Department of Medical Physics and Biomedical Engineering, University College London, London, UK.
Neuroradiology Unit, IRCCS Ospedale Policlinico San Martino, Genoa, Italy.
Magn Reson Med. 2025 Jan;93(1):183-198. doi: 10.1002/mrm.30267. Epub 2024 Aug 20.
Data for QSM are typically acquired using multi-echo 3D gradient echo (GRE), but EPI can be used to accelerate QSM and provide shorter acquisition times. So far, EPI-QSM has been limited to single-echo acquisitions, which, for 3D GRE, are known to be less accurate than multi-echo sequences. Therefore, we compared single-echo and multi-echo EPI-QSM reconstructions across a range of parallel imaging and multiband acceleration factors.
Using 2D single-shot EPI in the brain, we compared QSM from single-echo and multi-echo acquisitions across combined parallel-imaging and multiband acceleration factors ranging from 2 to 16, with volume pulse TRs from 21.7 to 3.2 s, respectively. For single-echo versus multi-echo reconstructions, we investigated the effect of acceleration factors on regional susceptibility values, temporal noise, and image quality. We introduce a novel masking method based on thresholding the magnitude of the local field gradients to improve brain masking in challenging regions.
At 1.6-mm isotropic resolution, high-quality QSM was achieved using multi-echo 2D EPI with a combined acceleration factor of 16 and a TR of 3.2 s, which enables functional applications. With these high acceleration factors, single-echo reconstructions are inaccurate and artefacted, rendering them unusable. Multi-echo acquisitions greatly improve QSM quality, particularly at higher acceleration factors, provide more consistent regional susceptibility values across acceleration factors, and decrease temporal noise compared with single-echo QSM reconstructions.
Multi-echo acquisition is more robust for EPI-QSM across parallel imaging and multiband acceleration factors than single-echo acquisition. Multi-echo EPI can be used for highly accelerated acquisition while preserving QSM accuracy and quality relative to gold-standard 3D-GRE QSM.
QSM 数据通常使用多回波 3D 梯度回波（GRE）采集，但 EPI 可用于加速 QSM 并缩短采集时间。到目前为止，EPI-QSM 仅限于单回波采集，而对于 3D GRE，单回波采集的准确性不如多回波序列。因此，我们比较了在不同并行成像和多带加速因子下的单回波和多回波 EPI-QSM 重建。
在大脑中使用 2D 单次激发 EPI，我们比较了单回波和多回波采集的 QSM，这些采集的并行成像和多带加速因子范围从 2 到 16，相应的体积脉冲 TR 从 21.7 到 3.2 s。对于单回波与多回波重建，我们研究了加速因子对局部磁化率值、时间噪声和图像质量的影响。我们引入了一种新的基于阈值局部场梯度幅度的掩蔽方法，以改善在具有挑战性的区域中的大脑掩蔽。
在 1.6-mm 各向同性分辨率下，使用具有 3.2 s TR 和 16 倍组合加速因子的多回波 2D EPI 可以实现高质量的 QSM，这使其能够用于功能应用。在这些高加速因子下，单回波重建不准确且存在伪影，因此无法使用。多回波采集极大地改善了 QSM 质量，特别是在更高的加速因子下，与单回波 QSM 重建相比，多回波采集提供了更一致的局部磁化率值，并降低了时间噪声。
与单回波采集相比，多回波采集在并行成像和多带加速因子下对 EPI-QSM 更稳健。多回波 EPI 可用于高度加速采集，同时保持相对于金标准 3D-GRE QSM 的准确性和质量。
