Numerically optimized hard pulse sequences for fast frequency selective excitation and inversion

Numerically optimized hard pulse sequences were developed providing a frequency selective response of the transverse and longitudinal magnetization for several applicationsin vivo. The sequences are based on pulse trains with binomial ratios of the pulse angles and constant time intervals between the pulses. These pulse trains were systematically optimized to obtain broad Larmor frequency ranges in which the magnetization is not markedly influenced by the pulse sequence. In addition, the sequences had to provide maximum transverse magnetization or complete inversion of the magnetization beside the suppression range. Such behaviour is needed for chemical shift selective imaging, pulsed magnetization transfer, and frequency selective spectroscopyin vivo. The phase of the magnetization response is shown and adequate rephasing conditions are discussed as well as the actual phase duration. Short optimized hard pulse sequences for water and fat-selective imaging provide lowT ₂-sensitivity of the resulting magnetization. The new optimized hard pulse sequences are suitable, if optimum suppression of signals at one single point in the Larmor frequency spectrum is less important than good suppression in a larger frequency range.     

An iterative system inversion technique

An inversion iterative algorithm for discrete‐time non‐linear systems is presented. The algorithm is tuned depending on system characteristics. Under the assumption that the system inverse exists, the iterative algorithm constructs the system inverse by attempting to achieve on‐line the perfect tracking. The inversion process is based on the observation of the input–output pairs. The resulting control scheme requires a very low a priori knowledge of the system dynamics. Moreover, low‐engineering efforts are needed to apply the control scheme to a particular control problem or to modify it in order to accommodate changes in the physical system. Copyright © 2001 John Wiley & Sons, Ltd.     

基于SFCW雷达的多场景墙体参数反演

穿墙探测所处的外界环境内通常存在多种散射体,是否存在金属物品、有无人体对墙体参数反演具有不同的影响,进而影响穿墙定位与成像的准确性.将不同的外界环境分为不同场景,根据电磁波传播理论和菲涅尔公式建立电磁波穿墙传播模型,并详细阐述了用于提高步进扫频连续波(stepped frequency continuous wave,SFCW)穿墙雷达分辨率的Extend Prony算法和墙体参数快速反演算法,得出一种反演墙体介电常数和厚度的方法,并在墙后无散射体场景、墙后存在金属物体场景和墙后存在人体场景多场景中进行实验,最后将真实值与实验结果对比,表明在不同场景中具有较强适应性,反演出的墙体参数是现场穿墙探测的重要参考依据.     

The soliton-lattice algorithm and selective pulses

The soliton-lattice algorithm, an exact algebraic method of inverting the Bloch equation to obtain frequency-selective radio-frequency pulses is described. Some general properties of pulses are described that were obtained with the help of this algorithm. It is used to obtain two new pulses. A highly prefocused pulse is shown that can be used in short-echo-timein vivo ³¹P spectroscopic imaging, and an adiabatic selective inversion pulse is obtained.     

基于L1正则化反演的电压行波高精度检测方法

针对电压行波传感器二次侧故障行波信号不能真实反映电网一次行波波形特征的问题,提出了一种基于L1正则化反演的电压行波高精度检测方法。首先,分析了行波传感器的非理想传变特性,揭示了一、二次行波信号的波形差异性。在此基础上,提出利用小波包变换对观测信号进行多尺度分解,并对各频段信号分别进行反演的方法,从而减小由行波传感器引起的畸变误差。其次,在反演模型中引入L1正则化约束对模型进行稀疏性刻画,使反演结果更能体现真实故障波形特征。最后,利用快速迭代收缩阈值算法(fast iterative shrinkage-thresholding algorithm, FISTA)进行迭代求解,将各分量的反演波形线性叠加,实现故障行波信号的精确还原。仿真和实验结果表明：与直接反演相比,所提方法能够实现故障行波在时域和频域上的高精度真实测量,在微弱故障和噪声环境下也能获得较为精确的反演结果,具有一定的工程应用价值。     

Charge density variation with fin width in FinFETs: an application of supersymmetric quantum mechanics

FinFETs have become of great interest since it appears that they will be implemented within the next generation of integrated circuits. One point of interest is the transition from surface inversion on the faces of the fin to bulk inversion as the fin width is reduced. We have found that a good scaling approach can give estimates of this transition, with the scaling theory based upon supersymmetric quantum mechanics. For this, a double well potential is chosen whose supersymmetric partner is the harmonic oscillator potential. The depths and the separation between the wells were varied to simulate the change in fin widths. The fraction of charge residing inside the bulk, calculated as a function of fin width and electron density, was determined by calculating overlap area between the squared wave function magnitude of any state and that of a pure bulk state. More charges reside in the bulk for any electron density as the fin width decreases. On the other hand, for a fixed fin width, increasing electron density moves charge to the surface. It was found that in a Si FinFET, bulk inversion occurs at a fin width of about 8 nm for an inversion density of 3×10¹² cm⁻².     

Calculation of Fin width for bulk inversion in Si FinFET by applying supersymmetry

The transition from surface inversion to bulk inversion in a Si FinFET is investigated in this paper using supersymmetric quantum mechanics. A double quantum well potential, which is the supersymmetric partner of a harmonic oscillator potential, was chosen. The fraction of charge residing inside the bulk was calculated as a function of fin width and electron density. For any electron density, more charge resides in the bulk as the fin width decreases. On the other hand, for a fixed fin width, charges move to the surface as the electron density increases. It was found that in Si FinFET for the electron density of 3×10¹² cm⁻² bulk inversion occurs when the fin width is about 8 nm.     

直升机航空瞬变电磁Occam反演及软件实现

直升机航空瞬变电磁系统对于地形复杂、环境恶劣地区的资源勘探具有十分重要的意义.由于航空瞬变电磁系统空间采样密集,数据量很大,在工程实践中普遍采用一维反演进行数据解释.研究了以圆形回线为模型的直升机航空瞬变电磁Occam反演算法,详细推导了圆形回线正演函数的雅克比矩阵的解析解,比用数值差分计算的雅克比矩阵有更高的反演精度.为适用于野外实验,基于直升机航空瞬变电磁系统用C++语言开发了具有自主知识产权的一维Occam反演软件,该软件能实现坐标选取、数据预处理和反演等功能.将该软件应用实测数据的反演中取得了较好的效果.     

Signal enhancement through heteronuclear polarisation transfer in in-vivo <Superscript>31</Superscript>P MR spectroscopy of the human brain

Significant ³¹P NMR signal enhancement through heteronuclear polarisation transfer was obtained in model solutions and in vivo on a 1.5-T whole-body MR scanner equipped with two RF channels. The much higher population differences involved in proton Zeeman energy levels can be transferred to the ³¹P levels with the refocused INEPT (insensitive nucleus enhancement by polarisation transfer) double-resonance experiment by means of a series of simultaneously applied broadband RF pulses. INEPT achieves a polarisation transfer from ¹H to ³¹P spin states by directly reordering the populations in spin systems with heteronuclear scalar coupling. Thus, only the ³¹P NMR signal of metabolites with scalar ¹H–³¹P coupling is amplified, while the other metabolite signals in the spectra are suppressed. Compared to Ernst-angle excitation, a repetition-time-dependent signal enhancement of η=(29±3)% for methylene diphosphonic acid (MDPA) and η=(56±1)% for phosphorylethanolamine (PE) was obtained on model solutions through optimisation of the temporal parameters of the pulse experiment. The results are in good agreement with numerical calculations of the theoretical model for the studied spin systems. With optimised echo times, in-vivo ³¹P signal enhancement of the same order was obtained in studies of the human brain.     

Improved short tau inversion recovery (iSTIR) for increased tumor conspicuity in the abdomen

Object

To develop an improved short tau inversion recovery (iSTIR) technique with simultaneous suppression of fat, blood vessels and fluid to increase tumor conspicuity in the abdomen for cancer screening.

Materials and methods

An adiabatic spectrally selective inversion pulse was used for fat suppression to overcome the reduced signal to noise ratio associated with chemically non-selective inversion pulse of STIR. A motion-sensitizing driven equilibrium was used for blood vessel suppression and a dual-echo single-shot fast spin echo acquisition was used for fluid suppression. The technique was optimized on four normal subjects and later tested on five patients referred for metastatic tumor evaluation.

Results

A velocity encoding of 2 cm/s achieved effective blood suppression even in small vessels. Subtraction of two images (one with 60 ms and the other with 280 ms echo time) acquired in the same echo train achieved excellent fluid suppression (>70 % reduction). Simultaneous suppression of fat, blood vessels and fluid improved the tumor conspicuity compared to corresponding fat-suppressed (STIR) image.

Conclusion

This technique generated two complementary images from a single scan: one that is equivalent to a STIR image and the other that qualitatively resembles a diffusion-weighted image and may have potential for magnetic resonance imaging cancer screening.

     

Pulsed magnetization transfer for imaging and spectroscopic applications on whole-body imagers

Magnetization transfer contrast is a tool to obtain additional information on tissue using standard whole-body magnetic resonance (MR) units without any hardware extension. Short hard pulse sequences with a total duration of about 1 ms can provide selective saturation of protons with very short relaxation timesT ₂ of 1–10 µs. Comparatively high amplitude of the radio-frequency field is advantageous; on the other hand, this amplitude is limited for whole-body units. The presented approach to achieve adapted hard pulse sequences is mainly based on maximum RF amplitudes corresponding with pulse angles per millisecond of 360 °, or 720°. The pulse sequences must not influence the magnetization of free protons with longer relaxation timesT ₂>10 ms in a frequency range that depends on the circumstances of the application. This frequency range has to be markedly broader for imaging techniques than for localized spectroscopy. The number of pulses, the pulse durations and pulse angles, and the interpulse delays were systematically varied. The time intervals between repetitions of the hard pulse sequences in order to obtain stronger magnetization transfer contrast were also optimized experimentally for human skeletal muscle and brain.Additional reprints of this chapter may be obtained from the Reprints Department, Chapman & Hall, One Penn Plaza, New York, NY 10119.     

Young Investigator Award Presentation at the 13th Annual Meeting of the ESMRMB, September 1996, Prague Quantification of pulmonary water compartments by magnetic resonance

The purpose of this study was to quantify pulmonary water compartments of total, intravascular, and extravascular lung water in excised and perfused sheep lungs with the use of magnetic resonance imaging techniques. Total lung water was measured by proton density maps calculated from multi-spin-echo images. Intravascular lung water was evaluated by magnetic resonance angiography before and after injection of gadolinium diethylenetriamine penta-acetic acid polylysine, a macromolecular paramagnetic contrast agent. Intravascular lung water was calculated from signal intensity histogram changes comparing pre- and postcontrast angiograms. Extravascular water was calculated as the difference between total and intravascular lung water. Quantities of total and intravascular lung water measured by magnetic resonance techniques were compared to reference results obtained from wet/dry weight gravimetry and Evans blue dilution performed after imaging. Magnetic resonance and reference results correlated significantly (total lung water:r=0.93,p<0.001; intravascular lung water:r=0.80,p<0.001; extravascular lung water:r=0.89,p<0.001). Therefore, we conclude that quantitative magnetic resonance techniques are potentially useful for the clinical evaluation of pulmonary water compartments.     

Ultra-fast three dimensional imaging of hyperpolarized 13C in vivo

Objective: PASADENA, a chemical method of enhancing nuclear spin polarization has demonstrated ¹³C polarizations of order unity for the nascent products of molecular addition by parahydrogen. The extreme brevity of signal enhancement obtained by hyperpolarization requires improved ¹³C MR in vivo imaging techniques for their optimum utility. Materials and Methods: ¹³C imaging sequences, including ¹³C 3D FIESTA, were compiled for a GE LX 1.5 T clinical MR scanner. Two water soluble ¹³C imaging agents were hyperpolarized utilizing parahydrogen and an automated polarizer. ¹³C polarization was quantified in flow phantoms and in rats with jugular vein catheters. Results: Fast 3D FIESTA ¹³C MR imaging technique acquired sequential 3D images (3.66 s/acquisition) with superior SNR. Hyperpolarized ¹³C solutions and vascular phantoms achieved a maximum signal of 26,624±593. In vivo ¹³C MR images of the cardiopulmonary circulation showed maximum ¹³C signal of 2,402±158. ¹³C images acquired within 3.66 s showed signal enhancement over 10,000 compared to equilibrium polarization. Conclusion: 3D-FIESTA was effective for sub-second in vivo imaging of hyperpolarized ¹³C reagents produced in a custom-built parahydrogen polarizer. Application to ¹³C hyperpolarized by parahydrogen is demonstrated in vitro and in vivo     

Optical signal inversion phenomena due to negative nonlinear absorption effect

A negative input-output characteristic was obtained in an erbium-doped yttrium aluminum garnet crystal which has multiple-level energy structure: absorption can occur between a ground level and an excited level (⁴I−⁴I) and between two other excited levels (⁴I−²H) in Er³⁺. It occurred at a wavelength between 786.6 and 788.4 nm by variation of the incident laser intensity in the intensity range of 60 nW/cm² to 100 W/cm². It is considered that an enhanced absorption occurs due to the excited-state absorption from ⁴I to ²H, its spectra matching the wavelength dependence of the incident laser In addition, optical signal inverter phenomenon was observed using a laser diode modulated at 10 MHz. By considering multiple excited-state absorption in detail, active functions such as an optical signal inverter could be derived from the negative nonlinear absorption effect. © 1997 Scripta Technica, Inc. Electr Eng Jpn, 120(4): 74–80, 1997     

Measurement of activation-related changes in cerebral blood volume: VASO with single-shot HASTE acquisition

Object The recently developed vascular space occupancy (VASO) fMRI technique is gaining popularity as it facilitates the measurement of cerebral blood volume (CBV) changes concomitant with brain activation, without the use of contrast agents. Thus far, VASO fMRI has only been used in conjunction with a GE-EPI (gradient-echo echo planar imaging) sequence, which is proceeded by an inversion recovery (IR) experiment to selectively null the blood signal. The use of GE-EPI has potential disadvantages: (a) the non-zero TE may lead to BOLD contamination and (b) images suffer from the EPI-typical inhomogeneity artefacts. Materials and methods Here, we propose the use of VASO based on an IR-HASTE (inversion recovery half-Fourier acquisition single-shot turbo spin echo) sequence. Results Results from a visual stimulation study (n = 8) show a 43% higher functional contrast-to-noise (CNR) of HASTE compared to EPI, with a strongly increased count of active voxels at the same significance threshold. Sensitivity to inflow effects was investigated and found to be similar for both methods. Conclusion As HASTE VASO yields essentially artefact-free images, it appears to be the method of choice for measuring relative CBV changes with VASO.     

Dynamic interleaved 1H/31P STEAM MRS at 3 Tesla using a pneumatic force-controlled plantar flexion exercise rig

Objective: To develop a measurement method for interleaved acquisition of ¹H and ³¹ STEAM localised spectra of exercising human calf muscle. Materials and Methods: A non-magnetic exercise rig with a pneumatic piston and sensors for force and pedal angle was constructed to enable plantar flexion measured in the 3 T MR scanner, which holds the dual tuned (¹H ,³¹P) surface coil used for signal transmission and reception. Results: ³¹ spectra acquired in interleaved mode benefit from higher Signal to noice ratio (factor of 1.34 ±0.06 for PCr) compared to standard acquisition due to the Nuclear Overhauser effect and substantial PCr/P_i changes during exercise can be observed in ³¹P spectra. ¹H spectral quality is equal to that in single mode experiments and allows Cr2 changes to be monitored. Conclusion: The feasibility of dynamic interleaved localised ¹H and ³¹P spectroscopy during plantar flexion exercise has been demonstrated using a custom-built pneumatic system for muscle activation. This opens the possibility of studying the dynamics of metabolism with multi nuclear MRS in a single run.     

漏磁检测的混合正则化反演方法研究

漏磁检测作为无损检测的主要方法之一已得到了广泛关注和应用。漏磁检测对缺陷的识别问题属于电磁场计算的反问题,因其不适定性难以直接求解,因此目前对于漏磁检测方法的研究和应用大部分都避免了对反问题直接求解,而是采用搜索匹配式的方法。本文尝试使用混合正则化LSQR-Tikhonov的方法对基于磁偶极子单元积分模型的漏磁检测反演问题进行处理,对LSQR方法添加Tikhonov正则化来获得更多的主奇异值信息减小误差,通过混合正则化得到的近似解较为逼近真实值,得到的二维反演结果也可以判断出缺陷的位置和形状,通过仿真及实验验证了算法的准确性。     

Minimizing the effects of magnetization transfer asymmetry on inhomogeneous magnetization transfer (ihMT) at ultra-high magnetic field (11.75 T)

Objectives

The recently reported inhomogeneous magnetization transfer technique (ihMT) has been proposed for specific imaging of inhomogeneously broadened lines, and has shown great promise for characterizing myelinated tissues. The ihMT contrast is obtained by subtracting magnetization transfer images obtained with simultaneous saturation at positive and negative frequency offsets (dual frequency saturation experiment, MT ^+/?) from those obtained with single frequency saturation (MT ⁺) at the same total power. Hence, ihMT may be biased by MT-asymmetry, especially at ultra-high magnetic field. Use of the average of single positive and negative frequency offset saturation MT images, i.e., (MT ⁺+MT ^?) has been proposed to correct the ihMT signal from MT-asymmetry signal.

Materials and methods

The efficiency of this correction method was experimentally assessed in this study, performed at 11.75 T on mice. Quantitative corrected ihMT and MT-asymmetry ratios (ihMTR and MTRasym) were measured in mouse brain structures for several MT-asymmetry magnitudes and different saturation parameter sets.

Results

Our results indicated a “safe” range of magnitudes (/MTRasym/<4 %) for which MT-asymmetry signal did not bias the corrected ihMT signal. Moreover, experimental evidence of the different natures of both MT-asymmetry and inhomogeneous MT contrasts were provided. In particular, non-zero ihMT ratios were obtained at zero MTRasym values.

Conclusion

MTRasym is not a confounding factor for ihMT quantification, even at ultra-high field, as long as MTRasym is restricted to ±4 %.

     

A linear response Monte Carlo algorithm for inversion layers and magnetotransport

Certain simulation problems require the solution of the Boltzmann equation for small driving forces. Due to the bad signal to noise ratio of the standard Monte Carlo algorithm in the linear regime these simulations are extremely CPU intensive. A linear response Monte Carlo algorithm is proposed which is orders of magnitude faster than the standard approach. It is used to extract transport parameters from magnetotransport measurements and to match semiempirical inversion layer models to experimental data.     

MDEFT imaging of the human brain at 8 T

T ₁-weighted images of the human brain obtained with the MDEFT sequence at 8 T are presented. These images are characterized by an excellent contrast and good signal to noise ratio. Importantly, results were obtained with adiabatic spin inversion and demonstrate that such pulses can be used event in the ultra high frequency (>300 MHz) range. It is thus possible to obtain high quality results at this field strength without violating SAR guidelines.