Purpose

To develop schemes that deliver faithful 2D slices near field heterogeneities of the kind arising from non-ferromagnetic metal implants, with reduced artifacts and shorter scan times.

Methods

An excitation scheme relying on cross-term spatio-temporal encoding (xSPEN) was used as basis for developing the new inhomogeneity-insensitive, slice-selective pulse scheme. The resulting Fully refOCUSED cross-term SPatiotemporal ENcoding (FOCUSED-xSPEN) approach involved four adiabatic sweeps. The method was evaluated in silico, in vitro and in vivo using mice models, and compared against a number of existing and of novel alternatives based on both conventional and swept RF pulses, including an analogous method based on LASER's selectivity spatial selectivity.

Results

Calculations and experiments confirmed that multi-sweep derivatives of xSPEN and LASER can deliver localized excitation profiles, centered at the intended positions and endowed with enhanced immunity to B₀ and B₁ distortions. This, however, is achieved at the expense of higher SAR than non-swept counterparts. Furthermore, single-shot FOCUSED-xSPEN and LASER profiles covered limited off-resonance ranges. This could be extended to bands covering arbitrary off-resonance values with uniform slice widths, by looping the experiments over a number of scans possessing suitable transmission and reception offsets.

Conclusions

A series of novel approaches were introduced to select slices near metals, delivering robustness against B_o and B₁⁺ field inhomogeneities.

中文翻译：

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在存在强烈的金属来源的不均匀性的情况下提高MRI的切片选择性。

目的

开发可在非铁磁金属注入引起的近场异质性附近交付真实2D切片的方案，从而减少伪像并缩短扫描时间。

方法

一种基于交叉项时空编码（xSPEN）的激励方案被用作开发新的非均质性不敏感，切片选择脉冲方案的基础。最终产生的完全重新交叉跨时空时空编码（FOCUSED-xSPEN）方法涉及四个绝热扫描。使用小鼠模型在计算机上，体外和体内对该方法进行了评估，并与许多基于常规和扫频RF脉冲的现有方法和新颖方法进行了比较，包括基于LASER选择性空间选择性的类似方法。

结果

计算和实验证实，xSPEN和LASER的多扫频导数可以提供局部激发曲线，其中心位于预期位置，并具有增强的B ₀和B ₁失真抗扰性。但是，这要以比未扫描的同类产品更高的SAR为代价来实现。此外，单次FOCUSED-xSPEN和LASER配置文件涵盖了有限的非共振范围。通过在具有合适的发送和接收偏移量的多个扫描上循环实验，可以将其扩展到覆盖具有均匀切片宽度的任意非共振值的频带。

结论

引入了一系列新颖的方法来选择金属附近的切片，从而对B _o和B ₁ ⁺场不均匀性具有鲁棒性。