Modulation Transfer Function¶
The modulation transfer function (MTF) is used in CBCT and planar imaging metrics to describe high-contrast characteristics of the imaging system. An excellent introduction is here. In pylinac, MTF is calculated using equation 3 of the above reference, which is also the Michelson contrast definition.
Then, all the contrasts are normalized to the largest one, resulting in a normalized MTF or rMTF (relative). Pylinac only reports rMTF values. This is the first of two inputs. The other is the line pair spacing. The spacing is usually provided by the phantom manufacturer. The rMTF is the plotted against the line pair/mm values. Also from this data the MTF at a certain percentage (e.g. 50%) can be determined in units of lp/mm.
However, it’s important to know what