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This study reports the development of a completely new class of MRI contrast agents, displaying remarkable relaxation effects in the absence of paramagnetic metal ions. Their detection requires the acquisition of images at variable magnetic field strength as provided by fast field cycling imaging scanners. They contain poly-histidine chains (poly-His), whose imidazole groups generate 14 N-quadrupolar-peaks that cause a relaxation enhancement of water protons at a frequency (1.38±0.3 MHz) that is readily detectable from the frequencies associated with endogenous proteins. The poly-His quadrupolar peaks are detectable only when the polymer is in a solid-like form, that is, at pH>6.6. Above this value, their intensity is pH dependent and can be used to report on the occurring pH changes. On this basis, the poly-His moieties were conjugated to biocompatible polymers, such as polylactic and glycolic acid, in order to form stable nanoparticles able to encapsulate structured water in their core. FFC images were acquired to assess their contrast-generating ability.

1H spin−lattice nuclear magnetic resonance relaxation experiments were performed for five kinds of dermal fillers based on hyaluronic acid. The relaxation data were collected over a broad frequency range between 4 kHz and 40 MHz, at body temperature. Thanks to the frequency range encompassing four orders of magnitude, the dynamics of water confined in the polymeric matrix was revealed. It is demonstrated that translation diffusion of the confined water molecules exhibits a two‐dimensional character and the diffusion process is slower than diffusion in bulk water by 3–4 orders of magnitude. As far as rotational dynamics of the confined water is concerned, it is shown that in all cases there is a water pool characterized by a rotational correlation time of about 4×10−9 s. In some of the dermal fillers a fraction of the confined water (about 10 %) forms a pool that exhibits considerably slower (by an order of magnitude) rotational dynamics. In addition, the water binding capacity of the dermal fillers was quantitatively compared., Mechanism of water diffusion: Profiting from the potential of NMR relaxometry to reveal mechanism of molecular motion in condensed matter, we demonstrate the 2D character of the translation diffusion of water in hyaluronic acid gels. The diffusion process is slower than diffusion in bulk water by 3–4 orders of magnitude.

T 1 relaxation times can be measured at a range of magnetic field strengths by Fast Field-Cycling (FFC) NMR relaxometry to provide T 1 -dispersion curves. These are valuable tools for the investigation of material properties as they provide information about molecular dynamics non-invasively. However, accessing information at fields below 230 μT (10 kHz proton Larmor frequency) requires careful correction of unwanted environmental magnetic fields. In this work a novel method is proposed that compensates for the environmental fields on a FFC-NMR relaxometer and extends the acquisition of Nuclear Magnetic Relaxation Dispersion profiles to 2.3 μT (extremely low field region), with direct application in the study of slow molecular motions. Our method is an improvement of an existing technique, reported by Anoardo and Ferrante in 2003, which exploits the non-adiabatic behaviour of the magnetisation in rapidly-varying magnetic fields and makes use of the oscillation of the signal amplitude to estimate the field strength. This increases the accuracy in measuring the environmental fields and allows predicting the optimal correction values by applying simple equations to fit the data acquired. Validation of the method is performed by comparisons with well-known dispersion curves obtained from polymers and benzene.

Proton-electron double-resonance imaging (PEDRI) employs electron paramagnetic resonance irradiation with low-field magnetic resonance imaging so that the electron spin polarization is transferred to nearby protons, resulting in higher signals. PEDRI provides information about free radical distribution and, indirectly, about the local microenvironment such as partial pressure of oxygen (pOHigh-power radio frequency irradiation is needed for optimum signal enhancement, which may be harmful to living tissue by unwanted heat deposition. Free radical probes differ depending on the purpose of PEDRI. Some probes are less effective for enhancing signal than others, which can reduce image quality. It is so far not possible to image endogenous radicals by PEDRI because low concentrations and broad line widths of the radicals lead to negligible signal enhancement.PEDRI has similarities with electron paramagnetic resonance imaging (EPRI) because both techniques observe the EPR signal, directly in the case of EPRI and indirectly with PEDRI. PEDRI provides information that is vital to research on homeostasis, development of diseases, or treatment responses in vivo. It is expected that the development of new EPR techniques will give insights into novel PEDRI applications and vice versa. Antioxid. Redox Signal. 28, 1345-1364.

Purpose Fast field-cycling MRI (FFC-MRI) is a technique that promises to expand upon the diagnostic capabilities of conventional MRI by allowing the main field, B0, to be varied during a pulse sequence, thus allowing access to new types of endogenous contrast. However, this necessitates longer scan times, which can limit the technique's application to clinical research. In this paper, an adaptation of the fast spin-echo (FSE) pulse sequence for use with FFC-MRI is presented, known as field-cycling fast spin-echo (FC-FSE). This technique allows much faster image acquisition, thus shortening scan times significantly. Methods Image quality and relaxometric accuracy were assessed by comparison of phantom images with data obtained using conventional techniques. As proof of principle, relaxometric images were obtained from the thighs of a human volunteer. Results Image quality remains good for speedup factors of up to 4-fold. The accuracy of relaxometry data is in good agreement with conventional techniques. Results from a volunteer study were encouraging, demonstrating that the technique is sensitive enough to detect quadrupole peaks in vivo. Conclusion The technique has been demonstrated in phantom experiments with little loss of image quality or relaxometric accuracy. Initial in-vivo results pave the way for future clinical studies. Magn Reson Med 73:1120–1124, 2015. © 2014 Wiley Periodicals, Inc.

1H Nuclear magnetic resonance (NMR) relaxometry was exploited to investigate the dynamics of solid proteins. The relaxation experiments were performed at 37 &deg, C over a broad frequency range, from approximately 10 kHz to 40 MHz. Two relaxation contributions to the overall 1H spin&ndash, lattice relaxation were revealed, they were associated with 1H&ndash, 1H and 1H&ndash, 14N magnetic dipole&ndash, dipole interactions, respectively. The 1H&ndash, 1H relaxation contribution was interpreted in terms of three dynamical processes occurring on timescales of 10&minus, 6 s, 10&minus, 7 s, and 10&minus, 8 s, respectively. The 1H&ndash, 14N relaxation contribution shows quadrupole relaxation enhancement effects. A thorough analysis of the data was performed revealing similarities in the protein dynamics, despite their different structures. Among several parameters characterizing the protein dynamics and structure (e.g., electric field gradient tensor at the position of 14N nuclei), the orientation of the 1H&ndash, 14N dipole&ndash, dipole axis, with respect to the principal axis system of the electric field gradient, was determined, showing that, for lysozyme, it was considerably different than for the other proteins. Moreover, the validity range of a closed form expression describing the 1H&ndash, 14N relaxation contribution was determined by a comparison with a general approach based on the stochastic Liouville equation.

Purpose The T1 of tissue has a strong dependence on the measurement magnetic field strength. T1-dispersion could be a useful contrast parameter, but is unavailable to clinical MR systems which operate at fixed magnetic field strength. The purpose of this work was to implement a removable insert magnet coil for field-cycling T1-dispersion measurements on a vertical-field MRI scanner, by offsetting the static field over a volume of interest. Methods An insert magnet coil was constructed for use with a whole-body sized 59 milli-Tesla (mT) vertical-field, permanent-magnet based imager. The coil has diameter 38 cm and thickness 6.1 cm and a homogeneous region (± 5%) of 5 cm DSV, offset by 5 cm from the coil surface. Surface radiofrequency (RF) coils were also constructed. Results The insert coil was used in conjunction with a surface RF coil and a volume-localized inversion-recovery pulse sequence to plot T1-dispersion in a human volunteer's forearm over a range of field strengths from 1 mT to 70 mT. Conclusion T1-dispersion measurements were demonstrated on a fixed-field MRI scanner, using an insert coil. This demonstrates the feasibility of relaxation dispersion measurements on an otherwise conventional MR imager, facilitating the exploitation of T1-dispersion contrast for enhanced diagnosis. Magn Reson Med 72:1492–1497, 2014. © 2013 Wiley Periodicals, Inc.

It is known that in the early stages of osteoarthritis, the concentration of glycan proteins decreases in articular cartilage. This phenomenon is under active research to develop a means to characterize osteoarthritis accurately in the early stages of the disease, when still reversible. However, no method of quantification has yet shown clear success in this area. In this article, we propose a novel approach to detect glycan depletion using fast field-cycling NMR. This technique was previously reported to allow noninvasive measurement of protein concentration via the (14)N quadrupolar relaxation in certain amide groups. We have demonstrated that the articular cartilage exhibits clear quadrupolar peaks that can be measured by a benchtop fast field-cycling NMR device and which changes significantly between normal and diseased tissues (P < 0.01). This signal is probably glycan specific. The method may have potential for early evaluation of osteoarthritis in patients on fast field-cycling-MRI scanners currently under evaluation in the authors' laboratory.

The relaxation of 1H nuclei due to their interaction with quadrupolar 14N nuclei in gel structures is measured using fast field-cycling NMR. This phenomenon called quadrupolar dips has been reported in different 1H-14N bond-rich species. In this study, we have studied quadrupolar dips in fibrin, an insoluble protein that is the core matrix of thrombi. Fibrin was formed by the addition of thrombin to fibrinogen in 0.2% agarose gel. T1-dispersion curves were measured using fast field-cycling NMR relaxometry, over the field range of 1.5–3.5 MHz (proton Larmor frequency), and were analyzed using a curve-fitting algorithm. A linear increase of signal amplitude with increasing fibrin concentration was observed. This agrees with the current theory that predicts a linear relationship of signal amplitude with the concentration of contributing 14N spins in the sample. Interestingly, fibrin formation gave rise to the signal, regardless of crosslinking induced by the transglutaminase factor XIIIa. To investigate the effect of proteins that might be trapped in the thrombi in vivo, the plasma protein albumin was added to the fibrin gel, and an increase in the quadrupolar signal amplitude was observed. This study can potentially be useful for thrombi classification by fast field-cycling MRI techniques. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.

Magnetisation Transfer Contrast (MTC) is an important MR contrast-generating mechanism to characterise the MR-invisible macromolecular protons using an off-resonance pre-saturation RF irradiation pulse (or MT pulse). MTC MRI is normally implemented at a fixed magnetic field; however, it may be useful to evaluate changes of the MT effect as a function of external magnetic field strength (B₀). In order to conduct field-dependent MTC experiments with a single MR system, two techniques are crucially needed. B₀ should be able to be switched between levels during irradiation of the MT pulse. At the same time, the resonance frequency of the RF coil (f₀) should also be able to be shifted to the corresponding value. Switching B₀ is attained by the fast field-cycling technique, while in order to switch f₀, a specially designed multi-tunable RF coil is required. Here, we designed and constructed an actively frequency-switchable RF coil for frequencies at and below 2.5 MHz. The design employed PIN diodes, and enabled switching f₀ between five different values, with excellent impedance matching (approximately -37 dB S₁₁ reflection) and Q-factor of about 100 at each configuration.

Magnetic resonance imaging (MRI) and fast field-cycling (FFC) NMR are both well-developed methods. The combination of these techniques, namely fast field-cycling magnetic resonance imaging (FFC-MRI) is much less well-known. Nevertheless, FFC-MRI has a number of significant applications and advantages over conventional techniques, and is being pursued in a number of laboratories. This article reviews the progress in FFC-MRI over the last two decades, particularly in the areas of Earth's field and pre-polarised MRI, as well as free radical imaging using field-cycling Overhauser MRI. Different approaches to magnet design for FFC-MRI are also described. The paper then goes on to discuss recent techniques and applications of FFC-MRI, including protein measurement via quadrupolar cross-relaxation, contrast agent studies, localised relaxometry and FFC-MRI with magnetisation-transfer contrast.

Theoretical Overhauser-detected EPR spectra of 14 N and 15 N nitroxide systems in low magnetic field by field-cycled dynamic nuclear polarization (FC-DNP) were described by a combination of DNP theory and a model of FC-DNP. Spectra were simulated at magnetic fields between 0 and 8 mT. The simulations were able to predict both the EPR peak positions and their amplitudes, corresponding to those from FC-DNP experiments with 14 N and 15 N TEMPOL solutions. EPR irradiation was in the 45–133 MHz range while NMR signal detection occurred at a field of 59 mT. At this frequency range, four π transitions of a 14 N system and three π transitions of a 15 N system were observed. The simulation programmes were also used to predict the spectral amplitudes of the FC-DNP with EPR irradiation power in the 1–15 W range. Theoretical FC-DNP systems were in good agreement with experimental results; however, at low magnetic fields the inhomogeneity of our magnet system resulted in the EPR peaks being left-shifted and somewhat broader than those from the theoretical prediction.

Along with significant progress in low-frequency electron spin resonance (ESR, also called electron paramagnetic resonance, EPR), other techniques such as Longitudinally-Detected ESR (LODESR), proton electron double-resonance imaging (PEDRI) and field-cycled dynamic nuclear polarization (FC-DNP) have been developed forin vivoapplications. However their potential is still far from maximally defined, in part, because of the need for new specific function-directed spin probes. An application of stable nitroxides of imidazoline and imidazolidine types provides unique possibility to measure local values of pH and thiol content in various biological systems, includingin vivostudies. These applications are based on the observation of specific chemical reactions of these nitroxides with protons or thiols, followed by significant changes in their EPR spectra. To increase sensitivity of pH probes for low-frequency EPR spectroscopy we evaluated two alternative approaches: (i) application of isotopically substituted labels, and (ii) acquisition of EPR spectra at high modulation amplitude. Spatial and spectral-spatial imaging (pH-mapping) using PEDRI and L-band EPR imagers were performed both on phantom samples andin vivo. The applications of the pH and SH probes in model systems, biological fluids, andin vivoin living animals are discussed.

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 T1e. 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. © 2002 Wiley-Liss, Inc.

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.

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.

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.

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.

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.

We describe a submicrosecond resonator and receiver system for use in pulsed magnetic resonance at 220 MHz. This new resonator and receiver system design enables a reduction of the dead time, and in principle its complete elimination. We show experimentally that the resonator and receiver system permits the detection of free induction decay signals 300 ns from the end of the transmitting pulse, even with large (55-ml) saline samples. This apparatus was specifically developed for in vivo Fourier transform electron paramagnetic resonance detection of free radicals; however, it can also be used in pulsed nuclear magnetic resonance imaging of solids with applications in materials research and the polymer industry.

We report the modification of a low-field whole-body NMR imager to allow radio frequency EPR spectroscopy. The instrument is designed to give optimum sensitivity for in vivo detection of free radicals. The RF circuit is able to operate over a wide frequency range (240 - 320 MHz) and is designed to handle input power levels of up to 12.5 W. The EPR resonator is of the loop - gap type suitable for samples up to 400 ml. A remote resonator coupling method has been developed enabling convenient matching adjustment. An automatic frequency control circuit is able to adjust for frequency deviations caused by animal motion. Where possible, existing imager hardware and commercially available instruments have been used. The instrument is controlled from a central computer via an IEEE 488 instrumentation bus. Here we present sensitivity measurements obtained from a variety of large aqueous samples containing nitroxide free radicals. We show that the instrument is suitable for the detection of exogenous free radicals in 200 g rats.

The birdcage resonator, well appreciated for its high signal-to-noise ratio and its magnetic field uniformity characteristics, operates efficiently in mid- to high-field MRI systems but, unfortunately not for low-field (

CT has long been the modality of choice for radiotherapy treatment planning, because of its inherent lack of spatial distortion and its natural ability to provide electron-density information about tissues, directly feeding in to dose calculations. Nevertheless, MRI offers much better soft-tissue contrast than CT, with the added benefit of zero radiation dose. Advances in MRI technology have improved its geometric accuracy, while image segmentation allows dose-related parameters to be inferred, making MRI a viable option for radiotherapy planning. In MRI-guided radiotherapy, MR images are used to delineate tumour boundaries and guide the radiation distribution in real time. MRI uses signals generated by nuclear magnetic resonance (NMR) of magnetic hydrogen nuclei (protons). The frequency of the signal depends on the strength of the magnetic field, typically 1.5 tesla giving an NMR frequency of 64 MHz. Spatial information is obtained by magnetic field gradients, which alter the local magnetic field (and hence the resonant frequency) as a function of position; analysis of the frequency and phase of the measured NMR signals allows images to be produced in 2- or 3-dimensions. The geometric accuracy of the images is crucial to MR-based treatment planning, and depends on the homogeneity of the main magnetic field and the linearity and homogeneity of the magnetic field gradients. A plethora of MRI pulse sequences exist, which can be optimised to improve the results of image segmentation for dose calculations. This presentation will cover MRI concepts and how they impact on MR-based treatment-planning and on MR-guided therapy.

We have used tuned receiver coils coupled to a dc SQUID preamplifier in a small scale magnetic resonance imaging (MRI) system operating at 425 kHz (B/sub 0/=0.01 T). The coil and SQUID are cooled to 4.2 K in a modified biomagnetic cryostat. The modifications provide transparency to rf signals originating outside the cryostat while maintaining an acceptably low liquid helium boiloff rate. By applying negative feedback we have damped low loss, high Q-factor SQUID input circuits to achieve a useful imaging bandwidth of over 2 kHz. >

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.

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.

Fast field-cycling (FFC) MRI allows switching of the magnetic field during an imaging scan. FFC-MRI takes advantage of the T(1) dispersion properties of contrast agents to improve contrast, thus enabling more sensitive detection of the agent. A new contrast agent designed specifically for use with FFC was imaged using both a homebuilt FFC-MRI system and a 3 T Philips clinical MRI scanner. T(1) dispersion curves were obtained using a commercial relaxometer which showed large changes in relaxation rate between fields. A model of magnetization behaviour was used to predict optimum evolution times for the maximum T(1) contrast between samples at each field. Images were processed and analysed to create maps of R(1) values using a set of images at each field. The R(1) maps produced at two different fields were then subtracted from each other in order to create a map of ΔR(1) in which pixel values depend on the change in R(1) of the sample between the two fields. The dispersion properties of the agent resulted in higher contrast in a ΔR(1) image compared with a standard T(1)-weighted image.

Fast field-cycling MRI offers access to sources of endogenous information not available from conventional fixed-field imagers. One example is the T1 dispersion curve: a plot of T1 versus field strength. We present a pulse sequence that combines saturation-recovery/inversion-recovery T1 determination with field cycling and point-resolved spectroscopy localization, enabling the measurement of dispersion curves from volumes selected from a pilot image. Compared with a nonselective sequence, our method of volume selection does not influence measurement accuracy, even for relatively long echo times and in the presence of amplitude of radiofrequency field nonuniformity. The measured voxel profile, while not ideal, corresponds with that expected from the image slice profile. On a whole-body fast field-cycling scanner with 59-mT detection, the sensitivity of the experiment is sufficient to reveal distinctive ‘‘quadrupole dips’’ in dispersion curves of protein-rich human tissue in vivo. Magn Reson Med 000:000– 000, 2009. V C 2009 Wiley-Liss, Inc.

Dynamic nuclear polarization (DNP) is an NMR-based technique which enables detection and spectral characterization of endogenous and exogenous paramagnetic substances measured via transfer of polarization from the saturated unpaired electron spin system to the NMR active nuclei. A variable field system capable of performing DNP spectroscopy with NMR detection at any magnetic field in the range 0–0.38 T is described. The system is built around a clinical open-MRI system. To obtain EPR spectra via DNP, partial cancellation of the detection field B 0 NMR is required to alter the evolution field B 0 EPR at which the EPR excitation is achieved. The addition of resistive actively shielded field cancellation coils in the gap of the primary magnet provides this field offset in the range of 0–100 mT. A description of the primary magnet, cancellation coils, power supplies, interfacing hardware, RF electronics and console are included. Performance of the instrument has been evaluated by acquiring DNP spectra of phantoms with aqueous nitroxide solutions (TEMPOL) at three NMR detection fields of 97 G, 200 G and 587 G corresponding to 413 kHz, 851.6 kHz and 2.5 MHz respectively and fixed EPR evolution field of 100 G corresponding to an irradiation frequency of 282.3 MHz. This variable-field DNP system offers great flexibility for the performance of DNP spectroscopy with independent optimum choice of EPR excitation and NMR detection fields.

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.

Magnetisation transfer contrast (MTC) is an important MR contrast generating mechanism to characterise the undetectable bound protons indirectly using the decreased signal intensity of the observable free protons. MTC imaging typically employs a range of off-resonance RF pre-saturation pulse with maintaining the RF magnetic field (B(1)) at a specified value. However, this presents a technical difficulty, particularly at low field, because the larger offset frequencies tend to be outside the bandwidth of the RF transmit system, causing B(1) to vary with the frequency offset. Here, we demonstrate a novel off-resonance irradiation method using fast field-cycling which allows switching of the external magnetic field between several chosen strengths, while holding constant the RF frequency and B(1) level. This permits one to avoid the problem of B(1) variation as a function of frequency offset. The results obtained by this new technique are in excellent agreement with those obtained by the conventional technique.

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

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.

The sections in this article are 1 Introduction 2 Pertinent Results From the Theory of the Electronic Overhauser Effect 3 PEDRI Techniques 4 Hardware Requirements 5 Applications and Potential of PEDRI 6 Biographical Sketch Related Articles

New improved pH-sensitive nitroxides were applied for in vivo studies. An increased stability of the probes towards reduction was achieved by the introduction of the bulky ethyl groups in the vicinity of the paramagnetic N O fragment. In addition, the range of pH sensitivity of the approach was extended by the synthesis of probes with two ionizable groups, and, therefore, with two p K a values. Stability towards reduction and spectral characteristics of the three new probes were determined in vitro using 290 MHz radiofrequency (RF)- and X-band electron paramagnetic resonance (EPR), longitudinally detected EPR (LODEPR), and field-cycled dynamic nuclear polarization (FC-DNP) techniques. The newly synthesized probe, 4-[bis(2-hydroxyethyl)amino]-2-pyridine-4-yl-2,5,5-triethyl-2,5-dihydro-1 H -imidazol-oxyl, was found to be the most appropriate for the application in the stomach due to both higher stability and convenient pH sensitivity range from pH 1.8 to 6. LODEPR, FC-DNP and proton–electron double resonance imaging (PEDRI) techniques were used to detect the nitroxide localization and acidity in the rat stomach. Improved probe characteristics allowed us to follow in vivo the drug-induced perturbation in the stomach acidity and its normalization afterwards during 1 h or longer period of time. The results show the applicability of the techniques for monitoring drug pharmacology and disease in the living animals.

In this work we show the feasibility of sequential, co-registered fluorine and proton field-cycled Overhauser imaging at a detection field of 59 mT. To this purpose we have built an RF coil assembly comprising an Alderman-Grant resonator for EPR irradiation at 127.7 MHz (evolution field of 4.5 mT) and a solenoidal coil for (19)F or (1)H MRI acquisition at the detection field of 59 mT. A removable tuning/matching circuit that allows the solenoid to be tuned to the (19)F frequency (2.346 MHz, FEDRI) or the (1)H frequency (2.494 MHz, PEDRI) without removing the sample was built and tested. Switching of the solenoid between the (19)F and (1)H frequency is thus achieved in less than 1 min. The co-registered FC-FEDRI and FC-PEDRI images show higher enhancement in the sample regions with higher free radical concentration. This work is the first methodological step towards the development of an MRI scanner capable of acquiring morphological ((1)H) and physiological ((19)F) images in animal models at very low fields.

A novel nuclear magnetic resonance (NMR) technique for imaging materials exhibiting extremely short T 2 relaxation times is described and some examples of its application to materials of scientific and industrial interest are presented. The technique employs continuous-wave (CW) radio-frequency irradiation of the sample together with continual detection, and thus the experimental dead-time inherent in pulsed techniques is effectively eliminated. The mechanisms which broaden the NMR linewidths of materials in the solid state and liquids in confined geometries are described, and the resulting difficulties which these broad linewidths pose to NMR imaging (NMRI) are outlined. A brief review of existing NMRI techniques is then presented. The CW-NMR approach to imaging is expounded, and the CW-NMRI system is described in detail. 1-D and 2-D images of the ingress of water ( 1 H imaging) and brine ( 23 Na imaging) into various cementitious materials are presented, together with images of the solid 27 Al phase of a high-temperature refractory cement. The inter- and self-diffusion coefficients of a clay mineral, bentonite, were successfully measured, and the 2-D and 3-D imaging performance of the system was demonstrated on phantoms of poly(methyl methacrylate) (PMMA; Perspex/Plexiglas).

This paper describes the design, construction and use of a field-cycled proton-electron double-resonance imaging (FC-PEDRI) system for the detection and imaging of free radicals. The unique feature of this imager is its use of a 450-mT detection magnetic field in order to achieve good image quality and sensitivity. The detection magnetic field is provided by a superconducting magnet, giving high stability and homogeneity. Field cycling is implemented by switching on and off the current in an internal, coaxial, resistive secondary magnet that partially cancels the superconducting magnet's field at the sample; the secondary magnet is actively shielded to avoid eddy currents. EPR irradiation takes place at approximately 5 mT, following which the field is switched to 450 mT in 40 ms for NMR signal detection. Full details of the imager's subsystems are given, and experiments to image the distribution of stable free radical contrast agents in phantoms and in anesthetized rats are described.

Electron Paramagnetic Resonance (EPR, or ESR) is a powerful non-invasive spectroscopic tool that can be used to monitor drug release processes in vitro and in vivo. Furthermore, spatial dissolution can be achieved by means of EPR-Imaging. The article introduces the basics of EPR and EPR-imaging. It discusses also the challenges of in vivo spectroscopy and imaging and presents information about new developments such as longitudinally detected EPR (LODEPR) and Proton Electron Double Resonance Imaging ((PEDRI). Examples of the usefulness of EPR in the field of drug delivery include the measurement of microviscosity and micropolarity, the direct detection of drug release mechanisms in vitro and in vivo, the monitoring of microacidity in biodegradable polymers and the characterisation of colloidal drug carriers.

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.

Proton-electron double-resonance imaging (PEDRI) is an alternative method to EPR for detecting and imaging free radicals in biological samples or animals. PEDRI uses the Overhauser effect, and involves collecting a proton NMR image while irradiating the EPR of the free radical under study. The spatial resolution in PEDRI is not affected by the linewidth of the free radical’s EPR resonance. An important development of the technique is field-cycled PEDRI (FC-PEDRI), in which the applied magnetic field is switched between two levels during the pulse sequence in order to decrease non-resonant RF power deposition and to increase sensitivity. A spectroscopic version of FC-PEDRI, called field-cycled dynamic nuclear polarization (FC-DNP) is useful for studying low concentrations of free radicals and enables EPR spectra to be obtained via the Overhauser effect. This Chapter describes the techniques and practical implementation of PEDRI and related techniques. Applications of the methods are described, and likely future uses of the techniques are discussed.

A numerical model of the behaviour of the magnetization in a field-cycled dynamic nuclear polarization (DNP) experiment is presented, with the aim of optimizing pulse sequence parameters in field-cycled proton–electron double-resonance free radical imaging. The model is used to predict the observed enhancement of the NMR signal as a function of the magnetic field strength, EPR irradiation frequency and pulse sequence timing, as well as the properties of the sample including the NMR and EPR relaxation times. The model allowed optimization of parameters in the field-cycled DNP experiment, in particular the EPR irradiation frequency, to find the value which would give the largest difference between NMR signals recorded with and without EPR irradiation. Experiments to verify the model were carried out using aqueous solutions of TEMPOL, which exhibits three hyperfine lines in its EPR spectrum and triarylmethyl (TAM), which has a single, narrow line. It was found that the model predicted very well the variation in DNP enhancement with EPR irradiation power for both samples. The behaviour of the NMR signal with EPR irradiation frequency in studies using TEMPOL was also accurately modelled, with the optimum frequency lying between 60 and 80 MHz, depending on the EPR irradiation power. The optimum frequency obtained from the model also agreed with the experimental data obtained using the TAM free radical, but with this sample the theoretical curves tended to deviate from the experimental data at irradiation frequencies below 70 MHz.

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 x 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.

The use of RF (100 to 300 MHz) PEDRI and CW-EPR techniques allows the in vivo study of large animals such as whole rats and rabbits. Recently a PEDRI instrument was modified to also allow CW-EPR spectroscopy with samples of similar size and under the same experimental conditions. In the present study, this CW-EPR and PEDRI apparatus was used to assess the feasibility of the detection of a pyrrolidine nitroxide free radical (2,2,5,5,-tetramethylpyrrolidine-1-oxyl-3-carboxylic acid, PCA) in the abdomen of rats. In particular, we have shown that after the PCA administration (4 mmol kg(-1) b.w.): (i) the PCA EPR linewidth does not show line broadening due to concentration effects; (ii) a similar PCA up-take phase is observed by EPR and PEDRI; and (iii) the PCA half-lives in the whole abdomen of rats measured with the CW-EPR (T1/2=26+/-4 min, mean+/-sd, n=10) and PEDRI (T1/2=29+/-4 min, mean+/-sd, n=4) techniques were not significantly different (p0.05). These results show, for the first time, that information about PCA pharmacokinetics obtained by CW-EPR is the same as that from PEDRI under the same experimental conditions.