Carr-Purcell sequences with composite pulses

We present novel Carr-Purcell-like sequences using composite pulses that exhibit improved performance in strongly inhomogeneous fields. The sequences are designed to retain the intrinsic error correction of the standard Carr-Purcell-Meiboom-Gill (CPMG) sequence. This is achieved by matching the excitation pulse with the refocusing cycle such that the initial transverse magnetization lies along the axis n(Beta) characterizing the overall rotation of the refocusing cycle. Such sequences are suitable for relaxation measurements. It is shown that in sufficiently inhomogeneous fields, the echo amplitudes have an initial transient modulation that is limited to the first few echoes and then decay with the intrinsic relaxation time of the sample. We show different examples of such sequences that are constructed from simple composite pulses. Sequences of the form 90 degrees (0)-(90 degrees (90-theta/2)-theta(180-theta/2)-90 degrees (90-theta/2))(n) with theta approximately 90 degrees and 270 degrees generate signal over a bandwidth larger than that of the conventional CPMG sequence, resulting in an improved signal-to-noise ratio in inhomogeneous fields. The new sequence 127 degrees (x,y)-(127 degrees (x)-127 degrees (-x))(n) only excites signal off-resonance with a spectrum that is bimodal, peaking at Delta omega(0)=+/-omega(1). Depending on the phase and exact timing of the first pulse, symmetric or antisymmetric excitation is obtained. We also demonstrate several new sequences with improved dependence on the RF field strength. The sequence (22.5 degrees (67.5)-90 degrees (-22.5))-(90 degrees (67.5)-45 degrees (157.5)-90 degrees (67.5))(n) has the property that the phase of the signal depends on B(1), allowing coarse B(1) imaging in a one-dimensional experiment.     

Simple two-pulse time-reversal sequence for dipolar and quadrupolar-coupled spin systems

We demonstrate, both theoretically and experimentally, that two-pulse sequence (2n+1) X 90 degrees(Y) - 90 degrees(X) -Acq(t) without delay between pulses, yields the reverse of time evolution of spin system with dipolar and quadrupole interactions. This process results in refocusing of the spin magnetization into magic echo at te = t1/2 after the second pulse, where t1 is the length of the first pulse.     

Magnetization recovery for signal enhancement: a fast imaging DEFT-based technique

This paper describes the development and application of a new fast MRI technique based on the DEFT principle. The sequence named MAgnetization RecoverY for Signal Enhancement (MARYSE) is composed of two completely symmetric gradient echoes separated by a 180 degrees refocusing pulse. The RF pulse scheme, 90 degrees x-180 degrees y-90 degrees -x enables restoration of the transverse magnetization along the longitudinal axis, and consequently artificially increases R1 relaxation rate. In this sequence, the period between the excitation pulse and the restoring pulse (Tem: transverse magnetization evolution time) is very short (< 10 ms). This makes possible a significant increase in signal-to-noise ratio, even with a relatively short repetition time (20 ms). Simulations were performed for different values of Tem and TR at definite T1 and T2 and for different values of T1 and T2 at constant Tem and TR. Relevant signal enhancement for species with long relaxation time constants as compared to classical gradient echo and fast spin-echo imaging was expected. In vitro studies on a fat/water phantom confirmed this simulation. Application of MARYSE to mouse brain imaging permitted to visualize almost completely cerebrospinal fluid of the ventricles, a signal usually partially saturated in fast gradient echo imaging.     

In vivo isotropic 3D diffusion tensor mapping of the rat brain using diffusion-weighted 3D MP-RAGE MRI

The purpose of this study was to examine the potential of diffusion-weighted (DW) three-dimensional (3D) MP-RAGE MRI for diffusion-tensor mapping of the rat brain in vivo. A DW-3D-MP-RAGE (3D-DWI) sequence was implemented at 2.0 T using six gradient orientations and a b value of 1000 s/mm2. In this sequence, the preparation sequence with a "90 degrees RF-motion proving gradient (MPG): MPG-180 degrees RF-MPG-90 degrees RF" pulse train (DW driven equilibrium Fourier transform) was used to sensitize the magnetization to diffusion. A centric k-space acquisition order was necessary to minimize saturation effects (T1 contamination) from tissues with short relaxation time. The image matrix was 128x128x128 (interpolated from 64x64x64 acquisitions), which resulted in small isotropic DW image data (voxel size: 0.273x0.273x0.273 mm3). Moreover, 3D-DWI-derived maps of the fractional anisotropy (FA), relative anisotropy (RA) and main-diffusion direction were completely free of susceptibility-induced signal losses and geometric distortions. Two well-known commissural fibers, the corpus callosum and anterior commissure, were indicated and shown to be in agreement with the locations of these known stereotaxic atlases. The experiment took 45 min, and shorter times should be possible in clinical application. The 3D-DWI sequence allows for in vivo 3D diffusion-tensor mapping of the rat brain without motion artifacts and susceptibility to distortion.     

抑制HMQC实验中与碳-12相连质子的信号

建议了一个新的HMQC实验的脉冲序列,其准备期和混合期分别为90°_x(¹H)-τ/2-180°^x(¹H,¹³C)-τ/2-90°_x(¹H,¹³C)和90°_x(¹³C)-τ/2-180°_x(¹H,¹³C)-τ/2-,以及τ=(2J_CH)^-1(J_CH为C-H偶合常数).在这个新的HMQC实验中,与碳-12相连质子的信号被有效地抑制,其抑制的机理与脉冲的相位循环无关.从而,它可以用于有效地抑制HMQC谱的噪音.特别是,在没有脉冲梯度场装置时,该方法显得非常实用     

J pulses for multiplet-selective NMR

Exact product operator solutions have been obtained for the evolution of weakly coupled spin-(1/2) I(m)S(n) systems during arbitrary RF irradiation of one spin. These solutions, which completely characterize the nature of J-coupling modulation during RF pulses, show that significant exchange occurs between single-spin magnetization and two-spin product operator states when the RF field strength is comparable to the coupling. In particular, a long (t(p) = [2J](-1) s), low-power (B(1) = J/2 Hz), constant amplitude pulse applied on resonance to one spin in an IS system completely interconverts the spinstates S(z) <--> 2S(x)I(z) and S(x) <--> 2S(z)I(z) when the RF is applied to the S spins, and interconverts S(x) <--> 2S(y)I(y) in 100% yield when the RF is applied to the I spins. Thus, these "J pulses," which select a bandwidth approximately equal to J Hz, may replace any combination of a (2J)(-1) delay period and a consecutive hard 90 degrees pulse in any polarization transfer or multiple quantum sequence. Although these rectangular pulses are highly frequency selective, in general they increase the replaced (2J)(-1) period by only a modest 40%, a time saving of a factor of 5 compared to existing pulses exhibiting the same selectivity. In favorable cases, there is no increase in duration of a pulse sequence using a particular type of J pulse, the 90(J) variety, which accomplishes the third spin state transformation listed above. J pulses will be advantageous for systems subject to rapid signal loss from relaxation and more generally for the enhanced operation of pulse sequences via the use of J modulation during RF irradiation.     

一种改进的核磁共振测井测量脉冲序列

提出了一种可以提高核磁共振测井测量速度的新型测量脉冲序列,它基于对原有CPMG测量脉冲序列的改进.在原有的测量序列中,在一个CPMG脉冲序列结束之后,直接进行等待(T₁延时),以使磁化强度矢量能够完全恢复到平衡位置,然后再进行下一个CPMG脉冲序列的测量.由于测井仪器的连续运动,在测量含有长T₁成分的地层时,等待时间需要取得较长,使得测井速度很低.但在实际地层中,恰恰又是油气的T₁较长,这无疑给核磁共振测井带来了麻烦.作者发现,如果在一个CPMG脉冲序列结束之后在-x方向先施加一个90°脉冲(或在x方向施加一个270°脉冲),再进行等待,就能更好地使长T₁成分得到极化,从而缩短等待时间、提高测量速度.文中对这种改进后的脉冲序列进行了计算和分析,证明它确实具有一定的优越性.     

Quantitative evaluation of the lactate signal loss and its spatial dependence in press localized (1)H NMR spectroscopy

Localized (1)H NMR spectroscopy using the 90 degrees -t(1)-180 degrees -t(1)+t(2)-180 degrees -t(2)-Acq. PRESS sequence can lead to a signal loss for the lactate doublet compared with signals from uncoupled nuclei which is dependent on the choice of t(1) and t(2). The most striking signal loss of up to 78% of the total signal occurs with the symmetrical PRESS sequence (t(1)=t(2)) at an echo time of 2/J (approximately 290 ms). Calculations have shown that this signal loss is related to the pulse angle distributions produced by the two refocusing pulses which leads to the creation of single quantum polarization transfer (PT) as well as to not directly observable states (NDOS) of the lactate AX(3) spin system: zero- and multiple-quantum coherences, and longitudinal spin orders. In addition, the chemical shift dependent voxel displacement (VOD) leads to further signal loss. By calculating the density operator for various of the echo times TE=n/J, n=1, 2, 3,..., we calculated quantitatively the contributions of these effects to the signal loss as well as their spatial distribution. A maximum signal loss of 75% can be expected from theory for the symmetrical PRESS sequence and TE=2/J for Hamming filtered sinc pulses, whereby 47% are due to the creation of NDOS and up to 28% arise from PT. Taking also the VOD effect into account (2 mT/m slice selection gradients, 20-mm slices) leads to 54% signal loss from NDOS and up to 24% from PT, leading to a maximum signal loss of 78%. Using RE-BURP pulses with their more rectangular pulse angle distributions reduces the maximum signal loss to 44%. Experiments at 1.5 T using a lactate solution demonstrated a maximum lactate signal loss for sinc pulses of 82% (52% NDOS, 30% PT) at TE=290 ms using the symmetrical PRESS sequence. The great signal loss and its spatial distribution is of importance for investigations using a symmetrical PRESS sequence at TE=2/J.     

Enhanced suppression of residual water in a "270" WET sequence

In certain water suppression experiments, the residual water, which comes from a region away from the center of the RF coil and experiences a much smaller flip angle than the designed one, may appear. The residual water in the WET sequence can be reduced significantly by using a composite 90(x)( degrees )90(y)( degrees )90(-x)( degrees )90(-y)( degrees ) pulse, which de-excites molecules experiencing a small flip angle. The composite pulse, however, has two null excitation points near on resonance, causing a severe loss of spectrum intensity and baseline distortion toward the null points. Since the residual water experiences a very small flip angle, it can be treated as a linear spin system; i.e., the intensity of the residual water is proportional to the pulse strength and width. Based on this principle, the residual water can be reduced dramatically by replacing the 90 degrees pulse in the "270" WET sequence with a 270 degrees pulse for one out of every four scans, without noticeable loss of intensity and baseline distortion.     

Optimization and validation of a rapid high-resolution T1-w 3D FLASH water excitation MRI sequence for the quantitative assessment of articular cartilage volume and thickness

In view of follow up, survey and development of therapeutic strategies for osteoarthritis where cartilage deterioration plays an important role, a non invasive, reliable and quantitative assessment of the articular cartilage is desirable. The currently available high resolution T(1)-weighted (T1-w) 3D FLASH pulse sequences with frequency selective fat suppression are very time consuming. We have 1) optimized a high resolution T1-w 3D FLASH water excitation (WE) sequence for short acquisition time and cartilage visualization, and 2) validated this sequence for cartilage volume and thickness quantification. The spectral fat presaturation was replaced by selective water excitation. The flip angle of the WE sequence was optimized for the contrast to noise (C/N(cart)) ratio of cartilage. Sagittal datasets (voxel size: 0.31 x 0.31 x 2 mm(3)) of the knees of nine healthy volunteers were acquired both, with the 3D FLASH WE (17.2/6.6/30 degrees ) sequence (WE) and a previously validated 3D FLASH fat saturated (42/11/30 degrees ) sequence (FS). For validation of the WE sequence, cartilage volume, mean and maximal cartilage thickness of the two sequences were compared. Reproducibility was assessed by calculating the coefficient of variation (COV %) of 4 consecutive WE data sets in the volunteers. The acquisition time was reduced from 16'30" (FS) down to 7'14" for the WE sequence. Image contrast and visualization of the cartilage was very similar, but delineation of the basal layer of the cartilage was slightly improved with the WE sequence. A flip angle of 30 degrees provided the best C/N(cart) ratios (WE). Reproducibility (COV) was between 1.9 and 5.9%. Cartilage volume and thickness agreed within 4% between FS and WE sequence. The WE sequence allows for rapid, valid and reproducible quantification of articular cartilage volume and thickness, prerequisites for follow-up examinations. The reduced acquisition time (50% of FS) enables routine clinical application and thus may contribute to a broader assessment of osteoarthritis.     

Satellite transitions in natural abundance solid-state (33)S MAS NMR of alums--sign change with zero-crossing of C(Q) in a variable temperature study

Experiences obtained from recent improvements in the performance of solid-state (14)N MAS NMR spectroscopy have been used in a natural abundance (33)S MAS NMR investigation of the satellite transitions for this interesting spin I=3/2 isotope. This study reports the first observation of manifolds of spinning sidebands for these transitions in (33)S MAS NMR as observed for the two alums XAl(SO(4))(2) x 12H(2)O with X=NH(4) and K. For the NH(4)-alum a variable temperature (33)S MAS NMR study, employing the satellite transitions, shows that the (33)S quadrupole coupling constant (C(Q)) exhibits a linear temperature dependence (in the range -35 degrees C to 70 degrees C) with a temperature gradient of 3.1 kHz/ degrees C and undergoes a sign change with zero-crossing for C(Q) at 4 degrees C (277 K). For the isostructural K-alum a quite similar increase in the magnitude of C(Q) with increasing temperature is observed, and with a temperature gradient of 2.3 kHz/ degrees C. Finally, for optimization purposes, a study on the effect of the applied pulse widths at constant rf field strength on the intensity and variation in second-order quadrupolar lineshape for the central (1/2<-->-1/2) transition of the K-alum has been performed.     

Use of Carr-Purcell pulse sequence with low refocusing flip angle to measure T₁ and T₂ in a single experiment

The Carr-Purcell pulse sequence, with low refocusing flip angle, produces echoes midway between refocusing pulses that decay to a minimum value dependent on T(2). When the refocusing flip angle was π/2 (CP(90)) and τ>T(2), the signal after the minimum value, increased to reach a steady-state free precession regime (SSFP), composed of a free induction decay signal after each pulse and an echo, before the next pulse. When τ相似文献   

Analytical analysis of multi-pulse NMR

It is well known that in multi-pulse applications in high-resolution NMR and MRI a steady state is reached for the magnetisation vector by the effect of relaxation in combination with the pulse repetition time. In this paper, a mathematical model is developed to understand how the parameters of the pulse sequence and relaxation times T(1) and T(2) affect the behaviour of the magnetisation vector. It will be shown that even under strong simplifying conditions an analytical analysis becomes very complex and only an analytical solution can be found for 90 degrees pulses and T(1)=T(2). For other cases a numerical approach is needed. Nevertheless, the basic approach of the mathematical analysis provides a general tool for analytical multi-operator applications. Our results provide a quantitative insight in the process by which the magnetisation relaxes towards the steady-state situation in a multi-pulse sequence.     

GF-AAS法测定处理污水灌溉土壤中铅最佳基体改进剂的选择及其应用

研究了石墨炉原子吸收法（GF-AAS法）测定一级处理污水灌溉土壤中铅最佳基体改进剂的选择及在最佳改进剂条件下测定污水灌溉土壤中铅的GF-AAS法。试验了磷酸二氢铵, 磷酸铵, 氯化铵, Pd-Mg和磷酸二氢铵+硝酸镁+硝酸铵等基体改进剂对测定一级处理污水灌溉土壤铅的基体改进作用,并根据基体改进剂的改进效果,最佳灰化温度和原子化温度, 选定了最佳基体改进剂和测定条件。40 g·L-1的磷酸二氢铵作为基体改进剂,灰化温度为850 ℃,原子化温度为1 600 ℃时,用氘灯校正背景GF-AAS法进行测定,测定方法的RSD为2.6% ,回收率在92.4%～104%之内。     

Rapid MRI method for mapping the longitudinal relaxation time

A novel method for mapping the longitudinal relaxation time in a clinically acceptable time is developed based on a recent proposal [J.-J. Hsu, I.J. Lowe, Spin-lattice relaxation and a fast T1-map acquisition method in MRI with transient-state magnetization, J. Magn. Reson. 169 (2004) 270-278] and the speed of the spiral pulse sequence. The method acquires multiple curve-fitting samples with one RF pulse train. It does not require RF pulses of specific flip angles (e.g., 90 degrees or 180 degrees ), nor are the long recovery waiting time and the measurement of the magnetization at thermal equilibrium needed. Given the value of the flip angle, the curve fitting is semi-logarithmic and not computationally intensive. On a heterogeneous phantom, the average percentage difference between measurements of the present method and those of an inversion-recovery method is below 2.7%. In mapping the human brain, the present method, for example, can obtain four curve-fitting samples for five 128 x 128 slices in less than 3.2s and the results are in agreement with other studies in the literature.     

原子吸收光谱法测定鸡蛋中营养元素的前处理方法研究

本文分别采用０．１ｍｏｌ·Ｌ－１盐酸、６ｍｏｌ·Ｌ－１盐酸、ｐＨ７．０１ｍｏｌ·Ｌ－１ＮＨ４Ａｃ振荡提取及６ｍｏｌ·Ｌ－１盐酸煮沸法测定了鸡蛋中Ｋ、Ｎａ、Ｍｇ、Ｃａ、Ｚｎ、Ｆｅ、Ｍｎ、Ｃｕ八种元素,并与经典前处理方法即现在普遍采用的浓硝酸－高氯酸全消化法及干灰法进行比较,结果表明,对于大量元素Ｋ、Ｎａ、Ｍｇ几种提取方法与经典法接近,回收率达９０％～１１５％;微量元素Ｚｎ、Ｍｎ、Ｆｅ也可分别用ｐＨ７．０１ｍｏｌ·Ｌ－１ＮＨ４Ａｃ,０．１ｍｏｌ·Ｌ－１盐酸、６ｍｏｌ·Ｌ－１盐酸振荡提取,回收率在１００％～１１５％之间,Ｃｕ则可用６ｍｏｌ·Ｌ－１盐酸煮沸法测定,回收率较理想。而Ｃａ这几种方法提取结果均不理想,主要因增感作用,造成结果偏高。经典干灰化结果偏低,灰化温度达４５０℃,即可造成多种元素损失,且繁琐费时。本文采用的几种提取方法简单、快速,结果准确性、精度可达到分析要求。     

SAR reduced pulse sequences

Three techniques were considered for reducing the RF (radiofrequency) power deposition in the body while maintaining scan time efficiency: reducing the RF peak amplitude while increasing the pulse width, substituting gradient echoes for spin echoes, and reducing the flip angle of the phase reversal pulse. The use of gradient echoes was found to be the most efficient means to reduce the power delivered to the patient and to obtain rapid data acquisition. The effect upon SAR (specific absorption rate) and SNR (signal-to-noise ratio) was demonstrated on a phantom when the phase reversal pulse was reduced from the standard 180 degrees to 90 degrees. Data in the body indicated a fairly constant SNR down to a refocusing flip angle between 110 degrees and 135 degrees. An initial clinical evaluation was performed at three institutions using the method of reducing the flip angle of the phase reversal pulse. The scan with theta = 120 degrees was rated by readers in a blinded study as having acceptable diagnostic image quality while the 135 degrees scan had comparable image quality to a conventional 90 degrees - 180 degrees pulse sequence. The use of reduced phase reversal pulses was seen as an efficient protocol to obtain T1-weighted images at rapid data rates while reducing the power delivered to the body by about 40%.     

SOGGY: solvent-optimized double gradient spectroscopy for water suppression. A comparison with some existing techniques

Excitation sculpting, a general method to suppress unwanted magnetization while controlling the phase of the retained signal [T.L. Hwang, A.J. Shaka, Water suppression that works. Excitation sculpting using arbitrary waveforms and pulsed field gradients, J. Magn. Reson. Ser. A 112 (1995) 275-279] is a highly effective method of water suppression for both biological and small molecule NMR spectroscopy. In excitation sculpting, a double pulsed field gradient spin echo forms the core of the sequence and pairing a low-power soft 180 degrees (-x) pulse with a high-power 180 degrees (x) all resonances except the water are flipped and retained, while the water peak is attenuated. By replacing the hard 180 degrees pulse in the double echo with a new phase-alternating composite pulse, broadband and adjustable excitation of large bandwidths with simultaneous high water suppression is obtained. This "Solvent-Optimized Gradient-Gradient Spectroscopy" (SOGGY) sequence is a reliable workhorse method for a wide range of practical situations in NMR spectroscopy, optimizing both solute sensitivity and water suppression.     

弱碱性阴离子交换树脂富集ICP-AES测定环境水体中Cr(Ⅵ)和有机态Cr(Ⅲ)的研究

选用331弱碱性阴离子交换树脂,对微污染水体中的Cr(Ⅵ)和有机态Cr(Ⅲ)进行了分离富集作用研究,考察了酸度、富集时间、洗脱液类型、洗脱液浓度及溶液中共存离子对分离富集过程的影响。研究表明,分别用1.0 mol·L-1 HCl和2.0 mol·L-1 NH₄NO₃＋0.5 mol·L-1 NH₃·H₂O可以很好分步洗脱有机态Cr(Ⅲ)和Cr(Ⅵ),利用ICP-AES测定, 该方法对Cr(Ⅲ)和Cr(Ⅵ)的检出限分别为1.1和 1.4 μg·L-1,相应的相对标准偏差RSD(n＝6)平均值分别为3.8％和5.6％。该方法适用于自来水、地下水、地表水及生活污水中痕量Cr(Ⅵ)和有机态Cr(Ⅲ)的分离富集及测定。     

掺Co钨酸镉单晶体的坩埚下降生长及光谱特性

应用坩埚下降法,采用70℃.cm-1的固液界面温度梯度与0.50 mm.h-1生长速度,生长出了初始Co掺杂摩尔百分浓度为0.5 mol%、尺寸为Ф25 mm×120 mm的钨酸镉单晶体。晶体呈透明深蓝色。用XRD表征了获得的晶体。观测到518,564和655 nm三个可见波段吸收带以及发光中心1 863 nm(1 323～2 220 nm)的超宽吸收带。可见波段的吸收归属于Co2+在八面晶格场中4T1(4F)→4A2(4F),4T1(4F)→4T1(4P)跃迁叠加,而红外波段的宽带吸收则为八面体场Co2+的4T1(4F)→4T2(4F)能级跃起所致。从吸收特性表明,Co离子掺杂于畸变的氧八面体中,呈现+2价态。根据吸收光谱计算出晶格场参数Dq=990 cm-1,Racah参数BC=726.3 cm-1。在520 nm波长光的激发下,观察到778 nm波段的荧光发射,归属于八面体晶格场中Co2+的4T1(4P)→4T1(4F)能级的跃迁。从吸收与荧光强度的变化推断:沿着晶体生长方向,Co2+浓度逐渐增加,有效分凝系数小于1。