Laboratory for Structural NMR Imaging, Department of Radiology, University of Pennsylvania School of Medicine, Philadelphia, PA, USA.
NMR Biomed. 2011 Aug;24(7):855-64. doi: 10.1002/nbm.1631. Epub 2011 Jan 28.
Bone contains a significant fraction of water that is not detectable with ordinary Cartesian imaging sequences. The advent of ultra-short echo-time (UTE) methods has allowed the recovery of this submillisecond T(2)* water. In this work, we have developed a new three-dimensional hybrid-radial ultra-short echo-time (3D HRUTE) imaging technique based on slab selection by means of half-sinc pulses, variable-TE slice encoding and algorithms for quantification. The protocol consists of collecting two datasets differing in TR, from which T(1) is extracted, which is needed for quantification. Unlike T(2), which has been found to vary within a narrow range and does not require individual correction, T(1) is critically subject dependent (range, 100-350 ms). No soft-tissue suppression was used to preserve the signal-to-noise ratio of the short-T(2) bone water protons or to minimize the loss of relatively mobile water in large pores. Critical for quantification is correction for spatial variations in reception field and selection of the endosteal boundary for inclusion of pixels in the bone water calculation, because of the ruffled boundary stemming from trabecularization of the endosteal surface. The reproducibility, evaluated in 10 subjects covering the age range 30-80 years, yielded an average coefficient of variation of 4.2% and an intraclass correlation coefficient of 0.95, suggesting that a treatment effect on the order of 5% could be detected in as few as 10 subjects. Lastly, experiments in specimens by means of graded deuterium exchange showed that approximately 90% of the detected signal arises from water protons, whose relaxation rates (1/T(1) and 1/T(2)) scale linearly with the isotopic volume fraction of light water after stepwise exchange with heavy water. The data thus show conclusively that the method quantifies water even though, in vivo, no distinction can be made between various fractions, such as collagen-bound vs pore-resident water.
骨组织中含有相当一部分普通笛卡尔成像序列无法检测到的水。超短回波时间 (UTE) 方法的出现使得亚毫秒级 T(2)*水得以恢复。在这项工作中，我们开发了一种新的三维混合径向超短回波时间 (3D HRUTE) 成像技术，该技术基于半正弦脉冲的切片选择、可变 TE 切片编码以及定量算法。该方案包括采集两个在 TR 上不同的数据集，从中提取 T(1)，这是定量所必需的。与 T(2)*不同，T(2)的变化范围很窄，不需要单独校正，而 T(1)则严重依赖于个体（范围为 100-350ms）。没有使用软组织抑制来保持短 T(2)骨水质子的信噪比，也没有最小化大孔中相对移动水的损失。定量的关键是校正接收场的空间变化，并选择骨水计算中包含的骨内边界的像素，因为源于骨内膜表面骨小梁化的波纹边界。在 10 名年龄在 30-80 岁之间的受试者中进行的重复性评估得出，平均变异系数为 4.2%，组内相关系数为 0.95，这表明在 10 名受试者中就可以检测到大约 5%的治疗效果。最后，通过分级氘交换实验在标本中进行的实验表明，大约 90%的检测信号来自水质子，其弛豫率（1/T(1)和 1/T(2)）在与重水逐步交换后与轻水的同位素体积分数呈线性关系。这些数据明确表明，该方法即使在体内无法区分各种水分数（如胶原结合水与孔驻留水）的情况下也能定量水。
