Department of Radiology, University of Pittsburgh, Pittsburgh, Pennsylvania, USA.
Magn Reson Med. 2022 Jul;88(1):254-265. doi: 10.1002/mrm.29211. Epub 2022 Mar 28.
Endogenous CEST signal usually has low specificity due to contamination from the magnetization transfer effect and from fast exchanging labile protons with close Larmor frequencies. We propose to improve CEST signal specificity with an average saturation efficiency filter (ASEF).
ASEF measures the difference between CEST signals acquired with similar average irradiation power but largely different duty cycles (DC), for example, a continuous wave or a high DC pulse train versus a low DC one. Simulation and Cr phantom studies were performed to evaluate the characteristics of ASEF for CEST.
Theoretical and simulation studies show that ASEF can suppress fast exchanging processes, with only a small loss of chemical exchange contrast for slow-to-intermediate exchange rates if the difference in DC is large. In the RF offset range of 2 to 5 ppm with an averaged saturation power of 0.8 and 1.6 microteslas, there is a mismatch of ∼0.1% to 2% in the magnetization transfer signal between saturation by continuous wave and a pulse train with DC = 15% and pulse duration of 24 ms, respectively. This mismatch can be minimized by careful selection of saturation power, pulse duration, and DC differences or by applying a small fudge factor between the 2 irradiation powers. Phantom studies of Cr confirmed that ASEF can minimize the magnetization transfer effect and reduce sensitivity to fast exchange processes.
ASEF can improve the specificity of slow-to-intermediate exchanging CEST signal with a relatively small loss of sensitivity.
由于磁化传递效应和具有相近拉莫尔频率的快速交换的不稳定质子的污染，内源性 CEST 信号通常特异性较低。我们提出使用平均饱和效率滤波器 (ASEF) 来提高 CEST 信号的特异性。
ASEF 测量在相似平均辐照功率但具有显著不同占空比（DC）下获得的 CEST 信号之间的差异，例如连续波或高 DC 脉冲串与低 DC 脉冲串之间的差异。模拟和 Cr 体模研究用于评估 ASEF 对 CEST 的特性。
理论和模拟研究表明，ASEF 可以抑制快速交换过程，对于慢至中等交换率，如果 DC 差异较大，则仅会对缓慢交换对比度产生较小的损失。在平均饱和功率为 0.8 和 1.6 微特斯拉、RF 偏移范围为 2 至 5 ppm 的情况下，连续波饱和和 DC = 15%、脉冲持续时间为 24 ms 的脉冲串之间的磁化转移信号存在约 0.1%至 2%的不匹配。通过仔细选择饱和功率、脉冲持续时间和 DC 差异，或在两种辐照功率之间应用小的调整因子，可以最小化这种不匹配。Cr 体模研究证实，ASEF 可以最小化磁化传递效应并降低对快速交换过程的敏感性。
ASEF 可以在不损失灵敏度的情况下提高慢至中等交换 CEST 信号的特异性。
