Cunningham Charles H, Dominguez Viqueira William, Hurd Ralph E, Chen Albert P
Department of Medical Biophysics and Sunnybrook Health Sciences Centre, Toronto, Canada; Imaging Research, Sunnybrook Health Sciences Centre, Toronto, Canada.
NMR Biomed. 2014 Feb;27(2):212-8. doi: 10.1002/nbm.3055. Epub 2013 Dec 19.
Blip-reversed echo-planar imaging (EPI) is investigated as a method for measuring and correcting the spatial shifts that occur due to bulk frequency offsets in (13)C metabolic imaging in vivo. By reversing the k-space trajectory for every other time point, the direction of the spatial shift for a given frequency is reversed. Here, mutual information is used to find the 'best' alignment between images and thereby measure the frequency offset. Time-resolved 3D images of pyruvate/lactate/urea were acquired with 5 s temporal resolution over a 1 min duration in rats (N = 6). For each rat, a second injection was performed with the demodulation frequency purposely mis-set by +35 Hz, to test the correction for erroneous shifts in the images. Overall, the shift induced by the 35 Hz frequency offset was 5.9 ± 0.6 mm (mean ± standard deviation). This agrees well with the expected 5.7 mm shift based on the 2.02 ms delay between k-space lines (giving 30.9 Hz per pixel). The 0.6 mm standard deviation in the correction corresponds to a frequency-detection accuracy of 4 Hz. A method was presented for ensuring the spatial registration between (13)C metabolic images and conventional anatomical images when long echo-planar readouts are used. The frequency correction method was shown to have an accuracy of 4 Hz. Summing the spatially corrected frames gave a signal-to-noise ratio (SNR) improvement factor of 2 or greater, compared with the highest single frame.
研究了回波平面成像（EPI）反转脉冲作为一种测量和校正体内¹³C代谢成像中由于体频率偏移而出现的空间位移的方法。通过每隔一个时间点反转k空间轨迹，给定频率的空间位移方向会反转。在此，互信息用于找到图像之间的“最佳”对齐，从而测量频率偏移。在大鼠（N = 6）中，以5 s的时间分辨率在1分钟内采集丙酮酸/乳酸/尿素的时间分辨三维图像。对于每只大鼠，进行第二次注射，故意将解调频率设置错误+35 Hz，以测试对图像中错误位移的校正。总体而言，35 Hz频率偏移引起的位移为5.9±0.6 mm（平均值±标准差）。这与基于k空间线之间2.02 ms延迟（每个像素30.9 Hz）预期的5.7 mm位移非常吻合。校正中的0.6 mm标准差对应于4 Hz的频率检测精度。提出了一种在使用长回波平面读出时确保¹³C代谢图像与传统解剖图像之间空间配准的方法。频率校正方法的精度为4 Hz。与最高单帧相比，对空间校正后的帧求和得到的信噪比（SNR）提高因子为2或更大。
