Sensitivity to local dipole fields in the CRAZED experiment: An approach to bright spot MRI
Sensitivity to local dipole fields in the CRAZED experiment: An approach to bright spot MRI
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DOI：
10.1016/j.jmr.2006.05.002
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发表时间：
2006-10-01
影响因子：
2.2
通讯作者：
Bowtell, Richard
中科院分区：
文献类型：
--
作者：
Faber, Cornelius;Heil, Carolin;Bowtell, Richard
局部偶极子场，如由小氧化铁颗粒产生的偶极子场，在许多分子磁共振成像应用中用于产生低强度（暗对比度）区域。通过计算机模拟和17.6 T的实验，我们研究了如何用非对称7梯度回波检测（CRAZED）实验改进COSY，选择分子间双量子相干，也可以用来可视化这样的局部偶极子场。平行于主磁场的相干选择梯度脉冲的应用产生了与传统梯度回波成像相似的暗对比度。沿魔角梯度的应用导致均匀样品中信号强度的全部损失。在局部偶极子场存在的情况下，对比被颠倒，偶极子发出的明亮信号在一个非常低的背景下被观测到。仿真和实验结果表明，采用相周期抑制单量子相干时，信号明显减弱。因此，我们得出结论，大多数信号来自直接重聚焦磁化或分子间单量子相干。最后，我们证明了当对相干选择梯度脉冲故意错配时，也可以获得局部偶极子场的明亮对比度。两种方法都可以在实验时间约为3分钟的幻影中可视化局部偶极子场。(c) 2006爱思唯尔公司版权所有。
Local dipole fields such as those created by small iron-oxide particles are used to produce regions of low intensity (dark contrast) in many molecular magnetic resonance imaging applications. We have investigated, with computer simulations and experiments at 17.6 T, how the COSY revamped with asymmetric 7-gradient echo detection (CRAZED) experiment that selects intermolecular double-quantum coherences can also be used to visualize such local dipole fields. Application of the coherence-selection gradient pulses parallel to the main magnetic field produced similar, dark contrast as conventional gradient echo imaging. Application of the gradient along the magic angle leads to total loss of signal intensity in homogeneous samples. In the presence of local dipole fields, the contrast was inverted and bright signals from the dipoles were observed over a very low background. Both simulations and experiments showed that the signal strongly decreased when a phase-cycle suppressing single-quantum coherences was employed. Therefore, we conclude that most of the signal comes from directly refocused magnetization or intermolecular single-quantum coherences. Finally, we demonstrate that bright contrast from local dipole fields can also be obtained, when the pair of coherence-selection gradient pulses is deliberately mismatched. Both methods allowed visualization of local dipole fields in phantoms in experimental times of about 3 min. (c) 2006 Elsevier Inc. All rights reserved.