Rizzo Rudy, Stamatelatou Angeliki, Heerschap Arend, Scheenen Tom, Kreis Roland
MR Methodology, Department for Diagnostic and Interventional Neuroradiology, University of Bern, Bern, Switzerland.
Translational Imaging Center (TIC), Swiss Institute for Translational and Entrepreneurial Medicine (sitem-insel), Bern, Switzerland.
NMR Biomed. 2025 Feb;38(2):e5318. doi: 10.1002/nbm.5318.
The purpose of this study was to produce metabolite-specific T and concentration maps in a clinically compatible time frame. A multi-TE 2D MR spectroscopic imaging (MRSI) experiment (multi-echo single-shot MRSI [MESS-MRSI]) deployed truncated and partially sampled multi-echo trains from single scans and was combined with simultaneous multiparametric model fitting. It was tested in vivo for the brain in five healthy subjects. Cramér-Rao lower bounds (CRLB) were used as the measure of performance. The novel method was compared with (1) traditional multi-echo multi-shot (MEMS) MRSI and, as proof of concept, with (2) a truncated version of MEMS, which mimics the MESS acquisition (MESS-mocked) on the original fully sampled MEMS dataset. MESS-MRSI simultaneously yields concentration and T maps with a nominal voxel size of ~2 cm with a 16 × 16 FOV matrix in 7 min scan time. The estimated values not only align well with the equivalent mocked experiment but are also in good agreement with the traditional threefold longer MEMS acquisition. The MESS-MRSI scheme extends former findings for single-voxel MESS, with improvements in CRLB ranging from 17% to 45% for concentrations and 10% to 23% for Ts when compared to traditional MEMS. This finding suggests that concentrations and T times can be reliably estimated in a multi-echo spectroscopic imaging exam by trading off spectral resolution (for some of the acquired TEs) with a significant reduction in scan time, as long as (1) an appropriate bidimensional frequency-TE model is deployed and (2) one TE is sampled in full. Thus, high spectral resolution information can be injected to the partially sampled TEs during fitting by prior knowledge from the one fully sampled TE. Tissue-type and regional distributions of 16 metabolite concentrations align well with the literature, and T distributions for five major metabolites are described by region and tissue. The novel MRSI acquisition strategy, based on partially sampled single-shot multi-echo trains twinned to multiparametric fitting, is optimally suited to provide simultaneous 2D concentration and T maps in clinic-compatible scan times. MESS principles allow embedding advanced MRSI techniques to further improve speed, coverage, or resolution. Preliminary findings from a cohort of five subjects reveal correlations between T relaxation times and the relative fraction of gray/white matter, suggesting tissue-type-dependent microstructural changes.
本研究的目的是在临床可接受的时间范围内生成代谢物特异性的T和浓度图谱。一种多回波二维磁共振波谱成像（MRSI）实验（多回波单次激发MRSI [MESS-MRSI]）采用了来自单次扫描的截断和部分采样的多回波序列，并与同步多参数模型拟合相结合。该方法在5名健康受试者的大脑中进行了体内测试。使用克拉美罗下界（CRLB）作为性能指标。将该新方法与（1）传统的多回波多次激发（MEMS）MRSI进行比较，并作为概念验证，与（2）MEMS的截断版本进行比较，该版本在原始的全采样MEMS数据集中模拟MESS采集（MESS模拟）。MESS-MRSI在7分钟的扫描时间内，以16×16的视野矩阵，同时生成标称体素大小约为2 cm的浓度和T图谱。估计值不仅与等效的模拟实验吻合良好，而且与传统的三倍长的MEMS采集结果也高度一致。与传统的MEMS相比，MESS-MRSI方案扩展了单体素MESS的先前研究结果，浓度的CRLB改善范围为17%至45%，T的改善范围为10%至23%。这一发现表明，在多回波波谱成像检查中，只要（1）采用合适的二维频率-回波模型，（2）对一个回波进行全采样，通过牺牲部分采集回波的光谱分辨率来显著缩短扫描时间，就可以可靠地估计浓度和T时间。因此，在拟合过程中，可以通过来自一个全采样回波的先验知识，将高光谱分辨率信息注入到部分采样的回波中。16种代谢物浓度的组织类型和区域分布与文献吻合良好，5种主要代谢物的T分布按区域和组织进行了描述。基于与多参数拟合配对的部分采样单次激发多回波序列的新型MRSI采集策略，最适合在临床可接受的扫描时间内同时提供二维浓度和T图谱。MESS原理允许嵌入先进的MRSI技术，以进一步提高速度、覆盖范围或分辨率。来自5名受试者队列的初步结果揭示了T弛豫时间与灰质/白质相对比例之间的相关性，表明存在组织类型依赖性的微观结构变化。
