Li Cheng, Magland Jeremy F, Seifert Alan C, Wehrli Felix W
IEEE Trans Med Imaging. 2014 Apr;33(4):961-9. doi: 10.1109/TMI.2014.2300500.
Zero-echo Time (ZTE) imaging is a promising technique for magnetic resonance imaging (MRI) of short-T2 tissue nuclei in tissues. A problem inherent to the method currently hindering its translation to the clinic is the presence of a spatial encoding gradient during excitation, which causes the hard pulse to become spatially selective, resulting in blurring and shadow artifacts in the image. While shortening radio-frequency (RF) pulse duration alleviates this problem the resulting elevated RF peak power and specific absorption rate (SAR) in practice impede such a solution. In this work, an approach is described to correct the artifacts by applying quadratic phase-modulated RF excitation and iteratively solving an inverse problem formulated from the signal model of ZTE imaging. A simple pulse sequence is also developed to measure the excitation profile of the RF pulse. Results from simulations, phantom and in vivo studies, demonstrate the effectiveness of the method in correcting image artifacts caused by inhomogeneous excitation. The proposed method may contribute toward establishing ZTE MRI as a routine 3D pulse sequence for imaging protons and other nuclei with quasi solid-state behavior on clinical scanners.
零回波时间（ZTE）成像对于组织中短T2组织核的磁共振成像（MRI）来说是一项很有前景的技术。目前阻碍该方法应用于临床的一个固有问题是，在激发过程中存在空间编码梯度，这会使硬脉冲具有空间选择性，从而在图像中产生模糊和阴影伪影。虽然缩短射频（RF）脉冲持续时间可以缓解这个问题，但在实际应用中，由此导致的射频峰值功率和比吸收率（SAR）升高会阻碍这种解决方案。在这项工作中，描述了一种通过应用二次相位调制的RF激发并迭代求解由ZTE成像信号模型构建的反问题来校正伪影的方法。还开发了一个简单的脉冲序列来测量RF脉冲的激发轮廓。模拟、体模和体内研究的结果证明了该方法在校正由不均匀激发引起的图像伪影方面的有效性。所提出的方法可能有助于将ZTE MRI确立为一种常规的3D脉冲序列，用于在临床扫描仪上对具有准固态行为的质子和其他原子核进行成像。
