den Harder J Chiel, van Yperen Gert H, Blume Ulrike A, Bos Clemens
Medical Physics Department, Reinier de Graaf Groep, Delft, the Netherlands.
Technology Development Department, Philips Healthcare, Best, the Netherlands.
Magn Reson Med. 2015 Jan;73(1):233-43. doi: 10.1002/mrm.25126. Epub 2014 Jan 31.
Metal artifact reduction in MRI within clinically feasible scan-times without through-plane aliasing.
Existing metal artifact reduction techniques include view angle tilting (VAT), which resolves in-plane distortions, and multispectral imaging (MSI) techniques, such as slice encoding for metal artifact correction (SEMAC) and multi-acquisition with variable resonances image combination (MAVRIC), that further reduce image distortions, but significantly increase scan-time. Scan-time depends on anatomy size and anticipated total spectral content of the signal. Signals outside the anticipated spatial region may cause through-plane back-folding. Off-resonance suppression (ORS), using different gradient amplitudes for excitation and refocusing, is proposed to provide well-defined spatial-spectral selectivity in MSI to allow scan-time reduction and flexibility of scan-orientation. Comparisons of MSI techniques with and without ORS were made in phantom and volunteer experiments.
Off-resonance suppressed SEMAC (ORS-SEMAC) and outer-region suppressed MAVRIC (ORS-MAVRIC) required limited through-plane phase encoding steps compared with original MSI. Whereas SEMAC (scan time: 5'46") and MAVRIC (4'12") suffered from through-plane aliasing, ORS-SEMAC and ORS-MAVRIC allowed alias-free imaging in the same scan-times.
ORS can be used in MSI to limit the selected spatial-spectral region and contribute to metal artifact reduction in clinically feasible scan-times while avoiding slice aliasing.
在临床可行的扫描时间内减少MRI中的金属伪影，同时避免层面间的混叠。
现有的金属伪影减少技术包括视角倾斜（VAT），它可解决平面内的畸变，以及多光谱成像（MSI）技术，如用于金属伪影校正的切片编码（SEMAC）和可变共振图像组合的多采集（MAVRIC），这些技术可进一步减少图像畸变，但会显著增加扫描时间。扫描时间取决于解剖结构的大小和信号预期的总光谱内容。预期空间区域之外的信号可能会导致层面间的折返。提出了使用不同梯度幅度进行激发和重聚焦的失谐抑制（ORS），以在MSI中提供明确的空间光谱选择性，从而减少扫描时间并实现扫描方向的灵活性。在体模和志愿者实验中对有无ORS的MSI技术进行了比较。
与原始MSI相比，失谐抑制的SEMAC（ORS-SEMAC）和外部区域抑制的MAVRIC（ORS-MAVRIC）所需的层面间相位编码步骤有限。虽然SEMAC（扫描时间：5分46秒）和MAVRIC（4分12秒）存在层面间混叠，但ORS-SEMAC和ORS-MAVRIC在相同扫描时间内实现了无混叠成像。
ORS可用于MSI，以限制选定的空间光谱区域，并在临床可行的扫描时间内减少金属伪影，同时避免层面混叠。
