7.1. Some Important Terminologies and Quantities 439
the overall effect of not only quantum efficiency but also of resolution and noise on
the system. In essence, it gives a measure of the overall system performance and
how well it produces a quality image.
For imaging systems,DQEis a function of spatial frequency and is therefore
sometimes referred to asspatialdetective quantum efficiency. Mathematically, it is
defined as
DQE(f)=SNR^2 out(f)
SNR^2 in(f). (7.1.23)
whereSNRoutis the signal-to-noise ratio in the output image andSNRinrepresents
the signal-to-noise ratio of the incident radiation, which can be written as
SNRin =N 0
σN=N 0
√
N 0
=
√
N 0
⇒SNR^2 in = N 0. (7.1.24)Hence the detective quantum efficiency can be evaluated from
DQE(f)=SNR^2 out(f)
N 0. (7.1.25)
DQE(f) is a frequency dependent parameter and is perhaps the most widely
used one for imaging detectors.
7.1.C Sensitivity.............................
Sensitivity of a position sensitive detector or an imaging device characterizes the
minimum signal that can be faithfully detected and measured.
7.1.D DynamicRange..........................
Dynamic range gives an idea of the range of the signal that the detector is able to
cover. It is defined by
D=Nmax
Nmin, (7.1.26)
whereNmaxandNmin represent the maximum and minimum detectable signals
respectively.
7.1.E Uniformity
Most imaging systems are two dimensional devices consisting of an array of pixels.
Each of these pixels can be thought of as an individual detector having its own
sensitivity, gain, and quantum efficiency. However if the material of all these pixels is
the same, as is normally the case, then the variations in these parameters is not very
large and the offsets can be corrected in the data acquisition and analysis software.
Large variations, on the other hand, are not acceptable in practical systems as they