Ohno Naoki, Miyati Tosiaki, Niwa Yoshiaki, Kan Hirohito, Ishida Shota, Kasai Harumasa, Shibamoto Yuta, Gabata Toshifumi
1 Faculty of Health Sciences, Institute of Medical, Pharmaceutical and Health Sciences, Kanazawa University , Ishikawa, Kanazawa , Japan.
2 Department of Radiology, Nagoya City University Hospital , Nagoya, Aichi , Japan.
Br J Radiol. 2018 Jun;91(1086):20170652. doi: 10.1259/bjr.20170652. Epub 2018 Apr 4.
We devised a practical method using double echo with the longest second echo time (DELSET) for simple and accurate signal-to-noise ratio (SNR) measurement of MRIs.
The DELSET method is based on the double-echo sequence in which the first and second echo times (TE) are set, respectively, as the clinically acceptable time for the signal image and as the longest time for the noise image. The second TE needs to be at least 8 times longer than T (for spin-echo) or T* (for gradient-echo) of the objective tissue. For example, second TE > 560 ms for the case of T = 70 ms: the real part of signal intensity theoretically reaches the same order of magnitude as the quantization limit, due to the T relaxation process. SNR was calculated by dividing mean signal intensity in the first echo image by signal standard deviation (SD) in the second echo image in identical regions of interest after necessary noise correction. We determined the SNRs of cylindrical phantom images with different coils [quadrature (QD) and array coils] and sequences (spin-echo and spoiled gradient-echo sequences) and compared them between the DELSET and subtraction or background methods. The ratio of the mean signal intensity and SD in the second echo image with QD coil was determined to confirm whether the signal intensity had reached noise level.
There were no significant differences in the phantom SNRs with both coils and sequences when DELSET was compared with the other two methods (p > 0.05 for all). The ratios of mean signal intensity and SD in the second echo images with both sequences were found to be in general agreement with a theoretical value. It was possible to obtain SNR images of the phantom, brain and abdomen with the DELSET method.
The DELSET method enables simple and accurate SNR quantification. This practical method is applicable to in vivo parallel imaging. Advances in knowledge: Practical SNR quantification based on the DELSET method is feasible for application in MRI systems used clinically.
我们设计了一种使用最长第二回波时间的双回波（DELSET）的实用方法，用于简单且准确地测量磁共振成像（MRI）的信噪比（SNR）。
DELSET方法基于双回波序列，其中第一和第二回波时间（TE）分别设置为信号图像的临床可接受时间和噪声图像的最长时间。第二TE需要比目标组织的T（自旋回波）或T*（梯度回波）至少长8倍。例如，当T = 70 ms时，第二TE > 560 ms：由于T弛豫过程，信号强度的实部理论上达到与量化极限相同的数量级。在进行必要的噪声校正后，通过将第一回波图像中的平均信号强度除以相同感兴趣区域中第二回波图像中的信号标准差（SD）来计算SNR。我们确定了使用不同线圈[正交（QD）线圈和阵列线圈]和序列（自旋回波和扰相梯度回波序列）的圆柱形体模图像的SNR，并在DELSET与减法或背景方法之间进行比较。确定了使用QD线圈的第二回波图像中平均信号强度与SD的比值，以确认信号强度是否已达到噪声水平。
将DELSET与其他两种方法进行比较时，两种线圈和序列的体模SNR均无显著差异（所有p > 0.05）。发现两种序列的第二回波图像中平均信号强度与SD的比值总体上与理论值一致。使用DELSET方法可以获得体模、大脑和腹部的SNR图像。
DELSET方法能够实现简单且准确的SNR量化。这种实用方法适用于体内并行成像。知识进展：基于DELSET方法的实用SNR量化可应用于临床使用的MRI系统。
