Optimization of the irradiation power in chemical exchange dependent saturation transfer experiments

In chemical exchange dependent saturation transfer imaging experiments, exchangeable solute protons are saturated and the transfer of saturation to water is subsequently detected. When the applied irradiation power is comparable to the resonance frequency difference between the water protons and saturated solute protons, the proton transfer (PT) efficiency is reduced due to concomitant direct saturation effects. In this study, the PT process is modeled using a two-pool system. An empirical general proton transfer ratio (PTR) equation for arbitrary RF irradiation power is derived, and its optimal power to maximize the PTR is analyzed. The results are confirmed experimentally on 4.7 T using a poly-L-lysine solution. The theory provides a useful tool for optimizing the irradiation power of the PT sequences in the presence of direct saturation effects.     

Simultaneous determination of labile proton concentration and exchange rate utilizing optimal RF power: Radio frequency power (RFP) dependence of chemical exchange saturation transfer (CEST) MRI

Chemical exchange saturation transfer (CEST) MRI is increasingly used to probe mobile proteins and microenvironment properties, and shows great promise for tumor and stroke diagnosis. However, CEST MRI contrast mechanism is complex, depending not only on the CEST agent concentration, exchange and relaxation properties, but also varying with experimental conditions such as magnetic field strength and RF power. Hence, it remains somewhat difficult to quantify apparent CEST MRI contrast for properties such as pH, temperature and protein content. In particular, CEST MRI is susceptible to RF spillover effects in that RF irradiation may directly saturate the bulk water MR signal, leading to an optimal RF power at which the CEST contrast is maximal. Whereas RF spillover is generally considered an adverse effect, it is noted here that the optimal RF power strongly varies with exchange rate, although with negligible dependence on labile proton concentration. An empirical solution suggested that optimal RF power may serve as a sensitive parameter for simultaneously determining the labile proton content and exchange rate, hence, allowing improved characterization of the CEST system. The empirical solution was confirmed by numerical simulation, and experimental validation is needed to further evaluate the proposed technique.     

A new class of contrast agents for MRI based on proton chemical exchange dependent saturation transfer (CEST)

It has been previously shown that intrinsic metabolites can be imaged based on their water proton exchange rates using saturation transfer techniques. The goal of this study was to identify an appropriate chemical exchange site that could be developed for use as an exogenous chemical exchange dependent saturation transfer (CEST) contrast agent under physiological conditions. These agents would function by reducing the water proton signal through a chemical exchange site on the agent via saturation transfer. The ideal chemical exchange site would have a large chemical shift from water. This permits a high exchange rate without approaching the fast exchange limit at physiological pH (6.5-7.6) and temperature (37 degrees C), as well as minimizing problems associated with magnetic field susceptibility. Numerous candidate chemicals (amino acids, sugars, nucleotides, heterocyclic ring chemicals) were evaluated in this preliminary study. Of these, barbituric acid and 5, 6-dihydrouracil were more fully characterized with regard to pH, temperature, and concentration CEST effects. The best chemical exchange site found was the 5.33-ppm indole ring -NH site of 5-hydroxytryptophan. These data demonstrate that a CEST-based exogenous contrast agent for MRI is feasible.     

基于CEST机制的pH成像方法、原理和应用

化学交换饱和转移（Chemical Exchange Saturation Transfer,CEST）技术作为一种新型的磁共振成像（Magnetic Resonance Imaging,MRI）技术.它的原理为溶质池中被激发饱和的质子与游离水中未被饱和的质子间的化学交换,能够引起水质子磁共振信号的下降,从而获得组织内生物分子的相关信息.由于质子间的交换速率k_ex与组织微环境的pH值之间存在直接联系,因而可以通过溶质质子的CEST信号对活体组织进行pH成像.目前用于pH成像的溶质分子既包括内源性游离的蛋白质、多肽分子,还包括一系列的外源性小分子和金属螯合物.通过不同类型的比率法、内源性胺和酰胺浓度-独立检测（Amine and Amide Concentration-independent Detection,AACID）等成像方法,能够获得肾脏、中风脑组织以及肿瘤组织的pH图谱.本文详细总结了2000年以来利用CEST技术进行pH成像方面的研究进展,包括对比剂、成像方法和相关应用,展望了活体组织pH成像的发展趋势和应用前景.     

1.5 T磁共振化学交换饱和转移成像的影响因素分析

本文探讨1.5 T磁共振化学交换饱和转移（Chemical Exchange Saturation Transfer,CEST）成像的影响因素.通过试管模型和临床病例,采用GE Signa HDe 1.5 T磁共振成像（Magnetic Resonance Imaging,MRI）扫描仪分别进行不同矩阵、激励次数、翻转角、磁化传递翻转角的CEST成像对比分析,以及不同激励次数、磁化传递翻转角的Z谱分析,并从成像组织、成像设备、成像技术等方面对原始图信号、酰胺质子转移（Amide Proton Transfer,APT）信号及Z谱进行分析研究.实验结果表明1.5 T MRI扫描仪的CEST图像信噪比相对较低,且磁场稳定性及均匀度影响了CEST成像的效果.在其他参数不变的情况下,降低采集矩阵和增加激励次数与翻转角可以增加原始图像信噪比.磁化传递翻转角为105°时,CEST成像效果最好.激励次数为2、磁化传递翻转角为105°时,所得数据符合组织Z谱情况.模型Z谱在磁化传递频率为-294~-194 Hz范围可显示30%谷氨酸（Glu）、碘剂（I₃₂₀）、纯水（H₂O）、肌酸（Cr）的信号差异,与H₂O差异最大处在-244~-214 Hz.原始图像信号30% I₃₂₀明显高于Glu、H₂O、Cr,Cr略低于Glu,APT图Cr略低于Glu.25例脑肿瘤的APT图呈高信号、12例脑梗塞的APT图呈低信号,CEST原始图像均可区分病变区域.有12例因采集时间、患者配合情况、环境及室温等影响导致CEST成像的失败.由此得出1.5 T场强下,CEST技术受到成像组织、设备、技术等因素的影响,需要进行多方面优化.在保证磁场稳定性及均匀度的情况下,优化参数的CEST成像和Z谱成像可以区分代谢物及其浓度.     

Exchange-mediated contrast in CEST and spin-lock imaging

Purpose

Magnetic resonance images of biological media based on chemical exchange saturation transfer (CEST) show contrast that depends on chemical exchange between water and other protons. In addition, spin–lattice relaxation rates in the rotating frame (R_1ρ) are also affected by exchange, especially at high fields, and can be exploited to provide novel, exchange-dependent contrast. Here, we evaluate and compare the factors that modulate the exchange contrast for these methods using simulations and experiments on simple, biologically relevant samples.

Methods

Simulations and experimental measurements at 9.4 T of rotating frame relaxation rate dispersion and CEST contrast were performed on solutions of macromolecules containing amide and hydroxyl exchanging protons.

Results

The simulations and experimental measurements confirm that both CEST and R_1ρ measurements depend on similar exchange parameters, but they manifest themselves differently in their effects on contrast. CEST contrast may be larger in the slow and intermediate exchange regimes for protons with large resonant frequency offsets (e.g. > 2 ppm). Spin-locking techniques can produce larger contrast enhancement when resonant frequency offsets are small (< 2 ppm) and exchange is in the intermediate-to-fast regime. The image contrasts scale differently with field strength, exchange rate and concentration.

Conclusion

CEST and R_1ρ measurements provide different and somewhat complementary information about exchange in tissues. Whereas CEST can depict exchange of protons with specific chemical shifts, appropriate R_1ρ-dependent acquisitions can be employed to selectively portray protons of specific exchange rates.     

纳米静态随机存储器低能质子单粒子翻转敏感性研究

针对65, 90, 250 nm三种不同特征尺寸的静态随机存储器基于国内和国外质子加速器试验平台, 获取了从低能到高能完整的质子单粒子翻转截面曲线. 试验结果表明, 对于纳米器件1 MeV以下低能质子所引起的单粒子翻转截面比高能质子单粒子翻转饱和截面最高可达3个数量级. 采用基于试验数据和器件信息相结合的方法, 构建了较为精确的复合灵敏体积几何结构模型, 在此基础上采用蒙特卡罗方法揭示了低能质子穿过多层金属布线层, 由于能量岐离使展宽能谱处于布拉格峰值的附近, 通过直接电离方式将能量集中沉积在灵敏体积内, 是导致单粒子翻转截面峰值的根本原因. 并针对某一轨道环境预估了低能质子对空间质子单粒子翻转率的贡献.     

CEST成像不同量化方式在急性帕金森氏病小鼠模型研究中的应用比较

本文通过比较7 T场强下化学交换饱和转移（chemical exchange saturation transfer,CEST）成像技术不同量化方式在急性帕金森氏病小鼠模型研究中的应用效果,探讨了客观无创的帕金森氏病研究方案.使用Bruker PharmaScan 7 T小动物磁共振成像（Magnetic Resonance Imaging,MRI）扫描仪,对经1-甲基-4-苯基-1,2,3,6-四氢吡啶（1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine,MPTP）急性造模前及造模后第3、10天的小鼠黑质、皮层及海马进行扫描,计算弛豫时间T₁、T₂、MTR_asym（magnetization transfer ratio based on asymmetry analysis）、MTR_rex（magnetization transfer ratio yielding Rex）、AREX（apparent exchange-dependent relaxation）及5池拟合后的胺峰面积Area_amine、酰胺峰面积Area_amide.结果显示仅黑质中融合了倒Z谱分析和5池洛仑兹拟合所得的量化指标MTR_rex、AREX及Area_amine在造模后显著减小,与黑质免疫组化结果一致,而T₁、T₂以及基于Z谱非对称性分析的MTR_asym未见统计学差异,这表明此量化方式消除了直接饱和效应及磁化转移效应的影响,准确性上要优于Z谱非对称分析法,更能正确地提示帕金森氏病黑质的变化.     

Quantitative magnetization transfer by trains of radio frequency pulses in human brain: extension of a free evolution model to continuous-wave-like conditions

A theoretical model of free evolution between repeated magnetic transfer (MT) pulses was extended to continuous-wave (CW)-like conditions showing that only the repetitive "direct" saturation of bulk water changes the transient and stationary behavior. The influence of the pulse repetition period (PR) on progressive saturation was studied in cortical gray matter (GM) and central white matter (WM) under conditions of short periods of free evolution and strong macromolecular saturation. Interpulse delays of 3 ms were achieved in vivo on a 1.5-T MR system with bell-shaped MT pulses of 12-ms duration and nominal flip angles of up to 1440 degrees and single-shot readout by a stimulated echo acquisition mode localization sequence. The frequency offset was chosen between 1 and 3 kHz to avoid excessive direct saturation. The stationary MT ratio (MTR) followed an inverse linear PR dependence, showing a consistent partial saturation of about 90% at zero PR for both WM and GM. Comparison to a relaxation-matched liquid indicated the presence of MT, but not necessarily of direct saturation. The transient behavior indicated considerable direct saturation, but this could also be explained by MT. These inconsistencies showed that the intervals of time evolution in our experiments were too long to be modeled by CW-like conditions. Free evolution takes place during the whole PR rather than during the interpulse delay only. Quantification using the rates of free evolution theory yielded the saturations and rate constants necessary to explain the observed behavior. The theory of rapid CW-like pulsing provides an upper limit for the rate of progressive saturation. This limit is approached at PR below an estimated value of 5 ms. The phenomenological PR dependence of the steady-state MTR may indicate that MT exceeded the direct saturation. Unlike to an idealized CW experiment, the extrapolated value at zero PR is subject to direct effects and not a physically meaningful constant.     

Z-spectroscopy with Alternating-Phase Irradiation

Magnetization transfer (MT) MRI and Z-spectroscopy are tools to study both water-macromolecule interactions and pH-sensitive exchange dynamics between water and the protons of mobile chemical groups within these macromolecules. Both rely on saturation of frequencies offset from water and observation of the on-resonance water signal. In this work, an RF saturation method called Z-spectroscopy with Alternating-Phase Irradiation (ZAPI) is introduced. Based on the T(2)-selectivity of the irradiation pulse, ZAPI can be used to separate the different contributions to a Z-spectrum, as well as to study the T(2) distribution of the macromolecules contributing to the MT signal. ZAPI can be run at resonance for water and with low power, thus minimizing problems with specific absorption rate (SAR) limits in clinical applications. In this paper, physical and practical aspects of ZAPI are discussed and the sequence is applied in vitro to sample systems and in vivo to rat head to demonstrate the method.     

基于神经网络拟合的腹部化学交换饱和转移成像

磁共振成像（Magnetic Resonance Imaging，MRI）化学交换饱和转移（Chemical Exchange Saturation Transfer，CEST）技术在临床诊断中展现了巨大的潜力，但在腹部成像中受到主磁场偏移量大的挑战，而且利用传统的非对称性分析法得到的酰胺质子转移（Amide Proton Transfer，APT）成像对比度受到核奥氏增强（Nuclear Overhauser Enhancement，NOE）效应的干扰.本文提出了一种基于神经网络拟合的CEST后处理方法，对每个像素采集得到的Z谱特征进行识别，不需要额外序列扫描即可得到背景参考Z谱与主磁场偏移量，用以校正和获得理想的Z谱，并进一步分离得到源自APT效应与NOE效应的信号.鸡蛋清和健康志愿者腹部成像结果显示，本文提出的基于神经网络的CEST后处理方法效果较好.     

Magnetization transfer contrast (MTC) in FLASH MR imaging

Magnetization transfer between bound and free protons was used as a source of contrast in high speed MR imaging using the FLASH technique. Contrast in FLASH MR images was found to depend upon the reduced magnetization and the spin lattice relaxation rate of free protons in the presence of bound proton radio-frequency saturation. MTC FLASH imaging was thus used to estimate the variation with saturation frequency of free proton spin-lattice relaxation during magnetization transfer.     

Numerical solutions to the time-dependent Bloch equations revisited

The purpose of this study was to demonstrate a simple and fast method for solving the time-dependent Bloch equations. First, the time-dependent Bloch equations were reduced to a homogeneous linear differential equation, and then a simple equation was derived to solve it using a matrix operation. The validity of this method was investigated by comparing with the analytical solutions in the case of constant radiofrequency irradiation. There was a good agreement between them, indicating the validity of this method. As a further example, this method was applied to the time-dependent Bloch equations in the two-pool exchange model for chemical exchange saturation transfer (CEST) or amide proton transfer (APT) magnetic resonance imaging (MRI), and the Z-spectra and asymmetry spectra were calculated from their solutions. They were also calculated using the fourth/fifth-order Runge-Kutta-Fehlberg (RKF) method for comparison. There was also a good agreement between them, and this method was much faster than the RKF method. In conclusion, this method will be useful for analyzing the complex CEST or APT contrast mechanism and/or investigating the optimal conditions for CEST or APT MRI.     

Dynamic imaging with MRI contrast agents: quantitative considerations

Time-resolved MRI has had enormous impact in cognitive science and may become a significant tool in basic biological research with the application of new molecular imaging agents. In this paper, we examine the temporal characteristics of MRI contrast agents that could be used in dynamic studies. We consider "smart" T1 contrast agents, T2 agents based on reversible aggregation of superparamagnetic nanoparticles and sensors that produce changes in saturation transfer effects (chemical exchange saturation transfer, CEST). We discuss response properties of several agents with reference to available experimental data, and we develop a new theoretical model that predicts the response rates and relaxivity changes of aggregation-based sensors. We also perform calculations to define the extent to which constraints on temporal resolution are imposed by the imaging methods themselves. Our analysis confirms that some small T1 agents may be compatible with MRI temporal resolution on the order of 100 ms. Nanoparticle aggregation T2 sensors are applicable at much lower concentrations, but are likely to respond on a single second or slower timescale. CEST agents work at high concentrations and temporal resolutions of 1-10 s, limited by a requirement for long presaturation periods in the MRI pulse sequence.     

Measurement of APT using a combined CERT-AREX approach with varying duty cycles

The goal is to develop an imaging method where contrast reflects amide-water magnetization exchange, with minimal signal contributions from other sources. Conventional chemical exchange saturation transfer (CEST) imaging of amides (often called amide proton transfer, or APT, and quantified by the metric MTR_asym) is confounded by several factors unrelated to amides, such as aliphatic protons, water relaxation, and macromolecular magnetization transfer. In this work, we examined the effects of combining our previous chemical exchange rotation (CERT) approach with the non-linear AREX method while using different duty cycles (DC) for the label and reference scans. The dependencies of this approach, named AREX_double,vdc, on tissue parameters, including T₁, T₂, semi-solid component concentration (f_m), relayed nuclear Overhauser enhancement (rNOE), and nearby amines, were studied through numerical simulations and control sample experiments at 9.4 T and 1 μT irradiation. Simulations and experiments show that AREX_double,vdc is sensitive to amide-water exchange effects, but is relatively insensitive to T₁, T₂, f_m, nearby amine, and distant aliphatic protons, while the conventional metric MTR_asym, as well as several other APT imaging methods, are significantly affected by at least some of these confounding factors.     

Chemical exchange saturation transfer MRI using intermolecular double-quantum coherences with multiple refocusing pulses

Chemical exchange saturation transfer (CEST) provides a new type of image contrast in MRI. Due to the intrinsically low CEST effect, new and improved experimental techniques are required to achieve reliable and quantitative CEST images. In the present work, we proposed a novel and more sensitive CEST acquisition approach, based on the intermolecular double-quantum coherence with a module of multiple refocusing pulses (iDQC-MRP). Experiments were performed on creatine and egg white phantoms using a Varian 7 T animal MRI scanner. The iDQC-MRP CEST technique showed a substantial enhancement in CEST and nuclear Overhauser enhancement (NOE) signal intensities, compared to the standard single-quantum coherence approach. In addition, the iDQC-MRP approach increased the signal-to-noise ratio of acquired saturation images, compared to the conventional iDQC approach. The new iDQC-MRP CEST sequence provides a promising way for exploiting in vivo CEST and NOE imaging applications.     

Measurement of regional cerebral glucose uptake by magnetic resonance spin-lock imaging

Purpose

The regional uptake of glucose in rat brain in vivo was measured at high resolution using spin-lock magnetic resonance imaging after infusion of the glucose analogue 2-deoxy-d-glucose (2DG). Previous studies of glucose metabolism have used ¹³C-labeled 2DG and NMR spectroscopy, ¹⁸F-labeled fluorodeoxyglucose (FDG) and PET, or chemical exchange saturation transfer (CEST) MRI, all of which have practical limitations. Our goal was to explore the ability of spin-lock sequences to detect specific chemically-exchanging species in vivo and to compare the effects of 2DG in brain tissue on CEST images.

Methods

Numerical simulations of R_1p and CEST contrasts for a variety of sample parameters were performed to evaluate the potential specificity of each method for detecting the exchange contributions of 2DG. Experimental measurements were made in tissue phantoms and in rat brain in vivo which demonstrated the ability of spin-lock sequences for detecting 2DG.

Results

R_1p contrast acquired with appropriate spin-lock sequences can isolate the contribution of exchanging protons in 2DG in vivo and appears to have better sensitivity and more specificity to 2DG–water exchange effects than CEST.

Conclusion

Spin-lock imaging provides a novel approach to the detection and measurement of glucose uptake in brain in vivo.     

Relaxation of protons by radicals in rotationally immobilized proteins

Proton spin-lattice relaxation by paramagnetic centers may be dramatically enhanced if the paramagnetic center is rotationally immobilized in the magnetic field. The details of the relaxation mechanism are different from those appropriate to solutions of paramagnetic relaxation agents. We report here large enhancements in the proton spin-lattice relaxation rate constants associated with organic radicals when the radical system is rigidly connected with a rotationally immobilized macromolecular matrix such as a dry protein or a cross-linked protein gel. The paramagnetic contribution to the protein-proton population is direct and distributed internally among the protein protons by efficient spin diffusion. In the case of a cross-linked-protein gel, the paramagnetic effects are carried to the water spins indirectly by chemical exchange mechanisms involving water molecule exchange with rare long-lived water molecule binding sites on the immobilized protein and proton exchange. The dramatic increase in the efficiency of spin relaxation by organic radicals compared with metal systems at low magnetic field strengths results because the electron relaxation time of the radical is orders of magnitude larger than that for metal systems. This gain in relaxation efficiency provides completely new opportunities for the design of spin-lattice relaxation based contrast agents in magnetic imaging and also provides new ways to examine intramolecular protein dynamics.     

Complete relaxation and conformational exchange matrix (CORCEMA) analysis of intermolecular saturation transfer effects in reversibly forming ligand-receptor complexes

A couple of recent applications of intermolecular NOE (INOE) experiments as applied to biomolecular systems involve the (i) saturation transfer difference NMR (STD-NMR) method and (ii) the intermolecular cross-saturation NMR (ICS-NMR) experiment. STD-NMR is a promising tool for rapid screening of a large library of compounds to identify bioactive ligands binding to a target protein. Additionally, it is also useful in mapping the binding epitopes presented by a bioactive ligand to its target protein. In this latter application, the STD-NMR technique is essentially similar to the ICS-NMR experiment, which is used to map protein-protein or protein-nucleic acid contact surfaces in complexes. In this work, we present a complete relaxation and conformational exchange matrix (CORCEMA) theory (H. N. B. Moseley et al., J. Magn. Reson. B 108, 243-261 (1995)) applicable for these two closely related experiments. As in our previous work, we show that when exchange is fast on the relaxation rate scale, a simplified CORCEMA theory can be formulated using a generalized average relaxation rate matrix. Its range of validity is established by comparing its predictions with those of the exact CORCEMA theory which is valid for all exchange rates. Using some ideal model systems we have analyzed the factors that influence the ligand proton intensity changes when the resonances from some protons on the receptor protein are saturated. The results show that the intensity changes in the ligand signals in an intermolecular NOE experiment are very much dependent upon: (1) the saturation time, (2) the location of the saturated receptor protons with respect to the ligand protons, (3) the conformation of the ligand-receptor interface, (4) the rotational correlation times for the molecular species, (5) the kinetics of the reversibly forming complex, and (6) the ligand/receptor ratio. As an example of a typical application of the STD-NMR experiment we have also simulated the STD effects for a hypothetical trisaccharide bound to a protein. The CORCEMA theory for INOE and the associated algorithm are useful in a quantitative interpretation of the intensity changes in the ligand in both the STD-NMR and ICS-NMR, provided the identity of the receptor protons experiencing direct RF saturation is known. The formalism presented here is likely to be useful in the design of bioactive ligands to a specific target protein and in the quantitative mapping of binding epitopes and interfaces between molecules in complexes.     

Proton transport through water-filled carbon nanotubes

Proton transfer along 1D chains of water molecules inside carbon nanotubes is studied by simulations. Ab initio molecular dynamics and an empirical valence bond model yield similar structures and time scales. The proton mobility along 1D water chains exceeds that in bulk water by a factor of 40, but is reduced if orientational defects are present. Excess protons interact with hydrogen-bonding defects through long-range electrostatics, resulting in coupled motion of protons and defects.