FLOW COMPENSATED SPECTRAL SPATIAL EXCITATION PULSES

简介：快速螺旋采集非常有用， 由于扫描时间短，因此可用于定量流量应用。 在 这些序列、频谱空间激励脉冲通常 用于减少脂质造成的模糊。 这些脉冲只激发 切片内的水质子，导致来自脂质的最小信号 质子 然而，移动的质子经历相位调制， 光谱-空间激发，导致信号降低， 流量量化的问题。 我们设计了流量补偿 频谱-空间脉冲作为解决这个问题的方案。 有了这些新 脉冲，以恒定速度运动的质子经历相同的 激发谱为静态质子，而脂质信号仍然是 显著下降。 方法和结果：实现流动 在频谱-空间激励期间，施加RF能量， 只有当振荡梯度的零阶和一阶矩 重新聚焦或归零。 这些脉冲的设计受到 系统的最大梯度振幅和转换速率。 与标准 梯度参数、流量补偿设计， 选择性仅对相对厚的空间板激发起作用。 使用我们更新的梯度系统（更高的梯度幅度和更快的速度 转换速率），可以保持光谱选择性，同时实现 更宽的切片厚度范围。 对于过平面流 测量，速度编码也可以被并入到流中 补偿脉冲。 这进一步降低了梯度占空比，并且 也缩短了序列的最小TE和TR。 总结和 结论：流量补偿频谱空间脉冲设计 具有更强的梯度子系统。 这些脉冲产生 以恒定速度运动的质子的期望激发轮廓 在激发期间，同时保持光谱和空间 选择性 与非流量补偿设计相比， 从移动的自旋结果更好地可视化更快的移动 血管中的流量，以及可能更准确地确定 定量流量应用中的过平面流量测量。

Introduction: Fast spiral acquisitions are very useful in quantitative flow applications because of their short scan times. In these sequences, spectral-spatial excitation pulses are typically employed to reduce blurring from lipids. These pulses excite only water protons within a slice, resulting in minimal signal from lipid protons. However, moving protons experience a phase modulation during spectral-spatial excitation, resulting in decreased signal and problems in flow quantification. We have designed flow compensated spectral-spatial pulses as a solution to this problem. With these new pulses, protons moving at constant velocities experience the same excitation profile as static protons, while lipid signal is still significantly decreased. Methods and Results: To achieve flow compensation during spectral-spatial excitations, rf energy is applied only when the zeroth and first moments of the oscillating gradient are refocused, or nulled. The design of these pulses is limited by the maximum gradient amplitude and slew rate of the system. With standard gradient parameters, flow compensated designs that maintain spectral selectivity only work for relatively thick spatial slab excitations. With our newer gradient system (higher gradient amplitude and faster slew rates), spectral selectivity can be maintained, while achieving a wider range of slice thicknesses. For through-plane flow measurements, velocity encoding can also be incorporated into the flow compensated pulses. This further reduces the gradient duty cycle, and also shortens the minimum TE and TR of the sequence. Summary and Conclusions: Flow compensated spectral-spatial pulse designs are feasible with stronger gradient subsystems. These pulses produce desired excitation profiles for protons moving at constant velocities during the excitation period, while maintaining spectral and spatial selectivity. Compared to non-flow compensated designs, higher signal from moving spins results in better visualization of faster moving flow in vessels, and potentially more accurate determination of through-plane flow measurements in quantitative flow applications.