Siemens Healthcare Ltd, Rosh Ha'ayin, Israel.
Imaging Centre of Excellence, University of Glasgow, Glasgow, United Kingdom.
Magn Reson Med. 2022 Dec;88(6):2339-2357. doi: 10.1002/mrm.29373. Epub 2022 Aug 17.
Fast proton ( H) MRSI is an important diagnostic tool for clinical investigations, providing metabolic and spatial information. MRSI at 7 T benefits from increased SNR and improved separation of peaks but requires larger spectral widths. RS-COKE (Readout-Segmented Consistent K-t space Epsi) is an echo planar spectroscopic imaging (Epsi) variant capable to support the spectral width required for human brain metabolites spectra at 7 T. However, mismatches between readout segments lead to artifacts, particularly when subcutaneous lipid signals are not suppressed. In this study, these mismatches and their effects are analyzed and reduced.
The following corrections to the data were performed: i) frequency-dependent phase corrections; ii) k-space trajectory corrections, derived from short reference scans; and iii) smoothing of data at segment transitions to mitigate the effect of residual mismatches. The improvement was evaluated by performing single-slice RS-COKE on a head-shaped phantom with a "lipid" layer and healthy subjects, using varying resolutions and durations ranging from 4.1 × 4.7 × 15 mm in 5:46 min to 3.1 × 3.3 × 15 mm in 13:07 min.
Artifacts arising from the readout-segmented acquisition were substantially reduced, thus providing high-quality spectroscopic imaging in phantom and human scans. LCModel fitting of the human data resulted in a relative Cramer-Rao lower bounds within 6% for NAA, Cr, and Cho images in the majority of the voxels.
Using the new reference scans and reconstruction steps, RS-COKE was able to deliver fast H MRSI at 7 T, overcoming the spectral width limitation of standard EPSI at this field strength.
快速质子（H）磁共振波谱成像（MRSI）是临床研究的重要诊断工具，提供代谢和空间信息。7T 下的 MRSI 得益于 SNR 的提高和峰的分离改善，但需要更大的谱宽。RS-COKE（读出分段一致 k 空间 Epsi）是一种能够支持 7T 下人脑代谢物谱所需谱宽的回波平面波谱成像（Epsi）变体。然而，读出段之间的不匹配会导致伪影，特别是当不能抑制皮下脂质信号时。在这项研究中，分析并减少了这些不匹配及其影响。
对数据进行了以下校正：i）频率相关相位校正；ii）来自短参考扫描的 k 空间轨迹校正；iii）在段过渡处平滑数据，以减轻残余不匹配的影响。通过在带有“脂质”层的头形体模和健康受试者上进行单层面 RS-COKE，使用从 4.1×4.7×15mm 到 3.1×3.3×15mm 的不同分辨率和持续时间（5：46min 到 13：07min）来评估改进。
从读出分段采集产生的伪影得到了很大程度的减少，从而在体模和人体扫描中提供了高质量的波谱成像。对人体数据进行 LCModel 拟合，在大多数体素中，NAA、Cr 和 Cho 图像的相对 Cramer-Rao 下限在 6%以内。
使用新的参考扫描和重建步骤，RS-COKE 能够在 7T 下提供快速 H MRSI，克服了该场强下标准 EPSI 的谱宽限制。
