目的:开发用于同时多层(SMS)成像的多光子激发技术,并评估其性能和比吸收率(SAR)的益处。通过新颖的CAIPIRINHA(并行成像中的受控混叠导致更高的加速度)设计来提高多光子SMS重建质量。
方法:当常规单片射频场与振荡梯度场一起施加时,两者可以结合在多个离散的空间位置产生多光子激发。因为传统的RF在多个空间位置被重用,多光子激发为SMS应用提供了降低的SAR。CAIPIRINHA移位通常用于提高并行成像加速度。有趣的是,通过更新每个相位编码的振荡梯度相位,可以获得多光子SMS的CAIPIRINHA型位移。在这项工作中,具有多光子CAIPIRINHA-SMS激发脉冲的梯度回波和自旋回波序列均用于3T的体内人体成像。
结果:对于三个切片,多光子SMS与传统叠加SMS相比,SAR降低了51%,而对于五片,SAR降低了66%。多光子SMS在SAR降低方面优于PINS(功率与切片数量无关)和MultiPINS,尤其是当脉冲持续时间短时,切片很薄,和/或切片间距较大。用于多光子SMS的自定义CAIPIRINHA相位编码设计显着提高了重建质量。
结论:多光子SMS激发可以通过将具有振荡梯度的常规单片RF脉冲组合来获得,并且与常规叠加SMS相比具有显着的SAR优势。新颖的CAIPIRINHA设计允许用于多光子SMS成像的更高的多频带因子。
OBJECTIVE: To develop
multiphoton excitation techniques for simultaneous multislice (SMS) imaging and evaluate their performance and specific absorption rate (SAR) benefit. To improve
multiphoton SMS reconstruction quality with a novel CAIPIRINHA (controlled aliasing in parallel imaging results in higher acceleration) design.
METHODS: When a conventional single-slice RF field is applied together with an oscillating gradient field, the two can combine to generate multiphoton excitation at multiple discrete spatial locations. Because the conventional RF is reused at multiple spatial locations, multiphoton excitation offers reduced SAR for SMS applications. CAIPIRINHA shifts are often used to improve parallel-imaging acceleration. Interestingly, CAIPIRINHA-type shifts can be obtained for
multiphoton SMS by updating the oscillating gradient phase at every phase encode. In this work, both a gradient-echo and a spin-echo sequence with
multiphoton CAIPIRINHA-SMS excitation pulses are implemented for in vivo human imaging at 3 T.
RESULTS: For three slices, multiphoton SMS provides a 51% reduction in SAR compared with conventional superposition SMS, whereas for five slices, SAR is reduced by 66%. Multiphoton SMS outperforms PINS (power independent of number of slices) and MultiPINS in terms of SAR reduction especially when the pulse duration is short, slices are thin, and/or the slice spacing is large. A custom CAIPIRINHA phase-encoding design for multiphoton SMS significantly improves reconstruction quality.
CONCLUSIONS: Multiphoton SMS excitation can be obtained by combining conventional single-slice RF pulses with an oscillating gradient and offers significant SAR benefits compared with conventional superposition SMS. A novel CAIPIRINHA design allows higher multiband factors for
multiphoton SMS imaging.