Tseitlin Mark, Dhami Amarjot, Eaton Sandra S, Eaton Gareth R
Department of Chemistry and Biochemistry, University of Denver, Denver, CO 80208, USA.
J Magn Reson. 2007 Jan;184(1):157-68. doi: 10.1016/j.jmr.2006.09.027. Epub 2006 Oct 27.
Reconstruction of two-dimensional images by filtered back-projection (FBP) and by the maximum entropy method (MEM) was compared for spectral-spatial EPR images with differing signal-to-noise ratios. Experimental projections were recorded using direct-detected rapid scans in the presence of a series of magnetic field gradients. The slow-scan absorption lineshapes were calculated by Fourier deconvolution. A Hamming filter was used in conjunction with FBP, but not for MEM. Imperfections in real experimental data, as well as random noise, contribute to discrepancies between the reconstructed image and experimental projections, which may make it impossible to achieve the customary MEM criterion for convergence. The Cambridge MEM algorithm, with allowance for imperfections in experimental data, produced images with more linear intensity scales and more accurate linewidths for weak signals than was obtained with another MEM method. The more effective elimination of noise in baseline regions by MEM made it possible to detect weak trityl (13)C trityl hyperfine lines that could not be distinguished from noise in images reconstructed by FBP. Another advantage of MEM is that projections do not need to be equally spaced. FBP has the advantages that computational time is less, the amplitude scale is linear, and there is less noise superimposed on peaks in images. It is useful to reconstruct images by both methods and compare results. Our observations indicate that FBP works well when the number of projections is large enough that the star effect is negligible. When there is a smaller number of projections, projections are unequally spaced, and/or signal-to-noise is lower MEM is advantageous.
针对具有不同信噪比的光谱空间电子顺磁共振（EPR）图像，比较了通过滤波反投影（FBP）和最大熵方法（MEM）进行二维图像重建的情况。在一系列磁场梯度存在的情况下，使用直接检测快速扫描记录实验投影。通过傅里叶反卷积计算慢扫描吸收线形状。汉明滤波器与FBP结合使用，但不用于MEM。实际实验数据中的缺陷以及随机噪声导致重建图像与实验投影之间存在差异，这可能使得无法达到MEM通常的收敛标准。考虑到实验数据中的缺陷，剑桥MEM算法生成的图像对于弱信号具有更线性的强度标度和更准确的线宽，这比另一种MEM方法得到的结果更好。MEM在基线区域更有效地消除噪声，使得能够检测到在FBP重建图像中无法与噪声区分开的弱三苯甲基（13）C三苯甲基超精细线。MEM的另一个优点是投影不需要等间距。FBP的优点是计算时间更短、幅度标度是线性的，并且图像峰值上叠加的噪声更少。通过两种方法重建图像并比较结果是很有用的。我们的观察表明，当投影数量足够大以至于星芒效应可忽略不计时，FBP效果良好。当投影数量较少、投影不等间距和/或信噪比更低时，MEM更具优势。
