Low duty cycle pulse trains for exchange rate insensitive chemical exchange saturation transfer MRI.
Low duty cycle pulse trains for exchange rate insensitive chemical exchange saturation transfer MRI.
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DOI：
10.1002/mrm.28896
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发表时间：
2021-11
影响因子：
3.3
通讯作者：
Jin T
中科院分区：
文献类型：
--
作者：
Chung JJ;Jin T
介绍并验证一种脉冲方案，该方案使用低占空比 π 脉冲序列来实现饱和度，该饱和度对汇率相对不敏感，但线性依赖于不稳定的质子浓度。进行模拟以探索 π 脉冲序列和连续波 CEST 信号的汇率敏感性。对具有不同 pH 值和不同浓度的肌酸模型进行成像，以证明低占空比 π 脉冲饱和度的 pH 不敏感性和浓度依赖性。仿真表明，降低 π 脉冲饱和度的占空比会降低对汇率的峰值敏感度，并且可以通过平均 B1 功率将这一不敏感范围调整到不同的汇率。不敏感范围不受弛豫和磁化转移变化的影响，而 CEST 信号的灵敏度保持对不稳定质子浓度的线性依赖。在 B1，avg = 0.48 μT 下，pH 范围在 6.36 和 8.21 之间的 30 mM 肌酸在连续波下表现出 ~6-11% 的 CEST 对比度，在使用 10% 占空比 π 脉冲的所有 pH 范围内表现出 ~4% 的 CEST 对比度。使用 5%、10%、25% 和 50% 的占空比对这些体模进行成像显示，pH 敏感性随着占空比的降低而降低。不同浓度和 pH 值的肌酸模型表明，π 脉冲序列饱和度与较低 DC 下的浓度表现出更严格的相关性。低 DC π 脉冲序列是一种易于实施的方法，可提供不稳定的质子浓度相关的 CEST MRI 信号，该信号对交换率不敏感。这种方法在化学交换率的变化可能干扰生理学或病理学的准确评估的研究中非常有用。
To introduce and validate a pulse scheme that uses low duty cycle trains of π-pulses to achieve saturation that is relatively insensitive to exchange rate yet linearly dependent on labile proton concentration. Simulations were performed to explore the exchange rate sensitivity of π-pulse trains and continuous wave CEST signals. Creatine phantoms with varying pH and varying concentrations were imaged to demonstrate pH insensitivity and concentration dependence of low duty cycle π-pulse saturation. Simulations show decreasing the duty cycle of π-pulse saturation decreases peak sensitivity to exchange rate and this range of insensitivity can be tuned to different exchange rates through average B1 power. The range of insensitivity is unaffected by changes in relaxation and magnetization transfer, while the sensitivity of CEST signal maintains linear dependence on labile proton concentration. Under B1,avg = 0.48 μT, 30 mM creatine with pHs ranging between 6.36 and 8.21 exhibited CEST contrast ranging between ~6–11% under continuous wave and ~4% across all pHs using 10% duty cycle π-pulses. Imaging these phantoms using duty cycles of 5, 10, 25, and 50% showed decreasing pH sensitivity with decreased duty cycle. Creatine phantoms with varied concentrations and pHs reveal that π-pulse train saturation exhibited stricter correlation to concentration at lower DCs. Low DC π-pulse train is an easy to implement way of providing labile proton concentration dependent CEST MRI signal that is insensitive to exchange rate. This approach can be useful in studies where change of chemical exchange rate may interfere with accurate assessments of physiology or pathology.