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Highlights • Fast Field-Cycling can assess proteoglycan content in vitro. • The T1 vs field strength profile of cartilage varies in osteoarthritis. • These biomarkers can potentially be accessed in vivo using Field-Cycling Imaging. • Potential application in clinical trials of new treatments for early osteoarthritis., Osteoarthritis in synovial joints remains a major cause of long-term disability worldwide, with symptoms produced by the progressive deterioration of the articular cartilage. The earliest cartilage changes are thought to be alteration in its main protein components, namely proteoglycan and collagen. Loss of proteoglycans bound in the collagen matrix which maintain hydration and stiffness of the structure is followed by collagen degradation and loss. The development of new treatments for early osteoarthritis is limited by the lack of accurate biomarkers to assess the loss of proteoglycan. One potential biomarker is magnetic resonance imaging (MRI). We present the results of a novel MRI methodology, Fast Field-Cycling (FFC), to assess changes in critical proteins by demonstrating clear quantifiable differences in signal from normal and osteoarthritic human cartilage for in vitro measurements. We further tested proteoglycan extracted cartilage and the key components individually. Three clear signals were identified, two of which are related predominantly to the collagen component of cartilage and the third, a unique very short-lived signal, is directly related to proteoglycan content; we have not seen this in any other tissue type. In addition, we present the first volunteer human scan from our whole-body FFC scanner where articular cartilage measurements are in keeping with those we have shown in tissue samples. This new clinical imaging modality offers the prospect of non-invasive monitoring of human cartilage in vivo and hence the assessment of potential treatments for osteoarthritis. Keywords: Fast Field-Cycling NMR; human hyaline cartilage; Osteoarthritis; T1 dispersion; quadrupolar peaks; protein interactions

This study is focused on the development of innovative sensors to non-invasively monitor the tissue implant status by Fast-Field-Cycling Magnetic Resonance Imaging (FFC-MRI). These sensors are based on oligo-histidine moieties that are conjugated to PLGA polymers representing the structural matrix for cells hosting scaffolds. The presence of 14N atoms of histidine causes a quadrupolar relaxation enhancement (also called Quadrupolar Peak, QP) at 1.39 MHz. This QP falls at a frequency well distinct from the QPs generated by endogenous semisolid proteins. The relaxation enhancement is pH dependent in the range 6.5–7.5, thus it acts as a reporter of the scaffold integrity as it progressively degrades upon lowering the microenvironmental pH. The ability of this new sensors to generate contrast in an image obtained at 1.39 MHz on a FFC-MRI scanner is assessed. A good biocompatibility of the histidine-containing scaffolds is observed after its surgical implantation in healthy mice. Over time the scaffold is colonized by endogenous fibroblasts and this process is accompanied by a progressive decrease of the intensity of the relaxation peak. In respect to the clinically used contrast agents this material has the advantage of generating contrast without the use of potentially toxic paramagnetic metal ions.

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.

Objectives. Low frequency coils present unique challenges as loop losses, component losses, and the supporting electronics can significantly degrade the signal-to-noise ratio (SNR). SNR may already be a limiting factor with MRI at low field (and frequency), therefore the minimization of additional loss is particularly important. If interactions between loops are managed, array coils can provide increased SNR, coverage, and potentially imaging speed. In this work, we investigate methods to characterise and preserve SNR from a low frequency coil array, allowing a more geometrically conforming array for quick, no-tune application with various anatomies. Approach. Single and multi-turn, 16.2 cm diameter litz wire loops were constructed and characterised for losses under various loading conditions. Low noise preamplifiers were acquired and characterized, as well as interfacing electronics were developed and evaluated. A bench level SNR test was implemented to observe the effects of tuning and loading on individual coils. The results were used to select a design for construction of a 6-channel, flex array coil. Main results. Ultra fine strand litz wire exhibited lower losses than equivalent diameter solid wire which should translate to improved SNR and provides the mechanical flexibility needed in a conforming array. Single turn loop losses were dominant under all loading conditions; however, 2 and 3 turn loops were body loss dominated under modest loading conditions. Preamplifier blocking achieved was well short of our design goal and critical overlaps became necessary for coil-to-coil interaction control. Our finished array, a 3-channel posterior array coil and a 3-channel anterior array coil, conforms nicely to various anatomies and is providing consistent results in various volunteer study trials. Significance. Receive coils are challenging at low fields as loop losses often limit the final SNR. This is exacerbated in an array coil as loops may be smaller and not coupled well to the body. In this work we have demonstrated that body loss dominance is possible with 16.2 cm loops at 8.5 MHz. We have optimized, built, and tested low noise interfacing electronics and characterized the SNR penalties as the tuning and loading is varied, a key parameter in a geometrically flexible array designed for rapid setup. The resultant 6-channel, general-purpose array is supporting various Field-Cycling Imaging studies where body habitus and anatomies require a flexible, adaptable array coil which can be quickly positioned and utilized.

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.

Fast Field-Cycling (FFC) is a well-established Nuclear Magnetic Resonance (NMR) technique that exploits varying magnetic fields to quantify molecular motion over a wide range of time scales, providing rich structural information from nanometres to micrometres, non-invasively. Previous work demonstrated great potential for FFC-NMR biomarkers in medical applications; our research group has now ported this technology to medical imaging by designing a whole-body FFC Magnetic Resonance Imaging (FFC-MRI) scanner capable of performing accurate measurements non-invasively over the entire body, using signals from water and fat protons. This is a unique tool to explore new biomarkers related to disease-induced tissue remodelling. Our approach required making radical changes in the design, construction and control of MRI hardware so that the magnetic field is switched within 12.5 ms to reach any field strength from 50 μT to 0.2 T, providing clinically useful images within minutes. Pilot studies demonstrated endogenous field-dependant contrast in biological tissues in good agreement with reference data from other imaging modalities, confirming that our system can perform multiscale structural imaging of biological tissues, from nanometres to micrometres. It is now possible to confirm ex vivo results obtained from previous clinical studies, offering applications in diagnosis, staging and monitoring treatment for cancer, stroke, osteoarthritis and oedema.

Simple Summary Breast cancer is the most diagnosed cancer for women, and clear surgical margins in breast-conserving surgery (BCS) are essential for preventing recurrence. In this study, the potential of fast field-cycling 1H-NMR relaxometry as a new tool for intraoperative margin assessment was evaluated. The technique allows the determination of the tissue proton relaxation rates as a function of the applied magnetic field on small tissue samples excised from surgical specimens, at the margins of tumour resection, prior to histopathological analysis. It was found that a good accuracy in margin assessment, i.e., a sensitivity of 92% and a specificity of 85%, can be achieved. The discriminating ability shown by the relaxometric assay relies mainly on the difference of fat/water content between healthy and tumour cells. The information obtained has the potential to support the surgeon in real-time margin assessment during BCS. Abstract As conserving surgery is routinely applied for the treatment of early-stage breast cancer, the need for new technology to improve intraoperative margin assessment has become increasingly important. In this study, the potential of fast field-cycling 1H-NMR relaxometry as a new diagnostic tool was evaluated. The technique allows the determination of the tissue proton relaxation rates (R1), as a function of the applied magnetic field, which are affected by the changes in the composition of the mammary gland tissue occurring during the development of neoplasia. The study involved 104 small tissue samples obtained from surgical specimens destined for histopathology. It was found that a good accuracy in margin assessment, i.e., a sensitivity of 92% and a specificity of 85%, can be achieved by using two quantifiers, namely (i) the slope of the line joining the R1 values measured at 0.02 and 1 MHz and (ii) the sum of the R1 values measured at 0.39 and 1 MHz. The method is fast, and it does not rely on the expertise of a pathologist or cytologist. The obtained results suggest that a simplified, low-cost, automated instrument might compete well with the currently available tools in margin assessment.

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.

Background Ultra-small superparamagnetic iron oxide (USPIO) nanoparticles are phagocytosed by macrophages and when subjected to magnetic resonance imaging (MRI) detect inflammation. We describe a novel MR technique where the magnetic field is rapidly cycled (Fast field-cycling MR, FFC-MR), assessing T1 dispersion over a range of low (100μT-0.2T) fields and offering superior T1-based iron quantification. AIM To quantify iron using the FFC-MR R1 (1/T1) dispersion profile and flow cytometry features of USPIO-laden cells, in comparison to colorimetric assays. Methods Murine J774 macrophage-like cells were incubated with 0–200μg/ml Fe as USPIO (ferumoxytol) for 16 hours, washed and suspended in 500μl PBS/2mM EDTA. Prussian blue staining confirmed USPIO phagocytosis. Relaxation was measured using a clinical, in-house built prototype 0.2T FFC-MR system with a custom test tube coil. R1 dispersion was derived from a saturation recovery sequence (Fig. 1A). R1 (s–1) values were calculated with a monoexponential curve fitting algorithm, R2 of fits were ≥0.999. R1 dispersion profiles were generated plotting R1 against the magnetic field (T). Quantification of side scatter (SCC) intensity and the USPIO-occupied fraction of total cell area was performed with imaging flow cytometry. A colorimetric assay provided validation of cell iron content. All data are mean±SEM, analysed with t-tests, Pearson correlation and linear regression; statistical significance set at p Results Table 1 shows quantitative data derived by all 3 modalities with increasing USPIO exposure. FFC-MR detection of intracellular iron was excellent (p≤0.001 vs. control for all), with separation of average R1 dispersion profiles (Fig. 1B), strong correlation with colorimetry (r=0.993 p USPIO exposure (μg/ml medium) 0 5 10 40 80 100 200 FFC-MR Mean R1 1x106 cell suspension (s–1) 0.308±0.014 0.356±0.013** 0.432±0.016** 0.706±0.021** 1.174±0.031** 1.239±0.033** 1.599±0.041** Flow cytometry USPIO area/cell area 0.034±0.001 0.036±0.001 0.037±0.001* 0.069±0.001** 0.085±0.001** 0.090±0.001** 0.097±0.001** Flow cytometry SSC intensity 26860 32815** 39573** 69285** 80967** 82693** 86373** Colorimetric assay Iron concentration 1x106 cells (ng/μg protein) 0.115±0.118 1.121±0.045** 2.074±0.084** 5.496±0.134** 8.421±0.269** 9.771±0.100** 12.398±0.233** SSC = side-scattered light; *p Conclusion Field-cycling MR is capable of highly accurate intracellular USPIO quantification, which has potential to non-invasively detect clinically relevant amounts of iron in inflammatory cardiovascular diseases. Acknowledgement/Funding NHS Grampian Endowment Fund

A model of spin-lattice relaxation for spin-1/2 nuclei in the presence of a residual dipole-dipole coupling has been presented. For slow dynamics the model predicts a bi-exponential relaxation at low frequencies, when the residual dipole-dipole interaction dominates the Zeeman coupling. Moreover, according to the model a frequency-specific relaxation enhancement, referred to as Dipolar Relaxation Enhancement (DRE) in analogy to the Quadrupole Relaxation Enhancement (QRE) is expected. The frequency position of the relaxation maximum is determined by the amplitude of the residual dipole-dipole interaction. Experimental examples of relaxation properties that might be attributed to the DRE are presented. The DRE effect has the potential to be exploited, in analogy to QRE, as a unique source of information about molecular dynamics and structure.

Graphical abstract, Highlights • FFC-MR can detect and quantify dependence of macrophage USPIO R1 dispersion in vitro. • Inflammation assessment in vivo and the utility of USPIO may be enhanced by FFC-MR. • Accessing tissue T1 dispersion properties may complement FFC-MR imaging techniques., Purpose Inflammation is central in disease pathophysiology and accurate methods for its detection and quantification are increasingly required to guide diagnosis and therapy. Here we explored the ability of Fast Field-Cycling Magnetic Resonance (FFC-MR) in quantifying the signal of ultra-small superparamagnetic iron oxide particles (USPIO) phagocytosed by J774 macrophage-like cells as a proof-of-principle. Methods Relaxation rates were measured in suspensions of J774 macrophage-like cells loaded with USPIO (0–200 μg/ml Fe as ferumoxytol), using a 0.25 T FFC benchtop relaxometer and a human whole-body, in-house built 0.2 T FFC-MR prototype system with a custom test tube coil. Identical non-imaging, saturation recovery pulse sequence with 90° flip angle and 20 different evolution fields selected logarithmically between 80 μT and 0.2 T (3.4 kHz and 8.51 MHz proton Larmor frequency [PLF] respectively). Results were compared with imaging flow cytometry quantification of side scatter intensity and USPIO-occupied cell area. A reference colorimetric iron assay was used. Results The T1 dispersion curves derived from FFC-MR were excellent in detecting USPIO at all concentrations examined (0–200 μg/ml Fe as ferumoxytol) vs. control cells, p ≤ 0.001. FFC-NMR was capable of reliably detecting cellular iron content as low as 1.12 ng/µg cell protein, validated using a colorimetric assay. FFC-MR was comparable to imaging flow cytometry quantification of side scatter intensity but superior to USPIO-occupied cell area, the latter being only sensitive at exposures ≥ 10 µg/ml USPIO. Conclusions We demonstrated for the first time that FFC-MR is capable of quantitative assessment of intra-cellular iron which will have important implications for the use of USPIO in a variety of biological applications, including the study of inflammation.

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.

Purpose Fast Field-Cycling MRI (FFC-MRI) is a novel MRI technique in which the external magnetic field ( B 0 ) is switched during the imaging experiment, always returning to the same value ( B 0 D ) for signal detection. By doing this, FFC-MRI grants access to information which is invisible to conventional MRI scanners, including the variation of T 1 with magnetic field. These measurements, known as T 1 -dispersion, exhibit great promise as a new form of endogenous image contrast, and may have application in the early diagnosis of a range of diseases. Construction of an MR imaging system capable of rapidly switching magnetic fields, and reaching ultra-low fields (200 μT or lower), requires novel magnets, power supplies and control electronics. Here we will describe progress on a whole-body human sized FFC imaging system and preliminary results from a clinical trial imaging acute stroke using FFC-MRI. Methods The magnet (Tesla Engineering Ltd, UK) is of a resistive design with a length of 2 m and an inner bore diameter of 500 mm; it is capable of achieving a maximum field strength ( B 0 D ) of 0.2 T (8.52 MHz proton Larmor frequency). The system can switch between zero and maximum field in 12 ms, corresponding to a maximum dB/dT of 16.7 T/s. The gradients and RF system are controlled by a commercial MRI console (MR Solutions Ltd, UK) while the main magnet coil, shim coils and earth’s-field cancellation coils (necessary for ultra-low field operation) are controlled by a dedicated embedded computer running in-house software written in Labview (National Instruments, US). Results The prototype system has been fully commissioned and is now operational. FFC-MRI imaging of patients (with full ethical permissions granted) using the system has now commenced as part of a clinical study on imaging acute stroke. It is hoped that the new information afforded by FFC-MRI will aid in the detection and assessment of stroke. Conclusions The novel system design described here will allow us to explore the unique T 1 dispersion contrast made available by FFC-MRI. Future work will concentrate on identifying how this newly accessible region of the T 1 dispersion curve can be exploited for clinical diagnosis.

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.

Introduction Diffusion refers to the random Brownian molecular motion. In a bulk liquid, water molecules are free to move and this is characterized by a large value of the diffusion constant, D. The motion of water molecules within cells may be restricted by the cell wall and other structures, resulting in a lower value of D; the same may be true of interstitial fluid in tissues if the cells are densely packed. Diffusion-Weighted Imaging (DWI) generates images based on the variation of D [1] . Diffusion-Weighted MRI DWI uses balanced pulses of magnetic field gradient, applied either side of a 180° pulse within a spin-echo pulse sequence, as introduced by Stejskal and Tanner in 1965 [2] . For stationary spins, any phase accrued during the first gradient pulse is unwound during the second pulse. Spins in random motion, however, become dephased, leading to a diffusion-dependent loss of signal. The signal can be given by S b , TE = M 0 exp - TE T 2 exp - bD where b is a factor which is dependent on the square of gradient strength ( G 2 ), its duration and spacing, known as the “b-value”; the stronger the value of b, the higher is the diffusion weighting [3] . b values in the range 0–1000 s/mm2 are typically used. In a DWI image with a large b-value, tissues containing freely-diffusing water molecules will appear dark (due to loss of signal), while tissues where diffusion is restricted will appear with greater intensity. Conclusions DWI is a standard part of the diagnostic armoury of clinical MRI, where one of its most important applications lies in the diagnosis of cancer. This lecture will describe the basic principles of DWI, with particular emphasis on its use in oncology [4] .

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.

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) 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.

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. >

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.

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.

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.

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.

PUBLISHED, A novel NMR technique for imaging materials exhibiting extremely short T2 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) radiofrequency irradiation of the sample together with continual detection, and thus the experimental deadtime 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 are outlined. A brief review of existing NMR imaging 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 (1H imaging) and brine (23Na imaging) into various cementitious materials are presented, together with images of the solid 27Al 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)., The authors would like to thank Dr. Gareth Davies and Prof. Jim Hutchison for help with various aspects of experimental design and construction, Prof. Fredrik Glasser for help and advice with the cement studies, Dr. Nikolaus Nestle (TU, Darmstadt) for collaboration with the bentonite work, and Dr. Yuanmu Deng (Ohio State University) for help with the reconstruction of the 3D dataset. We are also indebted to Mr. Eddie Stevenson and Mr. Peter Frew for workshop assistance. This work was funded by the UK Engineering and Physical Sciences Research Council (Grant Number GR/R02269/01).

The development of a 3-D, multi-nuclear continuous wave NMR imaging (CW-NMRI) system is described and its imaging capability is demonstrated on a range of materials exhibiting extremely short T2 relaxation values. A variety of radiofrequency resonators were constructed and incorporated into a new gradient and field offset coil assembly, while the overall system design was modified to minimise microphonic noise which was present in an earlier prototype system. The chemically combined 27Al in a high temperature refractory cement was imaged, and the CW-NMRI system was found to be sensitive to small differences in 27Al content in these samples. The penetration of 23Na in salt water into samples of ordinary Portland cement (OPC) was investigated, with enhanced uptake observed for samples with larger pore size distributions. The solid 13C component in a carbonated cement sample was also imaged, as were the 7Li nuclei in a sample of powdered Li2CO3. A spatial resolution of 1 mm was measured in an image of a rigid polymeric material exhibiting a principal T 2 ∗ value of 16.3 μs. Finally, a high-resolution 3-D image of this rigid polymer is presented.

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.

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.

PUBLISHED, A prototype continuous wave MRI system operating at 7T has been used successfully to study a variety of heterogeneous materials exhibiting T2 relaxation values ranging from 10 ls to 50 ms. Two-dimensional images of a poly(methly methacrylate) (PMMA) resolution phantom (T2 ?38 ls) exhibited a spatial resolution of approximately 1mm at a magneticfield gradient strength of 200 mT/m. The technique was used to study the hydration, drying, and subsequent water penetration properties of cement samples made from ordinary Portland cement, and revealed inhomogeneities arising from the cure conditions. Sandstone samples from an oil reservoir in the North Sea were also studied; structure within these materials, arising from the sedimentary bed layering in the reservoir, was found to have an effect on their water transport properties. A section from a confectionery bar (T#2;2 approximately 50?60 ms) was also imaged, and its internal structure could be clearly discerned., This work was funded by the UK Engineering and Physical Sciences Research Council (Grant No. GR/R02269/01). The authors thank Dr. H. Seton, Department of Biomedical Physics, for help with the measurements on the 4.7 T system, and Dr. S. Ogilvie, Department of Geology, University of Aberdeen, for supplying the sandstone samples.

Fast Field-Cycling (FFC) MRI is a novel technology that allows varying the main magnetic field BThe algorithm exploits the fact that ghosts caused by field instability manifest in image regions which should be signal free. The algorithm minimises the signal in the background by finding an optimum phase correction for each line of k-space and repeats the operation until the result converges, leaving the background free of signal.We showed the conditions for which the algorithm is robust and successfully applied it on images acquired on FFC-MRI scanners. The same algorithm can be used for various applications other than Fast Field-Cycling MRI.

Tumour-associated macrophages (TAM) are forced by cancer cells to adopt an anti-inflammatory phenotype and secrete factors to promote tumour invasion thus being responsible for poor patient outcome. The aim of this study is to develop a clinically applicable, non-invasive method to obtain a quantitative TAM detection in tumour tissue. The method is based on longitudinal proton relaxation rate (R1) measurements at low field (0.01–1 MHz) to assess the localization of ferumoxytol (clinical approved iron oxide particles) in TAM present in melanoma tumours, where R1 = 1/T1. R1 at low magnetic fields appears highly dependent on the intra or extra cellular localization of the nanoparticles thus allowing an unambiguous TAM quantification. R1 profiles were acquired on a Fast Field-Cycling relaxometer equipped with a 40 mm wide bore magnet and an 11 mm solenoid detection coil placed around the anatomical region of interest. The R1 values measured 3 h and 24 h after the injection were significantly different. At 24 h R1 exhibited a behavior similar to “in vitro” ferumoxytol-labelled J774A.1 macrophages whereas at 3 h, when the ferumoxytol distribution was extracellular, R1 exhibited higher values similar to that of free ferumoxytol in solution. This finding clearly indicated the intracellular localization of ferumoxytol at 24 h, as confirmed by histological analysis (Pearls and CD68 assays). This information could be hardly achievable from measurements at a single magnetic field and opens new horizons for cell tracking applications using FFC-MRI.

1H Nuclear Magnetic Resonance (NMR) relaxometry and Dielectric Spectroscopy (DS) have been exploited to investigate the dynamics of solid proteins. The experiments have been carried out in the frequency range of about 10 kHz-40 MHz for NMR relaxometry and 10−2Hz-20 MHz for DS. The data sets have been analyzed in terms of theoretical models allowing for a comparison of the correlation times revealed by NMR relaxometry and DS. The 1H spin–lattice relaxation profiles have been decomposed into relaxation contributions associated with 1H–1H and 1H-14N dipole – dipole interactions. The 1H–1H relaxation contribution has been interpreted in terms of three dynamical processes of time scales of 10−6s, 10−7s and 10−8s. It has turned out that the correlation times do not differ much among proteins and they are only weakly dependent on temperature. The analysis of DS relaxation spectra has also revealed three motional processes characterized by correlation times that considerably depend on temperature in contrast to those obtained from the 1H relaxation. This finding suggest that for solid proteins there is a contribution to the 1H spin–lattice relaxation associated with a kind of motion that is not probed in DS as it does not lead to a reorientation of the electric dipole moment.