Molecular Motions in the Supramolecular Complexes between Poly(ε‐caprolactone)‐Poly(ethylene oxide)‐Poly(ε‐caprolactone) and α‐ and γ‐Cyclodextrins

The structure and molecular motions of the triblock copolymer PCL‐PEO‐PCL and its inclusion complexes with α‐ and γ‐cyclodextrins (α‐ and γ‐CDs) have been studied by solid‐state NMR. Different cross‐polarization dynamics have been observed for the guest polymer and host CDs. Guest–host magnetization exchange has been observed by proton spin lattice relaxation T₁, proton spin lattice frame relaxation T_1ρ and 2D heteronuclear correlation experiments. A homogeneous phase has been observed for these complexes. Conventional relaxation experiments and 2D wide‐line separation NMR with windowless isotropic mixing have been used to measure the chain dynamics. The results show that for localized molecular motion in the megahertz regime, the included PCL block chains are much more mobile than the crystalline PCL blocks in the bulk triblock copolymer. However, the mobility of the included PEO block chains is not very different from the amorphous PEO blocks of the bulk sample. The cooperative, long chain motions in the mid‐kilohertz regime for pairs of PCL‐PEO‐PCL chains in their γ‐CD channels seem more restricted than for the single PCL‐PEO‐PCL chains in the α‐CD channels, however, they are not influencing the more localized, higher frequency megahertz motions.     

The solid state 13C NMR studies of intermolecular hydrogen bonding formation in a blend of phenolic resin and poly(hydroxyl ether) of bisphenol A

The formation of intermolecular hydrogen bonds in blends of novolac type phenolic and poly(hydroxyl ether) of bisphenol A was investigated by studying its T_g behavior, excess volume, and solid state ¹³C NMR spectra. The T_g and parameters of solid state ¹³C NMR, such as the T_CH and spin‐lattice relaxation time in the rotating frame T^H_1ρ, indicate that the London dispersion force (entropically favored) significantly affects the intermolecular hydrogen bonding of the blend. The phenoxy chain forces opening of the intra‐association of phenolic and thus creates more free OHs. This strong entropic effect reduces the total hydrogen bonding of the system, especially when one of the polymer is the minor component. This also results in the reduction of T_g and free volume expansion, reflecting in the increase of cross‐polarization (C—H) time and molecular mobility within the phenolic/phenoxy blend.     

Influence of configuration upon heterogeneity in anisotropic glasses as determined by thermally stimulated current

Thermally stimulated current (TSC) as well as differential scanning calorimetry (DSC) was used to determine the glass transition temperature of anisotropic glasses for semiflexible polyesters with polymethylene spacers. The highly sensitive technique of thermal-windowing polarization or relaxation map analysis (RMA) was used to further study the relaxation behavior of the anisotropic glasses as well as the heterogeneity of the samples. Experimental results indicate that phase separation is dependent upon the length of the polymethylene chain, i.e., the configuration of polymethylene segments and the number (n value) of methylene groups. The longer the polymethylene segment the larger the difference in the two transition temperatures, the higher transition temperature (T_g,H) and lower transition temperature (T_g,L). For a polymer with even value of n, ΔT_g (T_g,H minus T_g,L) of the anisotropic glass is lower than that for the neighboring two polymers with odd n values. In addition, the distortion effect of the polymethylene segment with even number of methylene groups on the polymer chains under polarization was studied by the Vogelian relaxation behavior of the ρ-peak. These results indicated that the maximum fractional volume of the polymethylene segments of the anisotropic glass for a polymer with even n value would be smaller than that for a polymer with odd n value under polarization.     

T1ρ magnetic resonance fingerprinting

T_1ρ relaxation imaging is a quantitative imaging technique that has been used to assess cartilage integrity, liver fibrosis, tumors, cardiac infarction, and Alzheimer's disease. T₁, T₂, and T_1ρ relaxation time constants have each demonstrated different degrees of sensitivity to several markers of fibrosis and inflammation, allowing for a potential multi‐parametric approach to tissue quantification. Traditional magnetic resonance fingerprinting (MRF) has been shown to provide quick, quantitative mapping of T₁ and T₂ relaxation time constants. In this study, T_1ρ relaxation is added to the MRF framework using spin lock preparations. An MRF sequence involving an RF‐spoiled sequence with T_R, flip angle, T_1ρ, and T₂ preparation variation is described. The sequence is then calibrated against conventional T₁, T₂, and T_1ρ relaxation mapping techniques in agar phantoms and the abdomens of four healthy volunteers. Strong intraclass correlation coefficients (ICC > 0.9) were found between conventional and MRF sequences in phantoms and also in healthy volunteers (ICC > 0.8). The highest ICC correlation values were seen in T₁, followed by T_1ρ and then T₂. In this study, T_1ρ relaxation has been incorporated into the MRF framework by using spin lock preparations, while still fitting for T₁ and T₂ relaxation time constants. The acquisition of these parameters within a single breath hold in the abdomen alleviates the issues of movement between breath holds in conventional techniques.     

Miscibility of an oligo(p-phenylenevinylene) with polystyrene studied by solid-state nuclear magnetic resonance

Molecular proximity of cis/trans mixtures of 2,5-dimethoxy-1,4-bis[2-(3,4,5-trimethoxyphenyl)vinyl]benzene (MPV, 1 ) and polystyrene (PS) in mixtures of different MPV/PS weight ratios up to 60/40 is shown by ¹H combined rotation and multiple pulse spectroscopy (CRAMPS) using 2D exchange experiments. The MPV/PS mixtures up to a weight ratio 60/40 are homogeneous, whereas the mixture with a weight ratio MPV/PS = 80/20 is heterogeneous. The comparison of the cross polarization/magic angle spinning (CP/MAS) ¹³C NMR spectra of pure MPV and of MPV mixed with PS shows significant linebroadening in case of intimate mixing, while the heterogeneous mixture shows some extra finestructure. The miscibility results were confirmed by proton spin-locking relaxation time measurements (T_1ρH) on the mixtures. On intimate mixing, T_1ρH is averaged out and varies linearly as a function of the composition of the mixture. CRAMPS-spectra and T_1ρH measurements show that iodine-doped MPV/PS mixtures (weight ratios 40/60 and 60/40) are heterogeneous. Furthermore, doping causes a slight proton chemical shift change and reduces the T_1ρH-values of those oligomer-polymer systems.     

T2, T1ρ and TRAFF4 detect early regenerative changes in mouse ischemic skeletal muscle

The identification of areas with regenerative potential in ischemic tissues would allow the targeting of treatments supporting tissue recovery. The regeneration process involves the activation of several cellular and molecular responses which could be detected using magnetic resonance imaging (MRI). However, to date, magnetic resonance (MR) relaxation parameters have received little attention in the diagnosis and follow‐up of limb ischemia. The purpose of this study was to evaluate the feasibility of different MRI relaxation and diffusion tensor imaging parameters in the detection of areas showing early signs of regeneration in ischemic mouse skeletal muscles. T₂ and T_1ρ relaxation time constants, together with T_RAFFn, T₁ and diffusion tensor imaging, were evaluated to differentiate areas of regeneration in a mouse hind limb ischemia model before and 0, 1, 4, 7, 14 and 30 days after ischemia. All the measured relaxation times were longer in the areas of early regeneration compared with normal muscle tissue. The relaxation times increased after ischemia in the ischemic muscles, reaching a maximum at 4–7 days after occlusion, coinciding with the appearance of early signs of regeneration. Fractional anisotropy decreased significantly (p < 0.05) on days 1–4, whereas mean diffusivity, λ₁ and λ₂ decreased later, starting at day 7 after ischemia compared with the pre‐operational time point. The percentages of areas with different tissue morphologies were determined based on histological analysis of the ischemic muscle cross‐sections, and correlations between the percentages obtained and different relaxation times were calculated. The highest correlation between relaxation times and histology was achieved with T₂, T_1ρ and T_RAFF4 (R² = 0.96, R² = 0.92 and R² = 0.84, respectively, p < 0.01)_. Early regenerative changes were visible using T₂, T_1ρ and T_RAFF4 MR relaxation time constants in skeletal muscle after ischemia. These markers could potentially be used for the identification of targets for therapies supporting muscle regeneration after ischemic injury.     

The follow‐up of progressive hypertrophic cardiomyopathy using magnetic resonance rotating frame relaxation times

Magnetic resonance rotating frame relaxation times are an alternative non‐contrast agent choice for the diagnosis of chronic myocardial infarct. Fibrosis typically occurs in progressive hypertrophic cardiomyopathy. Fibrosis has been imaged in myocardial infarcted tissue using rotating frame relaxation times, which provides the possibility to follow up progressive cardiomyopathy without contrast agents. Mild and severe left ventricular hypertrophy were induced in mice by transverse aortic constriction, and the longitudinal rotating frame relaxation times (T_1ρ) and relaxation along the fictitious field (T_RAFF2, T_RAFF3) were measured at 5, 10, 24, 62 and 89 days after transverse aortic constriction in vivo. Myocardial fibrosis was verified using Masson's trichrome staining. Increases in the relative relaxation time differences of T_1ρ, together with T_RAFF2 and T_RAFF3, between fibrotic and remote tissues over time were observed. Furthermore, T_RAFF2 and T_RAFF3 showed higher relaxation times overall in fibrotic tissue than T_1ρ. Relaxation time differences were highly correlated with an excess of histologically verified fibrosis. We found that T_RAFF2 and T_RAFF3 are more sensitive than T_1ρ to hypertrophic cardiomyopathy‐related tissue changes and can serve as non‐invasive diagnostic magnetic resonance imaging markers to follow up the mouse model of progressive hypertrophic cardiomyopathy.     

Simultaneous bilateral T1, T2, and T1ρ relaxation mapping of the hip joint with magnetic resonance fingerprinting

Quantitative MRI can detect early biochemical changes in cartilage, but its bilateral use in clinical routines is challenging. The aim of this prospective study was to demonstrate the feasibility of magnetic resonance fingerprinting for bilateral simultaneous T₁, T₂, and T_1ρ mapping of the hip joint. The study population consisted of six healthy volunteers with no known trauma or pain in the hip. Monoexponential T₁, T₂, and T_1ρ relaxation components were assessed in femoral lateral, superolateral, and superomedial, and inferior, as well as acetabular, superolateral, and superomedial subregions in left and right hip cartilage. Aligned ranked nonparametric factorial analysis was used to assess the side's impact on the subregions. Kruskal–Wallis and Wilcoxon tests were used to compare subregions, and coefficient of variation to assess repeatability. Global averages of T₁ (676.0 ± 45.4 and 687.6 ± 44.5 ms), T₂ (22.5 ± 2.6 and 22.1 ± 2.5 ms), and T_1ρ (38.2 ± 5.5 and 38.2 ± 5.5 ms) were measured in the left and right hip, and articular cartilage, respectively. The Kruskal–Wallis test showed a significant difference between different subregions’ relaxation times regardless of the hip side (p < 0.001 for T₁, p = 0.012 for T₂, and p < 0.001 for T_1ρ). The Wilcoxon test showed that T₁ of femoral layers was significantly (p < 0.003) higher than that for acetabular cartilage. The experiments showed excellent repeatability with CV_rms of 1%, 2%, and 4% for T₁, T₂, and T_1ρ, respectively. It was concluded that bilateral T₁, T₂, and T_1ρ relaxation times, as well as B₁⁺ maps, can be acquired simultaneously from hip joints using the proposed MRF sequence.     

Configurational dependences of carbon-13 and proton NMR spin-lattice relaxation times of various polymethacrylates

Isotactic and syndiotactic poly(alkyl methacrylate)s (R = CH₃, C₂H₅, isoC₃H₇, n-C₄H₉ and t-C₄H₉) were prepared by anionic mechanism and the spin-lattice relaxation times T₁ of individual carbons and protons were measured mainly in toluene-d₈ by the inversion-recovery Fourier transform (IRFT) method. The T₁'s of the carbons in the isotactic polymers were consistently longer than those of the comparable carbons in the syndiotactic polymers. The ratios of ¹³C-T₁'s for syndiotactic and isotactic polymers were always in the range of ca. 0,4–0,6 for all types of carbons. The ¹³C-T₁'s decreased with an increase in the bulkiness of the ester group in both polymers. From the measurements of the nuclear Overhauser enhancement (NOE) it was found that the spin-lattice relaxations of carbons in polymethacrylates are dominated by ¹³C-¹H dipolar interactions and that the extreme narrowing condition is satisfied at least above 70°C. The effect of solvent on ¹³C-T₁ was also studied. The results of the measurements of ¹H-T₁'s were parallel to those in the carbons' case. It was concluded that the dependence of T₁ upon the configuration of the polymer is mainly caused by the configurational dependence of the segmental mobility of the polymer.     

Biexponential T1ρ relaxation mapping of human knee cartilage in vivo at 3 T

The purpose of this study was to demonstrate the feasibility of biexponential T_1ρ relaxation mapping of human knee cartilage in vivo. A three‐dimensional, customized, turbo‐flash sequence was used to acquire T_1ρ‐weighted images from healthy volunteers employing a standard 3‐T MRI clinical scanner. A series of T_1ρ‐weighted images was fitted using monoexponential and biexponential models with two‐ and four‐parametric non‐linear approaches, respectively. Non‐parametric Kruskal–Wallis and Mann–Whitney U‐statistical tests were used to evaluate the regional relaxation and gender differences, respectively, with a level of significance of P = 0.05. Biexponential relaxations were detected in the cartilage of all volunteers. The short and long relaxation components of T_1ρ were estimated to be 6.9 and 51.0 ms, respectively. Similarly, the fractions of short and long T_1ρ were 37.6% and 62.4%, respectively. The monoexponential relaxation of T_1ρ was 32.6 ms. The experiments showed good repeatability with a coefficient of variation (CV) of less than 20%. A biexponential relaxation model showed a better fit than a monoexponential model to the T_1ρ relaxation decay in knee cartilage. Biexponential T_1ρ components could potentially be used to increase the specificity to detect early osteoarthritis by the measurement of different water compartments and their fractions.     

Capturing fast relaxing spins with SWIFT adiabatic rotating frame spin–lattice relaxation (T1ρ) mapping

Rotating frame spin–lattice relaxation, with the characteristic time constant T_1ρ, provides a means to access motion‐restricted (slow) spin dynamics in MRI. As a result of their restricted motion, these spins are sometimes characterized by a short transverse relaxation time constant T₂ and thus can be difficult to detect directly with conventional image acquisition techniques. Here, we introduce an approach for three‐dimensional adiabatic T_1ρ mapping based on a magnetization‐prepared sweep imaging with Fourier transformation (MP‐SWIFT) sequence, which captures signal from almost all water spin populations, including the extremely fast relaxing pool. A semi‐analytical procedure for T_1ρ mapping is described. Experiments on phantoms and musculoskeletal tissue specimens (tendon, articular and epiphyseal cartilages) were performed at 9.4 T for both the MP‐SWIFT and fast spin echo (FSE) read outs. In the phantom with liquids having fast molecular tumbling and a single‐valued T_1ρ time constant, the measured T_1ρ values obtained with MP‐SWIFT and FSE were similar. Conversely, in normal musculoskeletal tissues, T_1ρ values measured with MP‐SWIFT were much shorter than the values obtained with FSE. Studies of biological tissue specimens demonstrated that T_1ρ‐weighted SWIFT provides higher contrast between normal and diseased tissues relative to conventional acquisitions. Adiabatic T_1ρ mapping with SWIFT readout captures contributions from the otherwise undetected fast relaxing spins, allowing more informative T_1ρ measurements of normal and diseased states. Copyright © 2016 John Wiley & Sons, Ltd.     

Synthesis and characterization of poly(5-alkyl-2-norbornene)s by cationic polymerization. Effect of alkyl substituent length on monomer reactivity,polymer structure and thermal properties

Cationic polymerizations of various 5-alkyl-2-norbornenes (R? NB, where R?H, CH₃, C₂H₅, C₅H₁₁, C₇H₁₅, and C₁₀H₂₁) were carried out by the AlEtCl₂/tert-butyl chloride catalyst system, and the effect of the length of the alkyl substituents on the monomer reactivity and polymerizability, polymer structure, and softening point (SP) were investigated. The introduction of an alkyl substituent into 5-position of the norbornene ring remarkably reduces the monomer reactivity and polymerizability, although the length of alkyl group does not affect them. ¹³C NMR structural analysis of poly(R? NB)'s revealed that not only the normal 5-or 6-alkyl-2,3-unit but also other structures such as 5- or 6-alkyl-2,7-, 4-alkyl-3,6-units, etc., are the most plausible structures for the repeating units. Furthermore, the existence of the 4-alkyl-3,6-repeating unit and of the alkylidene type end-group indicates the presence of an intermediate tertiary cation (5-alkyl-5-norbornyl cation), which causes the low reactivity and polymerizability of the monomers. The SP of oligomers of equal molecular weight (DP _n = 3–5) decreases in the following order: H? NB, CH₃? NB,C₂H₅? NB>C₅H₁₁? NB>C₇H₁₅? NB>C₁₀H₂₁? NB. The effect the alkyl substituent length on the SP is discussed on the basis of the ¹³C spin-lattice relaxation times (T₁) for the carbons in the alkyl group.     

Solid Polymer Electrolytes, 9

Summary: A new hybrid polymer electrolyte system containing polysiloxane and polyether segments has been designed and prepared by epoxide crosslinking. The thermal behavior, structure and ionic conductivity of the hybrid materials were investigated and characterized by differential scanning calorimetry (DSC), ¹³C solid‐state NMR spectroscopy and alternating current (AC) impedance measurements. Two glass transition temperatures were observed, showing a dependence on the composition and LiClO₄/PC content. The miscibility of the polymer components in the hybrid was studied by examining the ¹H spin relaxation times in the laboratory frame (T₁(H)) and in the rotating frame (T_1ρ(H)) with various compositions. Multi‐relaxation T_1ρ(H) behavior has been observed, indicative of the presence of structural heterogeneity on the timescale of T_1ρ(H). These results are correlated and used to interpret the phenomenon of the conductivity of the lithium ion in the matrix of hybrid networks.

Schematic structure of polysiloxane/polyether networks.     

The morphology of poly(ethylene oxide)-barium perchlorate complexes

Poly(ethylene oxide) dissolves a wide variety of inorganic salts and forms solid complexes with multiphase structures. The interaction between polymer chains and ions directs the morphology of the sample. The structure is very sensitive to the preparation methods and the purity of the materials. In this work the morphology of poly(ethylene oxide)-Ba(ClO₄)₂ complexes has been analysed. The effect of the salt concentration and the variations in the molar mass of the polymer was studied. The multiphase structures were investigated by infra-red spectroscopy, differential scanning calorimetry, optical microscopy, ¹³C cross polarization and magic-angle spinning NMR spectroscopy, and wide angle X-ray scattering. Two different crystalline phases and changes in the amorphous phase were found for the complexes when the salt concentration was varied.     

Synthesis and thermotropic properties of new mesogenic diacrylate monomers

A new class of mesogenic α,ω-alkylene bis[4-(4-acryloyloxybenzoyloxy)benzoate]s 1a — 1g were synthesized and studied for their mesophase behaviour. The thermodynamic parameters of the isotropization transition T_i, ΔH_i and ΔS_i show distinct odd-even alternations and demonstrate that the thermotropic liquid crystalline properties in homologous series of mesogens can not be a continuous function of the length of the inner spacer segment. It is anticipated that the investigated diacrylates are of value as precursors of thermotropic semiflexible polymers, crosslinking agents for mesomorphic polymer networks and low molecular weight twin model compounds.     

Calorimetric study of tetrahydro-1,3-oxazin-2-one and poly(oxy-1,3-propanediyliminocarbonyl), and the polymerization/depolymerization equilibrium

In adiabatic vacuum and dynamic calorimeters the temperature dependence of the heat capacity C_p⁰ of tetrahydro-1,3-oxazin-2-one (THO) and poly(oxy-1,3-propanediyliminocarbonyl) (PTHO) was studied between 5 and 500 K. The melting temperature of the monomer and the polymer, the enthalpies of melting of THO and the glass transition temperature of PTHO were determined. In a calorimeter with a static bomb and an isothermal shield the energies of combustion ΔU_comb of the monomer and the polymer were measured. From the experimental data the thermodynamic functions C_p⁰, H⁰(T) – H⁰(O), S⁰(T), G⁰(T) – H⁰(0) were calculated in the range of 0 to 450 K, and enthalpies of combustion ΔH_comb⁰ and thermochemical parameters of formation ΔH_f⁰, ΔS_f⁰, ΔG_f⁰ of the compounds studied were estimated at T = 298.15 K and standard pressure. The results obtained were used to estimated the thermodynamic characteristics of the equilibrium of THO polymerization in bulk (ΔH_pol⁰, ΔS_pol⁰, ΔG_pol⁰), with opening of the six-membered ring and formation of the linear polymer PTHO, in the range 0 to 450 K. It was found that the ΔG_pol⁰ values are always negative and change from ?18 kJ · mol^?1 at 0 K to ?14 kJ · mol^?1 at 450 K. Thus, the process equilibrium is everywhere shifted towards the formation of the polymer. The upper (ceiling) limiting temperature of THO polymerization in bulk at standard pressure is T_ceil = 750 K, that is, considerably higher than the temperature of the onset of the thermal polymer decomposition (450 K).     

Calorimetric study of tetramethylene urethane and of poly(tetramethylene urethane) between 0 and 300 K

In an adiabatic vacuum calorimeter the temperature dependence of the heat capacity C_p⁰ of tetramethylene urethane (TMU) (2-oxo-hexahydro-1,3-oxazepine) and poly(tetramethylene urethane) (PTMU) was studied between 4.3 and 300 K with an uncertainty of about 0.2%. In a calorimeter with a static bomb and an isothermal shield the energies of combustion ΔU_comb of the monomer and the polymer were measured. From the experimental data the thermodynamic functions C_p⁰, H⁰(T)–H⁰(0), S⁰(T), G⁰(T)–H⁰(0) were calculated in the range of 0 to 300 K, and enthalpies of combustion ΔH⁰_comb and thermochemical parameters of formation ΔH⁰_f, ΔS⁰_f, ΔG⁰_f of TMU and PTMU were estimated at T = 298.15 K and standard pressure. The results obtained were used to calculate the thermodynamic characteristics of TMU polymerization in bulk (ΔH⁰_pol, ΔS⁰_pol, ΔG⁰_pol) with opening of the seven-membered ring and formation of the linear polymer PTMU, in the range of 0 to 300 K. It was found that the values of the standard Gibbs function of the process (ΔG⁰_pol = (–41)–(–42) kJ·mol^–1) are always negative and do not substantially depend upon temperature in the whole temperature interval studied. This implies that the equilibrium TMU ⇄ PTMU is far on the side of the polymer. The upper (ceiling) limiting temperature of TMU polymerization is T⁰_ceil > 1 000 K.     

Enthalpy Relaxation in Poly(4‐hydroxystyrene)/Poly(methyl methacrylate) Blends

Summary: The influence of blend composition on enthalpy relaxation behaviour was assessed for miscible blends of poly(4‐hydroxystyrene)/poly(methyl methacrylate) (PHS/PMMA). Values of enthalpy lost (ΔH(T_a, t_a)) were calculated from experimental data plotted against log₁₀(t_a) and modelled using the Cowie‐Ferguson (CF) semi‐empirical model. This gives a set of values for three adjustable parameters, ΔH_∞(T_a), log₁₀(t_c) and β. The blends relaxed more slowly than PMMA, but more quickly and less co‐operatively than PHS. Moreover, the blends released more enthalpy than PMMA, but less than PHS. The enthalpy lost by the fully relaxed glass (ΔH_∞(T_a)) was less than the theoretical amount possible on reaching the state defined by the liquid enthalpy line extrapolated into the glassy region (ΔH_max(T_a)). Infrared spectroscopy was used for assessing the hydrogen bonding interactions in the blends. The ageing results are discussed with reference to the hydrogen bonding interactions.

Dependence of ΔT (□) and ΔC_p (?) on PHS/PMMA blend composition.     

Rapid MR relaxometry using deep learning: An overview of current techniques and emerging trends

Quantitative mapping of MR tissue parameters such as the spin-lattice relaxation time (T₁), the spin-spin relaxation time (T₂), and the spin-lattice relaxation in the rotating frame (T_1ρ), referred to as MR relaxometry in general, has demonstrated improved assessment in a wide range of clinical applications. Compared with conventional contrast-weighted (eg T₁-, T₂-, or T_1ρ-weighted) MRI, MR relaxometry provides increased sensitivity to pathologies and delivers important information that can be more specific to tissue composition and microenvironment. The rise of deep learning in the past several years has been revolutionizing many aspects of MRI research, including image reconstruction, image analysis, and disease diagnosis and prognosis. Although deep learning has also shown great potential for MR relaxometry and quantitative MRI in general, this research direction has been much less explored to date. The goal of this paper is to discuss the applications of deep learning for rapid MR relaxometry and to review emerging deep-learning-based techniques that can be applied to improve MR relaxometry in terms of imaging speed, image quality, and quantification robustness. The paper is comprised of an introduction and four more sections. Section 2 describes a summary of the imaging models of quantitative MR relaxometry. In Section 3, we review existing “classical” methods for accelerating MR relaxometry, including state-of-the-art spatiotemporal acceleration techniques, model-based reconstruction methods, and efficient parameter generation approaches. Section 4 then presents how deep learning can be used to improve MR relaxometry and how it is linked to conventional techniques. The final section concludes the review by discussing the promise and existing challenges of deep learning for rapid MR relaxometry and potential solutions to address these challenges.     

Three‐dimensional ultrashort echo time cones T1ρ (3D UTE‐cones‐T1ρ) imaging

We report a novel three‐dimensional (3D) ultrashort echo time (UTE) sequence employing Cones trajectory and T_1ρ preparation (UTE‐Cones‐T_1ρ) for quantitative T_1ρ assessment of short T₂ tissues in the musculoskeletal system. A basic 3D UTE‐Cones sequence was combined with a spin‐locking preparation pulse for T_1ρ contrast. A relatively short TR was used to decrease the scan time, which required T₁ measurement and compensation using 3D UTE‐Cones data acquisitions with variable TRs. Another strategy to reduce the total scan time was to acquire multiple Cones spokes (N_sp) after each T_1ρ preparation and fat saturation. Four spin‐locking times (TSL = 0–20 ms) were acquired over 12 min, plus another 7 min for T₁ measurement. The 3D UTE‐Cones‐T_1ρ sequence was compared with a two‐dimensional (2D) spiral‐T_1ρ sequence for the imaging of a spherical CuSO₄ phantom and ex vivo meniscus and tendon specimens, as well as the knee and ankle joints of healthy volunteers, using a clinical 3‐T scanner. The CuSO₄ phantom showed a T_1ρ value of 76.5 ± 1.6 ms with the 2D spiral‐T_1ρ sequence, as well as 85.7 ± 3.6 and 89.2 ± 1.4 ms for the 3D UTE‐Cones‐T_1ρ sequences with N_sp of 1 and 5, respectively. The 3D UTE‐Cones‐T_1ρ sequence provided shorter T_1ρ values for the bovine meniscus sample relative to the 2D spiral‐T_1ρ sequence (10–12 ms versus 16 ms, respectively). The cadaveric human Achilles tendon sample could only be imaged with the 3D UTE‐Cones‐T_1ρ sequence (T_1ρ = 4.0 ± 0.9 ms), with the 2D spiral‐T_1ρ sequence demonstrating near‐zero signal intensity. Human studies yielded T_1ρ values of 36.1 ± 2.9, 18.3 ± 3.9 and 3.1 ± 0.4 ms for articular cartilage, meniscus and the Achilles tendon, respectively. The 3D UTE‐Cones‐T_1ρ sequence allows volumetric T_1ρ measurement of short T₂ tissues in vivo.