Nkongchu Ken, Santyr Giles
Department of Physics, Carleton University, Ottawa, ON, Canada K1S 5B6.
Magn Reson Imaging. 2007 Oct;25(8):1203-14. doi: 10.1016/j.mri.2007.02.020. Epub 2007 May 23.
The Look-Locker (LL) imaging method provides an accurate and efficient approach for mapping the spin-lattice relaxation time, T(1). However, the same recovery of signal during LL image acquisition required to estimate T(1) also results in unwanted modulation of k-space. This is particularly problematic with 3D LL imaging as the number of phase-encoding steps during the recovery interval (e.g., 16) increases in an effort to reduce imaging times. This modulation of k-space has the effect of introducing a point spread function (PSF), which can lead to either image blurring (if the earlier tip angles are assigned to the centre of k-space) or edge enhancement (if the earlier tip angles are assigned to the edges of k-space), thus corrupting T(1) estimation, particularly for small objects. In this study, the PSF and its effect on the acquired images for four different interleaved phase-encode schemes (centric-in, centric-out, sequential and hybrid-sequential) are simulated for a range of T(1), tip angle and 3D LL acquisition parameters expected in practice. It is shown by simulation and confirmed experimentally in phantoms that a hybrid sequential phase-encoding scheme reduces image blurring while maintaining T(1) accuracy ( approximately 2%) and precision (2%) over a range of object sizes down to 2 pixels (2 mm).
Look-Locker（LL）成像方法为绘制自旋晶格弛豫时间T(1)提供了一种准确且高效的途径。然而，在LL图像采集期间用于估计T(1)所需的相同信号恢复也会导致k空间出现不必要的调制。对于三维LL成像而言，这一问题尤为突出，因为在恢复间隔期间（例如16个）的相位编码步数会增加，以努力缩短成像时间。k空间的这种调制会引入点扩散函数（PSF），这可能导致图像模糊（如果较早的翻转角分配给k空间中心）或边缘增强（如果较早的翻转角分配给k空间边缘），从而破坏T(1)估计，特别是对于小物体。在本研究中，针对一系列实际中预期的T(1)、翻转角和三维LL采集参数，模拟了四种不同的交错相位编码方案（向心内、向心外、顺序和混合顺序）的PSF及其对采集图像的影响。通过模拟表明，并在体模中通过实验证实，混合顺序相位编码方案可减少图像模糊，同时在低至2像素（2毫米）的一系列物体尺寸范围内保持T(1)的准确性（约2%）和精度（2%）。
