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Dear All,<br>
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<p class="MsoNormal">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. <span
style="mso-spacerun:yes"> </span>So allow me to go over some
basic principles:</p>
<p class="MsoNormal">1)<span style="mso-spacerun:yes"> </span>The
<i style="mso-bidi-font-style:normal">instrumental resolution</i>
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.<span style="mso-spacerun:yes"> </span>The
classical case would be that of a light microscope where the
numerical aperture (NA) of the objective lens (<a
href="https://en.wikipedia.org/wiki/Numerical_aperture"
moz-do-not-send="true">https://en.wikipedia.org/wiki/Numerical_aperture</a>)
determines the “instrumental resolution” of the microscope (<a
href="https://en.wikipedia.org/wiki/Angular_resolution"
moz-do-not-send="true">https://en.wikipedia.org/wiki/Angular_resolution</a>).
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.</p>
<p class="MsoNormal">2) The <i style="mso-bidi-font-style:normal">resolution</i>
achieved in the <i style="mso-bidi-font-style:normal">results,</i><i>
</i>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?</p>
<p class="MsoNormal">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 (<a
href="https://en.wikipedia.org/wiki/Fourier_shell_correlation%29"
moz-do-not-send="true">https://en.wikipedia.org/wiki/Fourier_shell_correlation)</a>.
</p>
<p class="MsoNormal">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.<span style="mso-spacerun:yes"> </span>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. <span
style="mso-spacerun:yes"> </span>I never understood why it
took everybody so long to adapt to this straightforward
gold-standard metric.</p>
<p class="MsoNormal">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
…!</p>
<p class="MsoNormal">My TWO cents,</p>
<p class="MsoNormal">Marin</p>
<pre class="moz-signature" cols="72">--
==============================================================
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
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Emeritus Professor of Cryo-EM Data Processing
Leiden University
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Emeritus Professor of Structural Biology
Imperial College London
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