Modulation transfer function

Last revised by Dr Muhammad Jafar on 01 Apr 2022

The modulation transfer function (MTF) is the spatial frequency response of an imaging system or a component. It is the contrast at a given spatial frequency relative to low frequencies.

MTF = output signal amplitude / input signal amplitude

 

Discussion

On the radiogram, objects having different sizes and opacity are displayed with different gray-scale values. MTF is responsible for converting contrast values of different-sized objects (object contrast) into contrast intensity levels in the image (image contrast). For general imaging, the relevant details are in a range between 0 and 2 cycles/mm, which demands high MTF values.

In summary, MTF is the capacity of the detector to transfer the modulation of the input signal at a given spatial frequency to its output. 

MTF is a useful measure of true or effective resolution, since it accounts for the amount of blur and contrast over a range of spatial frequencies. 

At higher spatial  frequency MTF falls towards 0 , this corresponds to the poor visibility of the small structures. Conversely, at lower spatial frequency MTF is closer to 1 and it represents the ability to clearly visualize large structures.

MTF would be affected at a given spatial frequency as follows:

  • Movement unsharpness will degrade the MTF
  • Increasing the size of the focal spot will degrade the MTF (due to geometric reasons)
  • Magnification will degrade the MTF (due to geometric reasons)
  • Screens degrade the MTF because of the speed of light in the screen
  • Screens have a lower MTF than film
  • Fast screen-films have lower MTF than slower screen-film

 

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