[3dem] Instrumental Resolution versus Results Resolution
Marin van Heel
marin.vanheel at googlemail.com
Sat Jun 24 01:50:44 PDT 2017
Dear All,
From the lectures and discussions at the recent 3DEM GRC (Les
Diablerets, 2017) I noticed there are still huge misconceptions – even
among distinguished professors in Physics and Biology - on what
“resolution” means. So allow me to go over some basic principles:
1)The /instrumental resolution/ of an imaging device is given by the
physical properties of the microscope, telescope, of whatever your
favorite 1D-, 2D-, 3D-, 4D-imaging device is.The classical case would be
that of a light microscope where the numerical aperture (NA) of the
objective lens (https://en.wikipedia.org/wiki/Numerical_aperture)
determines the “instrumental resolution” of the microscope
(https://en.wikipedia.org/wiki/Angular_resolution). In the case of a
diffraction-limited telescope it is the diameter of the main lens that
determines the instrumental resolution. In the old days of Electron
Microscopy one would often see the first zero of the CTF being used to
define its instrumental resolution.
2) The /resolution/ achieved in the /results,///from images produced by
our imaging device, is a very different issue! Suppose, for example, you
forget to switch on the illumination of your light microscope! What good
will then the high-resolution (high NA) properties of your expensive
instrument do you? If, on the other hand, you do switch on the
illumination but only use a very low dose of say 10,000 photons to
generate an image, that image will be very noisy. How much better will
the image of your object be if the image is instead created accumulating
10,000,000,000 photons? The underlying question is: how do I define a
results-related quality metric that reflects the image information I
have collected in an experiment rather than what a specific instrument
can potentially collect?
The basic idea here is to take TWO images of the same object rather than
just ONE. Both images will contain the same signal (the object of your
affection) but a different realization of the random noise so we can
then compare the two images to each other in Fourier space using the FRC
(Fourier Ring Correlation). This suggestion first emerged in
single-particle EM in the early 1980s
(https://en.wikipedia.org/wiki/Fourier_shell_correlation)
<https://en.wikipedia.org/wiki/Fourier_shell_correlation%29>.
Strangely enough it took decades for the rest of the imaging scientists
to realize what they were missing. Only very recently “everybody”
suddenly started using the results-oriented FRC and FSC metrics in many
other imaging fields, including X-ray microscopy, X-ray crystallography,
light-microscopy, X-ray tomography, scanning microscopy, astronomical
imaging, etc.Instead of claiming “super resolution” by showing some nice
images from a given microscope, one can now objectively underpin that
claim through the experimental FRC/FSC curve. I never understood why it
took everybody so long to adapt to this straightforward gold-standard
metric.
Take home lesson: there is the “INSTRUMENTAL RESOLUTION” that the
imaging instrument has intrinsically, whether you actually use it or
just leave it in the cupboard, and there is the statistically
significant “RESULTS RESOLUTION” which reflects the quality of the final
results achieved within a given data collection experiment. TWO very
different concepts …!
My TWO cents,
Marin
--
==============================================================
Prof Dr Ir Marin van Heel
Research Professor
Laboratório Nacional de Nanotecnologia - LNNano
CNPEM/LNNano, Campinas, Brazil
Brazilian mobile phone +55-19-981809332
(041-19-981809332 TIM)
Skype: Marin.van.Heel
email: marin.vanheel(A_T)gmail.com
and: mvh.office(A_T)gmail.com
----------------------------------------------
Emeritus Professor of Cryo-EM Data Processing
Leiden University
----------------------------------------------
Emeritus Professor of Structural Biology
Imperial College London
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