TIME CONSTANT SENSITIVITY OF EDDY CURRENT CHARACTERIZING PULSE SEQUENCE

该子项目是利用NIH/NCRR资助的中心赠款提供的资源的许多研究子项目之一。子项目和研究者（PI）可能从另一个NIH来源获得主要资金，因此可以在其他CRISP条目中表示。所列机构为中心，不一定是研究者所在机构。简介：涡流感应场对所采集的信号施加相位，这可能使用于测量组织速度、位移和温度的幅度图像和相位估计失真。涡流预加重提供了一阶校正，但需要更复杂的技术来解释宽范围的短时间常数和长时间常数以及空间非线性。采用Bloch和蒙特-卡罗模拟方法对测量系统涡流响应的脉冲序列进行建模，以表征测量时间常数的灵敏度和带宽。方法：为了测量相移所产生的短？涡流脉冲序列被设计为在大的测试梯度之后立即采集62 ms的数据（DAQ）。模拟，稳态涡流诱导的单指数梯度产生0.01ms<？<200 ms，峰值涡流梯度强度为0.1Gmax/？。Bloch模拟被用来模拟信号响应在一个广泛的范围？蒙特-卡罗模拟被用来证明量化每个涡流相位响应的灵敏度。结果：与DG的脉冲序列不刺激，因此是不敏感的长？涡流效应，但敏感（相位累积>？）到25秒<？<25ms。误差与拟合的时间常数的单指数函数的相位响应的梯度感应涡流与一个单一的？最低点在哪里？5 ms，表明对估计该量级的时间常数的最大灵敏度。结论：布洛赫模拟表明，脉冲序列是最敏感的短？的~ 5 ms，但具有良好的灵敏度，以25秒<？<25ms。DAQ在梯度转换后立即开始的事实，结合序列的高SNR和短？效应对T2衰变不敏感，联合收割机使测量短路？可能的影响。

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.