Glaeser Robert M
Life Sciences Division, Lawrence Berkeley National Laboratory, University of California, Berkeley, CA 94720, USA.
J Struct Biol. 2008 Sep;163(3):271-6. doi: 10.1016/j.jsb.2008.06.001. Epub 2008 Jun 8.
The fact that radiation damage would limit the usefulness of electron microscopy with biological specimens was a concern in the earliest days of the field. Good estimates of what that limitation must be can be made by using Rose's empirical relationship between the inherent image contrast, the exposure used to record an image, and the smallest feature size that is detectable. Such estimates show that it is necessary to average many images in order to obtain statistically well-defined data at high resolution. Structures are now routinely obtained by averaging large numbers of shot-noise limited images, and some of these extend to atomic resolution. The signal level in current images is nevertheless far below what physics would allow it to be. A possible explanation is that beam-induced movement limits the quality of images recorded by electron microscopy. For specimens embedded in vitreous ice, beam-induced movement can even be severe enough to limit the resolution achieved during tomographic reconstruction. The fact that very high-quality images can nevertheless be obtained, although only unpredictably, suggests that it may be possible to devise new techniques of specimen preparation and/or data collection that at least partially overcome beam-induced movement. If so, the need for image averaging would be correspondingly reduced.
在电子显微镜学领域发展的早期，辐射损伤会限制其对生物标本的应用这一事实就已受到关注。通过运用罗斯提出的关于固有图像对比度、用于记录图像的曝光量以及可检测到的最小特征尺寸之间的经验关系，可以对这种限制的程度做出合理估计。这些估计表明，为了在高分辨率下获得统计上明确的数据，有必要对大量图像进行平均处理。目前，通过对大量受散粒噪声限制的图像进行平均处理，常规地获得了一些结构信息，其中一些甚至达到了原子分辨率。然而，当前图像中的信号水平仍远低于物理学所允许的水平。一种可能的解释是，电子束诱导的移动限制了电子显微镜记录图像的质量。对于嵌入玻璃态冰中的标本，电子束诱导的移动甚至可能严重到限制断层扫描重建过程中所达到的分辨率。尽管只能不可预测地获得非常高质量的图像，但这一事实表明，有可能设计出新的标本制备和/或数据收集技术，至少部分地克服电子束诱导的移动。如果是这样，对图像平均处理的需求将相应减少。
