Consistent non-Cartesian off-axis MRI quality: Calibrating and removing multiple sources of demodulation phase errors
Consistent non-Cartesian off-axis MRI quality: Calibrating and removing multiple sources of demodulation phase errors
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DOI：
10.1002/mrm.21092
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发表时间：
2007-01-01
影响因子：
3.3
通讯作者：
Block, Walter F.
中科院分区：
文献类型：
--
作者：
Jung, Youngkyoo;Jashnani, Yogesh;Block, Walter F.
大量扫描仪上的非笛卡尔序列离轴 MRI 的一致性变化很大。离轴成像所必需的梯度场、数据采集系统和实时频率解调参考信号的不正确的时序对准是这种变化的重要来源。此外，涡流和各向异性梯度延迟会导致k空间轨迹的偏差，进而使解调参考信号不准确。提出了一种快速测量频率解调参考信号中的定时误差并将其与各向异性梯度延迟分开的方法。使用先前的梯度校准技术测量的 k 空间偏差被证明是降低图像质量的解调相位误差的第二个来源。使用定时延迟和 k 空间偏差，在重建期间对数据重新网格化之前，对每个 k 空间样本应用回顾性相位校正。经测量，四台 MR 扫描仪的定时延迟比制造商建议的延迟低 4.2-7.5 μs。回顾性校正 3D 径向（3D 投影重建 (PR)）膝盖和乳房图像中的显着退化，同时对螺旋成像应用部分前瞻性校正。该方法允许在多个扫描仪上实现更一致的非笛卡尔序列性能，而无需操作员干预。
The consistency of off-axis MRI with non-Cartesian sequences across a large number of scanners is highly variable. Improper timing alignment of the gradient fields, data acquisition system, and real-time frequency demodulation reference signal, which are necessary for off-axis imaging, is an important source of this variability. In addition, eddy currents and anisotropic gradient delays cause deviations in k-space trajectories that in turn make the demodulation reference signals inaccurate. A method is presented to quickly measure the timing error in the frequency demodulation reference signal and separate it from anisotropic gradient delays. k-Space deviations, as measured with a previous gradient calibration technique, are shown to be a second source of demodulation phase errors that degrade image quality. Using the timing delay and k-space deviations, a retrospective phase correction is applied to each k-space sample before the data are regridded during reconstruction. The timing delays of four MR scanners were measured to be 4.2-7.5 mu s below the manufacturer's suggested delay. Significant degradation in 3D radial (3D projection reconstruction (PR)) knee and breast images are retrospectively corrected while a partial prospective correction is applied for spiral imaging. The method allows for more consistent performance of non-Cartesian sequences across multiple scanners without operator intervention.