Radiofrequency pulse designs for three-dimensional MRI providing uniform tipping in inhomogeneous B₁ fields.
Radiofrequency pulse designs for three-dimensional MRI providing uniform tipping in inhomogeneous B₁ fields.
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DOI：
10.1002/mrm.22913
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发表时间：
2011-11
影响因子：
3.3
通讯作者：
Matson, Gerald B.
中科院分区：
文献类型：
--
作者：
Liu, Hui;Matson, Gerald B.
尽管高场 MRI 提供了更高的信噪比 (S/N)，但传统 RF 脉冲产生的不均匀倾斜会导致空间依赖性对比度和次优 S/N，从而使 MR 图像的解释变得复杂。对于结构成像，不使用频率选择性 RF 脉冲的 3D 序列已变得流行。因此，本研究的目的是开发不受 B1 不均匀性和共振偏移影响的非切片选择性 (NSS) 射频脉冲。为了实现这一目标，使用基于最优控制理论的优化例程来设计新的 NSS 脉冲，该脉冲具有所需的 B1 不均匀性和共振偏移免疫范围。该设计允许脉冲产生的横向磁化的相位发生变化。虽然重点是浅尖端设计，但还提供了 30°、60°、90° 和 180° 度 NSS RF 脉冲的新设计。将这些较大的尖端角脉冲与最近发布的 NSS 脉冲进行比较。有证据表明，本文中提出的脉冲具有相同的性能，但比最近发布的脉冲更短。尽管 NSS 脉冲比它们所取代的矩形脉冲产生更高的比吸收率 (SAR) 和更大的磁化转移 (MT) 效应，但它们仍然显示出在高场 3D MRI 实验中的应用前景。
Although high-field MRI offers increased signal-to-noise (S/N), the non-uniform tipping produced by conventional RF pulses leads to spatially dependent contrast and sub-optimal S/N, thus complicating the interpretation of the MR images. For structural imaging, 3D sequences which do not make use of frequency-selective RF pulses have become popular. Therefore, the aim of this research was to develop non-slice-selective (NSS) RF pulses with immunity to both B1 inhomogeneity and resonance offset. To accomplish this, an optimization routine based on optimal control theory was used to design new NSS pulses with desired ranges of immunity to B1 inhomogeneity and resonance offset. The design allows the phase of transverse magnetization produced by the pulses to vary. While the emphasis is on shallow tip designs, new designs for 30°, 60°, 90° and 180° degree NSS RF pulses are also provided. These larger tip angle pulses are compared with recently published NSS pulses. Evidence is presented that the pulses presented in this article have equivalent performance but are shorter than the recently published pulses. Although the NSS pulses generate higher specific absorption rates (SAR) and larger magnetization transfer (MT) effects than the rectangular pulses they replace, they nevertheless show promise for 3D MRI experiments at high field.
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