TIME CONSTANT SENSITIVITY OF EDDY CURRENT CHARACTERIZING PULSE SEQUENCE

这个子项目是利用由NIH/NCRR资助的中心拨款提供的资源的许多研究子项目之一。子项目和调查员(PI)可能从另一个NIH来源获得了主要资金，因此可能会出现在其他CRISE条目中。列出的机构是针对中心的，而不一定是针对调查员的机构。前言：涡流感应场对采集到的信号施加相位，这会扭曲用于测量组织速度、位移和温度的幅度图像和相位估计。涡流预加重提供了一阶校正，但需要更复杂的技术来考虑大范围的短时间常数和长时间常数以及空间非线性。用Bloch和蒙特卡罗模拟方法模拟了用于测量S涡流响应的脉冲序列，以表征测量时间常数的灵敏度和带宽。方法：为了测量短时相移。涡流脉冲序列被设计为在大测试斜率之后立即采集数据(DAQ)62ms。模拟产生的稳态涡流诱导单指数梯度为0.01ms&lt；？？200ms，峰值涡流梯度强度为0.1Gmax/？使用Bloch模拟来模拟信号在很大范围内的响应。用蒙特卡罗模拟验证了对量化每个涡流的相位响应的敏感性。结果：含有DGS的脉冲序列不刺激，因此对长时间不敏感。涡流效应，但很敏感(相位积累？)至25？S？25毫秒。将单指数函数的时间常数与梯度感应涡流的相位响应进行拟合时产生的误差。？=5ms是最低的，表示对估算这种震级的时间常数的灵敏度最高。结论：Bloch模拟表明，脉冲序列对短？~5ms，但对25ms的S有很好的敏感性。DAQ在梯度摆动后立即开始的事实，再加上序列的高信噪比和短？影响是不是对T2衰减不敏感，结合起来就可以测量短了吗？可能产生的影响。

This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. INTRODUCTION: Eddy current induced fields impart phase on the acquired signal that can distort both magnitude images and phase estimates used to measure tissue velocities, displacements, and temperatures. Eddy current pre-emphasis provides first-order correction, but more sophisticated techniques are needed to account for a broad range of short and long time constants and spatial non-linearities. A pulse sequence designed to measure the system¿s eddy current response was modeled with Bloch and Monte-Carlo simulation to characterize the sensitivity and bandwidth of measured time constants (?). METHODS: In order to measure phase shifts that arise from short ? eddy currents the pulse sequence was designed to acquire data (DAQ) for 62ms immediately after a large test gradient. Simulated, steady-state eddy current induced mono-exponential gradients were generated with 0.01ms<?<200ms and peak eddy current gradient strengths of 0.1Gmax/?. Bloch simulation was used to simulate the signal response over a broad range of ?. Monte-Carlo simulation was used to demonstrate the sensitivity to quantifying the phase response of each eddy current ?. RESULTS: The pulse sequence with the DGs doesn¿t stimulate and is therefore insensitive to long ? eddy current effects, but is sensitive (phase accumulation >?) to 25¿s<?<25ms. Error associated with fitting the time constant of a mono-exponential function to the phase response of a gradient induced eddy-current with a single ? are lowest near ?=5ms indicating maximum sensitivity to estimating time constants of this magnitude. CONCLUSIONS: The Bloch simulation demonstrates that the pulse sequence is maximally sensitive to short ? of ~5ms, but with good sensitivity to 25¿s<?<25ms. The fact that DAQ begins immediately after a gradient slew, combined with the high SNR of the sequence and the fact that short ? effects are not sensitive to T2 decay combine to make measurement of short ? effects possible.