Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital, Harvard Medical School, 149 13th St, Suite 2301, Boston, MA 02129, USA.
Radiology. 2012 Feb;262(2):647-61. doi: 10.1148/radiol.11110277. Epub 2011 Dec 20.
To improve clinical three-dimensional (3D) MR spectroscopic imaging with more accurate localization and faster acquisition schemes.
Institutional review board approval and patient informed consent were obtained. Data were acquired with a 3-T MR imager and a 32-channel head coil in phantoms, five healthy volunteers, and five patients with glioblastoma. Excitation was performed with localized adiabatic spin-echo refocusing (LASER) by using adiabatic gradient-offset independent adiabaticity wideband uniform rate and smooth truncation (GOIA-W[16,4]) pulses with 3.5-msec duration, 20-kHz bandwidth, 0.81-kHz amplitude, and 45-msec echo time. Interleaved constant-density spirals simultaneously encoded one frequency and two spatial dimensions. Conventional phase encoding (PE) (1-cm3 voxels) was performed after LASER excitation and was the reference standard. Spectra acquired with spiral encoding at similar and higher spatial resolution and with shorter imaging time were compared with those acquired with PE. Metabolite levels were fitted with software, and Bland-Altman analysis was performed.
Clinical 3D MR spectroscopic images were acquired four times faster with spiral protocols than with the elliptical PE protocol at low spatial resolution (1 cm3). Higher-spatial-resolution images (0.39 cm3) were acquired twice as fast with spiral protocols compared with the low-spatial-resolution elliptical PE protocol. A minimum signal-to-noise ratio (SNR) of 5 was obtained with spiral protocols under these conditions and was considered clinically adequate to reliably distinguish metabolites from noise. The apparent SNR loss was not linear with decreasing voxel sizes because of longer local T2* times. Improvement of spectral line width from 4.8 Hz to 3.5 Hz was observed at high spatial resolution. The Bland-Altman agreement between spiral and PE data is characterized by narrow 95% confidence intervals for their differences (0.12, 0.18 of their means). GOIA-W(16,4) pulses minimize chemical-shift displacement error to 2.1%, reduce nonuniformity of excitation to 5%, and eliminate the need for outer volume suppression.
The proposed adiabatic spiral 3D MR spectroscopic imaging sequence can be performed in a standard clinical MR environment. Improvements in image quality and imaging time could enable more routine acquisition of spectroscopic data than is possible with current pulse sequences.
通过更精确的定位和更快的采集方案来提高临床三维（3D）MR 波谱成像质量。
本研究获得了机构审查委员会的批准和患者的知情同意。数据采集在体模、五名健康志愿者和五名胶质母细胞瘤患者中使用 3T MR 成像仪和 32 通道头部线圈完成。激发采用局部绝热自旋回波重聚焦（LASER）技术，使用带宽为 20kHz、幅度为 0.81kHz、持续时间为 3.5ms、回波时间为 45ms 的绝热梯度偏移独立绝热宽带均匀率和平滑截断（GOIA-W[16,4]）脉冲进行。交错的等密度螺旋同时编码一个频率和两个空间维度。在 LASER 激发后进行常规相位编码（PE）（1cm3 体素），作为参考标准。比较了具有相似和更高空间分辨率且采集时间更短的螺旋编码采集的光谱与 PE 采集的光谱。使用软件拟合代谢物水平，并进行 Bland-Altman 分析。
与低空间分辨率（1cm3）的椭圆 PE 方案相比，螺旋方案可将临床 3D MR 波谱图像的采集速度提高四倍。与低空间分辨率的椭圆 PE 方案相比，高空间分辨率（0.39cm3）图像的采集速度提高了两倍。在这些条件下，螺旋方案可获得至少 5 的最小信噪比（SNR），被认为足以可靠地区分代谢物与噪声。由于局部 T2*时间延长，表观 SNR 损失与体素尺寸减小不成线性关系。在高空间分辨率下观察到谱线宽度从 4.8Hz 改善到 3.5Hz。螺旋和 PE 数据之间的 Bland-Altman 一致性特征为其差异的 95%置信区间较窄（其平均值的 0.12、0.18）。GOIA-W(16,4)脉冲将化学位移位移误差最小化至 2.1%，将激发不均匀性降低至 5%，并消除对外围体积抑制的需求。
所提出的绝热螺旋 3D MR 波谱成像序列可在标准临床 MR 环境中进行。与当前脉冲序列相比，图像质量和采集时间的改进可以实现更多常规的光谱数据采集。
