Department of Radiology, University of California San Diego, San Diego, California.
Institute of Electrical Engineering, Chinese Academy of Science, Beijing, China.
Magn Reson Med. 2019 Mar;81(3):1907-1914. doi: 10.1002/mrm.27515. Epub 2018 Oct 16.
In this study, we explore the feasibility of a new imaging scheme for quantitative susceptibility mapping (QSM): continuous single-point imaging (CSPI), which uses a pure phase encoding strategy to achieve true phase imaging and improve QSM accuracy.
The proposed CSPI is a modification of conventional SPI to allow acquisition of multiple echoes in a single scan. Immediately following a phase encoding gradient, the free induction decay is continuously sampled with extremely high temporal resolution to obtain k-space data at a fixed spatial frequency (i.e., at a fixed k-space coordinate). By having near-0 readout duration, CSPI results in a true snapshot of the transverse magnetization at each TE. Additionally, parallel imaging with autocalibration is utilized to reduce scan time, and an optional temporal averaging strategy is presented to improve signal-to-noise ratio for objects with low proton density or short T2* decay. The reconstructed CSPI images were input to a QSM framework based on morphology enabled dipole inversion.
In an experiment performed using iron phantoms, susceptibility estimated using CSPI showed high linearity (R = 0.9948) with iron concentration. Additionally, reconstructed CSPI phase images showed much reduced ringing artifact compared with phase images obtained using a frequency encoding strategy. In an ex vivo experiment performed using human tibia samples, estimated susceptibilities ranged from -1.6 to -2.1 ppm, in agreement with values reported in the literature (ranging from -1.2 to -2.2 ppm).
We have demonstrated the feasibility of using CSPI to obtain true phase images for QSM.
本研究探索了一种新的定量磁敏感图（QSM）成像方案的可行性：连续单点成像（CSPI），它使用纯相位编码策略实现真实的相位成像并提高 QSM 的准确性。
所提出的 CSPI 是对传统 SPI 的一种改进，允许在单次扫描中采集多个回波。在相位编码梯度之后，立即以极高的时间分辨率连续采样自由感应衰减，以在固定空间频率（即固定 k 空间坐标）处获得 k 空间数据。由于读取时间接近 0，CSPI 可在每个 TE 获得横向磁化的真实快照。此外，使用自动校准的并行成像来减少扫描时间，并提出了一种可选的时间平均策略，以提高质子密度低或 T2*衰减短的物体的信噪比。将重建的 CSPI 图像输入基于形态学启用偶极子反演的 QSM 框架。
在使用铁体模进行的实验中，使用 CSPI 估计的磁化率表现出与铁浓度的高度线性（R = 0.9948）。此外，与使用频率编码策略获得的相位图像相比，重建的 CSPI 相位图像显示出明显减少的振铃伪影。在使用人胫骨样本进行的离体实验中，估计的磁化率范围为-1.6 至-2.1 ppm，与文献中报道的值（-1.2 至-2.2 ppm）一致。
我们已经证明了使用 CSPI 获得 QSM 真实相位图像的可行性。
