Proton-electron Double-resonance Imaging Research Articles

Proton electron double resonance imaging (PEDRI) of the isolated beating rat heart.

Proton electron double resonance imaging (PEDRI) is a double resonance technique where proton MRI is performed with irradiation of a paramagnetic solute. A low-field PEDRI system was developed at 20.1 mT suitable for imaging free radicals in biological samples. With a new small dual resonator, PEDRI was applied to image nitroxide free radicals in isolated beating rat hearts. Experiments with phantoms showed maximum image enhancement factors (IEF) of 42 or 28 with TEMPONE radical concentrations of 2-3 mM at EPR irradiation powers of 12W or 6W, respectively. In the latter case, image resolution better than 0.5 mm and radical sensitivity of 5 microM was obtained. For isolated heart studies, EPR irradiation power of 6W provided optimal compromise of modest sample heating with good SNR. Only a small increase in temperature of about 1 degrees C was observed, while cardiac function remained within 10% of control values. With infusion of 3 mM TEMPONE an IEF of 15 was observed enabling 2D or 3D images to be obtained in 27 sec or 4.5 min, respectively. These images visualized the change in radical distribution within the heart during infusion and clearance. Thus, PEDRI enables rapid and high-quality imaging of free radical uptake and clearance in perfused hearts and provides a useful technique for studying cardiac radical metabolism.

Proton electron double resonance imaging of the in vivo distribution and clearance of a triaryl methyl radical in mice.

Proton electron double resonance imaging (PEDRI) measures the spatial distribution of paramagnetic species in biological samples using the Overhauser effect. Triaryl methyl (TAM) free radicals have been developed as a spin probe for PEDRI since they have very long T(1e). Therefore, low RF power levels are sufficient to saturate the electron spin system with resultant high NMR enhancement facilitating application of PEDRI in living animals. In this report, PEDRI studies were performed at 0.02 T. The power-dependent image enhancement was studied using phantoms of TAM in saline, then the distribution and pharmacokinetics of TAM in living mice were measured. Following intravenous administration of 0.7 mmol/kg of TAM, enhancements of up to -34 were observed enabling visualization of its distribution within the body. Maximum uptake of TAM in the vascular compartment was seen 1 min postadministration with half-clearance within 5 min. Maximum uptake in the kidneys occurred at 10 min with half-clearance at 26 min and maximum accumulation in the bladder after 50 min. Thus, TAM is initially compartmentalized in the vasculature and this is followed by rapid uptake and excretion by the kidneys. PEDRI enabled rapid imaging of the distribution and clearance of this paramagnetic probe and this information should facilitate the use of TAM as a label for oximetry and other applications. Magn Reson Med 48:530-534, 2002.

Fluorine electron double resonance imaging for 19F MRI in low magnetic fields.

This work demonstrates the feasibility of generating fluorine NMR images at a very low magnetic field of 0.015 T by making use of the Overhauser enhancement of (19)F NMR signal brought about by a stable, water-soluble, narrow-line paramagnetic contrast agent. The enhancement in the (19)F NMR images depends on the concentration of the single electron contrast agent, the pO(2), and the electron paramagnetic resonance (EPR) irradiation power. The applicability of this technique for (19)F NMR imaging is demonstrated with phantom samples, where a time resolution of 4-10 min is achieved. Proton electron double resonance imaging (PEDRI) and fluorine electron double resonance imaging (FEDRI) images were also obtained from rat kidneys ex vivo, perfused with 10 mM Oxo63 and 10 M trifluoroacetic acid. The spatial and temporal resolutions of these images are comparable to those obtained at magnetic fields 2-3 orders of magnitude larger. Constant NMR frequency (628 kHz) operation permits both FEDRI and PEDRI of identical slices without removing the object under investigation. This feasibility of coregistration of proton-based anatomical PEDRI image with physiological FEDRI image offers good potential for studying fluorine-containing tracers.

Rapid imaging of free radicals in vivo using hybrid FISP field-cycled PEDRI

A new pulse sequence for rapid imaging of free radicals is presented which combines snapshot imaging methods and conventional field-cycled proton electron double resonance imaging (FC-PEDRI). The new sequence allows the number of EPR irradiation periods to be optimized to obtain an acceptable SNR and spatial resolution of free radical distribution in the final image while reducing the RF power deposition and increasing the temporal resolution. Centric reordered phase encoding has been employed to counter the problem of rapid decay of the Overhauser-enhanced signal. A phase-correction scheme has also been used to correct problems arising from instability of the magnetic field following field-cycling. In vivo experiments were carried out using triaryl methyl free radical contrast agent, injected at a dose of 0.214 mmol kg−1 body weight in anaesthetized adult male Sprague–Dawley rats. Transaxial images through the abdomen were collected using 1, 2, 4 and 8 EPR irradiation periods. Using 4 EPR irradiation periods it was possible to generate free radical distributions of acceptable SNR and resolution. The EPR power deposition is reduced by a factor of 16 and the acquisition time is reduced by a factor of 4 compared to an acquisition using the conventional FC-PEDRI pulse sequence.

Low field Overhauser images of the formation process of a hydrogel

The gelation of sodium alginate by calcium ions is studied by proton-electron double resonance imaging in a magnetic field of only 16 mT using a newly developed free radical. The high sensitivity of the Overhauser enhancement to small variations in water mobility permits the detection of minute changes in gel composition. Spatial inhomogeneity and temporal evolution of this, and possibly other processes involving diffusion and chemical reaction, could be accurately monitored by this technique.

Development of a PEDRI free-radical imager using a 0.38 T clinical MRI system.

Proton electron double resonance imaging (PEDRI) uses the Overhauser effect to image the distribution of free-radicals in biological samples and animals. Standard MRI hardware and software is used, with the addition of hardware to irradiate the free-radical-of-interest's EPR resonance. For in vivo applications it must be implemented at a sufficiently low magnetic field to result in an EPR irradiation frequency that will penetrate the sample but will not cause excessive nonresonant power deposition therein. Many clinical MRI systems use resistive magnets that are capable of operating at 10-20 mT, and which could thus be used as PEDRI imagers with the addition of a small amount of extra hardware. This article describes the conversion of a 0.38 T whole-body MRI system for operation as a 20.1 mT small-animal PEDRI imager. The magnet power supply control electronics required a small modification to operate at the lower field strength, but no permanent hardware changes to the MRI console were necessary, and no software modification was required. Frequency down- and up-conversion was used on the NMR RF system, together with a new NMR/EPR dual-resonance RF coil assembly. The system was tested on phantoms containing free-radical solution, and was also used to image the distribution of a free-radical contrast agent injected intravenously into anesthetized mice.

Rapid imaging of free radicals in vivo using field cycled PEDRI

Imaging of free radicals in vivo using an interleaved field-cycled proton-electron double-resonance imaging (FC-PEDRI) pulse sequence has recently been investigated. In this work, in order to reduce the EPR (electron paramagnetic resonance) irradiation power required and the imaging time, a centric reordered snapshot FC-PEDRI pulse sequence has been implemented. This is based on the FLASH pulse sequence with a very short repetition time and the use of centric reordering of the phase-encoding gradient, allowing the most significant free induction decay (FID) signals to be collected before the signal enhancement decays significantly. A new technique of signal phaseshift correction was required to eliminate ghost artefacts caused by the instability of the main magnetic field after field cycling. An FID amplitude correction scheme has also been implemented to reduce edge enhancement artefacts caused by the rapid change of magnetization population before reaching the steady state. Using the rapid pulse sequence, the time required for acquisition of a 64 × 64 pixel FC-PEDRI image was reduced to 6 s per image compared with about 2.5 min with the conventional pulse sequence. The EPR irradiation power applied to the sample was reduced by a factor of approximately 64. Although the resulting images obtained by the rapid pulse sequence have a lower signal to noise than those obtained by a normal interleaved FC-PEDRI pulse sequence, the results show that rapid imaging of free radicals in vivo using snapshot FC-PEDRI is possible.

Detection of nitrosyl–iron complexes by proton-electron–double-resonance imaging

The nitrogen monoxide radical (NO ) forms paramagnetic mono- and dinitrosyl–iron complexes in biologic tissues. To establish a noninvasive technique for in vivo NO imaging, we evaluated the suitability of these complexes as magnetic resonance (MR) contrast agents, making use of the ability of the unpaired electrons of the complexes to enter into dynamic nuclear polarization with water protons and hence produce enhancement on images generated by the technique of proton-electron–double-resonance imaging (PEDRI). Phantom solutions of synthetic nitrosyl–iron complexes (NICs) altered the signal intensity of PEDRI images. The dinitrosyl–iron complex (DNIC) with serum albumin induced a significantly larger signal alteration than the mononitrosyl–iron complex (MNIC) with dithiocarbamate. Exposure of rat liver to sodium nitroprusside (SNP) by ex vivo and in situ perfusion induced a composite X-band electron spin resonance (ESR) spectrum of the isolated liver characteristic of a MNIC and DNIC. On storage of the tissue, the MNIC signal disappeared and the DNIC signal intensity increased. Correspondingly, in cross-sectional PEDRI images taken at room temperature, the SNP-exposed livers initially exhibited a weak signal that strongly increased with time. In conclusion, NICs can be detected using PEDRI and could be exploited for in vivo NO imaging.

Mobility and Free Radical Concentration Effects in Proton–Electron Double-Resonance Imaging

The dependence of the enhancement of proton–electron double-resonance images upon the mobility of the proton bearing molecules, of the concentration of free radicals, and of the pulsed saturating RF power is studied in a magnetic field of 16 mT. The data exhibit a behavior which, in the potentially interesting region of small free radical concentration, may differ substantially from the high-concentration regime depending upon experimental conditions. The results permit a clearer understanding of the factors determining enhancement and contrast in images obtained by dynamic nuclear polarization.

Chemically induced analgesic nephropathy in the rat monitored by proton-electron double-resonance imaging (PEDRI).

Proton-electron double-resonance imaging (PEDRI) was used to assess renal function by monitoring the flow of the exogenous nitroxide free radical proxyl carboxylic acid (PCA) through normal and injured kidneys in the living rat. Kidney damage was induced by treatment with 2-bromoethylamine (BEA), which provides a well established model for human analgesic nephropathy. PCA clearance rates for liver, abdominal blood vessels, and renal tissues were determined from serial PEDRI images of normal rats (n = 6) and rats treated with BEA (n = 21). Different groups of BEA-treated animals were imaged on day 4 (n = 6), day 6 (n = 6), and day 9 (n = 9) after treatment. In BEA-treated rats, there was an increase in PCA half-life in all tissues studied. This increase was greatest in the kidney tissues and the effect progressed with time after treatment. The effect is probably due to BEA-induced damage to the tubules in the renal cortex and may not be related to the primary lesions in the renal medulla.

The application of PEDRI to the study of free radicals in vivo

Proton-electron double-resonance imaging (PEDRI) has considerable value for study of the distribution and elimination pathways of nitroxide free radicals (NFRs). This has been illustrated by its use in studies of kidney function in the living rat in which the NFR proxyl carboxylic acid (PCA) has been employed as a `tracer'. The technique, at its present stage of development, can demonstrate location of PCA in enough detail to observe the passage through kidney cortex and medulla differentially, and to see the NFR within the major abdominal blood vessels. These studies are helping towards an understanding of the metabolic fate of PCA, as well as providing information about kidney performance after challenge with a nephrotoxin. In addition, nitric oxide complexes, formed in vivo by providing rats with a nitrite-rich diet, have been observed ex vivo using PEDRI and field-cycled DNP.

Design, construction and use of a large-sample field-cycled PEDRI imager

The design, construction and use of a large-scale field-cycled proton-electron double-resonance imaging (FC-PEDRI) imager is described. The imager is based on a whole-body sized, vertical field, 59 mT permanent magnet. Field cycling is accomplished by the field compensation method, and uses a secondary, resistive magnet with an internal diameter of 52 cm. The magnetic field can be switched from zero to 59 mT or vice versa in 40 ms. It is used with a double-resonance coil assembly (NMR/EPR) comprising a solenoidal NMR transmit/receive coil and a coaxial, external birdcage resonator for EPR irradiation. Experiments to image the distribution of an exogenous nitroxide free radical in anaesthetized rabbits are described.

The excretion mechanism of the spin label proxyl carboxylic acid (PCA) from the rat monitored by X‐band ESR and PEDRI

Proton electron double resonance imaging (PEDRI) was used for monitoring in vivo the distribution, metabolism and, in particular, the excretion mechanism of the exogenous nitroxide free radical proxyl carboxylic acid (PCA) in the rat. PCA clearance half-lives through liver, abdominal vessels, and renal tissues were determined from a series of PEDRI images for normal rats (n = 5) and rats treated with probenecid (n = 5), a competitive inhibitor of the tubular secretion process. The approximately doubled renal half-lives of the treated animals suggest that tubular secretion accounts for about 50% of PCA renal loss in the normal rat and reabsorption is insignificant. PCA binding to bovine serum albumin was investigated by X-band ESR and the bound fraction was found to be less than 10% of the total PCA. Most probably, PCA binds to hydrophilic sites. Blood PCA concentration investigated by X-band ESR exhibited biphasic behavior and PEDRI results confirmed the in vivo metabolic reduction of PCA by rat liver cells.

Young Investigator Award presentation at the 13th Annual Meeting of the ESMRMB, September 1996, Prague. A proton-electron double-resonance imaging apparatus with simultaneous multiple electron paramagnetic resonance irradiation at 10 mT.

The detection of free radicals in vivo is very important for the study of many physiologic and pathologic conditions. Free radicals have been implicated in a number of diseases such as ischemia, inflammation, kidney damage, and cancer. Proton-electron double-resonance imaging (PEDRI) allows the indirect detection of free radicals via the Overhauser effect. Nitroxide free radicals used for in vivo PEDRI studies present spectra with two or three lines, but most PEDRI experiments performed to date have used only single-line electron paramagnetic resonance (EPR) irradiation. There is theoretical evidence that simultaneous irradiation of multiple EPR transitions could increase the maximum achievable PEDRI enhancement. From the experimental point of view, this requires the combined use of a suitable multiple-frequency EPR source and a multiple-tuned EPR resonator. A novel radiofrequency (RF) triple-tuned loop-gap resonator for use in PEDRI has recently been developed, and dynamic nuclear polarization (DNP) data were reported. In the present study we describe a new PEDRI apparatus, equipped with a triple-tuned resonator, that is suitable for simultaneous double- or triple-EPR irradiation of nitroxide free radicals. In particular, the details of the EPR hardware used to generate the two or three EPR frequencies are given, and PEDRI images obtained with simultaneous multiple EPR irradiation are shown. Moreover, DNP experimental results showing the increase of the enhancement as a function of the EPR power for single and simultaneous double EPR irradiation are presented. The main goal of this apparatus is to improve the sensitivity and/or to reduce EPR irradiation power in a PEDRI experiment. This is likely to be particularly important in future biologic applications of PEDRI where the applied power must be optimized to reduce sample heating.

Manipulation of free radical relaxivity for PEDRI

In free radical containing solutions Proton Electron Double Resonance Imaging (PEDRI) can be used to realize useful imaging applications at a low magnetic field strength. Because of toxicity reasons the allowed radical concentration will always be limited in the case of in vivo applications. This means that the relaxivity of the radicals should be as large as possible. The parameters that determine relaxivity are discussed. It is shown that it is difficult to obtain a considerable increase in relaxivity. Some gain in relaxivity can be realized by using sizeable radicals with strongly off-centred electron spins.

Progress toward whole-body proton—electron double-resonance imaging of free radicals

Proton—electron double-resonance imaging (PEDRI) has been developed recently for imaging free radicals in biological samples and small animals. This article summarizes the techniques of PEDRI and the related field-cycled method, FC-PEDRI, and discusses the difficulties in scaling the techniques up to whole-body size. Imaging free radicals with broad EPR lines in humans would require excessive radiofrequency (RF) power, but the use of magnetic field cycling alleviates this problem and improves the signal-to-noise ratio. The results of computer simulations of field-cycled PEDRI are presented, which show that optimum EPR irradiation frequencies exist, depending on the free radical's electron relaxation times and on the applied RF power.

The Application of Proton-Electron Double-Resonance Imaging Techniques to Proton Mobility Studies

A method of analyzing NMR images obtained using proton-electron double-resonance imaging (PEDRI) to image free-radical distributions is presented. The dependence of image intensity on proton mobility within the sample is demonstrated for samples of the nitroxide free radical TEMPOL in water/glycerol mixtures of different viscosities. From the images obtained at a fixed magnetic field strength of 10 mT, the parameters which are responsible for the image dependence on viscosity are determined. Field-cycling techniques are used to measure this dependence as a function of magnetic field strength with a view to defining a single parameter reflecting changes in proton mobility within a sample of unknown concentration.

Proceedings of the British Institute of Radiology. Progress in MRI: scientific and clinical developments in the United Kingdom, 14 October 1991

Endogenous free radicals such as hydroxyl (OH) are widely acknowledged to be implicated in many disease states. In this work we have used proton-electron double-resonance imaging (PEDRI) together with a technique called spin-trapping to image the hydroxyl free radicals formed by the action of UV light on phantoms containing hydrogen peroxide. In PEDRI a nuclear magnetic resonance (NMR) image is collected while the erythrocyte sedimentation rate (ESR) resonance of a free radical solute is irradiated. The result is an enhancement of the NMR signal in regions of the sample containing free radical; these regions then exhibit greater intensity in the final image. A quartz tube was filled with a solution of H2O2 and DMPO spin-trap. An ultraviolet source was placed 15 cm from the sample inside the NMR imager, which operated at 0.01 T. The ESR resonance of the free radical was irradiated at 223 MHz while a series of images was obtained. After switching on the UV source, the images showed that the free radical concentration increased to a peak of 200 μM and then decayed slowly We have demonstrated that PEDRI can be used to imag the Production of endogenous type free radicals. This work paves the way for both biological and in vivo studies.

Production of UV-generated hydroxyl free radicals imaged by proton-electron double-resonance imaging with spin trapping

Production of UV-generated hydroxyl free radicals imaged by proton-electron double-resonance imaging with spin trapping

EPR spectral information obtained from field-cycled proton-electron double-resonance images

The techniques of proton-electron double-resonance imaging (PEDRI) ( I ) and field-cycled PEDRI (FC-PEDRI) (2) have recently been developed to allow the distribution of free radicals in large aqueous samples to be imaged. In this Communication we describe a development of FC-PEDRI which enables spatially resolved EPR spectral information to be obtained from a series of FC-PEDRI images, in a manner analogous to chemical-shift NMR imaging. We call this development spectral FC-PEDRI. PEDRI relies on dynamic nuclear polarization: a proton NMR image of the sample is collected while an EPR resonance of a free radical solute is irradiated. Under favorable conditions, the NMR signal is enhanced in those parts of the sample where the free radical is influencing the proton relaxation rate, and these regions exhibit greater intensity in the final image. In FC-PEDRI the applied magnetic field B0 is switched rapidly between three values during the course of the pulse sequence, which comprises three distinct periods. The field starts off at a high value of Bz during the polarization period and is then lowered to BE for the evolution period, during which the EPR irradiation is applied, usually for a time of the order of the proton T, . The field is then increased to Bg for the detection period, during which the NMR signal is read out by a 90” pulse and the imaging gradients are applied (2). The enhancement of the NMR signal upon irradiation of the solute’s EPR may be defined as