High spatial resolution multi-site EPR oximetry. The use of convolution-based fitting method

Clinical EPR: unique opportunities and some challenges

Electron paramagnetic resonance (EPR) spectroscopy has been well established as a viable technique for measurement of free radicals and oxygen in biological systems, from in vitro cellular systems to in vivo small animal models of disease. However, the use of EPR in human subjects in the clinical setting, although attractive for a variety of important applications such as oxygen measurement, is challenged with several factors including the need for instrumentation customized for human subjects, probe, and regulatory constraints. This article describes the rationale and development of the first clinical EPR systems for two important clinical applications, namely, measurement of tissue oxygen (oximetry) and radiation dose (dosimetry) in humans. The clinical spectrometers operate at 1.2 GHz frequency and use surface-loop resonators capable of providing topical measurements up to 1 cm depth in tissues. Tissue pO2 measurements can be carried out noninvasively and repeatedly after placement of an oxygen-sensitive paramagnetic material (currently India ink) at the site of interest. Our EPR dosimetry system is capable of measuring radiation-induced free radicals in the tooth of irradiated human subjects to determine the exposure dose. These developments offer potential opportunities for clinical dosimetry and oximetry, which include guiding therapy for individual patients with tumors or vascular disease by monitoring of tissue oxygenation. Further work is in progress to translate this unique technology to routine clinical practice.

Compressed sensing of spatial electron paramagnetic resonance imaging

PURPOSE

To improve image quality and reduce data requirements for spatial electron paramagnetic resonance imaging (EPRI) by developing a novel reconstruction approach using compressed sensing (CS).

METHODS

EPRI is posed as an optimization problem, which is solved using regularized least-squares with sparsity promoting penalty terms, consisting of the l1 norms of the image itself and the total variation of the image. Pseudo-random sampling was employed to facilitate recovery of the sparse signal. The reconstruction was compared with the traditional filtered back-projection reconstruction for simulations, phantoms, isolated rat hearts, and mouse gastrointestinal (GI) tracts labeled with paramagnetic probes.

RESULTS

A combination of pseudo-random sampling and CS was able to generate high-fidelity EPR images at high acceleration rates. For three-dimensional (3D) phantom imaging, CS-based EPRI showed little visual degradation at nine-fold acceleration. In rat heart datasets, CS-based EPRI produced high quality images with eight-fold acceleration. A high resolution mouse GI tract reconstruction demonstrated a visual improvement in spatial resolution and a doubling in signal-to-noise ratio (SNR).

CONCLUSION

A novel 3D EPRI reconstruction using compressed sensing was developed and offers superior SNR and reduced artifacts from highly undersampled data.

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).

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 oxygenation in a murine SCC VII tumor after X-ray irradiation as determined by EPR spectroscopy

PURPOSE

The goal of this study was to clarify the dynamics of oxygenation (partial pressure of oxygen, pO(2)) in SCC VII murine tumors in mice after X-ray irradiation.

MATERIALS AND METHODS

Changes in pO(2) in tumors were measured by 1.2-GHz electron paramagnetic resonance (EPR) spectroscopy after they were exposed to various doses of irradiation. The pO(2) in tumors was followed for up to six days after irradiation at doses of 0, 5, 10, 15, and 20 Gy. Paramagnetic crystals were used as an oximetry probe and implanted into normal or tumor tissues in mice for prolonged periods.

RESULTS

The pattern of tumor oxygen after a single dose of radiation with the 5-Gy dose was different from those with other doses (10, 15, and 20 Gy). After 5 Gy, pO(2) increased rapidly (P<0.01, Student's t test) and then returned to the level observed before irradiation by 12h (P<0.01). In contrast, after 10, 15, or 20 Gy, pO(2) increased rapidly by 6h after irradiation, continued to increase until at least 24h (P<0.01), and then gradually decreased.

CONCLUSIONS

In tumors that received 5 Gy, post-irradiation increases in pO(2) at 4h after irradiation were detected by EPR oximetry (P<0.01) noninvasively.

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

In vivo electron paramagnetic resonance (EPR) spectroscopy can provide direct noninvasive, continuous, and repeatable measurements of oxygen in tissues. High-spatial-resolution multisite (HSRMS) oximetry is an EPR technique that uses applied magnetic field gradients to extend this capability to multiple implanted probes within the sample and accurately to estimate their respective local pO(2) values. These capabilities are crucial in experiments in which pO(2) varies across space and time and in which information about these variations is needed to describe physiologic and pathophysiologic phenomena and evaluate their responses to interventions such as therapy. One important application is the investigation of transient focal ischemia in the rat brain and the effects of treatment with hyperoxygenation. We used HSRMS oximetry with overmodulation to measure brain tissue oxygenation in a rat stroke model using lithium phthalocyanine as the oxygen probe. Oxygen measurements were made in a small cohort of rats at four implant sites during ischemia and reperfusion after transient focal ischemia initiated by occlusion of the middle cerebral artery. These measurements demonstrate the capabilities of the HSRMS oximetry technique and set the stage for more extensive physiologic studies.

Brain tissue oxygen concentration measurements

Brain function depends exquisitely on oxygen for energy metabolism. Measurements of brain tissue oxygen tension, by a variety of quantitative and qualitative techniques, going back for >50 years, have led to a number of significant conclusions. These conclusions have important consequences for understanding brain physiology as it is now being explored by techniques such as blood-oxygen-level-dependent functional magnetic resonance imaging (BOLD fMRI) and near-infrared spectroscopy (NIRS). It has been known for some time that most of the measured oxygen tensions are less than venous pO2 and are distributed in a spatially and temporally heterogeneous manner on a microregional scale. Although certain large-scale methods can provide reproducible average brain pO2 measurements, no useful concept of a characteristic oxygen tension or meaningful average value for brain tissue oxygen can be known on a microregional level. Only an oxygen field exists with large local gradients due to local tissue respiration, and the most useful way to express this is with a pO2 distribution curve or histogram. The neurons of the brain cortex normally exist in a low-oxygen environment and on activation are oxygenated by increases in local capillary blood flow that lead to increases in hemoglobin saturation and tissue oxygen.

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.

Using EPR to measure a critical but often unmeasured component of oxidative damage: oxygen

Oxygen is a critical variable in oxidative damage. It can be a direct reactant in one or more of the pertinent reactions that result in oxidative damage. It also is an essential substrate for mitochondrial respiration and many other essential synthetic and degradative reactions. The level of oxygen can have a regulatory role, affecting the rate and direction of metabolic processes and physiological functions that are germane to the pathophysiological processes that are being studied. Its supply to tissue and to cells is therefore a critical parameter governing normal homeostasis. The level of oxygen at specific sites may affect cell signaling. It therefore seems clear that it can be very useful to measure oxygen when studying oxidative damage. In order for the measurements of oxygen to be most useful, it often is essential to measure the amount of oxygen at particular sites and under appropriate conditions. These needs require methodology that can make sensitive and localized measurements of oxygen. Electron paramagnetic resonance (EPR) oximetry is such a technique, plus it has the capability of making repeated measurements from the same site non-invasively. The principles and applications of EPR oximetry to viable systems, including cell suspensions and intact animals, are described in this paper.