Development of a resonator with automatic tuning and coupling capability to minimize sample motion noise for in vivo EPR spectroscopy

Development of an L-band rapid scan EPR digital console

The development of a digital console for in-vivo rapid scan electron paramagnetic resonance (RS-EPR) spectroscopy and imaging is described in detail. The console was build using field programmable gate array (FGPA) technology that permits real-time control of the resonator and scanning magnetic fields during the measurements. Automatic resonator tuning and matching are achieved by implementing a digital feedback control system and using voltage-tunable capacitors. A band-pass subsampling method is used to directly digitize EPR signals at the carrier frequencies of about 1.2 GHz. The magnetic field scan waveforms, excitation EPR frequency, and sampling clock are all internally synchronized. Full-cycle RS-EPR signals are accumulated in the FPGA in real time without any time gaps. The result is the elimination of the re-arm time, during which data are not acquired. The proposed design in this manuscript has a small footprint and is relatively low cost. The FPGA-based RS-EPR system was tested using standard LiNc-BuO and tempone-d16 samples. The RS-EPR linewidth of the LiNc-BuO sample was consistent with an independent pulsed EPR measurement.

Design and testing of a 750MHz CW-EPR digital console for small animal imaging

This paper describes the development of a digital console for three-dimensional (3D) continuous wave electron paramagnetic resonance (CW-EPR) imaging of a small animal to improve the signal-to-noise ratio and lower the cost of the EPR imaging system. A RF generation board, an RF acquisition board and a digital signal processing (DSP) & control board were built for the digital EPR detection. Direct sampling of the reflected RF signal from a resonator (approximately 750MHz), which contains the EPR signal, was carried out using a band-pass subsampling method. A direct automatic control system to reduce the reflection from the resonator was proposed and implemented in the digital EPR detection scheme. All DSP tasks were carried out in field programmable gate array ICs. In vivo 3D imaging of nitroxyl radicals in a mouse's head was successfully performed.

A 750-MHz Electronically Tunable Resonator Using Microstrip Line Couplers for Electron Paramagnetic Resonance Imaging of a Mouse Tumor-Bearing Leg

OBJECTIVE

The purpose of this work was to develop an electronically tunable resonator operating at 750 MHz for continuous-wave electron paramagnetic resonance (CW-EPR) imaging of a mouse tumor-bearing leg.

METHODS

The resonator had a multi-coil parallel-gap structure with a sample space of 16 mm in diameter and 20 mm in length. Microstrip line couplers were used in conjunction with varactor diodes to enable resonance frequency adjustment and to reduce the nonlinear effects of the varactor diodes. The resonator was modeled by the finite-element method and a microwave circuit simulation was performed to clarify its radiofrequency characteristics.

RESULTS

A tunable resonator was evaluated in terms of its resonance frequency, tunable frequency band, and conversion efficiency of the RF magnetic field. The developed resonator provided a tunable frequency band of 4 MHz at a central frequency of 747 MHz and a conversion efficiency of 34 μT/W^1/2. To demonstrate the application of this tunable resonator to EPR imaging, three-dimensional EPR images of a sample solution and a mouse tumor-bearing leg were obtained.

CONCLUSION

The developed tunable resonator satisfied our initial requirements for in vivo EPR imaging and may be able to be further improved using the present finite-element and circuit models if any problems arise during future practical applications.

SIGNIFICANCE

This work may help to promote EPR imaging of tumor-bearing mice in cancer-related studies.

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.

A loop resonator for slice-selective in vivo EPR imaging in rats

A loop resonator was developed for 300 MHz continuous-wave electron paramagnetic resonance (CW-EPR) spectroscopy and imaging in live rats. A single-turn loop (55 mm in diameter) was used to provide sufficient space for the rat body. Efficiency for generating a radiofrequency magnetic field of 38 microT/W(1/2) was achieved at the center of the loop. For the resonator itself, an unloaded quality factor of 430 was obtained. When a 350 g rat was placed in the resonator at the level of the lower abdomen, the quality factor decreased to 18. The sensitive volume in the loop was visualized with a bottle filled with an aqueous solution of the nitroxide spin probe 3-carbamoyl-2,2,5,5-tetramethyl-3-pyrrolin-1-yloxy (3-CP). The resonator was shown to enable EPR imaging in live rats. Imaging was performed for 3-CP that had been infused intravenously into the rat and its distribution was visualized within the lower abdomen.

Transverse oriented electric field re-entrant resonator (TERR) with automatic tuning and coupling control for EPR spectroscopy and imaging of the beating heart

Sample motion, particularly that of a beating heart, induces baseline noise and spectral distortion on an EPR spectrum. In order to quench motional noise and restore the EPR signal amplitude and line-width, an L-band transverse oriented electric field re-entrant resonator (TERR) was designed and constructed with provisions for automatic tuning control (ATC) and automatic coupling control (ACC) suited for studies of isolated beating rat hearts. Two sets of electronic circuits providing DC biased voltage to two varactor diodes were implemented to electronically adjust coupling and tuning. The resonator has a rectangular cross-sectional sample arm of 25 mm diameter with a Q value of 1100 without sample. Once inserted with lossy aqueous samples of 0.45% NaCl, Q value drops to 400 with a volume of 0.5 ml and 150 with 5 ml. The ATC/ACC functions were tested with a moving phantom and isolated beating rat hearts with the improvement of signal to noise ratio (S/N, peak amplitude of signal over peak amplitude of baseline noise) of 6.7-, and 4 to 6-fold, respectively. With these improvements, EPR imaging could be performed on an isolated beating rat heart. Thus, this TERR resonator with ATC/ACC enables application of EPR spectroscopy and imaging for the measurement and imaging of radical metabolism, redox state, and oxygenation in the isolated beating rat heart.

Endothelial nitric oxide synthase (NOS3) knockout decreases NOS2 induction, limiting hyperoxygenation and conferring protection in the postischemic heart

Although it has been shown that endothelial nitric oxide synthase (eNOS)-derived nitric oxide downregulates mitochondrial oxygen consumption during early reperfusion, its effects on inducible NOS (iNOS) induction and myocardial injury during late reperfusion are unknown. Wild-type (WT) and eNOS(-/-) mice were subjected to 30 min of coronary ligation followed by reperfusion. Expression of iNOS mRNA and protein levels and peroxynitrite production were lower in postischemic myocardium of eNOS(-/-) mice than levels in WT mice 48 h postreperfusion. Significantly improved hemodynamics (+/-dP/dt, left ventricular systolic pressure, mean arterial pressure), increased rate pressure product, and reduced myocardial infarct size (18 +/- 2.5% vs. 31 +/- 4.6%) were found 48 h after reperfusion in eNOS(-/-) mice compared with WT mice. Myocardial infarct size was also significantly decreased in WT mice treated with the specific iNOS inhibitor 1400W (20.5 +/- 3.4%) compared with WT mice treated with PBS (33.9 +/- 5.3%). A marked reperfusion-induced hyperoxygenation state was observed by electron paramagnetic resonance oximetry in postischemic myocardium, but Po(2) values were significantly lower from 1 to 72 h in eNOS(-/-) than in WT mice. Cytochrome c-oxidase activity and NADH dehydrogenase activity were significantly decreased in postischemic myocardium in WT and eNOS(-/-) mice compared with baseline control, respectively, and NADH dehydrogenase activity was significantly higher in eNOS(-/-) than in WT mice. Thus deficiency of eNOS exerted a sustained beneficial effect on postischemic myocardium 48 h after reperfusion with preserved mitochondrial function, which appears to be due to decreased iNOS induction and decreased iNOS-derived peroxynitrite in postischemic myocardium.

Endothelium-derived nitric oxide regulates postischemic myocardial oxygenation and oxygen consumption by modulation of mitochondrial electron transport

BACKGROUND

Nitric oxide (NO) production is increased in postischemic myocardium, and NO can control mitochondrial oxygen consumption in vitro. Therefore, we investigated the role of endothelial NO synthase (eNOS)-derived NO on in vivo regulation of oxygen consumption in the postischemic heart.

METHODS AND RESULTS

Mice were subjected to 30 minutes of coronary ligation followed by 60 minutes of reperfusion. Myocardial oxygen tension (Po2) was monitored by electron paramagnetic resonance oximetry. In wild-type, N-nitro-L-arginine methyl ester (L-NAME)-treated (with 1 mg/mL in drinking water), and eNOS knockout (eNOS-/-) mice, no difference was observed among baseline myocardial Po2 values (8.6+/-0.7, 10.0+/-1.2, and 10.1+/-1.2 mm Hg, respectively) or those measured at 30 minutes of ischemia (1.4+/-0.6, 2.3+/-0.9, and 3.1+/-1.4 mm Hg, respectively). After reperfusion, myocardial Po2 increased markedly (P<0.001 versus baseline in each group) but was much lower in L-NAME-treated and eNOS-/- mice (17.4+/-1.6 and 20.4+/-1.9 mm Hg) than in wild-type mice (46.5+/-1.7 mm Hg; P<0.001). A transient peak of myocardial Po2 was observed at early reperfusion in wild-type mice. No reactive hyperemia was observed during early reperfusion. Endothelial NO decreased the rate-pressure product (P<0.05), upregulated cytochrome c oxidase (CcO) mRNA expression (P<0.01) with no change in CcO activity, and inhibited NADH dehydrogenase (NADH-DH) activity (P<0.01) without alteration of NADH-DH mRNA expression. Peroxynitrite-mediated tyrosine nitration was higher in hearts from wild-type mice than in eNOS-/- or L-NAME-treated hearts.

CONCLUSIONS

eNOS-derived NO markedly suppresses in vivo O2 consumption in the postischemic heart through modulation of mitochondrial respiration based on alterations in enzyme activity and mRNA expression of NADH-DH and CcO. The marked myocardial hyperoxygenation in reperfused myocardium may be a critical factor that triggers postischemic remodeling.

Safety and whole-body antioxidant potential of a novel anthocyanin-rich formulation of edible berries

Edible berry extracts rich in anthocyanins possess a broad spectrum of therapeutic, pharmacologic and anti-carcinogenic properties. Six berry extracts (wild blueberry, bilberry, cranberry, elderberry, raspberry seeds and strawberry), singly and in combination, were studied in our laboratories for antioxidant efficacy, cytotoxic potential, cellular uptake and anti-angiogenic properties. Combinations of edible berry extracts were evaluated to develop a synergistic formula, OptiBerry, which exhibited high oxygen radical absorbance capacity (ORAC) value, low cytotoxicity and superior anti-angiogenic properties compared to the other combinations tested. The current study sought to determine the broad spectrum safety and antioxidant potential of OptiBerry in vivo. Acute oral LD(50) of OptiBerry was greater than 5 g/kg in rats. Acute dermal LD(50) of OptiBerry was greater than 2 g/kg. No changes in the body weight or adverse effects were observed following necropsy. Primary skin and eye irritation studies were conducted in New Zealand albino rabbits. OptiBerry was classified as slightly irritating to the skin (primary skin irritation index 0.3) and minimally irritating to the eye (maximum mean total score 6.0). The antioxidant potential of OptiBerry was investigated in rats and mice by assessing GSH redox status in tissues as well as by a unique state-of-the-art electron paramagnetic resonance (EPR) imaging of whole-body redox status. A clinically relevant hyperbaric oxygen (HBO) exposure system (2 atm, 2 h) was employed to study the antioxidant properties of OptiBerry. OptiBerry feeding (8 weeks) significantly prevented HBO-induced GSH oxidation in the lung and liver of vitamin E-deficient Sprague Dawley rats. Furthermore, OptiBerry-fed mice, when exposed to HBO, demonstrated significant protection in whole-body HBO-induced oxidation compared to the unfed controls by EPR imaging. Taken together, these results indicate that OptiBerry is reasonably safe and possess antioxidant properties.

Oxygen: from the benefits of inducing VEGF expression to managing the risk of hyperbaric stress

Hypoxia limits wound healing. Both normobaric (1 atm) and hyperbaric oxygen (HBO) approaches have been used clinically to oxygenate wound tissue. Recently, we reported that HBO ameliorates stress-induced impairment of dermal healing. We examined the effect of pressure on oxygen-induced vascular endothelial growth factor (VEGF) expression by human HaCaT keratinocytes. Next, we investigated the effect of HBO on whole-body redox and on the ratio of oxidized to reduced glutathione (GSSG/GSH) in the liver, heart, lung, and brain of rats. Superoxygenation (90% O2) of keratinocytes partially arrested cell growth. G2-M growth arrest was substantially augmented by HBO. HBO also caused apoptosis in a small subpopulation. Normobaric oxygen, but not HBO (2 atm), potently induced the expression of VEGF165 and 189. In vivo electron paramagnetic resonance spectroscopy imaging revealed a clear shift of the whole-body redox status toward oxidation in response to HBO. The standard diet of laboratory rats contains excessive (17x human recommended dietary allowance) alpha-tocopherol (E++), which confers exceptional resistance to oxidant insults. People with chronic wounds commonly suffer from under- or malnutrition. We generated vitamin E-deficient (E-) rats by long-term dietary vitamin E restriction. HBO did not raise GSSG/GSH in E++ rats, but post-HBO GSSG/GSH was significantly higher in E- compared with E++. Thus, rats on antioxidant-enriched diet were well protected against HBO. The risk of oxidative stress may negatively impact the net benefits of HBO. This is of special concern for people with inadequate intake of dietary antioxidants. Nutritional antioxidant supplementation may offset HBO-induced oxidative stress.

EPR imaging: the relationship between CW spectra acquired from an extended sample subjected to fixed stepped gradients and the Radon transform of the resonance density

EPR spectroscopy can be extended to a spectroscopic imaging modality by applying magnetic field gradients across the sample to encode spatial information in the measured spectra. In this work, we present a mathematical model of the EPR imaging process in terms of the Radon transform. We describe a model for electron paramagnetic resonance imaging, derive its explicit relationship to the Radon transform, and discuss several options for reconstructing the sample absorption and dispersion densities. An important extension to previous descriptions is the incorporation of large amplitude magnetic field modulation, which can be used to improve the signal-to-noise ratio for continuous wave signal acquisition. Magnetic field modulation is shown to cause well understood changes in the shapes of spectra in the reconstructed images, but does not affect the spatial resolution achieved in these images. Since many of the novel image reconstruction strategies and noise filtering algorithms that have been developed for other modalities start from this formalism, this work allows for their direct application to EPR imaging. This promises to lead to further improvements in EPR imaging techniques.

Nonequivalent spectra of unpaired electrons in field and frequency modulation

We report a difference in the spectral lineshapes of continuous-wave (CW) electron paramagnetic resonance (EPR) spectroscopy between field and frequency modulation. This finding addresses the long-standing question of the effect of modulation in EPR absorption. We compared the first-derivative EPR spectra at 1.1 GHz for lithium phthalocyanine crystals, which have a single narrow linewidth in the EPR absorption spectrum, using field and frequency modulation. The experimental findings suggest that unpaired electrons have different behaviors under perturbation due to field and frequency modulation.

Control characteristics of an automatic matching control system for in vivo EPR spectroscopy

A method for estimating the characteristics of an automatic matching control (AMC) system for in vivo electron paramagnetic resonance (EPR) spectroscopy is described. AMC is also known as automatic coupling control (ACC) and is used to compensate for perturbation due to movement of the subject animal. The open-loop function of the AMC system is derived and the characteristics (gain, phase margin, steady-state error, system bandwidth, and disturbance rejection) of the system are examined. The proposed method is supported by experimental findings. The frequency response of perturbation due to the movement of an anesthetized rat was measured with a 1.1-GHz continuous-wave EPR spectrometer. Disturbance rejection with the AMC system was also considered for respiratory motion and the heartbeat of the anesthetized rat, and these perturbations were reduced by about 40 dB in the power spectrum of the output of the diode detector in the spectrometer.

Mapping of the B1 field distribution of a surface coil resonator using EPR imaging

Surface coil resonators have been widely used to perform topical EPR spectroscopy. They are usually positioned adjacent to or implanted within the body. For EPR applications these resonators have a number of important advantages over other resonator designs due to their ease of sample accessibility, mechanical fabrication, implementation of electronic tuning and coupling functions, and low susceptibility to sample motions. However, a disadvantage is their B(1) field inhomogeneity, which limits their usefulness for 3D imaging applications. We show that this problem can be addressed by mapping and correcting the B(1) field distribution. We report the use of EPR imaging (EPRI) to map the B(1) distribution of a surface coil resonator. We show that EPRI provides a fast, accurate, and reliable technique to evaluate the B(1) distribution. 3D EPRI was performed on phantoms, prepared using three different saline concentrations, to obtain the B(1) distribution. The information obtained from the phantoms was used to correct the images of living animals. With the use of this B(1) correction technique, surface coil resonators can be applied to perform 3D mapping of the distribution of free radicals in biological samples and living systems.

1.1-GHz continuous-wave EPR spectroscopy with a frequency modulation method

Continuous-wave EPR spectroscopy using a frequency modulation (FM) scheme was developed. An electronically tunable resonator and an automatic tuning control (ATC) system were used. Using the FM scheme instead of magnetic field modulation, we detected EPR absorption at the first derivative mode. We used a microwave frequency of 1.1 GHz in the present experiment. Similar signal-to-noise ratios were obtained with conventional field modulation and the FM method, and a low-quality factor EPR resonator was not necessary to suppress the significant microwave reflection from the resonator. The FM method with a tunable resonator may be an alternative solution to achieving phase-sensitive detection, when the side-effects of magnetic field modulation, such as microphonic noise and mechanical vibration, are detrimental for EPR detection.

EPR/NMR co-imaging for anatomic registration of free-radical images

While electron paramagnetic resonance imaging (EPRI) enables spatial mapping of free radicals in the whole body of small animals, it solely visualizes the free-radical distribution and does not typically provide anatomic visualization of the body. However, anatomic registration is often required for meaningful interpretation of the EPRI-derived free-radical images. An approach is reported for whole-body, EPRI-based, free-radical imaging along with proton MRI in mice. EPRI instrumentation with a 750-MHz narrow band microwave bridge and transverse oriented electric field reentrant resonator, with automatic coupling control and automatic tuning control capability, was used to map the spatial distribution of nitroxide free radicals in phantoms and in living mice, while low-field proton MRI at 16 MHz was used to define the anatomic structure to register the EPR images. Small capillary tubes containing an aqueous radical label were used as markers to enable image superimposition. With this coregistration technique, the EPRI free-radical images were precisely registered, enabling assignment of the location of the observed free-radical distribution within the organs of the mice. This technique enabled differentiation of the distribution and metabolism of nitroxide radicals within the major organs and body sites of living mice.