Lu D, Joseph P M
Department of Radiology, University of Pennsylvania, Philadelphia 19104-6086, USA.
Magn Reson Imaging. 1995;13(2):241-9. doi: 10.1016/0730-725x(94)00110-o.
A technique is presented to increase the signal-to-noise ratio (SNR) in two-dimensional (2D), phase-encoded imaging at low SNR. The essence of this technique is to combine multiple echoes in the time domain. As analyzed in the paper, phase discrepancies exist among different echoes and may deteriorate the combined echo. In particular, extraneous phase shifts can be created if unshielded gradient coils are used. To overcome these phase discrepancies, a matched filter was derived from the k = 0 component of image. This matched filter has the same phase discrepancies among its echoes as the imaging signal and its magnitude decays with an average T2. In the echo summation with the matched filter, the phase of the matched filter was subtracted from the imaging signal and the magnitude of the matched filter was used as the weighting function. We have shown that this matched filter echo summation technique has better SNR than the case of 2D, phase-encoded imaging in both simulation and experiment. The SNR improvement is up to 60% in a phantom experiment. This technique is mostly useful in low SNR imaging that requires long imaging time, such as spectroscopic imaging and 19F imaging.
本文提出了一种在低信噪比情况下提高二维（2D）相位编码成像中信噪比（SNR）的技术。该技术的核心是在时域中组合多个回波。正如本文所分析的，不同回波之间存在相位差异，这可能会使组合后的回波质量下降。特别是，如果使用未屏蔽的梯度线圈，可能会产生额外的相移。为了克服这些相位差异，从图像的k = 0分量推导出了一个匹配滤波器。该匹配滤波器在其回波之间具有与成像信号相同的相位差异，并且其幅度随平均T2衰减。在使用匹配滤波器进行回波求和时，从成像信号中减去匹配滤波器的相位，并将匹配滤波器的幅度用作加权函数。我们已经表明，在模拟和实验中，这种匹配滤波器回波求和技术的信噪比均优于二维相位编码成像。在体模实验中，信噪比提高了60%。该技术在需要长时间成像的低信噪比成像中最为有用，例如光谱成像和19F成像。
