[3dem] [ccpem] on increasing EM map resolution by modifying the b-factor level

[Marin van Heel]

Dear Smith Liu

Solving a 3D reconstruction problem is an optimization problem over a 
relevant resolution range ('RRR' :) ): finding the best possible 3D 
reconstruction, given a set of noisy molecular images. The very low 
frequency information in the cryo-EM images are not relevant to the 3D 
structure and can be removed by a high-pass filter. Similarly, the very 
high frequency information may be associated with noise only and may 
interfere with the structure determination; this can be suppressed by a 
low-pass filter. One can thus concentrate the analysis on the RRR by 
band-pass filtering the input data.  However, even within the RRR, the 
power distribution associated with the structural information we are 
after is not distributed homogeneously. Typically the low-frequency side 
of the structural information is over-represented compared to the 
high-frequency side of things due to all kinds of experimental reasons. 
This effect is described by an ad-hoc Gaussian low-pass filter called 
"the B-factor". Compensating for an estimated B-factor implies boosting 
the high frequency data components with respect to the low-frequency 
data components within the RRR. In any refinement/optimization of the 
structure, after this inverse B-factor filter, the relative influence of 
the high frequency components increases, which, in turn, results in a 3D 
reconstruction solution that is more focused on the fine structural details.

How to make use of this knowledge? That depends on the software being 
used and above all on the level of understanding that the user has over 
what is going on in the processing.

Hope this helps,

Marin van Heel


On 24/05/2015 07:15, Smith Liu wrote:
> Dear All,
> It was said the resolution of the EM map can be improved by modifying 
> the b-factor level. Will you please tell me the reason behind it and 
> how to process it?
> Smith
>
>


-- 
================================================================

     Prof Dr Ir Marin van Heel

     Professor of Cryo-EM Data Processing

     Leiden University
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     Imperial College London
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