Transmit Array Spatial Encoding (TRASE) using broadband WURST pulses for RF spatial encoding in inhomogeneous B0 fields.
Transmit Array Spatial Encoding (TRASE) using broadband WURST pulses for RF spatial encoding in inhomogeneous B0 fields.
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DOI：
10.1016/j.jmr.2016.04.005
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发表时间：
2016-07
期刊：
影响因子：
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通讯作者：
Wald LL
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文献类型：
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作者：
Stockmann JP;Cooley CZ;Guerin B;Rosen MS;Wald LL
发射阵列空间编码（TRASE）是一种很有前途的新MR编码方法，它利用视场内的发射RF（）相位梯度来执行傅立叶空间编码。采集使用自旋回波序列，其中发射线圈相位斜坡被调制为从一个k空间点跳到下一个。这项工作通过使用扫频射频（RF）脉冲和二次相位去除方法扩展了TRASE的能力，使TRASE能够用于最需要的地方：具有非均匀B 0场的便携式成像系统。该方法特别适合于便携式MR扫描仪，其中（a）不均匀的B 0场是轻质磁体设计的副产品，（B）重的、高功耗的梯度线圈系统是将系统定位在非常规位置的限制，以及（c）需要与自旋回波序列的使用的协同作用来克服不均匀场中的体素内失相（短）。TRASE不使用B 0场的调制来编码，但它确实受到不均匀场的二次效应的影响。当RF脉冲未覆盖成像FOV中自旋共振的全部带宽时，由于非共振效应，TRASE图像中会出现严重伪影。因此，对于高度不均匀的B 0场，除了需要能够承受数千伏的RF线圈之外，高带宽重聚焦硬脉冲所需的峰值RF功率变得非常昂贵。在这项工作中，我们使用扫频WURST RF脉冲回波序列在高度不均匀的磁场（Δ B 0/B 0 ~ 0.33%）中实现TRASE成像。通过精确地激励和重新聚焦自旋的全带宽，WURST脉冲消除了由先前TRASE成像实现中使用的硬脉冲的有限带宽引起的伪影。我们引入了一个校正方案，以消除不必要的二次相位调制所造成的扫频脉冲。此外，采用相位交替方案来减轻由于重聚焦脉冲中的缺陷而由偶数回波相干路径和奇数回波相干路径的混合引起的伪影。在本文中，我们描述了这种所需的方法，并证明了TRASE的傅立叶编码的能力，在一个不均匀的领域（Δ B 0/B 0 ~ 1%，在整个FOV）。
Transmit Array Spatial Encoding (TRASE) is a promising new MR encoding method that uses transmit RF () phase gradients over the field-of-view to perform Fourier spatial encoding. Acquisitions use a spin echo train in which the transmit coil phase ramp is modulated to jump from one k-space point to the next. This work extends the capability of TRASE by using swept radiofrequency (RF) pulses and a quadratic phase removal method to enable TRASE where it is arguably most needed: portable imaging systems with inhomogeneous B0 fields. The approach is particularly well-suited for portable MR scanners where (a) inhomogeneous B0 fields are a byproduct of lightweight magnet design, (b) heavy, high power-consumption gradient coil systems are a limitation to siting the system in non-conventional locations and (c) synergy with the use of spin echo trains is required to overcome intra-voxel dephasing (short ) in the inhomogeneous field. TRASE does not use a modulation of the B0 field to encode, but it does suffer from secondary effects of the inhomogeneous field. Severe artifacts arise in TRASE images due to off-resonance effects when the RF pulse does not cover the full bandwidth of spin resonances in the imaging FOV. Thus, for highly inhomogeneous B0 fields, the peak RF power needed for high-bandwidth refocusing hard pulses becomes very expensive, in addition to requiring RF coils that can withstand thousands of volts. In this work, we use swept WURST RF pulse echo trains to achieve TRASE imaging in a highly inhomogeneous magnetic field (ΔB0/B0 ~ 0.33% over the sample). By accurately exciting and refocusing the full bandwidth of spins, the WURST pulses eliminate artifacts caused by the limited bandwidth of the hard pulses used in previous realizations of TRASE imaging. We introduce a correction scheme to remove the unwanted quadratic phase modulation caused by the swept pulses. Also, a phase alternation scheme is employed to mitigate artifacts caused by mixture of the even and odd-echo coherence pathways due to defects in the refocusing pulse. In this paper, we describe this needed methodology and demonstrate the ability of TRASE to Fourier encode in an inhomogeneous field (ΔB0/B0 ~ 1% over the full FOV).