一种新的用于磁共振扩散加权波谱术的扩散敏感梯度磁场施加方案

目的 提出一种基于PRESS的用于磁共振扩散加权波谱的扩散敏感梯度磁场施加方案.方法 在PRESS序列中的2个π射频脉冲两侧各施加一对交错的扩散敏感梯度磁场.结果 本方案有效减小涡流,利用PRESS序列中2个π射频脉冲之间的时间,延长了扩散梯度磁场脉冲的持续时间.即使在扩散权重较大的情况下,Naa,Cr及Cho的峰仍十...     

肘部尺神经扩散张量成像技术参数优化的研究

目的：优化肘部尺神经扩散张量成像(DTI)参数。方法使用5组不同 b 值和扩散梯度方向数量(NDGDs)DTI 序列采集13名志愿者肘部尺神经图像并建立扩散示踪图(DTT)。比较不同成像参数条件下,尺神经各向异性分数(FA)、表观扩散系数(ADC)、神经纤维束长度和 DTI 图像质量的差异性。结果18个正常尺神经 DTI 结果纳入研究。不同成像条件下,尺神经 FA 值无明显差异。当 NDGDs 一定时,b 值升高,图像质量下降,尺神经 ADC 值减低;而 NDGDs 对 ADC 值和图像质量无显著影响。b=1000 s/mm2,NDGDs=20时,测得尺神经纤维束长度最长,且 DTT 的主观评分最高。结论以 b=1000 s/mm2,NDGDs=20用于肘部尺神经 DTI,可获得良好的图像质量和稳定的观测指标。     

Oscillating and pulsed gradient diffusion magnetic resonance microscopy over an extended b‐value range: Implications for the characterization of tissue microstructure

Oscillating gradient spin‐echo (OGSE) pulse sequences have been proposed for acquiring diffusion data with very short diffusion times, which probe tissue structure at the subcellular scale. OGSE sequences are an alternative to pulsed gradient spin echo measurements, which typically probe longer diffusion times due to gradient limitations. In this investigation, a high‐strength (6600 G/cm) gradient designed for small‐sample microscopy was used to acquire OGSE and pulsed gradient spin echo data in a rat hippocampal specimen at microscopic resolution. Measurements covered a broad range of diffusion times (TDeff = 1.2–15.0 ms), frequencies (ω = 67–1000 Hz), and b‐values (b = 0–3.2 ms/μm2). Variations in apparent diffusion coefficient with frequency and diffusion time provided microstructural information at a scale much smaller than the imaging resolution. For a more direct comparison of the techniques, OGSE and pulsed gradient spin echo data were acquired with similar effective diffusion times. Measurements with similar TD_eff were consistent at low b‐value (b < 1 ms/μm²), but diverged at higher b‐values. Experimental observations suggest that the effective diffusion time can be helpful in the interpretation of low b‐value OGSE data. However, caution is required at higher b, where enhanced sensitivity to restriction and exchange render the effective diffusion time an unsuitable representation. Oscillating and pulsed gradient diffusion techniques offer unique, complementary information. In combination, the two methods provide a powerful tool for characterizing complex diffusion within biological tissues. Magn Reson Med 69:1131–1145, 2013. © 2012 Wiley Periodicals, Inc.     

The effect of concomitant gradient fields on diffusion tensor imaging

Concomitant gradient fields are transverse magnetic field components that are necessarily present to satisfy Maxwell's equations when magnetic field gradients are utilized in magnetic resonance imaging. They can have deleterious effects that are more prominent at lower static fields and/or higher gradient strengths. In diffusion tensor imaging schemes that employ large gradients that are not symmetric about a refocusing radiofrequency pulse (unlike Stejskal–Tanner, which is symmetric), concomitant fields may cause phase accrual that could corrupt the diffusion measurement. Theory predicting the error from this dephasing is described and experimentally validated for both Reese twice‐refocused and split gradient single spin‐echo diffusion gradient schemes. Bias in apparent diffusion coefficient values was experimentally found to worsen with distance from isocenter and with increasing duration of gradient asymmetry in both a phantom and in the brain. The amount of error from concomitant gradient fields depends on many variables, including the diffusion gradient pattern, pulse sequence timing, maximum effective gradient amplitude, static magnetic field strength, voxel size, slice distance from isocenter, and partial Fourier fraction. A prospective correction scheme that can reduce concomitant gradient errors is proposed and verified for diffusion imaging. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.     

Use of ethylene glycol to evaluate gradient performance in gradient-intensive diffusion MR sequences

Imaging a phantom of known dimensions is a widely used and simple method for calibrating MRI gradient strength. However, full-range characterization of gradient response is not achievable using this approach. Measurement of the apparent diffusion coefficient of a liquid with known diffusivity allows for calibration of gradient amplitudes across a wider dynamic range. An important caveat is that the temperature dependence of the liquid's diffusion characteristics must be known, and the temperature of the calibration phantom must be recorded. In this report, we demonstrate that the diffusion coefficient of ethylene glycol is well described by Arrhenius-type behavior across the typical range of ambient MRI magnet temperatures. Because of ethylene glycol's utility as an NMR chemical-shift thermometer, the same (1)H MR spectroscopy measurements that are used for gradient calibration also simultaneously "report" the sample temperature. The high viscosity of ethylene glycol makes it well-suited for assessing gradient performance in demanding diffusion-weighted imaging and spectroscopy sequences.     

Practical design of a high-strength breast gradient coil

A high-strength three-axis local gradient coil set was constructed for MRI of the breast. Gradient fields with good uniformity (<10% deviation from the desired gradient) over most of the volume required for breast imaging were generated with efficiencies of up to 3.3 mT/m/A. The coils will allow diffusion breast imaging in clinically acceptable examination times. The electrical design, water cooling system, and fabrication techniques are described. Preliminary tests of the coil included images of a grid phantom and diffusion measurements in a short-T₂ agarose gel phantom.     

A method for improving the performance of gradient systems for diffusion-weighted MRI.

The MR signal is sensitive to diffusion. This effect can be increased by the use of large, balanced bipolar gradients. The gradient systems of MR scanners are calibrated at installation and during regular servicing visits. Because the measured apparent diffusion constant (ADC) depends on the square of the amplitude of the diffusion sensitizing gradients, errors in the gradient calibration are exaggerated. If the error is varying among the different gradient axes, it will affect the estimated degree of anisotropy. To assess the gradient calibration accuracy in a whole-body MRI scanner, ADC values were calculated for a uniform water phantom along each gradient direction while monitoring the temperature. Knowledge of the temperature allows the expected diffusion constant of water to be calculated independent of the MRI measurement. It was found that the gradient axes (+/-x, +/-y, +/-z) were calibrated differently, resulting in offset ADC values. A method is presented to rescale the amplitude of each of the six principal gradient axes within the MR pulse sequence. The scaling factor is the square root of the ratio of the expected and observed diffusion constants. In addition, fiber tracking results in the human brain were noticeably affected by improving the gradient system calibration.     

Analytical error propagation in diffusion anisotropy calculations

PURPOSE: To develop an analytical formalism describing how noise and selection of diffusion-weighting scheme propagate through the diffusion tensor imaging (DTI) computational chain into variances of the diffusion tensor elements, and errors in the relative anisotropy (RA) and fractional anisotropy (FA) indices. MATERIALS AND METHODS: Singular-value decomposition (SVD) was used to determine the tensor variances, with diffusion-weighting scheme and measurement noise incorporated into the design matrix. Anisotropy errors were then derived using propagation of error. To illustrate the applications of the model, 12 data sets were acquired from each human subject, over a range of b-values (500-2500 seconds/mm2) and diffusion-weighting gradient directions (N = 6-55). The mean RA and FA values and their respective errors were calculated within a region of interest (ROI) in the splenium. The RA and FA errors as a function of b-value and N were evaluated, and a number of diffusion-weighting schemes were assessed based on a new metric, sum of diffusion tensor variances. RESULTS: When the acquisition time was held constant, the sum of the diffusion tensor variances decreased as N increased. The same trend was also observed for several diffusion-weighting schemes with constant condition number when noise in the diffusion-weighted (DW) images was assumed unity. Errors in both FA and RA increased with b-value and decreased with N. The FA error in the splenium was approximately threefold smaller than RA error, irrespective of b-value or N. CONCLUSION: The condition number may not adequately characterize the noise sensitivity for a given diffusion-weighting scheme. Signal averaging may not be as effective as increasing N, especially when N is small (e.g., N < 13). Due to its smaller error, FA is preferred over RA for quantitative DTI applications.