1H spectroscopy without solvent suppression: characterization of signal modulations at short echo times.

While most proton ((1)H) spectra acquired in vivo utilize selective suppression of the solvent signal for more sensitive detection of signals from the dilute metabolites, recent reports have demonstrated the feasibility and advantages of collecting in vivo data without solvent attenuation. When these acquisitions are performed at short echo times, the presence of frequency modulations of the water resonance may become an obstacle to the identification and quantitation of metabolite resonances. The present report addresses the characteristics, origin, and elimination of these sidebands. Sideband amplitudes were measured as a function of delay time between gradient pulse and data collection, as a function of gradient pulse amplitude, and as a function of spatial location of the sample for each of the three orthogonal gradient sets. Acoustic acquisitions were performed to demonstrate the correlation between mechanical vibration resonances and the frequencies of MR sidebands. A mathematical framework is developed and compared with the experimental results. This derivation is based on the theory that these frequency modulations are induced by magnetic field fluctuations generated by the transient oscillations of gradient coils.     

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.     

The rapid and automatic combination of proton MRSI data using multi-channel coils without water suppression

The use of multi-channel coils can efficiently increase the signal-to-noise ratio (SNR) of magnetic resonance spectroscopy data if the signals from multiple channels are optimally combined. Combining multi-channel signals requires proper alignment of the phases of the signals from each of the elements of the coil and then accurately weighting the summation of those signals. We present a procedure for acquiring proton magnetic resonance spectroscopic imaging (MRSI) data using an eight-channel coil without water suppression and a rapid and robust method that uses unsuppressed water signal as a reference both for aligning the phases and for weighting the summation of signals that originate in the multiple coil elements. We use both computer simulation and in vivo proton MRSI data to demonstrate the advantages of our method for optimizing the SNR of the combined signal compared with the SNRs of signals that were acquired either using a standard volume head coil or using an eight-channel coil with a metabolite signal as the reference for combination.     

Optimization of residual water signal removal by HLSVD on simulated short echo time proton MR spectra of the human brain

Suppression of the residual water signal from proton magnetic resonance (MR) spectra recorded in human brain is a prerequisite to an accurate quantification of cerebral metabolites. Several postacquisition methods of residual water signal suppression have been reported but none of them provide a complete elimination of the residual water signal, thereby preventing reliable quantification of brain metabolites. In the present study, the elimination of the residual water signal by the Hankel Lanczos singular value decomposition method has been evaluated and optimized to provide fast automated processing of spectra. Model free induction decays, reproducing the proton signal acquired in human brain localized MR spectroscopy at short echo times (e.g., 20 ms), have been generated. The optimal parameters in terms of number of components and dimension of the Hankel data matrix allowing complete elimination of the residual water signal are reported.     

利用正弦调频信号的宽带速度敏感特性抑制混响

有源声呐探测水中低速目标时,混响与目标回波的频率接近,目标回波与混响分离困难。根据运动目标与产生混响的散射体在速度域的差异,利用正弦调频信号(SFM)的速度敏感特性实现目标回波与混响的有效分离。推导了 SFM信号宽带混响抑制能力与速度分辨模糊之间的约束关系,建立了波形优化设计的搜索模型。通过多普勒滤波处理将数据从频率域统一转换至速度域,设定合适的速度门限,去除静止的混响成分。通过仿真分析了 SFM信号的混响抑制能力、调制指数和带宽的关系,在设定的速度范围内平均混响抑制能力较连续波(CW)信号可提高7.5 dB。动目标试验数据的处理结果表明了该方法能够抑制混响中强杂波造成的虚警,通过速度差异性可有效提高对动目标的检测能力。     

利用正弦调频信号的宽带速度敏感特性抑制混响

有源声呐探测水中低速目标时,混响与目标回波的频率接近,目标回波与混响分离困难。根据运动目标与产生混响的散射体在速度域的差异,利用正弦调频信号(SFM)的速度敏感特性实现目标回波与混响的有效分离。推导了 SFM信号宽带混响抑制能力与速度分辨模糊之间的约束关系,建立了波形优化设计的搜索模型。通过多普勒滤波处理将数据从频率域统一转换至速度域,设定合适的速度门限,去除静止的混响成分。通过仿真分析了 SFM信号的混响抑制能力、调制指数和带宽的关系,在设定的速度范围内平均混响抑制能力较连续波(CW)信号可提高7.5 dB。动目标试验数据的处理结果表明了该方法能够抑制混响中强杂波造成的虚警,通过速度差异性可有效提高对动目标的检测能力。     

Contributions of suppression and excitation to simultaneous masking: effects of signal frequency and masker-signal frequency relation

This study investigated the contributions of suppression and excitation to simultaneous masking for a range of masker frequencies both below and above three different signal frequencies (750, 2000, and 4850 Hz). A two-stage experiment was employed. In stage I, the level of each off-frequency simultaneous masker necessary to mask a signal at 10 or 30 dB sensation level was determined. In stage II, three different forward-masking conditions were tested: (1) an on-frequency condition, in which the signals in stage I were used to mask probes of the same frequency; (2) an off-frequency condition, in which the off-frequency maskers (at the levels determined in stage I) were used to mask the probes; and (3) a combined condition, in which the on- and off-frequency maskers were combined to mask the probes. If the off-frequency maskers simultaneously masked the signal via spread of excitation in stage I, then the off-frequency and combined maskers should produce considerable forward masking in stage II. If, on the other hand, they masked via suppression, they should produce little or no forward masking. The contribution of suppression was found to increase with increasing signal frequency; it was absent at 750 Hz, but dominant at 4850 Hz. These results have implications for excitation pattern analyses and are consistent with stronger nonlinear processing at high rather than at low frequencies.     

Localized Larmor frequency-guided fat and water proton MRI of the spine: a method to emphasize pathological findings.

A measurement protocol providing a correct adjustment of the irradiation frequencies for well separated fat and water images of the lumbar spine is presented. To determine accurately the Larmor frequencies of water and fat protons within the vertebral bodies, a volume selective spectrum of a volume element (13 mm)3 located in a lumbar vertebral body was acquired with the 90 degrees-180 degrees-180 degrees double spin-echo method. These Larmor frequencies are used to adjust the frequency-selective pulse of the SENEX chemical-shift imaging sequence. This procedure provides well separated fat and water images for a large field of view even in the inhomogeneous region of the vertebral column. Their clinical importance is demonstrated by localized Larmor frequency-guided (LLFG) SENEX 1H images of the lumbar spine in healthy persons of different age and in a patient with acute myeloid leukemia.     

Some features of phase transformations induced by selective removal of oxygen atoms from metal oxides under proton irradiation

It has been shown that oxygen atoms are selectively removed from metal oxides under ～ 1 keV proton irradiation. The reduced metallic layer thickness reaches several tens of nanometers. The reduction of metal from oxide is accompanied by significant volume changes, the value of which has been experimentally determined and compared to theoretical estimates. It has been found that the metal reduction process can be carried out through an additional upper (relative to the proton beam) layer. It has been shown that the metal reduction rate does not change in the case of a thin (～5 nm) upper layer; however, the coercive force of ferromagnetic metals increases strongly.     

Doppler-free spectroscopy using magnetically induced dichroism of atomic vapor: a new scheme for laser frequency locking

We demonstrate a Doppler-free spectroscopic method that we test on ⁸⁷Rb and ⁸⁵Rb vapor. By using a magnetic field to induce a dichroism on the sample, we generate a dispersive signal with low background, which allows us to lock a diode laser even on small spectroscopic features. We elaborate the advantages of this simple and easy method by comparing it to other methods. Received 15 October 2002 Published online 17 December 2002     

水体痕量重金属Ni的激光诱导击穿光谱测量研究

以1 064nm波长Nd∶YAG脉冲激光为激发光源,采用高分辨率、宽光谱段的中阶梯光栅光谱仪和ICCD为谱线分离与探测器件,实验研究了水体中痕量镍元素的激光诱导等离子体光谱发射特性和检测限;为了延长水体中金属元素的激光等离子体光谱发射时间、提高光谱检测灵敏度并减小ns激光作用下水滴溅射的影响,实验中以固体圆饼状石墨块为样品基体进行元素富集,通过在基体表面固定区域滴定固定量的已知不同浓度的硝酸镍溶液,烘干并制备待测样品。研究表明,镍的最佳探测延迟时间在700ns左右,元素谱线强度随着样品浓度的增加而增大并在较低浓度下呈现良好的线性关系,计算得到水体中Ni元素的痕量检测限为0.28,研究结果为进一步开展水体痕量重金属的激光诱导击穿光谱测量提供了方法。     

激光波长对水中金属元素激光诱导击穿光谱探测的影响

针对激光诱导击穿光谱(LIBS)在海洋应用中的问题,对1 064和532nm两个激发波长下水中LIBS光谱特性进行探测分析,以比较其烧蚀效果。通过激光诱导等离子体的时间分辨光谱,分析水下等离子体电子密度随时间的演化规律,1 064nm激光诱导等离子体寿命约为1 200ns,而532nm激光激发情况下等离子体寿命仅约为600ns。基于光在水中的传输特性和LIBS的实验结果,建立了获得最佳LIBS探测效果所需的入水前激光脉冲能量Eiopt(r)与探测距离r的关系,并应用到水下原位探测的模拟分析。结果表明,当探测距离不大于5cm时,所需的入水前1 064nm激光单脉冲能量小于100mJ,该激发波长可用于LIBS的水下探测;当探测距离增至10cm时,所需的入水前532nm激光单脉冲能量只需30mJ左右。因此,当原位探测距离增加时,则需考虑选择532nm激光作为烧蚀光源。     

Spike suppression and longitudinal mode selection in a 1.319 μm Nd:YAG laser by high-efficiency intracavity frequency doubling

We describe a flashlamp pumped 1.319 μm Nd:YAG ring laser in which a high-efficiency intracavity frequency doubler is used as a nonlinear output coupler, virtually eliminating all spiking behavior and providing longitudinal mode selection as well. A single frequency, long pulse (>50 μs) output at 660 nm with about 20 W peak power is produced, suitable as a source for visar and other applications where long coherent pulses at relatively high power are required. Experimental results and theoretical analyses are given.     

In vivo quantification of the metabolites in normal brain and brain tumors by proton MR spectroscopy using water as an internal standard

Metabolite concentrations in normal adult brains and in gliomas were quantitatively analyzed by in vivo proton magnetic resonance spectroscopy (MRS) using the fully relaxed water signal as an internal standard. Between January 1998 and October 2001, 28 healthy volunteers and 18 patients with gliomas were examined by in vivo proton MRS. Single-voxel spectra were acquired using the point-resolved spectroscopic (PRESS) pulse sequence with a 1.5 T scanner (TR/TE/Ave = 3000 ms/30 ms/64). The calculated concentrations of N-acetyl-aspartate (NAA), creatine (Cre), choline (Cho), and water(H(2)O) in the normal hemispheric white matter were 23.59 +/- 2.62 mM (mean +/- SD), 13.06 +/- 1.8 mM, 4.28 +/- 0.8 mM, and 47280.96 +/- 5414.85 mM, respectively. The metabolite concentrations were not necessarily uniform in different parts of the brain. The concentrations of NAA and Cre decreased in all gliomas (p < 0.001). The NAA/Cho and NAA/H(2)O ratios can distinguish the normal brain from gliomas and low-grade from high-grade astrocytoma (p < 0.001). The concentration of taurine (Tau) in medulloblastomas was 29.64 +/- 5.76 mM. This is the first quantitative analysis of Tau in medulloblastoma in vivo and confirms earlier in vitro findings.     

In vivo quantification of the metabolites in normal brain and brain tumors by proton MR spectroscopy using water as an internal standard

Metabolite concentrations in normal adult brains and in gliomas were quantitatively analyzed by in vivo proton magnetic resonance spectroscopy (MRS) using the fully relaxed water signal as an internal standard. Between January 1998 and October 2001, 28 healthy volunteers and 18 patients with gliomas were examined by in vivo proton MRS. Single voxel spectra were acquired using the point-resolved spectroscopic pulse sequence with a 1.5-T scanner (TR/TE/Ave = 3000 ms/30 ms/64). The calculated concentrations of N-acetyl-aspartate (NAA), creatine (Cre), choline (Cho), and water (H2O) in the normal hemispheric white matter were 23.59 +/- 2.62 mM (mean +/- SD), 13.06 +/- 1.8 mM, 4.28 +/- 0.8 mM, and 47280.96 +/- 5414.85 mM, respectively. The metabolite concentrations were not necessarily uniform in different parts of the brain. The concentrations of NAA and Cre decreased in all gliomas (p < 0.001). The NAA/Cho and NAA/H2O ratios can distinguish the normal brain from gliomas, and low-grade astrocytoma from high-grade group (p < 0.001). The concentration of taurine (Tau) in medulloblastomas was 29.64 +/- 5.76 mM. This is the first quantitative analysis of Tau in medulloblastoma in vivo and confirms earlier in vitro findings.     

Studies of intermolecular interactions by matrix isolation vibrational spectroscopy: Self-association of water

The mid- and far-infrared spectra of water in argon and nitrogen matrices have been reinvestigated. The water molecule rotates in an argon matrix, but in a nitrogen matrix two low-frequency bands are attributed to libration of the water monomer about its A and C rotational axes. Combinations of these modes with the stretching and bending modes of the water monomer were observed. A detailed concentration-dependence study enabled bands to be attributed to dimer, trimer, and tetramer or higher multimers. The dimer and trimer were both found to have open chain structures, whereas tetramer or higher multimer species appear to be predominantly cyclic. Intermolecular modes, observed for dimer and trimer in the far-infrared spectra, are tentatively assigned.     

In contrast to BOLD: signal enhancement by extravascular water protons as an alternative mechanism of endogenous fMRI signal change

Despite the popularity and widespread application of functional magnetic resonance imaging (fMRI) in recent years, the physiological bases of signal change are not yet fully understood. Blood oxygen level-dependant (BOLD) contrast — attributed to local changes in blood flow and oxygenation, and therefore magnetic susceptibility — has become the most prevalent means of functional neuroimaging. However, at short echo times, spin-echo sequences show considerable deviations from the BOLD model, implying a second, non-BOLD component of signal change. This has been dubbed “signal enhancement by extravascular water protons” (SEEP) and is proposed to result from proton-density changes associated with cellular swelling. Given that such changes are independent of magnetic susceptibility, SEEP may offer new and improved opportunities for carrying out fMRI in regions with close proximity to air–tissue and/or bone–tissue interfaces (e.g., the prefrontal cortex and spinal cord), as well as regions close to large blood vessels, which may not be ideally suited for BOLD imaging. However, because of the interdisciplinary nature of the literature, there has yet to be a thorough synthesis, tying together the various and sometimes disparate aspects of SEEP theory. As such, we aim to provide a concise yet comprehensive overview of SEEP, including recent and compelling evidence for its validity, its current applications and its future relevance to the rapidly expanding field of functional neuroimaging. Before presenting the evidence for a non-BOLD component of endogenous functional contrast, and to enable a more critical review for the nonexpert reader, we begin by reviewing the fundamental principles underlying BOLD theory.     

基频和三倍频Nd:YAG激光诱导熔石英损伤特性

 采用光电探测器和数字示波器检测散射光脉冲信号,研究了基频和三倍频Nd:YAG激光诱导熔石英损伤过程,给出了泵浦光和探针光的散射光光电信号;比较了基频和三倍频激光作用下熔石英烧蚀斑显微照片,并分析了其损伤机理。结果显示:在ns脉冲激光作用下,熔石英损伤均发生在泵浦激光脉冲峰值附近,且基频光作用下损伤开始时间点比三倍频作用下早;在多脉冲或高能量激光辐照下,检测到了等离子体闪光信号,等离子体闪光发生在时间延迟21 ns附近。基于Keldysh理论计算了基频光和三倍频光作用下,熔石英光致电离速率同激光强度的关系。