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

Field cycling imaging to characterise breast cancer at low and ultra-low magnetic fields below 0.2 T

BACKGROUND

This prospective feasibility study explores Field-Cycling Imaging (FCI), a new MRI technology that measures the longitudinal relaxation time across a range of low magnetic field strengths, providing additional information about the molecular properties of tissues. This study aims to assess the performance of FCI and investigate new quantitative biomarkers at low fields within the context of breast cancer.

METHODS

We conducted a study involving 9 people living with breast cancer (10 tumours in total, mean age, 54 ± 10 years). FCI images were obtained at four magnetic field strengths (2.3 mT to 200 mT). FCI images were processed to generate T1 maps and 1/T1 dispersion profiles from regions of tumour, normal adipose tissue, and glandular tissue. The dispersion profiles were subsequently fitted using a power law model. Statistical analysis focused on comparing potential FCI biomarkers using a Mann-Whitney U or Wilcoxon signed rank test.

RESULTS

We show that low magnetic fields clearly differentiate tumours from adipose and glandular tissues without contrast agents, particularly at 22 mT (1/T1, median [IQR]: 6.8 [3.9-7.8] s-1 vs 9.1 [8.9-10.2] s-1 vs 8.1 [6.2-9.2] s-1, P < 0.01), where the tumour-to-background contrast ratio was highest (62%). Additionally, 1/T1 dispersion indicated a potential to discriminate invasive from non-invasive cancers (median [IQR]: 0.05 [0.03-0.09] vs 0.19 [0.09-0.26], P = 0.038).

CONCLUSIONS

To the best of our knowledge, we described the first application of in vivo FCI in breast cancer, demonstrating relevant biomarkers that could complement diagnosis of current imaging modalities, non-invasively and without contrast agents.

Water Dynamics in Fish Collagen Gels-Insight from NMR Relaxometry

1H spin-lattice relaxation experiments have been performed for gels based on fish collagen in order to analyze water dynamics. The covered frequency range ranges from 10 kHz to 10 MHz; in some cases, the temperature has varied as well. The relaxation data have been reproduced in terms of two models of water motion-a model including two relaxation contributions associated with the diffusion of water molecules on the macromolecular surfaces and a second model being just a phenomenological power law. The concept of surface diffusion has led to a very good agreement with the experimental data and a consistent set of parameters, with the diffusion coefficients being about five orders of magnitude slower compared to bulk water for one of the pools and considerably faster for the second one (smaller by factors between 2 and 20 compared to bulk water). In some cases, the attempt to reproduce the data in terms of a power law has led to a good agreement with the experimental data (the power law factor varying between 0.41 and 0.57); however, in other cases, the discrepancies are significant. This outcome favors the concept of surface diffusion.

Field-Cycling MRI for Identifying Minor Ischemic Stroke Below 0.2 T

Background Field-cycling imaging (FCI) is a new technology developed at the University of Aberdeen that measures change in T1 relaxation time constant of tissues over a range of low magnetic field strengths (0.2-200 mT) by rapidly switching between different fields during the pulse sequence. This provides new sources of contrast, including some invisible to clinical MRI scanners, and may be a useful alternative imaging modality for stroke. Purpose To test whether a prototype whole-body FCI scanner can be used to identify infarct regions in patients with subacute ischemic stroke. Materials and Methods This prospective study screened consecutive adult patients admitted to a single center stroke unit from February 2018 to March 2020 and April to December 2021. Included participants with confirmed ischemic stroke underwent FCI 1-6 days after ictus. FCI scans were obtained at four to six evolution fields between 0.2 mT and 0.2 T, with five evolution times from 5 to 546 msec. T1 maps were generated. The Wilcoxon signed-rank test was used to compare infarct region and contralateral unaffected brain, and Spearman rank correlation was used to examine associations between infarct to contralateral tissue contrast ratio and field strengths. Two independent readers blinded to clinical images rated the FCI scans. Results Nine participants (mean age, 62 years ± 16 [SD]; all male) successfully completed FCI. FCI scans below 0.2 T exhibited hyperintense T1 regions corresponding to the infarct region identified at baseline imaging, visually confirmed with 86% interrater agreement (Cohen κ = 0.69). Infarct to contralateral tissue contrast ratio increased as magnetic field decreased between 0.2 mT and 0.2 T (r[24] = -0.68; P < .001). T1 dispersion slopes differed between infarct and unaffected tissues (median, 0.23 [IQR, 0.18-0.37] vs 0.35 [IQR, 0.27-0.43]; P = .03). Conclusion Whole-brain FCI can be used to identify subacute ischemic stroke by T1 relaxation mechanisms at field strengths as low as 0.2 mT. Research Registry no. 1813 Published under a CC BY 4.0 license. Supplemental material is available for this article.

Fast-field-cycling NMR at very low magnetic fields: water molecular dynamic biomarkers of glioma cell invasion and migration

Our objective was to study NMR relaxometry of glioma invasion/migration at very low field (<2 mT) by fast-field-cycling NMR (FFC-NMR) and to decipher the pathophysiological processes of glioma that are responsible for relaxation changes in order to open a new diagnostic method that can be extended to imaging. The phenotypes of two new glioma mouse models, Glio6 and Glio96, were characterized by T_2w -MRI, HE histology, Ki-67 immunohistochemistry (IHC) and CXCR4 RT-qPCR, and were compared with the U87 model. R₁ dispersions of glioma tissues were acquired at low field (0.1 mT-0.8 T) ex vivo and were fitted with Lorentzian and power-law models to extract FFC biomarkers related to the molecular dynamics of water. In order to decipher relaxation changes, three main invasion/migration pathophysiological processes were studied: hypoxia, H₂ O₂ function and the water-channel aquaporin-4 (AQP4). Glio6 and Glio96 were characterized with invasion/migration phenotype and U87 with high cell proliferation as a solid glioma. At very low field, invasion/migration versus proliferation was characterized by a decrease in the relaxation-rate constant (ΔR₁  ≈ -32% at 0.1 mT) and correlation time (≈-40%). These decreases corroborated the AQP4-IHC overexpression (Glio6/Glio96: +92%/+46%), suggesting rapid transcytolemmal water exchange, which was confirmed by the intracellular water-lifetime τ_IN decrease (Δτ_IN  ≈ -30%). In functional experiments, AQP4 expression, τ_IN and the relaxation-rate constant at very low field were all found to be sensitive to hypoxia and to H₂ O₂ stimuli. At very low field the role of water exchanges in relaxation modulation was confirmed, and for the first time it was linked to the glioma invasion/migration and to its main pathophysiological processes: hypoxia, H₂ O₂ redox signaling and AQP4 expression. The method appears appropriate to evaluate the effect of drugs that can target these pathophysiological mechanisms. Finally, FFC-NMR operating at low field is demonstrated to be sensitive to invasion glioma phenotype and can be straightforwardly extended to FFC-MRI as a new cancer invasion imaging method in the clinic.

Intracellular Water Lifetime as a Tumor Biomarker to Monitor Doxorubicin Treatment via FFC-Relaxometry in a Breast Cancer Model

This study aims to explore whether the water exchange rate constants in tumor cells can act as a hallmark of pathology status and a reporter of therapeutic outcomes. It has been shown, using 4T1 cell cultures and murine allografts, that an early assessment of the therapeutic effect of doxorubicin can be detected through changes in the cellular water efflux rate constant kio. The latter has been estimated by analyzing the magnetization recovery curve in standard NMR T1 measurements when there is a marked difference in the proton relaxation rate constants (R1) between the intra- and the extra-cellular compartments. In cellular studies, T1 measurements were carried out on a relaxometer working at 0.5 T, and the required difference in R1 between the two compartments was achieved via the addition of a paramagnetic agent into the extracellular compartment. For in-vivo experiments, the large difference in the R1 values of the two-compartments was achieved when the T1 measurements were carried out at low magnetic field strengths. This task was accomplished using a Fast Field Cycling (FFC) relaxometer that was properly modified to host a mouse in its probe head. The decrease in kio upon the administration of doxorubicin is the result of the decreased activity of Na+/K+-ATPase, as shown in an independent test on the cellular uptake of Rb ions. The results reported herein suggest that kio can be considered a non-invasive, early and predictive biomarker for the identification of responsive patients immediately from the first doxorubicin treatment.

Nuclear relaxation rate enhancement by a 14N quadrupole nucleus in a fluctuating electric-field gradient

We consider the longitudinal quadrupole relaxation rate enhancement (QRE) of a ¹H nucleus due to the time fluctuations of the local dipolar magnetic field created by a close quadrupole ¹⁴N nucleus, the electric-field gradient (EFG) Hamiltonian of which changes with time because of vibrations/distortions of its chemical environment. The QRE is analytically expressed as a linear combination of the cosine Fourier transforms of the three quantum time auto-correlation functions G_AA(t) of the ¹⁴N spin components along the principal axes A = X, Y, and Z of the mean (time-averaged) EFG Hamiltonian. Denoting the three transition frequencies between the energy levels of this mean Hamiltonian by ν_A, the functions G_AA(t) oscillate at frequencies ν_A + s_A/(2π) with mono-exponential decays of relaxation times τ_A, where the frequency dynamic shifts s_A and the relaxation times τ_A are closed expressions of the magnitude of the fluctuations of the instantaneous EFG Hamiltonian about its mean and of the characteristic fluctuation time. Thus, the theoretical QRE is the sum of three Lorentzian peaks centered at ν_A + s_A/(2π) with full widths at half maxima 1/(πτ_A). The predicted peak widths are nearly equal. The predicted dynamic shifts of the peaks are much smaller than their widths and amazingly keep proportional to the transition frequencies ν_A for reasonably fast EFG fluctuations. The theory is further improved by correcting the transition frequencies by the ¹⁴N Zeeman effects of second order. It is successfully applied to reinterpret the QRE pattern measured by Broche, Ashcroft, and Lurie [Magn. Reson. Med. 68, 358 (2012)] in normal cartilage.

Low-field and variable-field NMR relaxation studies of H2O and D2O molecular dynamics in articular cartilage

Osteoarthritis (OA) as the main degenerative disease of articular cartilage in joints is accompanied by structural and compositional changes in the tissue. Degeneration is a consequence of a reduction of the amount of macromolecules, the so-called proteoglycans, and of a corresponding increase in water content, both leading to structural weakening of cartilage. NMR investigations of cartilage generally address only the relaxation properties of water. In this study, two-dimensional (T1-T2) measurements of bovine articular cartilage samples were carried out for different stages of hydration, complemented by molecular exchange with D2O and treatment by trypsin which simulates degeneration by OA. Two signal components were identified in all measurements, characterized by very different T2 which suggests liquid-like and solid-like dynamics. These measurements allow the quantification of separate hydrogen components and their assignment to defined physical pools which had been discussed repeatedly in the literature, i.e. bulk-like water and a combination of protein hydrogens and strongly bound water. The first determination of 2H relaxation dispersion in comparison to 1H dispersion suggests intramolecular interactions as the dominating source for the pronounced magnetic field dependence of the longitudinal relaxation time T1.

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.

Joint multi-field T1 quantification for fast field-cycling MRI

Purpose

Recent developments in hardware design enable the use of fast field‐cycling (FFC) techniques in MRI to exploit the different relaxation rates at very low field strength, achieving novel contrast. The method opens new avenues for in vivo characterizations of pathologies but at the expense of longer acquisition times. To mitigate this, we propose a model‐based reconstruction method that fully exploits the high information redundancy offered by FFC methods.

Methods

The proposed model‐based approach uses joint spatial information from all fields by means of a Frobenius ‐ total generalized variation regularization. The algorithm was tested on brain stroke images, both simulated and acquired from FFC patients scans using an FFC spin echo sequences. The results are compared to three non‐linear least squares fits with progressively increasing complexity.

Results

The proposed method shows excellent abilities to remove noise while maintaining sharp image features with large signal‐to‐noise ratio gains at low‐field images, clearly outperforming the reference approach. Especially patient data show huge improvements in visual appearance over all fields.

Conclusion

The proposed reconstruction technique largely improves FFC image quality, further pushing this new technology toward clinical standards.

Towards applying NMR relaxometry as a diagnostic tool for bone and soft tissue sarcomas: a pilot study

This work explores what Fast Field-Cycling Nuclear Magnetic Resonance (FFC-NMR) relaxometry brings for the study of sarcoma to guide future in vivo analyses of patients. We present the results of an ex vivo pilot study involving 10 cases of biopsy-proven sarcoma and we propose a quantitative method to analyse 1H NMR relaxation dispersion profiles based on a model-free approach describing the main dynamical processes in the tissues and assessing the amplitude of the Quadrupole Relaxation Enhancement effects due to 14N. This approach showed five distinct groups of dispersion profiles indicating five discrete categories of sarcoma, with differences attributable to microstructure and rigidity. Data from tissues surrounding sarcomas indicated very significant variations with the proximity to tumour, which may be attributed to varying water content but also to tissue remodelling processes due to the sarcoma. This pilot study illustrates the potential of FFC relaxometry for the detection and characterisation of sarcoma.

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.

Design of a fast field-cycling magnetic resonance imaging system, characterization and methods for relaxation dispersion measurements around 1.5 T

The dependence of the nuclear magnetic resonance relaxation rate on the magnetic field has been widely studied, in particular, in biomedical areas with the objectives to better understand the underlying microscopic mechanisms in tissues and provide biomarkers of diseases. By combining fast-field cycling (FFC) and magnetic resonance imaging (MRI), it is possible to provide localized relaxation dispersion measurements in heterogeneous systems with recent demonstrations in solutions, biological samples, human beings, and small animals. We report here the developments and performances of a device designed for small animal FFC-MRI comprising a resistive insert technology operating inside a 1.5 T MRI system. Specific measurement methods were developed to characterize the system efficiency, response time, homogeneity, stability, and compensation. By adding a non-linear element in the system and using a dual amplifier strategy, it is shown that large field offsets can be produced during relaxation periods while maintaining precise field control during detection periods. The measurement of longitudinal nuclear magnetic relaxation dispersion (NMRD) profiles in the range of 1.08 T-1.92 T is reported, essentially displaying a linear variation in this range for common MRI contrast agents. The slopes of both the longitudinal and transverse relaxation dispersion profiles at 1.5 T are measured and validated, extending the capabilities of previous approaches. The performances of a longitudinal relaxation dispersion mapping method are finally reported, opening the way to quantitative preclinical dispersion imaging studies at a high FFC-MRI field.

In vivo assessment of tumour associated macrophages in murine melanoma obtained by low-field relaxometry in the presence of iron oxide particles

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.

Exploring the tumour extracellular matrix by in vivo Fast Field Cycling relaxometry after the administration of a Gadolinium-based MRI contrast agent

¹ H Fast Field Cycling NMR (FFC-NMR) relaxometry is proposed as a powerful method to investigate tumour stroma in vivo upon the administration of a Gd-based contrast agent. To perform this study, an FFC-NMR equipment endowed with a wide bore magnet was used for the acquisition of Nuclear Magnetic Resonance Dispersion profiles on healthy muscle and tumour tissue in living mice. At magnetic field strengths < of ca. 1 MHz, the differences in the relaxation rates of the intra and extracellular compartment become of the same order of magnitude of the exchange rate across the cellular membranes. Under this condition, the water exchange rate between the two compartments yields to a biexponential magnetization recovery that can be analysed by fitting the experimental data with the two-Site eXchange (2SX) model. Using this model, it was possible to obtain, for the two compartments, both relaxation properties and water kinetic constants for water exchange across cell membranes. The method allowed us to determine the effect of the "matrix" on the water proton relaxation times and, in turn, to get some insights of the composition of this compartment, till now, largely unknown.

A whole-body Fast Field-Cycling scanner for clinical molecular imaging studies

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.

FFC NMR Relaxometer with Magnetic Flux Density Control

This paper describes an innovative solution for the power supply of a fast field cycling (FFC) nuclear magnetic resonance (NMR) spectrometer considering its low power consumption, portability and low cost. In FFC cores, the magnetic flux density must be controlled in order to perform magnetic flux density cycles with short transients, while maintaining the magnetic flux density levels with high accuracy and homogeneity. Typical solutions in the FFC NMR literature use current control to get the required magnetic flux density cycles, which correspond to an indirect magnetic flux density control. The main feature of this new relaxometer is the direct control of the magnetic flux density instead of the magnet current, in contrast with other equipment available in the market. This feature is a great progress because it improves the performance. With this solution it is possible to compensate magnetic field disturbances and parasitic magnetic fields guaranteeing, among other possibilities, a field control below the earth magnetic field. Experimental results validating the developed solution and illustrating the real operation of this type of equipment are shown.

Multi-quantum quadrupole relaxation enhancement effects in 209Bi compounds

¹H spin-lattice nuclear magnetic resonance relaxation experiments have been performed for triphenylbismuth dichloride (C₁₈H₁₅BiCl₂) and phenylbismuth dichloride (C₆H₅BiCl₂) in powder. The frequency range of 20-128 MHz has been covered. Due to ¹H-²⁰⁹Bi dipole-dipole interactions, a rich set of pronounced Quadrupole Relaxation Enhancement (QRE) peaks (quadrupole peaks) has been observed. The QRE patterns for both compounds have been explained in terms of single- and double-quantum transitions of the participating nuclei. The analysis has revealed a complex, quantum-mechanical mechanism of the QRE effects. The mechanism goes far beyond the simple explanation of the existence of three quadrupole peaks for ¹⁴N reported in literature. The analysis has been supported by nuclear quadrupole resonance results that independently provided the ²⁰⁹Bi quadrupole parameters (amplitude of the quadrupole coupling constant and asymmetry parameter).

Use of FCC-NMRD relaxometry for early detection and characterization of ex-vivo murine breast cancer

Breast Cancer is the most diffuse cancer among women and the treatment outcome is largely determined by its early detection. MRI at fixed magnetic field is already widely used for cancer detection. Herein it is shown that the acquisition of proton T₁ at different magnetic fields adds further advantages. In fact, Fast Field Cycling Nuclear Magnetic Resonance Dispersion (FFC-NMRD) profiles have been shown to act as a high -sensitivity tool for cancer detection and staging in ex vivo murine breast tissues collected from Balb/NeuT mice. From NMRD profiles it was possible to extract two new cancer biomarkers, namely: (i) the appearance of ¹⁴N-quadrupolar peaks (QPs) reporting on tumor onset and (ii) the slope of the NMRD profile reporting on the progression of the tumor. By this approach it was possible to detect the presence of tumor in transgenic NeuT mice at a very early stage (5-7 weeks), when the disease is not yet detectable by using conventional high field (7 T) MRI and only minimal abnormalities are present in histological assays. These results show that, NMRD profiles may represent a useful tool for early breast cancer detection and for getting more insight into an accurate tumor phenotyping, highlighting changes in composition of the mammary gland tissue (lipids/proteins/water) occurring during the development of the neoplasia.

Multicomponent analysis of T1 relaxation in bovine articular cartilage at low magnetic fields

Purpose

The multi‐exponential character of T₁ relaxation in bovine articular cartilage was investigated at low magnetic fields below 0.5 T. The ultimate aim was to identify a parameter based on the T₁ relaxation time distribution as a biomarker to biochemical features of osteoarthritis.

Methods

Osteoarthritis conditions were simulated by enzymatic digestion of cartilage with trypsin. Fast‐field cycling NMR relaxometry was carried out in the magnetic field range B₀ = 70 μT to 600 mT. The data were analyzed in terms of T₁ distributions on a log‐time scale using inverse Laplace transform, whereas integral properties such as mean T₁s and distribution widths were obtained without data inversion from logarithmic moment analysis and a stretched‐exponential fit to the data. Attempts were also made to differentiate between water dynamic components through multi‐Lorentzian decomposition of average relaxation‐rate dispersions.

Results

T₁ distribution in bovine articular cartilage was found to be bimodal, with the dominating, long component shifting toward larger values following trypsin digestion. The effect is more prominent toward lower magnetic field strength. This shift leads to an overall increase of the distribution width and an equivalently more pronounced deviation from exponential behavior.

Conclusion

The logarithmic width of T₁ distribution functions at fields of 0.5 T and below, and the stretched‐exponential decay fit exponent β, show a significant trend after trypsin digestion of cartilage. These 2 parameters are suggested as possible biomarkers for osteoarthritis in humans and can be acquired entirely in vivo, with increasing significance for lower magnetic field strengths.

Evidence for the Role of Intracellular Water Lifetime as a Tumour Biomarker Obtained by In Vivo Field-Cycling Relaxometry

It was established through in vivo T1 measurements at low magnetic fields that tumour cells display proton T1 values that are markedly longer than those shown by healthy tissue. Moreover, it has been found that the elongation of T1 parallels the aggressiveness of the investigated tumour. The T1 lengthening is associated with an enhanced water exchange rate across the transcytolemmal membrane through an overexpression/upregulation of GLUT1 and Na+ /K+ ATPase transporters. It follows that the intracellular water lifetime represents a hallmark of tumour cells that can be easily monitored by measuring T1 at different magnetic field strengths ranging from 0.2 to 200 mT.

R1 dispersion contrast at high field with fast field-cycling MRI

Contrast agents with a strong R₁ dispersion have been shown to be effective in generating target-specific contrast in MRI. The utilization of this R₁ field dependence requires the adaptation of an MRI scanner for fast field-cycling (FFC). Here, we present the first implementation and validation of FFC-MRI at a clinical field strength of 3 T. A field-cycling range of ±100 mT around the nominal B₀ field was realized by inserting an additional insert coil into an otherwise conventional MRI system. System validation was successfully performed with selected iron oxide magnetic nanoparticles and comparison to FFC-NMR relaxometry measurements. Furthermore, we show proof-of-principle R₁ dispersion imaging and demonstrate the capability of generating R₁ dispersion contrast at high field with suppressed background signal. With the presented ready-to-use hardware setup it is possible to investigate MRI contrast agents with a strong R₁ dispersion at a field strength of 3 T.

209Bi quadrupole relaxation enhancement in solids as a step towards new contrast mechanisms in magnetic resonance imaging

²⁰⁹Bi containing species have the potential to become novel contrast agents for MRI.

Nuclear magnetic relaxation dispersion of murine tissue for development of T1 (R1 ) dispersion contrast imaging

This study quantified the spin-lattice relaxation rate (R₁ ) dispersion of murine tissues from 0.24 mT to 3 T. A combination of ex vivo and in vivo spin-lattice relaxation rate measurements were acquired for murine tissue. Selected brain, liver, kidney, muscle, and fat tissues were excised and R₁ dispersion profiles were acquired from 0.24 mT to 1.0 T at 37 °C, using a fast field-cycling MR (FFC-MR) relaxometer. In vivo R₁ dispersion profiles of mice were acquired from 1.26 T to 1.74 T at 37 °C, using FFC-MRI on a 1.5 T scanner outfitted with a field-cycling insert electromagnet to dynamically control B₀ prior to imaging. Images at five field strengths (1.26, 1.39, 1.5, 1.61, 1.74 T) were acquired using a field-cycling pulse sequence, where B₀ was modulated for varying relaxation durations prior to imaging. R₁ maps and R₁ dispersion (ΔR₁ /ΔB₀ ) were calculated at 1.5 T on a pixel-by-pixel basis. In addition, in vivo R₁ maps of mice were acquired at 3 T. At fields less than 1 T, a large R₁ magnetic field dependence was observed for tissues. ROI analysis of the tissues showed little relaxation dispersion for magnetic fields from 1.26 T to 3 T. Our tissue measurements show strong R₁ dispersion at field strengths less than 1 T and limited R₁ dispersion at field strengths greater than 1 T. These findings emphasize the inherent weak R₁ magnetic field dependence of healthy tissues at clinical field strengths. This characteristic of tissues can be exploited by a combination of FFC-MRI and T₁ contrast agents that exhibit strong relaxivity magnetic field dependences (inherent or by binding to a protein), thereby increasing the agents' specificity and sensitivity. This development can provide potential insights into protein-based biomarkers using FFC-MRI to assess early changes in tumour development, which are not easily measureable with conventional MRI.

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.

Correlations of low-field NMR and variable-field NMR parameters with osteoarthritis in human articular cartilage under load

NMR experiments carried out at magnetic fields below 1 T provide new relaxation parameters unavailable with conventional clinical scanners. Contrast of T₁ generally becomes larger towards low fields, as slow molecular reorientation processes dominate relaxation at the corresponding Larmor frequencies. This advantage has to be considered in the context of lower sensitivity and frequently reduced spatial resolution. The layered structure of cartilage is one example where a particularly strong variation of T₁ across the tissue occurs, being affected by degenerative diseases such as osteoarthritis (OA). Furthermore, the presence of ¹ H-¹⁴  N cross-relaxation, leading to so-called quadrupolar dips in the ¹ H relaxation time dispersion, provide insight into the concentration and mobility of proteoglycans and collagen in cartilage, both being affected by OA. In this study, low-field imaging and variable-field NMR relaxometry were combined for the first time for tissue samples, employing unidirectional load to probe the mechanical properties. 20 human knee cartilage samples were placed in a compression cell, and studied by determining relaxation profiles without and with applied pressure (0.6 MPa) at 50 μm in-plane resolution, and comparing with volume-averaged T₁ dispersion. Samples were subsequently stored in formalin, prepared for histology and graded according to the Mankin score system. Quadrupolar dips and thickness change under load showed the strongest correlation with Mankin grade. Average T₁ and change of maximum T₁ under load, as well as its position, correlate with thickness and thickness change. Furthermore, T₁ (ω) above 25 mT was found to correlate with thickness change. While volume-averaged T₁ is not a suitable indicator for OA, its change due to mechanical load and its extreme values are suggested as biomarkers available in low-field MRI systems. The shape of the dispersion T₁ (ω) represents a promising access to understanding and quantifying molecular dynamics in tissue, pointing toward future in vivo tissue studies.

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.

Dry-cured ham tissue characterization by fast field cycling NMR relaxometry and quantitative magnetization transfer

Fast field cycling (FFC) and quantitative magnetization transfer (qMT) NMR methods are two powerful tools in NMR analysis of biological tissues. The qMT method is well established in biomedical NMR applications, while the FFC method is often used in investigations of molecular dynamics on which longitudinal NMR relaxation times of the investigated material critically depend. Despite their proven analytical potential, these two methods were rarely used in NMR studies of food, especially when combined together. In our study, we demonstrate the feasibility of a combined FFC/qMT-NMR approach for the fast and nondestructive characterization of dry-curing ham tissues differing by protein content. The characterization is based on quantifying the pure quadrupolar peak area (area under the quadrupolar contribution of dispersion curve obtained by FFC-NMR) and the restricted magnetization pool size (obtained by qMT-NMR). Both quantities correlate well with concentration of partially immobilized, nitrogen-containing and proton magnetization exchanging muscle proteins. Therefore, these two quantities could serve as potential markers for dry-curing process monitoring. Copyright © 2016 John Wiley & Sons, Ltd.

Simple expressions of the nuclear relaxation rate enhancement due to quadrupole nuclei in slowly tumbling molecules

For slowly tumbling entities or quasi-rigid lattices, we derive very simple analytical expressions of the quadrupole relaxation enhancement (QRE) of the longitudinal relaxation rate R1 of nuclear spins I due to their intramolecular magnetic dipolar coupling with quadrupole nuclei of arbitrary spins S ≥ 1. These expressions are obtained by using the adiabatic approximation for evaluating the time evolution operator of the quantum states of the quadrupole nuclei S. They are valid when the gyromagnetic ratio of the spin S is much smaller than that of the spin I. The theory predicts quadrupole resonant peaks in the dispersion curve of R1 vs magnetic field. The number, positions, relative intensities, Lorentzian shapes, and widths of these peaks are explained in terms of the following properties: the magnitude of the quadrupole Hamiltonian and the asymmetry parameter of the electric field gradient (EFG) acting on the spin S, the S-I inter-spin orientation with respect to the EFG principal axes, the rotational correlation time of the entity carrying the S-I pair, and/or the proper relaxation time of the spin S. The theory is first applied to protein amide protons undergoing dipolar coupling with fast-relaxing quadrupole (14)N nuclei and mediating the QRE to the observed bulk water protons. The theoretical QRE agrees well with its experimental counterpart for various systems such as bovine pancreatic trypsin inhibitor and cartilages. The anomalous behaviour of the relaxation rate of protons in synthetic aluminium silicate imogolite nano-tubes due to the QRE of (27)Al (S = 5/2) nuclei is also explained.

The role of imaging in early hip OA

Hip osteoarthritis (OA) is characterized by cartilage degradation, subchondral bone sclerosis and osteophyte formation. Nowadays, OA is thought to develop via different etiologies that all lead to a similar form of end stage joint degradation. One of these subtypes is related to an abnormal shaped hip joint, like acetabular dysplasia and a cam deformity. These bony abnormalities are highly predictive for development of hip OA, but they are likely to already be present from childhood. This suggests that these deformations induce OA changes in the hip, well before extensive hip degradation becomes present three to four decades later. Accurate detection and successful characterization of these early OA events might lead to better treatment options for hip OA besides nowadays available invasive joint replacement surgery. However, current diagnostic imaging techniques like radiographs or plain magnetic resonance imaging (MRI), are not sensitive enough to detect these subtle early OA changes. Nor are they able to disentangle intertwined and overlapping cascades from different OA subtypes, and neither can they predict OA progression. New and more sensitive imaging techniques might enable us to detect first OA changes on a cellular level, providing us with new opportunities for early intervention. In this respect, shape analysis using radiography, MRI, computed tomography (CT), single photon emission computed tomography (SPECT)/CT, and positron emission tomography (PET) might prove promising techniques and be more suited to detect early pathological changes in the hip joint. A broad application of these techniques might give us more understanding what can be considered physiological adaptation of the hip, or when early OA really starts. With a more clear definition of early OA, more homogenous patient populations can be selected and help with the development of new disease modifying OA interventions.

NMR dispersion investigations of enzymatically degraded bovine articular cartilage

PURPOSE

Cross-relaxation of protons with (14) N nuclei in proteins enhances relaxivity in the quadrupolar dip range of typically 2-3 MHz Larmor frequency. The magnitude of these dips was suggested as a means of assessing the degeneracy of articular cartilage during osteoarthritis (OA). However, so far only proteoglycans have been considered whereas collagen nitrogen was neglected. This study addresses the relative importance of glycosaminoglycans (GAG), collagen, and water content for the cross-relaxation effect.

METHODS

Relaxation dispersion data were acquired for protons in samples of bovine articular cartilage, collagen, and GAG before and after the addition of trypsin or collagenase, and were compared with spatially resolved dGEMRIC experiments at 0.27 Tesla.

RESULTS

Both collagen as well as GAG show quadrupolar dips that strongly depend on hydration. For typical water concentrations in cartilage, the effect of enzymatic activity onto GAG is minor but a strong dependence on water concentration is found.

CONCLUSION

Quadrupolar dips in the (1) H relaxation dispersion of cartilage possess similar contributions from both GAG and collagen. The reduction of the cross-relaxation contribution observed in OA tissue is thus not directly proportional to GAG concentration, but maintains a collagen contribution and reflects predominantly the increase in water concentration during OA.

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.

Osteoarthritis year 2013 in review: biomarkers; reflecting before moving forward, one step at a time

In 2010, in Osteoarthritis and Cartilage, we published a comprehensive systematic review applying the consensus BIPED criteria (Burden of Disease, Investigative, Prognostic, Efficacy of Intervention and Diagnostic) criteria on serum and urinary biochemical markers for knee and hip osteoarthritis (OA) using publications that were available at that time. It appeared that none of the biochemical markers at that time were sufficiently discriminating to allow diagnosis and prognosis of OA in individual or limited numbers of patients, nor performed so consistently that they could function as primary outcome parameters in clinical trials. Also at present, almost 3 years later, this ultimate goal has not been reached (yet). Frankly, it might be questioned whether we are making the most adequate steps ahead and maybe we have to take a step back to reconsider our approaches. Some reflections are made and discussed: A critical review of molecular metabolism in OA and validation of currently investigated marker molecules in this may be vital and may lead to new and better markers. Creating cohorts in which synovial fluid (SF) is obtained in a systematic way, together with serum and urine, may also bring the field a further step ahead. Thirdly, better understanding of different phenotypes (subtypes) of OA may facilitate identification and validation of biochemical markers. Finally, the systems biology approach as discussed in the last years OA in review on biomarkers, although very complex, might provide steps forward. Looking ahead, we are optimistic but realistic in our expectations, we believe that the field can be brought forward by critically and cautiously reconsidering our approaches, and making changes forward, one step at a time.