Nuclear magnetic resonance relaxometry of the normal heart: Relationship between collagen content and relaxation times of the four chambers

The use of nuclear magnetic resonance (NMR) relaxation time measurements for characterization of abnormal cardiac tissue depends upon knowledge of variations of relaxation times of normal myocardium and determinants of these variations. We calculated in vitro NMR T₁ and T₂ relaxation times of canine myocardium from the four cardiac chambers, and determined hydroxyproline concentration (as a measure of collagen) and percent water content of the samples. We found both water content and T₁ relaxation time of the right ventricle to be significantly greater than the left atrium (p < 0.05). T₂ relaxation time of the left ventricle was found to be shorter than each of the other three chambers (p < 0.05). There were significant correlations between the spin-lattice relaxation time and both percent water content (r = 0.58) and hydroxyproline concentration (r = 0.45). A significant correlation was also found between T₂ relaxation time and hydroxyproline concentration (r = 0.49). When T₁ and T₂ were adjusted for water and hydroxyproline content, there was no longer any evidence for significant interchamber differences for either T₁ or T₂. These data suggest that differences in NMR relaxation times exist among the four chambers of the normal canine heart. Furthermore, a major determinant of myocardial spin-lattice relaxation time is tissue water content while both collagen content and percent water content significantly contribute to variability in cardiac chamber T₂ relaxation times.     

A new method for characterization of low grade gliomas using ultra low field magnetic resonance imaging

Low grade gliomas were studied with ultra low field magnetic resonance imaging (ULF MRI). The tumors exhibited high tissue contrast in both T₁ and T₂-weighted images as compared to normal brain tissue. Moreover they were sharply delineated towards the surrounding brain tissue. When compared with X-ray computed tomography the tumors were more readily detected and delineated by using ultra-low field magnetic imaging. A computerassisted classification procedure was used to define new regions of interest for relaxation time estimation. By using this procedure more accurate estimations of the T₁ and T₂ values were obtained.     

NMR relaxation of protons in tissues and other macromolecular water solutions

Nuclear magnetic resonance (NMR) longitudinal (T₁) and transverse (T₂) relaxation parameters have been evaluated for protein solutions, cellular suspensions and tissues using both data from our laboratory and the extensive literature. It is found that this data can be generalized and explained in terms of three water phases: free water, hydration water, and crystalline water. The proposed model which we refer to as the FPD model differs from similar models in that it assumes that free and hydration water are two phases with distinct relaxation times but that T₁ = T₂ in each phase. In addition there is a single correlation time for each rather than a distribution as assumed in most other models. Longitudinal decay is predicted to be single exponent in character resulting from a fast exchange between the free and hydration compartments. Transverse decay is predicted to be multiphasic with crystalline (T₂ 10 μsec), hydration (T₂ 10 sec) and free (T₂ 100 sec) water normally visible. The observed or effective transverse relaxation times for both the hydration and free water phases are greatly affected by the crystalline phase and are much shorter than the inherent relaxation times.     

Selection of pulse sequences producing maximum tissue contrast in magnetic resonance imaging

The importance of spin density [N(H)] and spin-lattice (T₁) and spin-spin (T₂) relaxation in the characterization of tissue by nuclear magnetic resonance (NMR) is clearly recognized. This work considers which optimized pulse sequences provide the best tissue discrimination between a given pair of tissues. The effects of tissue spin density and machine-imposed minimum rephasing echo times (TE_MIN) for achieving maximum signal tissue contrast are discussed. A long TE_MIN sacrifices T₁-dependent contrast in saturation recovery (SR) and inversion recovery (IR) pulse sequences so that spin-echo (SE) becomes the optimum sequence to provide tissue contrast, due to T₂ relaxation. Pulse sequences providing superior performance may be selected based on spin density and T₁ and T₂ ratios for a given pair of tissues. Selection of the preferred pulse sequence and interpulse delay times to produce maximum tissue contrast is strongly dependent on knowledge of tissue spin densities as well as T₁ and T₂ characteristics. As the spin density ratio increases, IR replaces SR as the preferred sequence and SE replaces IR and SR as the pulse sequence providing superior contrast. To select the optimal pulse sequence and interpulse delay times, an accurate knowledge of tissue spin density, T₁ and T₂ must be known for each tissue.     

Investigation of cerebral ischemia using magnetization transfer contrast (MTC) MR imaging

The effects of cerebral ischemia in rat brain were monitored as a function of time using proton MR imaging. Spinspin relaxation time (T₂), proton density, and magnetization transfer contrast (MTC) were measured by MR imaging at various time intervals during a 1-week period following the induction of ischemic damage. Ischemic injury was characterized by a maximization of both T₂ value and MTC appearance at 24 hr postischemic injury. These changes were accompanied by a gradual increase in MR observable water density over the first few days of ischemia. A reduction in the magnetization exchange rate between “free” and “bound” water protons as measured by MTC imaging is at least partially responsible for the elevation in T₂ values observed during ischemia, and may accompany breakdown of cellular structure.     

Myocardial proton spin-lattice relaxation times in vitro: Effect of elapsed time after excision

An assumption made in using excised tissue for in vitro nuclear magnetic resonance (NMR) studies is that variables of interest, such as spin-lattice (T₁) relaxation times, remain stable for periods of time after excision sufficient to perform NMR spectroscopy. In this study, we evaluated the changes in T₁ of rat myocardium, measured at two NMR field strengths, at serial time intervals up to 72 hours postmortem. Left ventricular myocardium from six male Sprague-Dawley rats was excised and stored at room temperature in sealed NMR sample tubes. Spin-lattice relaxation times were determined with a modified inversion-recovery pulse sequence immediately postmortem and at intervals up to 72 hours post-excision; NMR studies were performed using 90 MHz and 360 MHz spectrometers. A gradual decrease in T₁ was noted with increasing time post-excision; T₁ was not significantly shorter than baseline until 72 hours postmortem at either field strength. The rate of change of T₁ was similar at the two field strengths. At any given time post-excision, T₁ was significantly higher (p < 0.001) at 360 MHz than at 90 MHz. We conclude that, with proper tissue handling and storage techniques, rat myocardial T₁ is stable postmortem sufficiently long to permit meaningful NMR studies of excised tissue.     

基于多弛豫信号补偿的快速磁共振T1ρ散布成像

定量磁共振成像（MRI）可量化组织特性,是科学研究和临床研究的重要工具.旋转坐标系下的自旋-晶格弛豫时间（T_1ρ）能反映水与大分子之间的低频交互作用,在3 T及以上的高场环境下,T_1ρ受水和不稳定质子之间化学交换的影响较大,通过测量弛豫率随自旋锁定场强度的变化而得到其分布情况（T_1ρ散布）,可用于分析和量化质子的交换过程,因此T_1ρ散布是一种重要的定量MRI技术.然而,获得不同自旋锁定场强下T_1ρ加权图像的时间过长,限制了其应用范围.针对这一问题,本研究提出一种基于多弛豫信号补偿策略的快速T_1ρ散布成像方法.该方法将不同锁定频率下的T_1ρ加权图像补偿到同一信号强度水平,并结合低秩与稀疏建立重建模型.实验结果表明,该方法在加速倍数高达7倍时仍获得了较好的重建结果.     

Validity of NMR pore-size analysis of cultural heritage ancient building materials containing magnetic impurities

NMR relaxation time distributions, obtained with laboratory and portable devices, are utilized to characterize the pore-size distributions of building materials coming from the Roman remains of the Greek-Roman Theatre of Taormina. To validate the interpretation of relaxation data in terms of pore-size distribution, comparison of results from standard and in situ NMR experiments with results of the mercury intrusion porosimetry (MIP) has been made. Although the pore-size distributions can be obtained by NMR in terms of either longitudinal (T₁) or transverse (T₂) relaxation times distributions, the shorter duration of the T₂ measurement makes it, in principle, preferable, although the determination of T₂ distributions is not necessarily an easy alternative to finding T₁ distributions. Among other things, the T₁ distribution is almost independent of the inhomogeneity of the magnetic field, while the T₂ distribution is strongly influenced by it. This paper was aimed at answering two questions: what are the validity limits to interpret NMR data in terms of pore-size distributions and whether the portable device can successfully be applied as a non-destructive and non-invasive tool for in situ NMR analysis of building materials, particularly those of Cultural Heritage interest.     

回波间隔对核磁共振表观孔隙度的影响及矫正方法

本文对具有特定横向弛豫时间（T₂）的硫酸铜溶液进行了多回波间隔（T_E）的核磁共振（NMR）实验，并利用数值模拟对32组具有不同弛豫分量的模型进行了变T_E模拟实验，定量研究了T_E对致密油气、页岩气等低孔低渗储层NMR孔隙度的影响规律.实验结果表明，随着T_E的增大，各T₂弛豫组分NMR孔隙度先维持在100%左右，然后迅速衰减，当T_E增加到一定数值时，趋近于0；不同T₂弛豫组分NMR孔隙度开始迅速衰减及最后变为0的T_E值存在显著差异.根据不同T₂弛豫组分NMR孔隙度与T_E的关系，将整个NMR测量分为无损测量区、快速衰减区、无效参数区和仪器盲区4个区域.对特定弛豫组分而言，在快速衰减区弛豫组分损失量与T_E呈对数关系，本文还给出了该区域NMR孔隙度的校正公式及方法.     

Molecular dynamics in solid riboflavin as studied by 1H NMR

Spin–lattice relaxation times T₁ and T_1d as well as NMR second moment were employed to study the molecular dynamics of riboflavin (vitamin B₂) in the temperature range 55–350 K. The broad and flat T₁ minimum observed at low temperatures is attributed to the motion of two nonequivalent methyl groups. The motion of the methyl groups is interpreted in terms of Haupt's theory, which takes into account the tunneling assisted relaxation. An additional mechanism of relaxation in the high temperature region is provided by the motion of a proton in one of the hydroxyl groups. The Davidson–Cole distribution of correlation times for this motion is assumed.     

Spin fluctuations in weakly itinerant ferromagnets Y(Co1−xAlx)2

The nuclear spin-lattice relaxation rate, 1/T₁, has been measured in weak itinerant ferromagnets Y(Co_1−xAl_x)₂. The temperature and magnetic field dependence of 1/T₁T has been found to be well described by the self-consistent renormalization (SCR) theory of spin fluctuations. The parameters characterizing spin fluctuations in this system were estimated from NMR and magnetic measurements. The temperature dependence of susceptibility calculated from these parameters well reproduces the experimental results.     

C CPMAS studies of plant cell wall materials and model systems using proton relaxation-induced spectral editing techniques

The solid state ¹³C CPMAS NMR spectra of plant cell walls are often complex owing to superposition of resonances from different polysaccharides and the heterogeneity of the cell wall assembly. In this paper, we describe the application of a set of proton relaxation-induced spectral editing (PRISE) experiments which combine ¹H relaxation properties (T₁, T_1ρ, T₂) with ¹³C high resolution spectroscopy (CPMAS) to relate the dynamics of the plant cell walls and model systems to their domain structural details. With PRISE it has been found that in plant cell wall materials, cellulose is always associated with the long components of spin–lattice relaxation in both the laboratory and rotating frames whereas non-cellulose polysaccharides (pectin and hemicellulose) are associated with the short ones. For the proton T₂ relaxation, cellulose is only associated with the short component (below 20 μs), pectin contributes to both the short component and the long one.     

In vivo relaxation of N-acetyl-aspartate, creatine plus phosphocreatine, and choline containing compounds during the course of brain infarction: a proton MRS study.

Localized water suppressed proton spectroscopy has opened up a new field of pathophysiological studies of severe brain ischemia. The signals obtained with the pulse sequences used so far are both T₁ and T₂ weighted. In order to evaluate the extent to which changes in metabolite signals during the course of infarction can be explained by changes in T₁ and T₂ relaxation times, eight patients with acute stroke were studied. STEAM sequences with varying echo delay times and repetition times were used to measure T₁ and T₂ of N-acetyl-aspartate (NAA), creatine plus phosphocreatine (Cr+PCr) and choline containing compounds (CHO) in a 27-ml voxel located in the affected area of the brain. Ten healthy volunteers served as controls. We found no difference in T₁ or T₂ of the metabolites between the patients and the normal controls. The T₂ of CHO was longer than that of NAA and Cr+PCr. Our results indicate that spectra obtained in brain infarcts and normal tissue with the same acquisition parameters are directly comparable with respect to relative signal intensities as well as signals scaled with internal and external standards.     

Novel MR image contrast mechanisms in epilepsy

A range of magnetic resonance (MR) parameters are introduced, which can give rise to image contrast by using suitable pulse sequences, and that can be measured quantitatively. Their relationship to tissue pathology is given as far as possible. Techniques for their measurement, and results from multiple sclerosis, stroke, and epilepsy are given. The parameters are proton density, T₁, T₂, transverse magnetisation decay, which gives estimates of extracellular water and myelin concentrations, magnetisation transfer ratio and T_1sat, and diffusion (including trace and anisotropy measured from the tensor matrix).     

基于二维核磁共振弛豫谱的雷四段孔隙度计算方法

基于常规测井资料的体积模型和一维核磁共振（NMR）测井资料的固定截止值法，可以计算地层孔隙度，但是扩径段孔隙度计算结果偏大.本文通过川西海相雷四段岩心样品NMR实验和实测二维NMR测井资料，开展扩径段的T₂、T₁孔隙度计算方法研究.首先总结不同孔径流体在T₂谱和T₁谱上的响应特征，分析钻井液流体峰截止值的分布范围和影响因素，建立钻井液流体弛豫时间截止值的计算模型；然后根据未扩径段和扩径段的粘土束缚水弛豫时间截止值确定原则，确立变粘土束缚水弛豫时间截止值的有效孔隙度计算方法.多口井的应用效果表明，基于T₁谱的孔隙度计算结果精度更高、定量分析误差小，能有效解决扩径段孔隙度计算结果偏大的问题，满足储层评价的要求.     

The application of proton nuclear magnetic resonance imaging for the in vivo characterisation of chemically induced renal lesions in rats over a prolonged time study

Renal cortical and medullary spin-lattice (T₁) relaxation times were measured at various time points over a period of 56 days following the administration of a single i.p. injection of 100 mg/kg 2-bromoethanamine hydrobromide (BEA), 200 mg/kg hexachloro-1,3-butadiene (HCBD) or 100 mg/kg puromycin aminonucleoside (PAN) to male Wistar rats. Administration of a single injection of HCBD caused a dramatic, immediate rise in the cortical T₁ values above control values, and these levels remained elevated until, by Day 28 postinjection the levels were back to control values. Administration of BEA also caused an elevation in cortical T₁ values, but in this case these values remained above control values for the rest of the study. The administration of PAN did not produce any significant increases in cortical T₁ values until 14 days postinjection. The elevated T₁ values remained above control values for the rest of the study. These increases observed in cortical T₁ values appeared to be mirrored by decreases in medullary T₁ values. Increases in cortical T₁ values were accompanied by visual changes in the NMR images and enlargement of the kidneys. The histological findings were consistent with the NMR data, confirming that morphologically the tissues did show a full recovery by Day 28 in the HCBD-treated animals. This was not the case following injection of both BEA and PAN, where necrosis was not reversible and there was no recovery of the tissues.     

Detection of toxoplasmic lesions in mouse brain by USPIO-enhanced magnetic resonance imaging

The objective of this study was to examine the feasibility of detecting toxoplasmic brain lesions in a mouse model of cerebral toxoplasmosis by ultrasmall superparamagnetic particles of iron oxide (USPIO)-enhanced magnetic resonance imaging (MRI). Toxoplasmosis encephalitis was induced in Kunming mice by intracerebral injection of Toxoplasma gondii tachyzoites. T₂- and T₂*-weighted MRI was performed 1, 3, 4, 5 and 6 days after infection before USPIO injection; immediately after USPIO injection; and 24 h later. A comparison of USPIO enhancement and Gd-DTPA enhancement was made in three toxoplasmic mice 4 days after infection. Hematoxylin and eosin staining and Prussian blue staining were performed to detect inflammatory reactions and presence of iron in and around the toxoplasmic brain lesions. Nonenhanced T₂-/T₂*-weighted imaging detected few abnormalities in the brain up to 5 days. Most mice developed prominent hydrocephalus at 6 days. Gd-DTPA-enhanced imaging showed prominent enhancement of the cerebral ventricles but revealed only few space-occupying lesions in the parenchyma. USPIO-enhanced T₂*-weighted imaging showed improved detection of toxoplasmic brain lesions that were invisible to nonenhanced T₂-/T₂*-weighted imaging and gadolinium-enhanced imaging. Most of the enhancing lesions showed nodular enhancement immediately after USPIO injection, some of which changed appearance 24 h later, having a ring enhancement at the outer rim. It can be concluded that USPIO enhancement of the toxoplasmic lesions may reflect blood–brain barrier impairment and/or inflammatory reactions associated with these lesions. USPIO-enhanced imaging may be used in combination with gadolinium-enhanced imaging to provide better characterization of toxoplasmic brain lesions and, potentially, improve the differential diagnosis of toxoplasmosis encephalitis.     

Establishing norms for age-related changes in proton T1 of human brain tissue in vivo

The goal of this study was to determine the expected normal range of variation in spin-lattice relaxation time (T₁) of brain tissue in vivo, as a function of age. A previously validated precise and accurate inversion recovery method was used to map T₁ transversely, at the level of the basal ganglia, in a study population of 115 healthy subjects (ages 4 to 72; 57 male and 58 female). Least-squares regression analysis shows that T₁ varied as a function of age in pulvinar nucleus (R² = 56%), anterior thalamus (R² = 51%), caudate (R² = 50%), frontal white matter (R² = 47%), optic radiation (R² = 39%), putamen (R² = 36%), genu (R² = 22%), occipital white matter (R² = 20%) (all p < 0.0001), and cortical gray matter (R² = 53%) (p < 0.001). There were no significant differences in T₁ between men and women. T₁ declines throughout adolescence and early adulthood, to achieve a minimum value in the fourth to sixth decade of life, then T₁ begins to increase. Quantitative magnetic resonance imaging provides evidence that brain tissue continues to change throughout the lifespan among healthy subjects with no neurologic deficits. Age-related changes follow a strikingly different schedule in different brain tissues; white matter tracts tend to reach a minimum T₁ value, and to increase again, sooner than do gray matter tracts. Such normative data may prove useful for the early detection of brain pathology in patients.     

Comparative analysis of the 1H NMR relaxation enhancement produced by iron oxide and core-shell iron-iron oxide nanoparticles

Physicochemical and magnetorelaxometric characterization of the colloidal suspensions consisting of Fe-based nanoparticles coated with dextran have been carried out. Iron oxide and iron core/iron oxide shell nanoparticles were obtained by laser-induced pyrolysis of Fe(CO)₅ vapours. Under different magnetic field strengths, the colloidal suspension formed by iron oxide nanoparticles showed longitudinal (R₁) and transverse (R₂) nuclear magnetic relaxation suspension (NMRD) profiles, similar to those previously reported for other commercial magnetic resonance imaging (MRI) contrast agents. However, colloidal suspension formed by ferromagnetic iron-core nanoparticles showed a strong increase of the R₁ values at low applied magnetic fields and a strong increase of the R₂ measured at high applied magnetic field. This behaviour was explained considering the larger magnetic aggregate size and saturation magnetization values measured for this sample, 92 nm and 31 emu/g Fe, respectively, with respect to those measured for the colloidal suspensions of iron oxide nanoparticles (61 nm and 23 emu/g Fe). This suspension can be used both as T₁ and T₂ contrast agent.     

基于T2截止值的幂函数构建毛管压力曲线

岩心核磁共振（NMR）T₂谱和毛管压力曲线都在一定程度上反映了岩石孔隙结构，理论分析表明可利用T₂谱构建毛管压力曲线，由此快速获取储层孔隙结构的信息．本文对18块岩样T₂分布和毛管压力曲线进行了分析，提出将T₂截止值作为幂函数的分段点，采用分段幂函数方法构建岩样的NMR毛管压力曲线，并与采用不分段幂函数方法获得的毛管压力曲线进行了对比．研究结果表明，与不分段幂函数方法相比，分段幂函数方法得到的结果和压汞实验测定的毛管压力曲线吻合度更高，平均拟合度（R²）达到0.943 1，并且论证了T₂截止值作为分段点进行分段幂函数法构建NMR毛管压力曲线的合理性和可靠性．T₂截止值的引入提高了幂函数法构建NMR毛管压力曲线的精度，是利用NMR T₂谱构建岩样毛管压力曲线的有价值探索．