Contrast transfer function

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Typical contrast transfer function observed from an electron micrograph

The contrast transfer function[1][2][3] is the equivalent of the optical transfer function in light that affects images collected in a transmission electron microscope. The contrast transfer function must be corrected in the images in order to obtain high resolution structures in three-dimensional electron microscopy, especially cryo-electron microscopy.

The oscillations of contrast transfer functions have the form (not including the envelope function):


\operatorname{CTF}(\vec{s}) \; = \sqrt{1 - A^2 \,} \cdot \sin{ \left( \gamma(\vec{s}) \right)} \, + \, A \cdot \cos{ \left( \gamma(\vec{s}) \right)}

where A is the amplitude contrast.[4] The amplitude contrast term can be converted into a phase shift, using the linear combination trigonometry rule:


\operatorname{CTF}(\vec{s}) \; = \sqrt{1 - A^2 \,} \cdot \sin{ \left( \gamma(\vec{s}) \right)} \, + \, A \cdot \cos{ \left( \gamma(\vec{s}) \right)} \; = \; \sin{ \left( \gamma(\vec{s}) + \varphi \right)}

where \varphi = \arcsin{(A)}. The function \gamma(\vec{s}) is defined as:


\gamma(\vec{s}) = \; \gamma(s, \theta) = \; -\frac{\pi}{2} \, C_s \, \lambda^3 \, s^4  \; + \; \pi \lambda \, z(\theta) \, s^2

where r is the radius from the center of the image, Cs is the spherical aberration, λ is the wavelength of the electron beam (usually converted from the potential difference voltage) and z is the amount of defocus (using the convention that underfocus is negative and overfocus is positive)[4][5]

Furthermore, if the CTF is astigmatic, the defocus becomes a function of the angle θ where the astigmatic angle, θast given by:[6][7]


z(\theta) \; = \; z_{\mathrm{avg}} + \frac{z_{\mathrm{diff}}}{2} \cos{\left( 2(\theta - \theta_{\mathrm{ast}}) \right)} \;
 = \; z_1 \!\cdot\! \cos^2{\left( \theta - \theta_{\mathrm{ast}} \right)} \; + \; z_2 \!\cdot\! \sin^2{\left( \theta - \theta_{\mathrm{ast}} \right)}

where z_{\mathrm{avg}} = \frac{z_1 + z_2}{2} is the average defocus and z_{\mathrm{diff}} = z_1 - z_2 is the difference between the maximal and minimal defocus in the CTF. Where the defocal difference is defined such that:

\left| z_2 \right| > \left| z_1 \right| \; or \; \frac{z_2}{z_1} > 1

See also[edit]

References[edit]

  1. ^ Spence, John C. H. (1988 2nd ed) Experimental high-resolution electron microscopy (Oxford U. Press, NY) ISBN 0195054059.
  2. ^ Ludwig Reimer (1997 4th ed) Transmission electron microscopy: Physics of image formation and microanalysis (Springer, Berlin) preview.
  3. ^ Earl J. Kirkland (1998) Advanced computing in electron microscopy (Plenum Press, NY).
  4. ^ a b Malick, S.P. (2005). "ACE: Automated CTF Estimation". Ultramicroscopy 104 (1): 8–29. doi:10.1016/j.ultramic.2005.02.004. 
  5. ^ Maxim V. Sidorov. "What Is CTF (Contrast Transfer Function)?". ctfExplorer. Retrieved July 29, 2011. 
  6. ^ Mindell, J. A.; Grigorieff, N. (2003). "Accurate determination of local defocus and specimen tilt in electron microscopy". Journal of structural biology 142 (3): 334–347. PMID 12781660.  edit
  7. ^  edit