Proton Longitudinal Relaxation Rates Research Articles

Optimization of cervical cord synthetic T1-weighted MRI for enhancing clinical application in neurodegenerative spinal cord disorders

Abstract Synthetic MRI offers the advantage of reducing acquisition time and enhancing flexibility through the reconstruction of various contrast weightings from a single set of MRI scans. However, the use of synthetic T1-weighted (synT1-w) MRI can lead to potentially biased measurements of the cross-sectional area (CSA) in the spinal cord when compared to conventionally acquired T1-weighted MRI. This disparity can have implications for comparability and sensitivity of MRI in assessing disease progression or treatment effects in neurodegenerative spinal cord disorders. Thus, this study aimed at improving the accuracy (i.e., difference between synthetic and acquired MRI) of cervical cord CSA measurements (C1-C3 level) based on synT1-w MRI implementing a longitudinal data set acquired from 23 acute spinal cord injury (SCI) patients and 21 healthy controls over 2 years. Moreover, the validity of using synT1-w MRI for tracking cervical cord atrophy following SCI over 2 years was verified. SynT1-w images were reconstructed from quantitative maps of proton density, longitudinal, and effective transverse relaxation rates derived from a multi-parameter mapping protocol. The results showed a minimal bias of -0.31 mm2 (-0.5%) in CSA measurements based on synT1-w compared to acquired MRI. Estimates of atrophy rates and average CSA were comparable between synthetic and acquired MRI. A sample size estimation for detecting treatment effects on CSA atrophy after 2 years following SCI revealed that the required sample size is reduced by 13.5% using synT1-w instead of acquired MRI. This study shows high accuracy of synT1-w MRI and demonstrates its applicability in clinical studies for optimizing long MRI protocols.

Multivariate white matter differences links to cognition in individuals with family history of Alzheimer’s disease and APOE4 genetic risk

AbstractBackgroundGrowing evidence suggests Alzheimer’s disease (AD) develops from a complex cascade of events that vary between individuals. Multivariate approaches have the potential to capture the complexity and heterogeneity of pathologies underlying AD in a more holistic manner compared to univariate approaches.MethodThe MRI data of 105 older adults with a family history of AD from the PREVENT‐AD dataset were analysed. Forty six of these individuals had the APOE4‐3 genotype (greater risk of developing AD), while 59 subjects had the APOE3‐3 genotype (normal risk). Diffusion‐weighted imaging and multi‐echo magnetization transfer, proton density and T1‐weighted data were used to compute several WM metrics (Fig1 b‐c). We computed the voxel‐wise Mahalanobis distance (D2) in WM between APOE4‐3 individuals and a reference group (APOE3‐3) (Fig1). D2 is a multivariate measure that combines several MRI metrics, accounting for their covariance, and yielding a score indicative of the degree of abnormality at each WM voxel (Fig2). Associations between WM D2 and cognitive performance were then investigated (Fig3). Independent component analysis (ICA) was performed on spatial dimensions and linear regression analyses were conducted between D2 in ICA components and scores on items of the Repeatable Battery for the Assessment of Neuropsychological Status (RBANS). We then extracted the contribution of each MRI feature to D2 in the regions where significant associations were found.ResultA significant negative association (R = ‐0.36, p = 0.014) was found between D2 in a component corresponding to the splenium of the corpus callosum (CC) and scores on RBANS‐immediate memory (Fig3), indicating a link between a greater degree of WM abnormality and poorer performance. Proton density (MPM_PD) and longitudinal relaxation rate (MPM_R1), potentially indicative of WM loss, contributed most to D2 in this region.ConclusionOur findings suggest that microstructural alterations in the posterior part of the CC may play a role in memory deficit in older adults who are at greater risk of developing AD. This is in line with other studies that showed a link between CC degeneration and memory impairments in early AD stages.1 1. Qiu et al. (2016). Inter‐hemispheric dysconnectivity in AD and aMCI. Sci.Rep.

Multivariate white matter differences links to cognition in individuals with family history of Alzheimer’s disease and APOE4 genetic risk

AbstractBackgroundGrowing evidence suggests Alzheimer’s disease (AD) develops from a complex cascade of events that vary between individuals. Multivariate approaches have the potential to capture the complexity and heterogeneity of pathologies underlying AD in a more holistic manner compared to univariate approaches.MethodThe MRI data of 105 older adults with a family history of AD from the PREVENT‐AD dataset were analysed. Forty six of these individuals had the APOE4‐3 genotype (greater risk of developing AD), while 59 subjects had the APOE3‐3 genotype (normal risk). Diffusion‐weighted imaging and multi‐echo magnetization transfer, proton density and T1‐weighted data were used to compute several WM metrics (Fig1 b‐c). We computed the voxel‐wise Mahalanobis distance (D2) in WM between APOE4‐3 individuals and a reference group (APOE3‐3) (Fig1). D2 is a multivariate measure that combines several MRI metrics, accounting for their covariance, and yielding a score indicative of the degree of abnormality at each WM voxel (Fig2). Associations between WM D2 and cognitive performance were then investigated (Fig3). Independent component analysis (ICA) was performed on spatial dimensions and linear regression analyses were conducted between D2 in ICA components and scores on items of the Repeatable Battery for the Assessment of Neuropsychological Status (RBANS). We then extracted the contribution of each MRI feature to D2 in the regions where significant associations were found.ResultA significant negative association (R = ‐0.36, p = 0.014) was found between D2 in a component corresponding to the splenium of the corpus callosum (CC) and scores on RBANS‐immediate memory (Fig3), indicating a link between a greater degree of WM abnormality and poorer performance. Proton density (MPM_PD) and longitudinal relaxation rate (MPM_R1), potentially indicative of WM loss, contributed most to D2 in this region.ConclusionOur findings suggest that microstructural alterations in the posterior part of the CC may play a role in memory deficit in older adults who are at greater risk of developing AD. This is in line with other studies that showed a link between CC degeneration and memory impairments in early AD stages.1 1. Qiu et al. (2016). Inter‐hemispheric dysconnectivity in AD and aMCI. Sci.Rep.

Reliability of spinal cord measures based on synthetic T1-weighted MRI derived from multiparametric mapping (MPM)

Short MRI acquisition time, high signal-to-noise ratio, and high reliability are crucial for image quality when scanning healthy volunteers and patients. Cross-sectional cervical cord area (CSA) has been suggested as a marker of neurodegeneration and potential outcome measure in clinical trials and is conventionally measured on T1-weigthed 3D Magnetization Prepared Rapid Acquisition Gradient-Echo (MPRAGE) images. This study aims to reduce the acquisition time for the comprehensive assessment of the spinal cord, which is typically based on MPRAGE for morphometry and multi-parameter mapping (MPM) for microstructure. The MPRAGE is replaced by a synthetic T1-w MRI (synT1-w) estimated from the MPM, in order to measure CSA. SynT1-w images were reconstructed using the MPRAGE signal equation based on quantitative maps of proton density (PD), longitudinal (R1) and effective transverse (R2*) relaxation rates. The reliability of CSA measurements from synT1-w images was determined within a multi-center test-retest study format and validated against acquired MPRAGE scans by assessing the agreement between both methods. The response to pathological changes was tested by longitudinally measuring spinal cord atrophy following spinal cord injury (SCI) for synT1-w and MPRAGE using linear mixed effect models. CSA measurements based on the synT1-w MRI showed high intra-site (Coefficient of variation [CoV]: 1.43% to 2.71%) and inter-site repeatability (CoV: 2.90% to 5.76%), and only a minor deviation of -1.65 mm2 compared to MPRAGE. Crucially, by assessing atrophy rates and by comparing SCI patients with healthy controls longitudinally, differences between synT1-w and MPRAGE were negligible. These results demonstrate that reliable estimates of CSA can be obtained from synT1-w images, thereby reducing scan time significantly.

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.

Repeatability and reproducibility of longitudinal relaxation rate in 12 small-animal MRI systems

BackgroundMany translational MR biomarkers derive from measurements of the water proton longitudinal relaxation rate R1, but evidence for between-site reproducibility of R1 in small-animal MRI is lacking. ObjectiveTo assess R1 repeatability and multi-site reproducibility in phantoms for preclinical MRI. MethodsR1 was measured by saturation recovery in 2% agarose phantoms with five nickel chloride concentrations in 12 magnets at 5 field strengths in 11 centres on two different occasions within 1–13 days. R1 was analysed in three different regions of interest, giving 360 measurements in total. Root-mean-square repeatability and reproducibility coefficients of variation (CoV) were calculated. Propagation of reproducibility errors into 21 translational MR measurements and biomarkers was estimated. Relaxivities were calculated. Dynamic signal stability was also measured. ResultsCoV for day-to-day repeatability (N = 180 regions of interest) was 2.34% and for between-centre reproducibility (N = 9 centres) was 1.43%. Mostly, these do not propagate to biologically significant between-centre error, although a few R1-based MR biomarkers were found to be quite sensitive even to such small errors in R1, notably in myocardial fibrosis, in white matter, and in oxygen-enhanced MRI. The relaxivity of aqueous Ni2+ in 2% agarose varied between 0.66 s−1 mM−1 at 3 T and 0.94 s−1 mM−1 at 11.7T. InterpretationWhile several factors affect the reproducibility of R1-based MR biomarkers measured preclinically, between-centre propagation of errors arising from intrinsic equipment irreproducibility should in most cases be small. However, in a few specific cases exceptional efforts might be required to ensure R1-reproducibility.

Hydration of MgO-Based Cement: Water Dynamics by 1H Fast Field-Cycling NMR Relaxometry

1H Fast Field-Cycling (FFC) NMR relaxometry was applied for the first time to monitor the state of water during the hydration reaction of MgO and silica that brings to the formation of magnesium silicate hydrate (M-S-H), the binder phase of innovative cements with promising applications in the containment of radioactive waste. To this aim water proton longitudinal relaxation rates (R1=1/T1) were measured in the Larmor frequency range between 10 kHz and 30 MHz at different hydration times ranging from 0.5 h to ~ 4 months. The obtained R1 vs frequency (NMRD) curves were analyzed considering fast exchange of water molecules between a hydration layer, where dynamics is affected by interactions with the surface of solids present in the reacting mixture, and a bulk phase. For the hydration layer, water molecules undergoing fast local molecular dynamics on the surface gave a constant contribution to R1 throughout the investigated frequency range; on the other hand, water molecules undergoing slow dynamics on the...

In vivo and in vitro MR imaging of glioma targeted block copolymer micelles

Objective To explore the in vivo/in vitro MR imaging effect of Angiopep2 modified glioma targeted block copolymer micelles(ANPs/CPT). Methods Angiopep2 modified DOTA-Gd conjugated polymeric micelles based on beta-cyclodextrin(β-CD) were successfully synthesized.Water proton longitudinal relaxation rate (1/T1) were calculated to compare the r1 of DOTA-Gd and ANPs/CPT. Different concentration of DOTA-Gd, NP/CPT, ANP/CPT were co-cultured with C6 cells, the intracellular content of Gd3+ was analyzed by ICP-AES;The signal intensity(SI) and contrast to noise ratio(CNR) were measured by 1.5 T MR;The CNR of ANPs/CPT was evaluated on different organs of normal rats. The enhancement feature of ANP/CPT in glioma as well as T/N and CNR between tumor and normal brain tissue were also investigated. Several comparisons were made between the 3 groups by one-way ANOVA, and the difference were significant when P<0.05. Results The r1 of ANPs/CPT(13.900 s-1·mmol/L-1) was 3 times as much as DOTA-Gd(4.927 s-1·mmol/L-1).C6 cells uptake different concentrations of Gd3+ had significant difference among groups(F=362.502, 1636.136, 386.880, 918.173, P<0.001).The difference of SI and CNR among groups were significant(F=55.240, 155.419,P<0.001).The CNR of the brain tissue had significant difference with other organs at all times after tail vein injection of ANPs/CPT(F=9.417, 21.808, 21.383, 107.318, 178.762, P<0.01).The ANPs/CPT showed excellent brain-targeting efficiency and theT/N ratio (30 min after) , CNR in different time point had significant difference among groups (F=5.349, 6.594, 24.078, 18.508 and 6.840, 6.780, 5.895, 12.620, 37.139, 368.893, P<0.05) . Conclusions ANPs/CPT shows nice agnetic relaxation performance,extended blood circulation duration,excellent brain-targeting efficieney, which will render the theranosticnano-carriers with important reference for the construction of new generation multifunctional nanomedicine. Key words: Glioma; Targeted; Block copolymer micelles; Magnetic resonance imaging

Oxygen-Enhanced MRI Accurately Identifies, Quantifies, and Maps Tumor Hypoxia in Preclinical Cancer Models.

There is a clinical need for noninvasive biomarkers of tumor hypoxia for prognostic and predictive studies, radiotherapy planning, and therapy monitoring. Oxygen-enhanced MRI (OE-MRI) is an emerging imaging technique for quantifying the spatial distribution and extent of tumor oxygen delivery in vivo. In OE-MRI, the longitudinal relaxation rate of protons (ΔR1) changes in proportion to the concentration of molecular oxygen dissolved in plasma or interstitial tissue fluid. Therefore, well-oxygenated tissues show positive ΔR1. We hypothesized that the fraction of tumor tissue refractory to oxygen challenge (lack of positive ΔR1, termed "Oxy-R fraction") would be a robust biomarker of hypoxia in models with varying vascular and hypoxic features. Here, we demonstrate that OE-MRI signals are accurate, precise, and sensitive to changes in tumor pO2 in highly vascular 786-0 renal cancer xenografts. Furthermore, we show that Oxy-R fraction can quantify the hypoxic fraction in multiple models with differing hypoxic and vascular phenotypes, when used in combination with measurements of tumor perfusion. Finally, Oxy-R fraction can detect dynamic changes in hypoxia induced by the vasomodulator agent hydralazine. In contrast, more conventional biomarkers of hypoxia (derived from blood oxygenation-level dependent MRI and dynamic contrast-enhanced MRI) did not relate to tumor hypoxia consistently. Our results show that the Oxy-R fraction accurately quantifies tumor hypoxia noninvasively and is immediately translatable to the clinic.

Acceleration of natural-abundance solid-state MAS NMR measurements on bone by paramagnetic relaxation from gadolinium-DTPA

Reducing the data collection time without affecting the signal intensity and spectral resolution is one of the major challenges for the widespread application of multidimensional nuclear magnetic resonance (NMR) spectroscopy, especially in experiments conducted on complex heterogeneous biological systems such as bone. In most of these experiments, the NMR data collection time is ultimately governed by the proton spin–lattice relaxation times (T1). For over two decades, gadolinium(III)-DTPA (Gd-DTPA, DTPA=Diethylene triamine pentaacetic acid) has been one of the most widely used contrast-enhancement agents in magnetic resonance imaging (MRI). In this study, we demonstrate that Gd-DTPA can also be effectively used to enhance the longitudinal relaxation rates of protons in solid-state NMR experiments conducted on bone without significant line-broadening and chemical-shift-perturbation side effects. Using bovine cortical bone samples incubated in different concentrations of Gd-DTPA complex, the 1H T1 values were calculated from data collected by 1H spin-inversion recovery method detected in natural-abundance 13C cross-polarization magic angle spinning (CPMAS) NMR experiments. Our results reveal that the 1H T1 values can be successfully reduced by a factor of 3.5 using as low as 10mM Gd-DTPA without reducing the spectral resolution and thus enabling faster data acquisition of the 13C CPMAS spectra. These results obtained from 13C-detected CPMAS experiments were further confirmed using 1H-detected ultrafast MAS experiments on Gd-DTPA doped bone samples. This approach considerably improves the signal-to-noise ratio per unit time of NMR experiments applied to bone samples by reducing the experimental time required to acquire the same number of scans.

Molecular MR Imaging of Neovascular Progression in the Vx2 Tumor with αvβ3-Targeted Paramagnetic Nanoparticles

To assess the dependence of neovascular molecular magnetic resonance (MR) imaging on relaxivity (r1) of αvβ3-targeted paramagnetic perfluorocarbon (PFC) nanoparticles and to delineate the temporal-spatial consistency of angiogenesis assessments for individual animals. Animal protocols were approved by the Washington University Animal Studies Committee. Proton longitudinal and transverse relaxation rates of αvβ3-targeted and nontargeted PFC nanoparticles incorporating gadolinium diethylenetrianime pentaacedic acid (Gd-DTPA) bisoleate (BOA) or gadolinium tetraazacyclododecane tetraacetic acid (Gd-DOTA) phosphatidylethanolamine (PE) into the surfactant were measured at 3.0 T. These paramagnetic nanoparticles were compared in 30 New Zealand White rabbits (four to six rabbits per group) 14 days after implantation of a Vx2 tumor. Subsequently, serial MR (3.0 T) neovascular maps were developed 8, 14, and 16 days after tumor implantation by using αvβ3-targeted Gd-DOTA-PE nanoparticles (n = 4) or nontargeted Gd-DOTA-PE nanoparticles (n = 4). Data were analyzed with analysis of variance and nonparametric statistics. At 3.0 T, Gd-DTPA-BOA nanoparticles had an ionic r1 of 10.3 L · mmol(-1) · sec(-1) and a particulate r1 of 927000 L · mmol(-1) · sec(-1). Gd-DOTA-PE nanoparticles had an ionic r1 of 13.3 L · mmol(-1) · sec(-1) and a particulate r1 of 1 197000 L · mmol(-1) · sec(-1). Neovascular contrast enhancement in Vx2 tumors (at 14 days) was 5.4% ± 1.06 of the surface volume with αvβ3-targeted Gd-DOTA-PE nanoparticles and 3.0% ± 0.3 with αvβ3-targeted Gd-DTPA-BOA nanoparticles (P = .03). MR neovascular contrast maps of tumors 8, 14, and 16 days after implantation revealed temporally consistent and progressive surface enhancement (1.0% ± 0.3, 4.5% ± 0.9, and 9.3% ± 1.4, respectively; P = .0008), with similar time-dependent changes observed among individual animals. Temporal-spatial patterns of angiogenesis for individual animals were followed to monitor longitudinal tumor progression. Neovasculature enhancement was dependent on the relaxivity of the targeted agent.

Water and Protein Dynamics in Sedimented Systems: A Relaxometric Investigation

Sedimented proteins have recently been shown to provide solid-state NMR spectra of high quality, suitable for structural investigation. This is ascribed to the strong self-crowding effect, which apparently increases the reorientation time up to the point that the protein can be considered immobile on the NMR timescale. Herein, a relaxometric investigation of sedimented bovine serum albumin is performed to obtain information on the dynamics of the system. The measurement of the proton longitudinal relaxation rates as a function of the applied magnetic field indicates that the sedimented protein has relaxation properties very different from those of the protein in concentrated water solutions, even in the presence of glycerol, and similar to those of slightly rehydrated lyophilized systems. This study confirms the hypothesis that the reorientation of the protein molecules is largely abolished in sediments.

Comparative in vitro studies of MR imaging probes for metabotropic glutamate subtype-5 receptor targeting

A series of magnetic resonance imaging probes has been evaluated to target selectively the metabotropic glutamate receptor subtype 5 (mGluR5). Eight imaging probes based on the contrast agent [Gd·DOTA], previously derived by linking it to a series of specific and selective mGluR5 antagonists, have been extensively tested for their functionality in vitro. The Nuclear Magnetic Relaxation Dispersion (NMRD) profiles of selected probes have been examined via field-cycling relaxometry in the presence and absence of a model protein. The properties of the targeted contrast agents were evaluated using a primary astrocyte model, as these cells mimic the in vivo situation effectively. The probes were non-toxic (up to 200 μM) to these mGluR5 expressing primary cells. Cellular proton longitudinal relaxation rate enhancements of up to 35% were observed by MRI at 200 μM of probe concentration. The antagonistic effect of all compounds was tested using an assay measuring changes of intracellular calcium levels. Furthermore, treatment at two different temperatures (4 °C vs. 37 °C) and of an mGluR5-negative cell line provided further insight into the selectivity and specificity of these probes towards cell surface mGluR5. Finally, two out of eight probes demonstrated an antagonistic effect as well as significant enhancement of receptor mediated cellular relaxation rates, strongly suggesting that they would be viable probes for the mapping of mGluR5 by MRI in vivo.

Nanocomposites Based on Layered Silicates and Epoxy Resins: Measurement of Clay Dispersion and Exfoliation using TEM, Solid-State NMR, and X-ray Diffraction Methods

In an epoxy/layered silicate nanocomposite containing different organically modified layered silicats, the effect of various organic modifications and the layered silicate content on the layer distances are investigated by transmission electron microscopy (TEM), X-ray diffraction (XRD) measurements, and 1H solid-state NMR. TEM micrographs were evaluated using a semi-quantitative method. The effect of paramagnetic impurities present in the clay is used to obtain overall sample information related to the degree of exfoliation by 1H solid-state NMR. A new model for determination of the degree of exfoliation by proton longitudinal relaxation rate measurements is developed.

Almost Ideal 1D Water Diffusion in Imogolite Nanotubes Evidenced by NMR Relaxometry

The longitudinal proton relaxation rates R(1) of water diffusing inside synthetic aluminium silicate imogolite nanotubes are measured by fast field-cycling NMR for frequencies between 0.02 and 35 MHz at 25, 37 and 50 degrees C. We give analytical expressions of the dominant intermolecular dipolar spin-spin contribution to R(1) and to the transverse relaxation rate R(2). A remarkable variation of R(1) by more than two orders of magnitude is observed and shown to be close to the theoretical law, inversely proportional to the square root of the resonance frequency, which is characteristic of perfect molecular 1D diffusion. The physics of diffusion is discussed.

Assessment of Blood Hemodynamics by USPIO-Induced R 1 Changes in MRI of Murine Colon Carcinoma

The objective of this study is to assess whether ultrasmall superparamagnetic iron oxide (USPIO)-induced changes of the water proton longitudinal relaxation rate (R 1) provide a means to assess blood hemodynamics of tumors. Two types of murine colon tumors (C26a and C38) were investigated prior to and following administration of USPIO blood-pool contrast agent with fast R 1 measurements. In a subpopulation of mice, R 1 was measured following administration of hydralazine, a well-known blood hemodynamic modifier. USPIO-induced R 1 increase in C38 tumors (ΔR 1 = 0.072 ± 0.0081 s−1) was significantly larger than in C26a tumors (ΔR 1 = 0.032 ± 0.0018 s−1, N = 9, t test, P < 0.001). This was in agreement with the immunohistochemical data that showed higher values of relative vascular area (RVA) in C38 tumors than in C26a tumors (RVA = 0.059 ± 0.015 vs. 0.020 ± 0.011; P < 0.05). Following administration of hydralazine, a decrease in R 1 value was observed. This was consistent with the vasoconstriction induced by the steal effect mechanism. In conclusion, R 1 changes induced by USPIO are sensitive to tumor vascular morphology and to blood hemodynamics. Thus, R 1 measurements following USPIO administration can give novel insight into the effects of blood hemodynamic modifiers, non-invasively and with a high temporal resolution.

Preparation, spectroscopic and high field NMR relaxometry studies of gadolinium(III) complexes with the asymmetric tetraamine 1,4,7,11-tetraazaundecane

The reaction of Gd(III) with asymmetric tetramine 1,4,7,11-tetraazaundecane (2,2,3-tet, L 1 ) ligand has been studied via NMR spectroscopy. The ligand proton longitudinal relaxation rates ( R 1) have been used to estimate the distances of these protons from the Gd(III) center, in Gd(III)– L 1 reaction solutions, in H 2O/D 2O 5/1 mixtures. Two Gd(III) complexes [Gd(III)( L 1 )(NH 3)(H 2O) 4](CH 3COO) 3·2H 2O ( 1) and [Gd(III)( L 1 )(NH 3)(H 2O) 2]Cl 3·EtOH ( 2) have been isolated and characterized by elemental analyses, TGA, IR, NMR and relaxometry measurements. The NMR relaxation measurements of 2 in aqueous solutions have been performed, under various temperature or concentration conditions, and compared with those of the commercial contrast agents Gd(III)–DTPA and Gd(III)–DTPA-BMA. It has also been studied the influence of (i) the Gd(III) inner-sphere water molecule number ( q) alteration and (ii) the steric constraint enhancement on the metal site, over the relaxation rate values of the parent aqueous solution of Gd(III)–2,2,3-tet, and of the aqueous solutions of 2.

Nuclear Relaxometry Helps Designing Systems for Solution DNP on Proteins

The measurement of water proton longitudinal relaxation rates in aqueous solutions of paramagnetic compounds (radicals, paramagnetic metal ions) as a function of magnetic field (relaxometry) provides two important pieces of information for predicting the efficacy of dynamic nuclear polarization (DNP) to be ultimately achieved on protein nuclei: the correlation time for the electron–nuclear interaction and the actual water proton relaxation rate at any magnetic field of interest. Likewise, relaxometry measurements on protein protons themselves, which can be performed in D2O solutions, provide information on the polarization loss occurring on protein protons in DNP-shuttling experiments during the shuttling of the sample from the electron paramagnetic resonance to the nuclear magnetic resonance field. A collection of water proton and protein proton relaxometry profiles, recently obtained in the course of the DNP Design Study project, is here reported, and the relevant parameters are discussed.

Multimodal MRI contrast agents

The advent of powerful new imaging tools such as confocal and two-photon microscopy, MRI, PET, SPECT, and ultrasound have provided scientists and clinicians with the ability to acquire in vivo images of anatomy and physiology in whole animals and humans [1–3]. Each of these in vivo imaging techniques possesses unique strengths and weaknesses with varying spatial and temporal resolution limits [4]. Further, the development of sophisticated reporter probes for cell tracking, gene expression, and gene monitoring therapeutic paradigms is rapidly changing the way these techniques are being applied. For example, advances in optical microscopy have revolutionized the way living cells and intact tissues are studied [5]. As a result, cellular events, biochemical processes, and tissue morphogenesis can be followed in developing embryos at submicron resolution. MRI employs nonionizing radiation and has become an integral tool in clinical radiology, and more recently in experimental settings [6–8]. The intensity of the signal in each voxel is dependent on several intrinsic parameters, such as the proton density, longitudinal (R1) and transverse (R2) NMR relaxation rates, and extrinsic parameters, such as magnetic field strength, pulse sequence, and the presence of contrast agents (CAs). CAs for a variety of imaging methods have been in use for decades in diagnostic radiology and developmental biology [9–13]. The application of these CAs is well documented and the fundamental chemical principles for the synthesis and characterization of these complexes are known [14–16]. A rapidly growing body of literature documents the clinical effectiveness of paramagnetic CAs [17–19]. Currently, eight such CAs are in clinical use or clinical trials and more than 30% of all clinical MRI examinations employ CAs [20]. Paramagnetic metal ions, as a result of their unpaired electrons, act as potent relaxation-enhancement agents and decrease the T1 and T2 relaxation times of nearby spins [21]. Some paramagnetic ions decrease T1 without causing substantial line broadening [T1 agents; e.g., Gd(III)], while others induce drastic line broadening (T2 agents; e.g., superparamagnetic iron oxide) [22]. The lanthanide ion Gd(III) has generally been chosen as the metal ion for CAs because it has a high magnetic moment (l = 63 lB ) and a symmetric electronic ground state (S) [23]. Transition metal ions such as high-spin Mn(II) and Fe(III) are candidates owing to their high magnetic moments. The mechanism of T1 relaxation is generally a through-space dipole–dipole interaction between the unpaired electrons of the paramagnetic metal ion and the protons of coordinated or diffusing water molecules. The longitudinal and transverse relaxation rates (Ri) of a CA solution are proportional to the CA’s relaxivity rip (Eq. 1) [20, 23]:

Mapping the Orientation of Helices in Micelle-Bound Peptides by Paramagnetic Relaxation Waves

Many antimicrobial peptides form alpha-helices when bound to a membrane. In addition, around 80% of residues in membrane-bound proteins are found in alpha-helical regions. The orientation and location of such helical peptides and proteins in the membrane are key factors determining their function and activity. Here we present a new solution state NMR method for obtaining the orientation of helical peptides in a membrane-mimetic environment (micelle-bound) without any chemical perturbation of the peptide-micelle system. By monitoring proton longitudinal relaxation rates upon addition of the freely water-soluble and inert paramagnetic probe Gd(DTPA-BMA) to an alpha-helical peptide, a wavelike pattern with a periodicity of 3.6 residues per turn is observed. The tilt and azimuth (rotation) angle of the helix determine the shape of this paramagnetic relaxation wave and can be obtained by least-square fitting of measured relaxation enhancements. Results are presented for the 15-residue antimicrobial peptide CM15 which forms an amphipathic helix almost parallel to the surface of the micelle. Thus, a few fast experiments enable the identification of helical regions and determination of the helix orientation within the micelle without the need for covalent modification, isotopic labeling, or sophisticated equipment. This approach opens a path toward the topology determination of alpha-helical membrane-proteins without the need for a complete NOE-based structure determination.