A flexible 8.5 MHz litz wire receive array for field-cycling imaging

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.

Low-Field NMR Relaxometry for Intraoperative Tumour Margin Assessment in Breast-Conserving Surgery

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.

A new method for investigating osteoarthritis using Fast Field-Cycling nuclear magnetic resonance

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.

Monitoring tissue implants by field-cycling 1H-MRI via the detection of changes in the 14N-quadrupolar-peak from imidazole moieties incorporated in a "smart" scaffold material

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.

1H spin-lattice NMR relaxation in the presence of residual dipolar interactions - Dipolar relaxation enhancement

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.

Slow dynamics of solid proteins - Nuclear magnetic resonance relaxometry versus dielectric spectroscopy

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.

Fast field-cycling magnetic resonance detection of intracellular ultra-small iron oxide particles in vitro: Proof-of-concept

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.

Mechanism of Water Dynamics in Hyaluronic Dermal Fillers Revealed by Nuclear Magnetic Resonance Relaxometry

1 H 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.

4324Fast-field cycling magnetic resonance detection of intracellular iron in the nanomolar range - A pre-requisite for in-vivo study of inflammation

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<0.05.

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<0.001) and good fit on linear regression model (R2=0.9222, Fig 1C). Flow cytometry quantification of SCC was comparable (p≤0.001 for all), whereas USPIO-occupied area was only sensitive at exposures ≥10μg/ml USPIO (r=0.967 p<0.001 & r=0.983 p<0.001 vs. colorimetric respectively). FFC-MR detected iron at ≥1.12ng/μg protein.

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<0.05 vs. control; **p<0.001 vs. control.

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

In Vivo Application of Proton-Electron Double-Resonance Imaging

SIGNIFICANCE

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 (pO2), tissue permeability, redox status, and acid-base balance. Recent Advances: Local acid-base balance can be imaged by exploiting the different resonance frequency of radical probes between R and RH+ forms. Redox status can also be imaged by using the loss of radical-related signal after reduction. These methods require optimized radical probes and pulse sequences.

CRITICAL ISSUES

High-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.

FUTURE DIRECTIONS

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.

Correction of environmental magnetic fields for the acquisition of Nuclear magnetic relaxation dispersion profiles below Earth's field

T₁ relaxation times can be measured at a range of magnetic field strengths by Fast Field-Cycling (FFC) NMR relaxometry to provide T₁-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 (10kHz 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 2.3μT (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.

Simple algorithm for the correction of MRI image artefacts due to random phase fluctuations

PURPOSE

Fast Field-Cycling (FFC) MRI is a novel technology that allows varying the main magnetic field B₀ during the pulse sequence, from the nominal field (usually hundreds of millitesla) down to Earth's field or below. This technique uses resistive magnets powered by fast amplifiers. One of the challenges with this method is to stabilise the magnetic field during the acquisition of the NMR signal. Indeed, a typical consequence of field instability is small, random phase variations between each line of k-space resulting in artefacts, similar to those which occur due to homogeneous motion but harder to correct as no assumption can be made about the phase error, which appears completely random. Here we propose an algorithm that can correct for the random phase variations induced by field instabilities without prior knowledge about the phase error.

METHODS

The 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.

CONCLUSION

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.

Rapid multi-field T(1) estimation algorithm for Fast Field-Cycling MRI

Fast Field-Cycling MRI (FFC-MRI) is an emerging MRI technique that allows the main magnetic field to vary, allowing probing T1 at various magnetic field strengths. This technique offers promising possibilities but requires long scan times to improve the signal-to-noise ratio. This paper presents an algorithm derived from the two-point method proposed by Edelstein that can estimate T1 using only one image per field, thereby shortening the scan time by a factor of nearly two, taking advantage of the fact that the equilibrium magnetisation is proportional to the magnetic field strength. Therefore the equilibrium magnetisation only needs measuring once, then T1 can be found from inversion recovery experiments using the Bloch equations. The precision and accuracy of the algorithm are estimated using both simulated and experimental data, by Monte-Carlo simulations and by comparison with standard techniques on a phantom. The results are acceptable but usage is limited to the case where variations of the main magnetic field are fast compared with T1 and where the dispersion curve is relatively linear. The speed-up of T1-dispersion measurements resulting from the new method is likely to make FFC-MRI more acceptable when it is applied in the clinic.

In vivo field-cycling relaxometry using an insert coil for magnetic field offset

PURPOSE

The T(1) of tissue has a strong dependence on the measurement magnetic field strength. T(1) -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 T(1) -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 T(1) -dispersion in a human volunteer's forearm over a range of field strengths from 1 mT to 70 mT.

CONCLUSION

T(1) -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 T(1) -dispersion contrast for enhanced diagnosis.

Detection of osteoarthritis in knee and hip joints by fast field-cycling NMR

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.

Measurement of fibrin concentration by fast field-cycling NMR

The relaxation of (1)H nuclei due to their interaction with quadrupolar (14)N nuclei in gel structures is measured using fast field-cycling NMR. This phenomenon called quadrupolar dips has been reported in different (1)H-(14)N 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. T(1)-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 (14)N 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.

The use of contrast agents with fast field-cycling magnetic resonance imaging

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.

Design and construction of an actively frequency-switchable RF coil for field-dependent Magnetisation Transfer Contrast MRI with fast field-cycling

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.

A novel variable field system for field-cycled dynamic nuclear polarization spectroscopy

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.

Off-resonance magnetisation transfer contrast (MTC) MRI using fast field-cycling (FFC)

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.

Real-time monitoring of drug-induced changes in the stomach acidity of living rats using improved pH-sensitive nitroxides and low-field EPR techniques

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 NO 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 pKa 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-1H-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.

Sequential, co-registered fluorine and proton field-cycled Overhauser imaging at a detection field of 59 mT

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.

Development of a 3-D, multi-nuclear continuous wave NMR imaging system

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 T(2) 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 (27)Al in a high temperature refractory cement was imaged, and the CW-NMRI system was found to be sensitive to small differences in (27)Al content in these samples. The penetration of (23)Na 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 (13)C component in a carbonated cement sample was also imaged, as were the (7)Li nuclei in a sample of powdered Li(2)CO(3). A spatial resolution of 1mm was measured in an image of a rigid polymeric material exhibiting a principal T( *)(2) value of 16.3 micros. Finally, a high-resolution 3-D image of this rigid polymer is presented.

Monitoring drug delivery processes by EPR and related techniques--principles and applications

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.

Continuous wave MRI diffusion study of water in bentonite clay

Three-layered clay minerals such as montmorillonite (bentonite) exhibit very short transverse relaxation times. This is especially true for samples with relatively low water contents in the region of 20% to 30%, which is the water content typically used in environmental technology applications (e.g., as a mineral liner material for landfills). The diffusion of water in samples with such short transverse relaxation times can be measured with NMR by observing the moisture gradients or isotope tracer fronts propagating through appropriately prepared samples by means of continuous wave MRI. The first results from such studies on bentonite clays are presented in this paper.

Field-cycled PEDRI imaging of free radicals with detection at 450 mT

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.

Proton electron double resonance imaging of free radical distribution in environmental science applications—first results and perspectives

In this contribution, we explore the potential of proton electron double resonance imaging (PEDRI) in environmental science (hydrogeological) applications. After a discussion of the hydrogeological motivation for studies of free radical transport in environmental matrices, we present results from first experiments that show the principal applicability of the PEDRI technique to sediment samples. Field-cycled (FC) relaxation time contrast is identified as a possible source of artifacts in samples in which strong concentration gradients of the free radical phase are present. Furthermore, an outlook is given on how PEDRI can help in observations of the local interplay among contaminants, water and nonaqueous liquid phases.

In vitro and in vivo measurement of pH and thiols by EPR-based techniques

In vitro and in vivo measurements of pH and thiols provide critical information on physiology and pathophysiology of living organisms, particularly related to oxidative stress. Stable nitroxides of imidazoline and imidazolidine types provide the unique possibility of measuring local values of pH and glutathione content in various biological systems, including in vivo studies. The basis for these applications is the observation of specific chemical reactions of these nitroxides with protons or thiols, followed by significant changes in the electron paramagnetic resonance (EPR) spectra of these probes, measured by low-frequency EPR techniques. The applications of some newly developed pH and SH probes in model systems of pharmacological interest, biological fluids, tissues, and cells as well as in vivo studies in isolated hearts and in the gut of living animals are discussed.

Continuous wave MRI of heterogeneous materials

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 micros to 50 ms. Two-dimensional images of a poly(methly methacrylate) (PMMA) resolution phantom (T2=38 micros) exhibited a spatial resolution of approximately 1mm at a magnetic field 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 (T2* approximately 50-60 ms) was also imaged, and its internal structure could be clearly discerned.

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

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

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

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

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

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

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

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

Optimization of field-cycled PEDRI for in vivo imaging of free radicals

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.

Continuous-wave magnetic resonance imaging of short T(2) materials

There is growing interest in the use of magnetic resonance imaging (MRI) to examine solid materials where the restricted motion of the probed spins leads to broad lines and short T(2) values, rendering many interesting systems invisible to conventional 2DFT pulsed imaging methods. In EPR T(2) seldom exceeds 0.1 mus and continuous-wave methods are adopted for spectroscopy and imaging. In this paper we demonstrate the use of continuous-wave MRI to obtain 2-dimensional images of short T(2) samples. The prototype system can image samples up to 50 mm in diameter by 60 mm long and has been used to image polymers and water penetration in porous media. Typical acquisition times range between 10 and 40 min. Resolution of 1 to 2 mm has been achieved for samples with T(2) values ranging from 38 to 750 mus. There is the possibility of producing image contrast that is determined by the material properties of the sample.

Biological applications of spin pH probes

The determination of pH is one of the most important problems in the biochemistry of living organisms, since many of the vital processes of cells and cellular organelles depend on the local pH value. Amongst currently used experimental approaches for the measurement of pH, the application of spin pH probes in combination with EPR spectroscopy is a comparatively new and rapidly developing field. In this article we describe the background, advantages and limitations of the method of spin pH probes, and discuss its recent applications. Availability of a wide variety of pH-sensitive nitroxides with different ranges of pH-sensitivity, labeling group and lipophilicity facilitates their application to a variety of biological systems from subcellular organelles to complex organisms. The recent progress in low-field EPR-based imaging and spectroscopy-based techniques allows spin pH probes to be used for non-invasive in vivo pH measurement and pH-sensitive imaging.

Rapid imaging of free radicals in vivo using field cycled PEDRI

Imaging of free radicals in vivo using an interleaved field-cycled proton-electron double-resonance imaging (FC-PEDRI) pulse sequence has recently been investigated. In this work, in order to reduce the EPR (electron paramagnetic resonance) irradiation power required and the imaging time, a centric reordered snapshot FC-PEDRI pulse sequence has been implemented. This is based on the FLASH pulse sequence with a very short repetition time and the use of centric reordering of the phase-encoding gradient, allowing the most significant free induction decay (FID) signals to be collected before the signal enhancement decays significantly. A new technique of signal phaseshift correction was required to eliminate ghost artefacts caused by the instability of the main magnetic field after field cycling. An FID amplitude correction scheme has also been implemented to reduce edge enhancement artefacts caused by the rapid change of magnetization population before reaching the steady state. Using the rapid pulse sequence, the time required for acquisition of a 64 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.