When the distribution of activity is such that spatial frequency increases,
the peaks and valleys come closer. When the peaks and valleys are too close,
the imaging device cannot delineate them, and the MTF tends to 0, yield-
ing the poorest spatial resolution of the system. The values of the MTF
between 0 and 1 give intermediate spatial resolutions. It is important to note
that small objects are better imaged at higher frequencies and large objects
at lower frequencies.
It has been demonstrated that the MTF is a normalized Fourier trans-
form of the LSF discussed previously. In practice, the source activity distri-
bution is assumed to be composed of line sources separated by infinitesimal
distances, and the MTFs are then calculated from the LSFs of all line
sources. The mathematical expression of these functions is quite complex
and can be found in reference physics books.
Plots of the MTFs against spatial frequencies are useful in determining
the overall spatial resolution of imaging devices and are presented in Figure
10.7 for three imaging systems. It is seen that, at very low frequencies (i.e.,
larger separation of sinusoidal cycles), the MTFs are almost unity for all
three systems; that is, they reproduce equally good images of the radiation
source. At higher frequencies (i.e., sinusoidal cycles are too close), system
A in Figure 10.7 gives the best resolution, followed in order by system B
and system C.
126 10. Performance Parameters of Gamma Cameras
Fig. 10.7. Plot of modulation transfer function (MTF) against spatial frequency.
System A gives better spatial resolution than systems B and C, and system B pro-
vides better resolution than system C.