genetic, epigenetic and somatic alterations promoting cell proliferation and survival. The maintenance and expansion of tumor cells rely on their ability to adapt to changes in their microenvironment, together with the acquisition of the ability to remodel their surroundings. Tumor cells interact with two types of interconnected microenvironments: the metabolic cell autonomous microenvironment and the nonautonomous cellular-molecular microenvironment comprising interactions between tumor cells and the surrounding stroma. Hypoxia is a central player in cancer progression, affecting not only tumor cell autonomous functions, such as cell division and invasion, resistance to therapy and genetic instability, but also nonautonomous processes, such as angiogenesis, lymphangiogenesis and inflammation, all contributing to metastasis. Closely related microenvironmental stressors affecting cancer progression include, in addition to hypoxia, elevated interstitial pressure and oxidative stress. Noninvasive imaging offers multiple means to monitor the tumor microenvironment and its consequences, and can thus assist in the understanding of the biological basis of hypoxia and microenvironmental stress in cancer progression, and in the development of strategies to monitor therapies targeted at stress-induced tumor progression.

In vivo multisite oximetry using EPR-NMR coimaging

Coimaging employing electron paramagnetic resonance (EPR) imaging and MRI is used for rapid in vivo oximetry conducted simultaneously across multiple organs of a mouse. A recently developed hybrid EPR-NMR coimaging instrument is used for both EPR and NMR measurements. Oxygen sensitive particulate EPR probe is implanted in small localized pockets, called sites, across multiple regions of a live mouse. Three dimensional MRI is used to generate anatomic visualization, providing precise locations of implant sites. The pO₂ values, one for every site, are then estimated from EPR measurements. To account for radio frequency (RF) phase inhomogeneities inside a large resonator carrying a lossy sample, a generalization of an existing EPR data model is proposed. Utilization of known spectral lineshape, sparse distribution, and known site locations reduce the EPR data collection by more than an order of magnitude over a conventional spectral-spatial imaging, enhancing the feasibility of in vivo EPR oximetry for clinically relevant models.

Effect of hyperoxygenation on tissue pO2 and its effect on radiotherapeutic efficacy of orthotopic F98 gliomas

PURPOSE

Lack of methods for repeated assessment of tumor pO(2) limits the ability to test and optimize hypoxia-modifying procedures being developed for clinical applications. We report repeated measurements of orthotopic F98 tumor pO(2) and relate this to the effect of carbogen inhalation on tumor growth when combined with hypofractionated radiotherapy.

METHODS AND MATERIALS

Electron paramagnetic resonance (EPR) oximetry was used for repeated measurements of tumor and contralateral brain pO(2) in rats during 30% O(2) and carbogen inhalation for 5 consecutive days. The T(1)-enhanced volumes and diffusion coefficients of the tumors were assessed by magnetic resonance imaging (MRI). The tumors were irradiated with 9.3 Gy x 4 fractions in rats breathing 30% O(2) or carbogen to determine the effect on tumor growth.

RESULTS

The pretreatment F98 tumor pO(2) varied between 8 and 16 mmHg, while the contralateral brain had 41 to 45 mmHg pO(2) during repeated measurements. Carbogen breathing led to a significant increase in tumor and contralateral brain pO(2); however, this effect declined over days. Irradiation of the tumors in rats breathing carbogen resulted in a significant decrease in tumor growth and an increase in the diffusion coefficient measured by MRI.

CONCLUSIONS

The results provide quantitative measurements of the effect of carbogen inhalation on intracerebral tumor pO(2) and its effect on therapeutic outcome. Such direct repeated pO(2) measurements by EPR oximetry can provide temporal information that could be used to improve therapeutic outcome by scheduling doses at times of improved tumor oxygenation. EPR oximetry is currently being tested for clinical applications.

Application of in vivo EPR for tissue pO2 and redox measurements

The technique of electron paramagnetic resonance (EPR) spectroscopy is more than 50 years old, but only recently it has been used for in vivo studies. Its limited application in the past was due to the problem of high nonresonant dielectric loss of the exciting frequency because of high water content in biological samples. However, with the development of spectrometers working at lower frequencies (1,200 MHz and below) during the last 15 years, it is now possible to conduct in vivo measurements on a variety of animals and isolated organs. This is further facilitated by the development of new resonators with high sensitivity and appropriate stability for in vivo applications. It now has become feasible to obtain new insights into the complex aspects of physiology and pathophysiology using in vivo EPR. Among several important applications of this technique, the in vivo tissue pO(2) (partial pressure of oxygen) and redox measurements seem to be the most extensive use of this technique. In this chapter, we describe the procedure for in vivo pO(2) and redox measurements in animal models.

Tissue pO2 of orthotopic 9L and C6 gliomas and tumor-specific response to radiotherapy and hyperoxygenation

PURPOSE

Tumor hypoxia is a well-known therapeutic problem; however, a lack of methods for repeated measurements of glioma partial pressure of oxygen (pO(2)) limits the ability to optimize the therapeutic approaches. We report the effects of 9.3 Gy of radiation and carbogen inhalation on orthotopic 9L and C6 gliomas and on the contralateral brain pO(2) in rats using a new and potentially widely useful method, multisite in vivo electron paramagnetic resonance oximetry.

METHODS AND MATERIALS

Intracerebral 9L and C6 tumors were established in the left hemisphere of syngeneic rats, and electron paramagnetic resonance oximetry was successfully used for repeated tissue pO(2) measurements after 9.3 Gy of radiation and during carbogen breathing for 5 consecutive days.

RESULTS

Intracerebral 9L gliomas had a pO(2) of 30-32 mm Hg and C6 gliomas were relatively hypoxic, with a pO(2) of 12-14 mm Hg (p < 0.05). The tissue pO(2) of the contralateral brain was 40-45 mm Hg in rats with either 9L or C6 gliomas. Irradiation resulted in a significant increase in pO(2) of the 9L gliomas only. A significant increase in the pO(2) of the 9L and C6 gliomas was observed in rats breathing carbogen, but this effect decreased during 5 days of repeated experiments in the 9L gliomas.

CONCLUSION

These results highlight the tumor-specific effect of radiation (9.3.Gy) on tissue pO(2) and the different responses to carbogen inhalation. The ability of electron paramagnetic resonance oximetry to provide direct repeated measurements of tissue pO(2) could have a vital role in understanding the dynamics of hypoxia during therapy that could then be optimized by scheduling doses at times of improved tumor oxygenation.

Repetitive tissue pO2 measurements by electron paramagnetic resonance oximetry: current status and future potential for experimental and clinical studies

Tissue oxygen plays a crucial role in maintaining tissue viability and in various diseases, including responses to therapy. Useful knowledge has been gained by methods that can give limited snapshots of tissue oxygen (e.g., oxygen electrodes) or evidence of a history of tissue hypoxia (e.g., EF5) or even indirect evidence by monitoring oxygen availability in the circulatory system (e.g., NMR methods). Each of these methods has advantages and significant limitations. EPR oximetry is a technique for direct measurement of tissue pO2, which has several advantages over the other existing methods for applications in which the parameter of interest is the pO2 of tissues, and information is needed over a time course of minutes to hours, and/or for repetitive measurements over days or weeks or years. The aim of this article is to provide an overview of EPR oximetry using particulates to readers who are not familiar with this technique and its potential in vivo and clinical applications. The data presented here are from the experiments currently being carried out in our laboratory. We are confident that in vivo EPR oximetry will play a crucial role in the understanding and clinical management of various pathologies in the years to come.