Ian Nicholson

Field-Cycled Proton-Electron Double-Resonance Imaging of Free Radicals in Large Aqueous Samples

We have recently published a new method of imaging free radicals in aqueous solutions called proton-electron double-resonance imaging (PEDRI) ( I ). In this technique a conventional proton NMR image is collected while the EPR resonance of a free radical solute is irradiated. If the EPR irradiation has sufficient power, the NMR signal from those protons being relaxed by the paramagnetic solute is enhanced, and the parts of the sample containing free radical exhibit greater intensity in the final image. Unlike EPR imaging (2) the sample size in PEDRI is not constrained by magnetic field gradient requirements. In this Communication we present the first results of an extension of PEDRI which uses magnetic field cycling, allowing larger samples to be imaged with lower levels of applied radiofrequency power. PEDRI is an imaging version of a dynamic nuclear polarization experiment (35). The enhancement of the NMR signal upon irradiation of the EPR of the solute may be written empirically as

Free radicals imaged in vivo in the rat by using proton-electron double-resonance imaging

A new technique called proton—electron double-resonance imaging is described for imaging free radicals in aqueous samples. The method is a combination of proton NMR imaging with nuclear electron double resonance. The results of using this technique to image free radicals in vivo in the rat are presented. Rats were injected intravenously with a nitroxide free radical solution and a series of images was obtained from which the clearance of the free radical through the liver and kidneys could be observed.

In vivo detection of a pH‐sensitive nitroxide in the rat stomach by low‐field ESR‐based techniques

A study was made of the in vivo detectability of a pH‐sensitive, imidazolidine spin probe, and the efficacy of low‐frequency electron spin resonance (ESR)‐based techniques for pH measurement in vitro and in vivo in rats. The techniques used were longitudinally‐detected ESR (LODESR) and field‐cycled dynamic nuclear polarization (FC‐DNP) for in vitro and in vivo measurements, and radiofrequency (RF)‐ and X‐band ESR for comparisons in vitro. The spin probe was hexamethyl imidazolidine (HMI) with a pK of 4.6. All techniques detected HMI. Detection by FC‐DNP implies coupling between the free radical and solvent water spins. Separations between the three spectral lines of the nitroxide radical, relative to measurement frequency, were consistent with theory. The overall spectrum width from unprotonated HMI (pH > pK) was greater than that from protonated agent (pH < pK). This was observed in vitro and in vivo. Longer‐term studies showed that HMI is detectable and has the same spectral width (i.e., is at the same pH) up to 2 hr after gavage into the stomach, although the magnitude of the signal decreases rapidly during the first hour. These findings demonstrate the suitability of LODESR and FC‐DNP for monitoring HMI and measuring pH in vivo. These techniques would be useful for monitoring disease and drug pharmacology in the living system. Magn Reson Med 49:558–567, 2003.

DC SQUID-based NMR detection from room temperature samples

The authors have shown that a DC SQUID with a tuned input circuit can be used as a low noise NMR preamplifier for small, room temperature samples at low field. They were interested to observe that the SQUID maintains its bias point over periods of several hours, despite the absence of a Q-spoiler in the input circuit. The 30 ms dead time following NMR excitation pulses would be unacceptable for imaging, but earlier experiments lead the authors to expect that by including a Q-spoiler junction array in the input circuit they would be able to reduce this to well below 1 ms to allow detection after much shorter echo times.

Recent developments in combining LODESR imaging with proton NMR imaging

We have designed and constructed RF coil assemblies and the appropriate instrumentation for combining proton NMR imaging with LODESR imaging. This has enabled us to collect sequential images from the same sample using both methods. The coil assembly consists of a crossed ellipse coil for LODESR and proton NMR signal detection and a saddle coil for excitation of the ESR resonance. Images have been collected of phantoms containing copper sulphate and Tempol solutions. NMR images were collected (4.3 min) and within 30 s LODESR data collection started (collection time 2.5 min). Only the Tempol solutions are visible in the LODESR images.

Spin-trapped hydroxyl free radicals studied at low field by Field-Cycled Dynamic Nuclear Polarization

The technique of Field-Cycled Dynamic Nuclear Polarization (FC-DNP) involves the EPR irradiation of a free radical solution and the subsequent observation of the NMR signal, the experiment being carried out at a range of magnetic field strengths in order to measure the free radical’s EPR spectrum. In this work FC-DNP has been used to study the EPR spectrum of DMPO spin-trapped hydroxyl free radicals at magnetic field strengths between 0.5 mT and 13.0 mT (5–130 Gauss). The low-field EPR spectrum contains six separate EPR lines, in contrast to the well-known X-band spectrum where only four are seen. Knowledge of the spin-adduct’s EPR spectrum will be of use to workers involved in low-field EPR, especially those conducting biological or in-vivo spin-trapping experiments.

T *1e and T *2e maps derived in vivo from the rat using longitudinally detected electron spin resonance phase imaging: Application to abdominal oxygen mapping

A novel imaging modality is introduced which uses radiofrequency longitudinally detected electron spin resonance (RF‐LODESR). It is capable of providing qualitative and semiquantitative information on a variety of parameters reflecting physiological function, the most significant being tissue oxygenation. Effective spin‐lattice (T  *1e ) and spin‐spin (T  *2e ) electronic relaxation time maps of the abdomen of living 200‐g rats were generated after intravenous administration of a triarylmethyl free radical (TAM). These maps were used to evaluate oxygen distribution. Differences between the liver, kidneys, and bladder were noted. Conclusions were made regarding the distribution, perfusion, and excretion rate of the contrast medium. Ligature‐induced anoxia in the kidney was also visualized. LODESR involves transverse ESR irradiation with a modulated excitation, and observing oscillations in the spin magnetization parallel to the main magnetic field. The T  *1e and T  *2e maps were calculated from a set of LODESR signal phase images collected at different detection frequencies. Each phase image also provides qualitative information on tissue oxygen levels without any further processing. This method presents an alternative to the conventional transverse ESR linewidth‐based oximetry methods, particularly for animal whole‐body imaging applications. Magn Reson Med 46:1223–1232, 2001.

Multimodality magnetic resonance systems for studying free radicals in vivo

The multimodality approach to in vivo detection of free radicals combines the relative benefits of three free radical detection modalities: conventional RF CW-ESR, LODESR and PEDRI. We have built apparatus capable of combining these modalities to allow sequential PEDRI and CW-ESR, sequential LODESR and proton NMR imaging and simultaneous LODESR and CW-ESR. These systems offer superior performance in terms of both the scope and quality of information over single-modality free radical detection systems.

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

Young investigator award presentation at the 13th annual meeting of the esmrmb, september 1996, prague

The detection of free radicalsin 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 forin 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.