Tao Shengzhen, Zhou Xiangzhi, Lin Chen, Patel Vishal, Westerhold Erin M, Middlebrooks Erik H
Department of Radiology, Mayo Clinic, Jacksonville, FL, USA.
Department of Radiology, Mayo Clinic, Jacksonville, FL, USA.
Magn Reson Imaging. 2023 Jul;100:55-63. doi: 10.1016/j.mri.2023.03.007. Epub 2023 Mar 15.
Deep brain stimulation (DBS) is an effective treatment of various neurological disorders. Due to higher intrinsic signal, 7 T MRI can potentially improve delineation of DBS targets. However, the severe RF transmit field (B1+) inhomogeneity at 7 T can compromise the image contrast of traditional single-contrast sequences for DBS targeting, leading to sub-optimal target visualization. The Magnetization Prepared 2 Rapid Acquisition Gradient Echo (MP2RAGE)-based T1 mapping provides an alternative to the traditional single-contrast techniques by allowing retrospective synthesis of images at arbitrary inversion times to aid in visualization of various DBS targets. With this approach, optimization of sequence parameters to create T1 maps with low noise and low quantification bias is critical, as these characteristics directly affect the noise and uniformity of the synthetic images. In this work, we perform sequence optimization for MP2RAGE-based T1 mapping using a radial view-ordering technique to improve image quality, and demonstrate the clinical utility of T1 mapping approach for DBS targeting.
We first introduce a systematic sequence optimization framework for 7 T MP2RAGE T1 mapping by formulating it into a constrained, multi-dimensional optimization process considering the effect of B1+ inhomogeneity on image noise, T1 quantification bias, and image blurring. With this framework, we investigate the use of radial view-order approach for T1 mapping, in lieu of the conventional linear view-ordering. Bloch's equation-based simulations were performed to compare the T1 maps generated using different approaches. Images of healthy volunteer and patients were acquired on a clinical 7 T MRI scanner for validation and to demonstrate the utility of T1 mapping for DBS targeting.
Numerical experiments demonstrated that the proposed framework allowed optimization of image SNR in T1 maps while controlling the quantification bias and image blurring, therefore facilitating the selection of optimal sequence parameters for visualizing DBS targets. The optimized sequence using radial view-ordering offered 40-60% noise reduction compared to the linear view-ordering. The improvement of SNR was confirmed in the in vivo examples. Clinical images showed that the synthetic images generated from the optimized T1 maps allowed clear visualization of DBS targets.
We demonstrated the optimization of MP2RAGE T1 mapping with radial view-ordering technique for DBS targeting at 7 T and showed that the optimized sequence allows retrospective generation of synthetic inversion time images commonly utilized in DBS targeting, such as fast gray matter acquisition T1 inversion recovery (FGATIR) and edge-enhancing gradient echo (EDGE) sequences.
脑深部电刺激（DBS）是治疗各种神经系统疾病的有效方法。由于具有更高的固有信号，7T磁共振成像（MRI）有可能改善DBS靶点的描绘。然而，7T时严重的射频发射场（B1+）不均匀性会损害用于DBS靶点定位的传统单对比度序列的图像对比度，导致靶点可视化效果欠佳。基于磁化准备快速采集梯度回波（MP2RAGE）的T1映射通过允许在任意反转时间进行图像的回顾性合成，为传统单对比度技术提供了一种替代方法，有助于各种DBS靶点的可视化。采用这种方法时，优化序列参数以创建低噪声和低定量偏差的T1映射至关重要，因为这些特性直接影响合成图像的噪声和均匀性。在这项工作中，我们使用径向视图排序技术对基于MP2RAGE的T1映射进行序列优化以提高图像质量，并展示T1映射方法在DBS靶点定位中的临床应用价值。
我们首先通过将其制定为一个考虑B1+不均匀性对图像噪声、T1定量偏差和图像模糊影响的约束多维优化过程，引入一种用于7T MP2RAGE T1映射的系统序列优化框架。利用该框架，我们研究使用径向视图排序方法进行T1映射，以替代传统的线性视图排序。进行了基于布洛赫方程的模拟，以比较使用不同方法生成的T1映射。在临床7T MRI扫描仪上采集了健康志愿者和患者的图像，以进行验证并展示T1映射在DBS靶点定位中的应用价值。
数值实验表明，所提出的框架允许在控制定量偏差和图像模糊的同时优化T1映射中的图像信噪比（SNR），从而便于选择用于可视化DBS靶点的最佳序列参数。与线性视图排序相比，使用径向视图排序的优化序列可降低40 - 60%的噪声。体内实例证实了SNR的提高。临床图像显示，从优化的T1映射生成的合成图像能够清晰地可视化DBS靶点。
我们展示了使用径向视图排序技术对7T下用于DBS靶点定位的MP2RAGE T1映射进行优化，并表明优化后的序列允许回顾性生成DBS靶点定位中常用的合成反转时间图像，如快速灰质采集T1反转恢复（FGATIR）和边缘增强梯度回波（EDGE）序列。
