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.     

MRI measurements of the dependence on T1 of the echo amplitudes using a multiple spin-echo scheme

Analytical calculations using the Bloch formalism were performed to assess the dependence on T₁ of the echo amplitudes for the Phase-Alternating Phase-Shift (PHAPS) multiple spin-echo protocol. Measurements in a 0.5 T MR imaging unit were performed to ratify the analytical results. especially for low T₂ values, the echo amplitudes were erroneous, with an increasing contribution from stimulated echo components with increasing T₁. Apart from affecting T₂ estimates, stimulated echoes generated a non-monoexponential signal decay of the echo trains. The results confirmed previous simulation studies as regards the dependence on T₁ of T₂ estimates from PHAPS.     

MRI of hepatic lymphoma

Thirteen patients with biopsy proven hepatic lymphoma (2 Hodgkin, 11 Non-Hodgkin) and a control group of 15 patients with hepatic metastases were analyzed quantitatively and qualitatively by MRI. Focal hepatic lymphoma was most reliably detected (eight of eight patients) and appeared hypointense relative to liver on T₁ weighted (CNR − 7.4 ± 2.3) and hyperintense on T₂ weighted (CNR + 8.4 ± 2.9) images. The mean T₁ and T₂ relaxation times of focal hepatic lymphoma (T₁ = 832 ± 234 msec, T₂ = 84 ± 16 ms) differed significantly from adjacent non-tumorous liver (T₁ = 420 ± 121 ms, T₂ = 51 ± 9 ms; p < 0.05), however CNR values and relaxation times were similar to those of hepatic metastases. Diffuse hepatic lymphoma (microscopic periportal infiltration) was undetectable by MRI in three patients by either morphologic features or quantitative criteria. A mixed pattern of hepatic lymphoma (focal lesions and diffuse infiltration) showed focal areas of slightly decreased signal intensity on T₁ weighted images (CNR = −1.7 ± 0.4) while T₂ weighted images revealed multiple regions of focal hyperintensity (CNR = +13.3 ± 8.4) superimposed on a diffusely hyperintense liver. Our experience demonstrates that either T₁ or T₂ weighted techniques are useful in detecting focal and that T₂ weighted techniques are useful in detecting mixed hepatic lymphoma. Conventional image derived relaxation time measurements and quantitative parameters were of no additional diagnostic value.     

Temporal fluctuations in proton relaxation times

We studied mouse liver, heart and kidney for possible diurnal fluctuations of T₁ and T₂. In a subgroup of animals, we attempted to relate T₁ and T₂ of the organ samples to their water and lipid content (and in the liver, also to glycogen content). Diurnal periodic fluctuation was found only in liver T₂ and was of a very minor degree. Regression analysis of organ T₂ estabilished relationships with chemical composition which explained 25%–40% of the observed variation in T₂. No relationship with T₁ could be established.     

Cross relaxation of the proton magnetization in ammonium compounds

Expressions are derived for the time constants T_1D and T_SD of the NH₄ protons in tunneling ammonium compounds below the line-width transition temperature. T_1D characterizes the speed of the spin-lattice relaxation of the dipolar energy and T_SD the speed of the cross relaxation between the A and T symmetry species. The expressions should be valid if all the tunnel splittings between the T species levels are larger than the magnetic dipolar interaction. Predictions are compared with new experimental results on T_SD in (NH₄)₂PbCl₆ and with some earlier results on T_SD and T_1D in (NH₄)₂ SnBr₆ and NH₄ClO₄. They support the conclusion that for T_1D>T_SD the T levels are nondegenerate, while the condition T_1D< T_SD refers to at least a partial degeneracy.     

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.     

Magnetization transfer MRI of mouse brain reveals areas of high neural density

Extending applications of magnetization transfer contrast (MTC) in magnetic resonance imaging (MRI) of the human central nervous system, this work quantitatively describes MTC of the murine brain. As a novel finding, complementing T₁- and T₂-weighted MRI, MTC allows for the distinction of densely packed gray matter from normal gray and white matter. Examples include the Purkinje cell layer and the granular cell layer in the mouse cerebellum as well as the delineation of the CA3 subfield of the hippocampus relative to surrounding hippocampal gray matter and white matter tracts such as the hippocampal fimbria. Using a kainate lesion model, the CA3 hyperintensities in MTC and T₁-weighted MRI are assigned to the densely packed somata of pyramidal cells.     

Normal state magnetism of the high Tc cuprate superconductors

Magnetic measurements of various types have played an essential role in establishing the novel normal state characteristics of high transition temperature (T_c) superconductors with T_c > 23 K. Among these materials, the highest T_c's ( 125 K) are exhibited by the layered cuprates. In this paper, the normal state magnetic susceptibilities of the cuprates are reviewed and interpreted in the context of magnetic neutron scattering and other magnetic measurements, using the La_2−xM_xCuO₄-type and YBa₂Cu₃O_6+x-type materials as prototypical examples. The evolution of the magnetism upon doping the insulating antiferromagnetic “parent” compounds with x = 0 to form the high temperature superconductors is described. A recurrent property which differentiates these materials from conventional superconductors is the existence of strong antiferromagnetic correlations in the metallic state on the same sublattice of the structure in which the itinerant carriers reside.     

Tissue characterization by image processing subtraction: windowing of specific T1 values.

A method for windowing specific T₁ values is presented. A 1.0 T imager with two routine pulse sequences was employed: A T₁-weighted spin echo (SE) sequence and a short tau inversion recovery STIR sequence (fat-suppressed IR). A T₁ window for fat was obtained by subtracting the STIR image from the SE image. Negative values were coded black. The method was tested on a normal human thigh, on a human liver with confirmed fatty infiltration, and on the livers of four live burbots. The fat-containing tissues of the two human volunteers were well depicted. The differences in fat concentration among the burbot livers were also clearly shown. The fat intensity seen in the images correlated well with the chemically measured fat concentration. This subtraction method for windowing T₁ values proved feasible for fat. The method could be used for tissues with other short T₁ values as well.     

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.     

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.     

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．     

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.     

Effects of magic-angle spinning on spin-lattice relaxations in talc

The spin-lattice relaxation times T₁ of ¹H and ²⁹Si spins in talc have been measured at room temperature with and without magic-angle spinning (MAS) of the sample. Paramagnetic impurities work as relaxation centers. ¹H T₁ depends on the spinning rate, whereas ²⁹Si T₁ is independent of the spinning rate. These facts demonstrate that spin diffusion plays an important role in ¹H relaxation but not in ²⁹Si relaxation. ²⁹Si spins relax through dipole-dipole interactions with electron spins directly, which mechanism is not affected by spinning. The relaxation rates have been analyzed theoretically.     

非自治物质畸形波的传播操控

研究了(1+1)维的变系数Gross-Pitaevskii方程, 获得了该方程的精确畸形波解. 基于该精确畸形波解, 深入研究了非自治物质畸形波在随时间指数变化的相互作用下的传播动力学行为, 发现非自治畸形波除具有“来无影、去无踪”的不可预测特性外, 也可实现完全激发、抑制激发以及维持激发等操控. 研究表明, 畸形波操控的关键是对累积时间的最大值T_max 与峰值位置T₀ (或T_I,T_II)值大小关系的调节. 当T_max > T₀ (或T_I,T_II)时畸形波被快速地完全激发, 热原子团中的原子增加到凝聚体中. 当T_max = T₀ (或T_I,T_II) 时畸形波激发到最大振幅, 可以维持相当长的时间而不消失, 热原子团中的原子增加到凝聚体中. 当T_max < T₀ (或T_I,T_II)时畸形波没有充足的时间来激发而被抑制甚至消失, 凝聚体中的原子减少. 这些结果在理论和实际应用上具有启迪意义.     

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.     

Radiation-induced changes in phosphorus T1values in human melanoma xenografts studied by P-MRS

³¹P-magnetic resonance spectroscopy (MRS) has been shown to be a promising method for monitoring tumor response to radiation therapy. The purpose of the work reported here was to investigate whether the usefulness of ³¹P-MRS might be enhanced by measurement of spin-lattice relaxation times (T₁s) in addition to resonance ratios. The work was based on the hypothesis that tumors having a high probability of being controlled locally would show shortened T₁s during the treatment course due to reoxygenation and development of necrosis. BEX-t human melanoma xenografts, which show efficient reoxygenation and development of necrosis following single dose irradiation, were used as tumor models. Tumors were treated with single doses of 5.0 or 15.0 Gy and the T₁s of the inorganic phosphate and nucleoside triphosphate β resonances were measured as a function of time after irradiation by using the superfast inversion recovery method. Fractional tumor water content was determined by drying excised tumors at 50°C until a constant weight was reached. The T₁s in irradiated tumors were either longer than or not significantly different from those in unirradiated control tumors. The increase in the T₁s following irradiation coincided in time with a radiation-induced increase in tumor water content, suggesting a causal relationship. The effects of reoxygenation and development of necrosis on T₁s were probably overshadowed by the effects of tumor water content. Consequently, the usefulness of ³¹P-MRS in monitoring tumor response to radiation therapy might not be significantly enhanced by measurement of T₁s.     

Measurement of the analysing powers iT11, T20, T21 and T22 in elastic deuteron-proton scattering

The analysing powers iT₁₁, T₂₀, T₂₁ and T₂₂ for the elastic scattering of polarized deuterons by protons have been measured at deuteron energies of 6, 8, 10 and 11.5 MeV with the polarized deuteron beam from a tandem accelerator. The measurements cover the c.m. angular range from 40° to 155° and show small but non-zero analysing powers at all energies.     

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.     

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.