Modulation transfer function

The modulation transfer function (MTF) describes the ability of a detector to retain the contrast (signal amplitude) of an object. It also measures how much spatial frequency is transferred from the object to an image ⁸. This is because each detector has its own limiting spatial resolution. Such limiting spatial resolution causes loss in contrast on a resulting image. As the object's spatial frequency increases, the signal amplitude detected by an imaging system goes to zero ¹⁰. The limiting spatial frequencies of two imaging systems can be compared by arbitrarily choosing an MTF of 10% ⁹.

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 enables full retention of the original spatial frequency of the object with high signal retention, thus showing high MTF conversion.

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 spread of light in the screen

• screens have a lower MTF than film

• fast screen-films have lower MTF than slower screen-film