Instituto de Biofísica e Engenharia Biomédica, Faculdade de Ciências, Universidade de Lisboa, Lisboa, Portugal.
Institute for Systems and Robotics and Department of Bioengineering, Instituto Superior Técnico, Universidade de Lisboa, Lisboa, Portugal.
Magn Reson Med. 2018 Jul;80(1):279-285. doi: 10.1002/mrm.26998. Epub 2017 Nov 8.
Fetal functional MRI studies using conventional 2-dimensional single-shot echo-planar imaging sequences may require discarding a large data fraction as a result of fetal and maternal motion. Increasing the temporal resolution using echo volumar imaging (EVI) could provide an effective alternative strategy. Echo volumar imaging was combined with inner volume (IV) imaging (IVEVI) to locally excite the fetal brain and acquire full 3-dimensional images, fast enough to freeze most fetal head motion.
IVEVI was implemented by modifying a standard multi-echo echo-planar imaging sequence. A spin echo with orthogonal excitation and refocusing ensured localized excitation. To introduce T2* weighting and to save time, the k-space center was shifted relative to the spin echo. Both single and multi-shot variants were tested. Acoustic noise was controlled by adjusting the amplitude and switching frequency of the readout gradient. Image-based shimming was used to minimize B inhomogeneities within the fetal brain.
The sequence was first validated in an adult. Eight fetuses were scanned using single-shot IVEVI at a 3.5 × 3.5 × 5.0 mm resolution with a readout duration of 383 ms. Multishot IVEVI showed reduced geometric distortions along the second phase-encode direction.
Fetal EVI remains challenging. Although effective echo times comparable to the T2* values of fetal cortical gray matter at 3 T could be achieved, controlling acoustic noise required longer readouts, leading to substantial distortions in single-shot images. Although multishot variants enabled us to reduce susceptibility-induced geometric distortions, sensitivity to motion was increased. Future studies should therefore focus on improvements to multishot variants. Magn Reson Med 80:279-285, 2018. © 2017 International Society for Magnetic Resonance in Medicine.
使用传统的二维单次激发 echo-planar 成像序列进行胎儿功能磁共振成像研究，由于胎儿和母体运动的原因，可能需要丢弃大量数据。使用回波容积成像 (EVI) 增加时间分辨率可以提供一种有效的替代策略。EVI 与内部容积成像 (IVEVI) 相结合，局部激发胎儿大脑并快速获取全 3 维图像，足以冻结大多数胎儿头部运动。
IVEVI 通过修改标准多回波 echo-planar 成像序列来实现。具有正交激励和重聚焦的自旋回波可确保局部激发。为了引入 T2*加权并节省时间，相对于自旋回波移动 k 空间中心。测试了单激发和多激发变体。通过调整读出梯度的幅度和切换频率来控制声噪声。基于图像的匀场用于最小化胎儿大脑内的 B 不均匀性。
该序列首先在成人中进行了验证。使用单激发 IVEVI 对 8 例胎儿进行扫描，分辨率为 3.5×3.5×5.0mm，读出时间为 383ms。多激发 IVEVI 显示沿第二相位编码方向的几何变形减小。
胎儿 EVI 仍然具有挑战性。尽管可以实现与 3T 下胎儿皮质灰质的 T2*值相当的有效回波时间，但控制声噪声需要更长的读出时间，导致单激发图像中的严重变形。尽管多激发变体使我们能够减少感测引起的几何变形，但对运动的敏感性增加。因此，未来的研究应集中在改进多激发变体上。磁共振医学 80:279-285, 2018. © 2017 国际磁共振医学学会。
