Relaxometry and magnetometry of ferritin

By combining nuclear magnetic relaxometry on 39 ferritin samples with different iron loading with magnetometry, results were obtained that suggest a new interpretation of the core structure and magnetic properties of ferritin. These studies provide evidence that, contrary to most earlier reports, the ferritin core is antiferromagnetic ( AFM ) even at body temperature and possesses a superparamagnetic (SPM) moment due to incomplete cancellation of antiparallel sublattices, as predicted by Néel's theory. This moment also provides a likely explanation for the anomalous T₂ shortening in ferritin solution. However, the number of SPM moments derived from this model is less than the number of ferritin molecules determined chemically, and a similar discrepancy was found by retrospectively fitting previously published magnetometry data. In other words, only a fraction of the ferritin molecules seem to be SPM. The studies also provide evidence for paramagnetic (PM) Curie-Weiss iron ions at the core surface, where the local Néel temperature is lower; these ions are apparently responsible for the weaker T₁ shortening. In fact, the conversion of uncompensated AFM lattice ions to PM i ons could explain the small number of SPM particles. The apparent Curie Law behavior of ferritin thus appears to be a coincidental result of different temperature dependences of the PM and SPM components.     

Hepatic hemosiderosis in non-human primates: Quantification of liver iron using different field strengths

Using a non-human primate model of idiopathic hemochro-matosis, hemosiderin-induced T₂ shortening of the liver was assessed at nine different field strengths over a range of 0.05 to 1.5 Tesla. The 1/T₂ values increased linearly with field strength, with all specimens having approximately the same zero-field intercept. The slope of the field increase, termed “field-dependent T₂ proton relaxation enhancement (PRE)”, appeared to be proportional to the chemically determined tissue iron content, viz. 10.8 s^?1T^?1(mg Fe/g wet tissue)^?1. The correlation between iron content and field-dependent T₂ PRE (r = 0.94) was better than the correlation between iron content and 1/T₂ values obtained at single field strengths. For livers containing ± 2 mg Fe/g wet weight, biexponential T₂ relaxation behavior emerged at higher field strengths, with the short T₂ component (intracellular water) exhibiting a linear dependence of 1/T₂ on field, while T₂ of the long component (extracellular/sinusoidal water) was nearly field-independent. After maceration of the specimens, all T₂ relaxation curves became monoexponential, including those for high iron content at high field strengths. The present data suggest that the use of double-field MR imaging to assess the field-dependent T₂ PRE has potential for specific quantification of (liver) tissue iron stores.     

White matter abnormalities in patients with treated hyperphenylalaninaemia: Magnetic resonance relaxometry and proton spectroscopy findings

In order to further clarify the pathogenesis and clinical significance of MRI white matter abnormalities in treated hyperphenylalaninaemia (HPA), ten patients (seven type I HPA, two type II and one type III) underwent T₂ relaxometry (n=8) and/or¹H spectroscopy (n=7) in addition to conventional MR spin-echo imaging at 1.5 T. Two patients with severe MRI abnormalities had repeat examinations during and after a 6-to 8-month period of strict diet control. The clinical evaluation included a detailed neurological examination. In nine out of ten patients visual evoked potentials (VEP) were obtained parallel to the MR examination. MR imaging demonstrated typical symmetrical areas of prolonged T₂ relaxation time predominantly in the posterior periventricular white matter in all but one of type I and II patients. There was no consistent relationship between MRI findings and time of diagnosis/initiation of therapy, IQ or visual evoked potential changes. MRI abnormalities tended to be more severe in patients with poor dietary control and high current plasma phenylalanine levels, whereas a normal MRI was found only in patients with plasma phenylalanine levels continuously below 0.36 mmol/l. There was marked regression of MRI abnormalities already after 3 months of strict diet control. T₂ relaxometry showed a bi-exponential behaviour of T₂ in the affected white matter, with a slow component of about 200–450 ms, indicating an increase in free (extracellular) water.¹H spectroscopy revealed no signs of severe neuronal damage. We conclude, that the observed white matter changes in treated HPA probably represent reversible structural myelin changes rather than permanent demyelination.     

Multi‐parametric MRI characterization of healthy human thigh muscles at 3.0 T – relaxation,magnetization transfer,fat/water,and diffusion tensor imaging

Muscle diseases commonly have clinical presentations of inflammation, fat infiltration, fibrosis, and atrophy. However, the results of existing laboratory tests and clinical presentations are not well correlated. Advanced quantitative MRI techniques may allow the assessment of myo‐pathological changes in a sensitive and objective manner. To progress towards this goal, an array of quantitative MRI protocols was implemented for human thigh muscles; their reproducibility was assessed; and the statistical relationships among parameters were determined. These quantitative methods included fat/water imaging, multiple spin‐echo T₂ imaging (with and without fat signal suppression, FS), selective inversion recovery for T₁ and quantitative magnetization transfer (qMT) imaging (with and without FS), and diffusion tensor imaging. Data were acquired at 3.0 T from nine healthy subjects. To assess the repeatability of each method, the subjects were re‐imaged an average of 35 days later. Pre‐testing lifestyle restrictions were applied to standardize physiological conditions across scans. Strong between‐day intra‐class correlations were observed in all quantitative indices except for the macromolecular‐to‐free water pool size ratio (PSR) with FS, a metric derived from qMT data. Two‐way analysis of variance revealed no significant between‐day differences in the mean values for any parameter estimate. The repeatability was further assessed with Bland–Altman plots, and low repeatability coefficients were obtained for all parameters. Among‐muscle differences in the quantitative MRI indices and inter‐class correlations among the parameters were identified. There were inverse relationships between fractional anisotropy (FA) and the second eigenvalue, the third eigenvalue, and the standard deviation of the first eigenvector. The FA was positively related to the PSR, while the other diffusion indices were inversely related to the PSR. These findings support the use of these T₁, T₂, fat/water, and DTI protocols for characterizing skeletal muscle using MRI. Moreover, the data support the existence of a common biophysical mechanism, water content, as a source of variation in these parameters. Copyright © 2014 John Wiley & Sons, Ltd.     

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, H₂O₂ 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 H₂O₂ 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, H₂O₂ 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.     

T1 nuclear magnetic relaxation dispersion of hyperpolarized sodium and cesium hydrogencarbonate‐13C

In vivo pH mapping in tissue using hyperpolarized hydrogencarbonate‐¹³C has been proposed as a method to study tumor growth and treatment and other pathological conditions related to pH changes. The finite spin–lattice relaxation times (T₁) of hyperpolarized media are a significant limiting factor for in vivo imaging. Relaxation times can be measured at standard magnetic fields (1.5 T, 3.0 T etc.), but no such data are available at low fields, where T₁ values can be significantly shorter. This information is required to determine the potential loss of polarization as the agent is dispensed and transported from the polarizer to the MRI scanner. The purpose of this study is to measure T₁ dispersion from low to clinical magnetic fields (0.4 mT to 3.0 T) of different hyperpolarized hydrogencarbonate formulations previously proposed in the literature for in vivo pH measurements. ¹³C–enriched cesium and sodium hydrogencarbonate preparations were hyperpolarized using dynamic nuclear polarization, and the T₁ values of different samples were measured at different magnetic field strengths using a fast field‐cycling relaxometer and a 3.0 T clinical MRI system. The effects of deuterium oxide as a dissolution medium for sodium hydrogencarbonate were also analyzed. This study finds that the cesium formulation has slightly shorter T₁ values compared with the sodium preparation. However, the higher solubility of cesium hydrogencarbonate‐¹³C means it can be polarized at greater concentration, using less trityl radical than sodium hydrogencarbonate‐¹³C. This study also establishes that the preparation and handling of sodium hydrogencarbonate formulations in relation to cesium hydrogencarbonate is more difficult, due to the higher viscosity and lower achievable concentrations, and that deuterium oxide significantly increases the T₁ of sodium hydrogencarbonate solutions. Finally, this work also investigates the influence of pH on the spin–lattice relaxation of cesium hydrogencarbonate‐¹³C measured over a pH range of 7 to 9 at 0.47 T.     

A study of T1 relaxation time as a measure of liver fibrosis and the influence of confounding histological factors

Liver biopsy is the standard test for the assessment of fibrosis in liver tissue of patients with chronic liver disease. Recent studies have used a non‐invasive measure of T₁ relaxation time to estimate the degree of fibrosis in a single slice of the liver. Here, we extend this work to measure T₁ of the whole liver and investigate the effects of additional histological factors such as steatosis, inflammation and iron accumulation on the relationship between liver T₁ and fibrosis. We prospectively enrolled patients who had previously undergone liver biopsy to have MR scans. A non‐breath‐holding, fast scanning protocol was used to acquire MR relaxation time data (T₁ and T₂*), and blood serum was used to determine the enhanced liver fibrosis (ELF) score. Areas under the receiver operator curves (AUROCs) for T₁ to detect advanced fibrosis and cirrhosis were derived in a training cohort and then validated in a second cohort. Combining the cohorts, the influence of various histology factors on liver T₁ relaxation time was investigated. The AUROCs (95% confidence interval (CI)) for detecting advanced fibrosis (F ≥ 3) and cirrhosis (F = 4) for the training cohort were 0.81 (0.65–0.96) and 0.92 (0.81–1.0) respectively (p < 0.01). Inflammation and iron accumulation were shown to significantly alter T₁ in opposing directions in the absence of advanced fibrosis; inflammation increasing T₁ and iron decreasing T₁. A decision tree model was developed to allow the assessment of early liver disease based on relaxation times and ELF, and to screen for the need for biopsy. T₁ relaxation time increases with advanced fibrosis in liver patients, but is also influenced by iron accumulation and inflammation. Together with ELF, relaxation time measures provide a marker to stratify patients with suspected liver disease for biopsy. Copyright © 2015 John Wiley & Sons, Ltd.