Department of Electrical Engineering and Computer Sciences, University of California, Berkeley, CA, USA.
Department of Electrical Engineering and Computer Sciences, University of California, Berkeley, CA, USA.
J Magn Reson. 2023 Mar;348:107376. doi: 10.1016/j.jmr.2023.107376. Epub 2023 Jan 11.
Excitation in MRI is traditionally done at the Larmor frequency, where the energy of each radiofrequency photon corresponds to the energy difference between two spin states. However, if multiple radiofrequencies are employed, then multiphoton excitation can also occur when the sum or difference of multiple photon frequencies equals the Larmor frequency. Although multiphoton excitation has been known since the early days of NMR, it has been relatively unexplored in MRI. In this work, equations and principles for multiphoton selective RF pulse design in imaging are presented and experimentally demonstrated. In particular, the case where there are radiofrequency fields in both the traditional xy-direction and non-traditional z-direction is considered. To produce the z-direction radiofrequency field, an additional uniform coil was added to a clinical MRI scanner. Using this coil, two-photon slice-selective pulses were designed to be equivalent to traditional pulses, producing similar excitation, slice profiles, and in vivo images. Being the result of a combination of multiple radiofrequency fields instead of just one, two-photon pulses have more flexibility in how their parameters can be changed. Although individual multiphoton excitations are less efficient than their traditional counterparts, when the z-direction radiofrequency field is spatially non-uniform, multiple multiphoton resonances can be simultaneously used at different locations to produce simultaneous multislice excitation with the same pulse duration but less tissue heating than a naive implementation. In particular, non-uniform z-direction radiofrequency fields with negligible added tissue heating provided by oscillating the MRI scanner's gradient fields at kilohertz frequencies were used to excite multiple slices simultaneously with less high-frequency xy-direction radiofrequency power. For an example three-slice excitation, we achieve half the xy-direction radiofrequency power compared to the naïve approach of adding three single-slice pulses. For conventional or unconventional applications, multiphoton excitation may be of interest when designing new MRI systems.
磁共振成像中的激发传统上是在拉莫尔频率下进行的，其中每个射频光子的能量对应于两个自旋态之间的能量差。然而，如果使用多个射频，则当多个光子频率的和或差等于拉莫尔频率时，也会发生多光子激发。尽管多光子激发自 NMR 的早期就已经为人所知，但在 MRI 中它的研究相对较少。在这项工作中，提出并实验证明了在成像中设计多光子选择性射频脉冲的方程和原理。特别是，考虑了在传统的 xy 方向和非传统的 z 方向都存在射频场的情况。为了产生 z 方向的射频场，在临床 MRI 扫描仪中添加了一个额外的均匀线圈。使用这个线圈，设计了双光子切片选择性脉冲，使其等效于传统脉冲，产生相似的激发、切片轮廓和体内图像。由于是多个射频场的组合而不是仅仅一个，双光子脉冲在其参数可以改变的方式上具有更大的灵活性。尽管单个多光子激发的效率低于其传统对应物，但当 z 方向的射频场空间不均匀时，可以同时在不同位置使用多个多光子共振，以相同的脉冲持续时间但比盲目实现更少的组织加热来产生同时的多切片激发。特别是，使用以千赫兹频率振荡 MRI 扫描仪的梯度场来提供几乎没有增加的组织加热的非均匀 z 方向射频场，可以同时用更少的高频 xy 方向射频功率激发多个切片。对于一个三切片激发的例子，与添加三个单切片脉冲的盲目方法相比，我们实现了 xy 方向射频功率的一半。对于传统或非常规应用，在设计新的 MRI 系统时，多光子激发可能会引起人们的兴趣。
