Modulation Transfer Function (MTF)

A range of radiographic image brightness levels (Grayscale) is displayed due to differences in intensity of radiation crossing the body tissue. Any anatomic detail is best displayed when there is a high contrast between the object's brightness level and the tissue level near it. 

Low spatial frequency object can be seen more easily. As the size of the object decreases, its spatial frequency increases, so it becomes more difficult to see. 

Modulation transfer function (MTF) is the ability of an imaging system to display the contrast of anatomic object of different size. 

Its value is between 0 (no diffrence in brightness level) and 1.0 (maximum diffrence in brightness level).  

$$MTF = (Maximum intensity-Minimum intensity) / (Maximum intensity + Minimum intensity)

$$MTF = 1

Here the difference between maximum and minimum brightness is maximum (100% diffrence). MTF = 1 can be easily obtained with Larger object (low spatial frequency). 

$$MTF = 0

$There\; is\; no\; difference\; between\; maximum\; and\; minimum\; brightness.$ Smaller object (High spatial frequency) is difficult to display.  

Therefore, the MTF value of most digital imaging systems is less than 1.0.